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Science of the Total Environment 599–600 (2017) 581–596

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Evaluating the impact of irrigation on surface water – groundwater interaction and stream temperature in an agricultural watershed Hedeff I. Essaid a,⁎, Rodney R. Caldwell b a b

U.S. Geological Survey, 345 Middlefield Rd., MS496, Menlo Park, CA 94025, USA U.S. Geological Survey, 3162 Bozeman Ave., Helena, MT 59601, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Watershed hydrology and stream temperature are modified by irrigation. • Irrigation with surface water can enhance groundwater discharge to streams. • Groundwater (GW) withdrawal for irrigation can decrease GW discharge to streams. • GW discharge to streams moderates stream temperature fluctuations. • Irrigation practices can enhance or degrade instream fish habitat.

a r t i c l e

i n f o

Article history: Received 30 January 2017 Received in revised form 17 April 2017 Accepted 27 April 2017 Available online xxxx Editor: D. Barcelo Keywords: Irrigation Surface water-groundwater interaction Stream temperature Fish refugia

a b s t r a c t Changes in groundwater discharge to streams caused by irrigation practices can influence stream temperature. Observations along two currently flood-irrigated reaches in the 640-square-kilometer upper Smith River watershed, an important agricultural and recreational fishing area in west-central Montana, showed a downstream temperature decrease resulting from groundwater discharge to the stream. A watershed-scale coupled surface water and groundwater flow model was used to examine changes in streamflow, groundwater discharge to the stream and stream temperature resulting from irrigation practices. The upper Smith River watershed was used to develop the model framework including watershed climate, topography, hydrography, vegetation, soil properties and current irrigation practices. Model results were used to compare watershed streamflow, groundwater recharge, and groundwater discharge to the stream for three scenarios: natural, pre-irrigation conditions (PreIrr); current irrigation practices involving mainly stream diversion for flood and sprinkler irrigation (IrrCurrent); and a hypothetical scenario with only groundwater supplying sprinkler irrigation (IrrGW). Irrigation increased groundwater recharge relative to natural PreIrr conditions because not all applied water was removed by crop evapotranspiration. Groundwater storage and groundwater discharge to the stream increased relative to natural PreIrr conditions when the source of irrigation water was mainly stream diversion as in the IrrCurrent scenario. The hypothetical IrrGW scenario, in which groundwater withdrawals were the sole source of irrigation water, resulted in widespread lowering of the water table and associated decreases in groundwater storage and groundwater discharge to the stream. A mixing analysis using model predicted groundwater discharge along the reaches suggests that stream diversion and flood irrigation, represented in the IrrCurrent scenario, has led to cooling of stream temperatures relative to natural PreIrr conditions improving fish thermal

⁎ Corresponding author. E-mail addresses: [email protected] (H.I. Essaid), [email protected] (R.R. Caldwell).

http://dx.doi.org/10.1016/j.scitotenv.2017.04.205 0048-9697/Published by Elsevier B.V.

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habitat. However, the decrease in groundwater discharge in the IrrGW scenario resulting from large-scale groundwater withdrawal for irrigation led to warmer than natural stream temperatures and possible degradation of fish habitat. Published by Elsevier B.V.

1. Introduction There is increased appreciation for the fact that surface water and groundwater resources are interconnected and must be managed together to sustain watershed resources and ecosystem health (Barlow and Leake, 2012; Barthel and Banzhaf, 2016; Gorelick and Zheng, 2015; Water Education Foundation, 2015; Winter et al., 1998). Understanding how agricultural activities associated with irrigation modify natural watershed hydrology and potentially degrade water quality and stream habitat requires examining the dynamic interaction between surface water and groundwater flow. Natural watershed hydrology is modified by both the withdrawal and application of irrigation water. Surface water, groundwater, or a combination of both may be the source of water that is applied to the land surface using flood, sprinkler and/or drip irrigation methods. Surface-water diversions for irrigation are often accompanied by transmission losses along leaking canals and the use of less efficient irrigation methods such as flood irrigation resulting in increased groundwater recharge, rising water tables and consequently increased groundwater discharge to streams (Kendy and Bredehoeft, 2006; Knight et al., 2005; Scanlon et al., 2007). Groundwater withdrawal may deplete streamflow due to reduced groundwater discharge to streams and/or induced stream water infiltration caused by reduced groundwater levels (Bredehoeft, 1997, 2007, 2011; Bredehoeft and Kendy, 2008; Konikow and Leake, 2014; Scanlon et al., 2007; Wen and Chen, 2006). Evaluating watershed-scale hydrologic changes caused by irrigation practices is challenging given the strong coupling of the surface water and groundwater systems through streambed exchange and groundwater recharge; the limitations of available data; and the impacts of climate variability. Hydrologic models that couple surface water and groundwater systems are useful tools that can be used to gain insight into the impact of agriculture on watershed hydrology and water quality (Condon and Maxwell, 2014a; Ferguson and Maxwell, 2011; Foglia et al., 2013; Konikow and Bredehoeft, 1974; Leng et al., 2015; Tian et al., 2015; Young and Bredehoeft, 1972). Agricultural activities associated with irrigation can impact instream habitat through changes in flow regime and stream temperature, both critical parameters for fish health (Caissie, 2006; Poff et al., 1997; Wenger et al., 2011). Salmon and trout are sensitive to temperature and their abundance has been found to decrease in stream reaches with elevated temperatures (Dunham et al., 2003a, 2003b; Ebersole et al., 2001). These salmonid fish may survive warm summer stream temperatures by congregating in relatively cold thermal refugia in streams (Ebersole et al., 2001; Kurylyk et al., 2015; Torgersen et al., 1999). Discharge of relatively cool groundwater to streams is recognized as a factor contributing to dampening of diurnal stream temperature variation, reduction in summer maximum stream temperatures; and development of thermal refugia for fish (Arora et al., 2016; Garner et al., 2014; Kurylyk et al., 2015; Luce et al., 2014; Mayer, 2012). Consequently, stream diversions and/or changes in groundwater discharge to streams induced by groundwater withdrawal and/or application of irrigation water can influence streamflow and stream temperature regimes (Condon and Maxwell, 2014b). Decreased summer streamflow and/or irrigation return flow from surface runoff can increase stream temperature (Tate et al., 2005; Wu et al., 2012). Increased groundwater discharge to the stream induced by the application of excess irrigation water can decrease warm-season stream temperature (Boyd et al., 2012; Stringham et al., 1998). Conversely, decreased groundwater discharge to the stream can increase warm-season stream temperature (Loinaz et al., 2013; Risley et al., 2010). Thus, an aspect of understanding

the potential impact of agricultural activities on stream temperature and fish habitat is characterization of the change in surface watergroundwater interaction induced by irrigation practices. Observations in the upper Smith River watershed (Caldwell and Eddy-Miller, 2013; Nilges and Caldwell, 2012), an irrigated agricultural and recreational fishing area in west-central Montana (Fig. 1), are combined with coupled surface water – groundwater flow modeling to investigate the influence of irrigation-induced hydrologic changes on stream temperature and streamflow. Observed upstream water temperature at SF1 on the South Fork Smith River is close to observed downstream temperature at the Smith River below Newlan Creek U. S. Geological Survey (USGS) gage (Fig. 2a) in the late summer and fall despite the greater than twenty-fold difference in streamflow at the two sites (Fig. 2b). This suggests that stream temperature is primarily determined by air temperature and climate conditions during this period. However, downstream temperatures at the USGS gage are cooler than upstream temperatures at SF1 during late spring and early summer. We hypothesize that this downstream cooling is primarily caused by the influx of cooler groundwater to the stream and that the magnitude of groundwater discharge and stream temperature cooling is affected by irrigation practices. Identifying and characterizing the influence of irrigation practices is important for sustainable management of stream fisheries located in agricultural regions. 2. Methods 2.1. The upper Smith River, Montana, watershed The watershed of the Smith River, a tributary of the Missouri River located in west-central Montana (Fig. 1), is valued for its agricultural lands, recreational uses and trout fishing. The drainage has been managed as a wild trout fishery since 1974 although trout were introduced in the past (Montana Fish, Wildlife and Parks, 2016). Brook and cutthroat trout occur in the steeper, higher elevation streams; brown and rainbow trout occur in the lower gradient and lower elevation stream reaches (Montana Fish, Wildlife and Parks, 2016). Studies of brown and rainbow trout movement suggest that the upper reach of the Smith River, downstream from the confluence of the North and South Forks, encompasses good trout spawning and habitat refuge sites (Grisak, 2010; Grisak et al., 2012). Brown and brook trout spawn in the fall with eggs hatching the following spring (Montana Natural Heritage Program and Montana Fish, Wildlife and Parks, 2016a, 2016b); rainbow and cutthroat trout spawn in the spring (Montana Natural Heritage Program and Montana Fish, Wildlife and Parks, 2016c, 2016d). Challenges to fish health and survival in the Smith River may result from unfavorable spawning conditions due to poor substrate and/or elevated stream temperature, low summer streamflow, high summer stream temperatures, and winter freezing. Groundwater is generally cooler than stream water during the warm season and warmer than stream water during the cool season (Fig. 2c). Therefore, groundwater discharge to the stream has the potential to moderate stream temperature and reduce summer heating and winter freezing. The upper portion of the Smith River watershed, excluding Big Birch and Newlan Creek drainage areas, where the North Fork Smith River and South Fork Smith River meet (Fig. 1) was used as the hydrologic framework for this study and is henceforth referred to as the upper Smith River watershed. Streamflow, stream stage, groundwater levels, and groundwater and stream water temperature were monitored at

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Fig. 1. Map of the study area showing: weather stations, surface water – groundwater (SW-GW) monitoring sites, GW wells, the long-term USGS stream gage at Smith River below Newlan Creek, Montana, temporary stream gages on the North Fork, Big Birch and Newlan Creeks, and the modeled area of the upper Smith River watershed. The upper Smith watershed in this study excludes Big Birch and Newlan Creek drainages.

six surface water – groundwater monitoring sites (Fig. 1) in this watershed during the period from 2006 to 2010 (Nilges and Caldwell, 2012). Analysis of the data indicated that groundwater - surface water interactions in the watershed were spatially and temporally variable and influenced by irrigation practices. Streamflow was observed to decrease with downstream distance along the Smith River during periods of intense irrigation withdrawal in the spring but increase downstream in early summer possibly due to irrigation return flow (Caldwell and Eddy-Miller, 2013). In this study these data are combined with watershed modeling to develop a better understanding of how irrigation influences streamflow, groundwater discharge to streams and stream temperature. The upper Smith River watershed has a semi-arid climate with elevation-dependent temperature and precipitation. Average water year (WY) 2005 through 2010 annual precipitation and maximum and minimum temperature at the White Sulphur Springs 2 (WSS2) weather station (Fig. 1, elevation 1537 m) were 332 mm, 13.0 °C and − 1.5 °C, respectively (National Oceanic and Atmospheric Administration, 2015). Average annual precipitation and maximum and minimum temperature for the same period at the Deadman Creek (DC) station (Natural Resources Conservation Service, 2015a) located in the mountains just north of the upper Smith watershed (Fig. 1, elevation 1966 m) were 708 mm, 9.8 °C and −5.1 °C, respectively. Average WY 2005 through WY2010 annual runoff at the long-term U. S. Geological Survey (USGS) gage at Smith River below Newlan Creek near White Sulphur Springs, Montana, (Fig. 1) was 1.98 × 105 m3/d (National Water Information System, 2015). Streamflow in the upper Smith River watershed is modified by irrigation diversions and reservoir operations. Reservoir releases are used to augment flow in the summer and fall and reservoir storage is replenished by spring and early summer high flows. The largest reservoir in the upper Smith River watershed, Lake Sutherland (1.8 × 107 m3 capacity), is located in the upper North Fork Smith River

drainage (Fig. 1). All other reservoirs in the upper Smith River watershed were relatively small (b 6.2 × 105 m3 capacity) (Montana Natural Resource Information System, 2010). The most productive and utilized aquifers in the watershed are located in the valley lowlands and include permeable Quaternary alluvium overlying less permeable weakly-consolidated Tertiary sedimentary rock (Caldwell and Eddy-Miller, 2013). These units overlie older, relatively low permeability bedrock. The upper Smith River watershed area considered in this study encompasses the primary area of irrigated agriculture. Current irrigated areas are located predominantly in the lower North Fork and South Fork drainages and below their confluence (Fig. 1). Irrigated land is used primarily to produce hay, alfalfa and grain with some pasture. Flood irrigation in riparian areas occurs during late spring to early summer with some late summer/early fall irrigation if water is available. Sprinkler irrigation occurs throughout the late-spring through summer growing season with short pauses for harvest. Surface water diversions currently supply the majority of water for irrigation and occur via numerous small diversions along stream channels for riparian flood irrigation as well as a major diversion from the North Fork Smith River near its inflow to the modeled area. Potential changes in irrigation practices in the watershed such as shifts from flood irrigation to sprinkler irrigation or from surface water use to groundwater use have been considered (Montana Department of Natural Resources and Conservation, 2003). 2.2. Estimating the influence of groundwater discharge on stream temperature Stream water temperature is determined by solar, atmospheric, and topographic radiation, evaporation and back radiation from the stream surface, conduction to the adjacent streambed and air, vegetation shading, and groundwater inflow (Theurer et al., 1984). Comparison of observed stream temperatures at SF1, NF1 and the USGS gage with

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groundwater and stream temperatures can be developed: T SW out ¼ f GW in T GW þ f SW in T SW in

ð1Þ

where TSWout is the temperature of stream water flowing out of the reach, fGWin is the fraction of streamflow exiting the downstream end of the reach (Qout) attributed to net groundwater inflow along the reach (groundwater inflow minus groundwater loss), TGW is the temperature of groundwater measured in the streambed, fSWin is the fraction of Qout attributed to net surface water inflow to the reach (surface water inflow minus stream diversion), TSWin is the temperature of surface water entering the stream reach, and fGWin +fSWin =1. The influence of groundwater discharge on stream temperature was compared for two stream reaches in the upper Smith River watershed. The reaches extended from SF1 to SF3 (10 km) on the South Fork and from SR1 to the USGS gage (9.2 km) on the Smith River (Fig. 1) and were both located within areas currently subject to riparian flood irrigation. The fraction of groundwater inflow along the South Fork and Smith River reaches for the period from May 1, 2008 through December 1, 2008 was estimated by rearranging Eq. (1) and using observed daily average stream and groundwater temperatures (Caldwell and Eddy-Miller, 2013; Nilges and Caldwell, 2012) for each reach: f GW in ¼

 T SW out −T SW in  : T GW −T SW in

ð2Þ

Groundwater temperature measured in the streambed was used for TGW, stream temperature measured at the downstream end of the reach was used for TSWout, and stream temperature measured at the upstream end of the reach was used for TSWin. Successful estimation of fGWin requires a contrast in stream and groundwater temperature, therefore fGWin was only estimated on days when the difference in observed TSWin and TGW was greater than 2 °C. Finally, the estimated groundwater (QGWin) and surface water (QSWin) inflows to the reach were calculated as: Q GW in ¼ f GW in Q out   Q SW in ¼ 1−f GW in Q Fig. 2. (a) Observed maximum (Max) and minimum (Min) air temperatures and observed stream temperatures at SF1, NF1 and the USGS gage; (b) observed streamflow at SF1, NF1 and the USGS gage; (c) observed maximum and minimum air, South Fork Smith River reach upstream (SF1), downstream (SF3) and groundwater (GW) at SF3 (USGS site 463124110572301) temperatures; (d) South Fork Smith River reach observed streamflow at SF3, estimates of groundwater fraction of downstream flow, and surface water (SW) and groundwater inflow along the reach; (e) observed maximum and minimum air, Smith River reach upstream (SR1), downstream (USGS gage) and groundwater at SR1 (USGS site 463207110594802) temperatures; (f) Smith River reach observed streamflow at the USGS gage, estimates of groundwater fraction of downstream flow, and surface water and groundwater inflow along the reach.

maximum and minimum air temperatures indicates that stream temperatures in the watershed are generally close to the average daily temperature (Fig. 2a) regardless of the magnitude of streamflow (Fig. 2b) suggesting that the effects of radiation, evaporation and conduction on stream temperature are relatively uniform throughout the watershed. The only deviation from this occurs during the late spring and early summer when temperatures at NF1 and the USGS gage (both located downstream of irrigated areas) are cooler than upstream at SF1. Most riparian vegetation is grass and therefore there is little potential for cooling due to shading along the stream channel. Furthermore, the cooling occurs when streamflow is decreasing. Therefore, we have assumed that downstream change in stream temperature is primarily the result of groundwater inflow to the stream. Applying this assumption to a stream reach and assuming no storage of water or heat within the reach (i.e. net inflow = outflow) a simple mixing analysis based on

ð3aÞ

out

ð3bÞ

where the observed average daily streamflow at the downstream end of the reach was used for Qout. The analysis of field observations only reflects watershed conditions under current irrigation practices and cannot identify changes in groundwater discharge to the stream or changes in stream temperature resulting from agricultural activities. A generalized model of the upper Smith River watershed was constructed to simulate surface watergroundwater interactions for three hydrologic scenarios described in Section 2.3.4. Model-predicted estimates of fGWin and fSWin along two stream reaches from SF1 to SF3 and SR1 to the model outflow were used to predict stream temperature change along each reach for the three scenarios. Eq. (1) was applied to estimate TSWout for the two modeled reaches using estimates of fGWin and fSWin (= 1 − fGWin) from the model results and observed TGW and TSWin. Model estimates of fGW for each reach were obtained by summing the model-predicted groundwater discharges to stream segments within the reach and dividing it by the model-predicted streamflow out of the reach (Qout). The impact of irrigation practices on stream temperature was then examined by comparing and contrasting the predicted stream temperature change along each reach for the three scenarios. 2.3. Simulating hydrologic response to irrigation practices The USGS coupled surface water – groundwater flow model GSFLOW (Markstrom et al., 2008), an integration of the USGS Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water

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Fig. 3. Conceptual framework of the coupled surface water-groundwater flow model.

Flow Model (MODFLOW), was used to simulate watershed hydrologic changes caused by irrigation. Processes represented in this model include: rainfall, snowfall, and snowmelt; canopy interception and evapotranspiration; streamflow, overland runoff, interflow, and infiltration; soil-zone evapotranspiration; vertical unsaturated zone flow and evapotranspiration, and groundwater flow and evapotranspiration (Fig. 3). Irrigation water is supplied by either groundwater withdrawal from wells and/or diversion of streamflow and is applied at the land surface in the model as additional precipitation. GSFLOW iteratively couples daily overland flow and streamflow, interflow and infiltration in the near-surface soil zone, vertical flow through the unsaturated zone, and three-dimensional groundwater flow in the saturated zone. Climate data are used to compute daily precipitation, snowmelt and potential evapotranspiration throughout the watershed. Actual evapotranspiration occurs first from canopy interception and water stored in the soil zone but if demand is not satisfied then evapotranspiration can occur

from the unsaturated zone and saturated zone depending on plant root and water table depths. Overland flow and interflow are routed toward stream channels based on land surface slope. Exchange of water between the stream and groundwater is dependent on streambed properties and the difference between stream stage and groundwater head. The simulated watershed must be discretized into hydrologic response units for surface and soil zone computations, a finite-difference grid for groundwater flow computations, and a stream network for streamflow routing. Rather than developing a completely hypothetical watershed model, the upper Smith River watershed was used as the basis for formulating the climate, topography, hydrography, vegetation, and soil properties of the modeled watershed to ensure a realistic model framework. The model was not designed as a detailed calibration of the specific conditions in the upper Smith River watershed, but rather as a simplified representation of the system. Model parameters were selected to provide simulation results that qualitatively reproduced observed

Fig. 4. Map of the modeled area of the upper Smith River watershed showing: topography, sprinkler and flood irrigated areas with the source of irrigation water, surficial geology, the stream network, the North Fork inflow, the watershed outflow, IrrCurrent scenario stream diversion points, existing irrigation wells, and IrrGW scenario hypothetical irrigation wells.

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spatial and temporal trends in streamflow, baseflow and groundwater depth and, thus, relatively realistic estimates of groundwater discharge to the stream. 2.3.1. Framework of the coupled surface water-groundwater flow model The coupled surface water – groundwater flow model of the upper Smith River watershed focused on the area above the confluence with Big Birch and Newlan Creek encompassing the majority of irrigated agricultural land thus excluding the relatively undeveloped upper North Fork watershed area (Fig. 4). The modeled watershed area was 640 km2 with elevations ranging from 1476 m to 2570 m. Discretized 200 m by 200 m groundwater grid blocks with coincident, overlying 200 m by 200 m surface water hydrologic response units were used to represent the model domain resulting in a grid of 180 rows by 160 columns. The subsurface was discretized into 6 layers (two 50-m thick layers overlying four 100-m thick layers). Watershed hydrogeology was simplified to three hydrogeologic units (Table 1) including bedrock overlain by Tertiary sedimentary rock overlain by Quaternary alluvium (Fig. 3) and the extent of the surface outcrop of each unit was approximated (Fig. 4). The thickness of alluvium ranged from 0 m at the outer extent of its surface outcrop to a maximum of 50 m in the basin interior. The thickness of Tertiary sedimentary rock ranged from 0 m at the outer extent of its surface outcrop to a maximum of 200 m in the basin interior. The bedrock thickness ranged from a maximum of 500 m at the outer edge of its outcrop along the model boundary to a minimum of 250 m in the basin interior. The effective horizontal hydraulic conductivity in a grid block crossing two hydrogeologic units was estimated using the thickness-weighted mean of the hydraulic conductivities; the effective vertical hydraulic conductivity was estimated using the thickness weighted harmonic mean of the hydraulic conductivities. The modeled time period, water years 2005 through 2010, was simulated using 2192 daily stress periods. Observed daily maximum/minimum temperature, and precipitation records at the White Sulphur Springs 2 (WSS2) weather station (elevation 1537 m) were used with temperature and precipitation lapse rates derived using weather records from the Deadman Creek SNOTEL (DC) station (elevation 1966 m) to distribute elevation-dependent temperature and precipitation across the watershed. Potential evapotranspiration was calculated as a function of air temperature and solar radiation using the JensenHaise approach (Jensen et al., 1970) as implemented in GSFLOW (p. 43 in Markstrom et al., 2008). Soil zone flow processes included in the model were minimized so that the fewest number of parameters were needed to reproduce the general behavior of the watershed flow system. This was accomplished by representing all overland flow as Dunnian (no Hortonion runoff or preferential flow), using only a nonlinear interflow-routing coefficient (GSFLOW parameter slowcoef_sq) for interflow (no linear coefficient), and using a linear gravity-drainage coefficient (GSFLOW parameter ssr2gw_rate) for infiltration from the soil zone to groundwater. Initial estimates of soil zone and climate-related parameters were developed using standard recommended approaches (U.S. Geological Survey, 2015); parameters used in a PRMS model of the entire Smith River watershed (Chase et al., 2014); and common default parameter values of GSFLOW (Markstrom et al., 2008). A limited group of model parameters (Table 1) were adjusted by trial-and-error during model development until the simulation results qualitatively reproduced the observed magnitude and timing of peak streamflow, baseflow, groundwater depth, and the difference between groundwater head and stream stage in the upper Smith River watershed. Parameters controlling soil zone storage, infiltration from the soil zone to groundwater, and interflow to the stream varied with land-surface slope and SSURGO soil properties (Natural Resources Conservation Service, 2015b). The maximum amount of water held in the soil zone (GSFLOW parameter sat_threshold) was larger in zones having greater soil porosity (Fig. 5a). The soil zone gravity drainage parameter controlling infiltration (ssr2gw_rate) was larger in zones with larger soil saturated hydraulic conductivity (Fig. 5b). The coefficient

Table 1 Select model parameters. Model parameter Soil zone Maximum amount of water that can be stored in the soil zone = sat_threshold in GSFLOW ∝ porosity − water content at 15 bar Maximum amount of water that can be held in the soil zone by capillary forces = soil_moist_max in GSFLOW Linear interflow routing coefficient = slowcoef_lin in GSFLOW Non-linear interflow routing coefficient = slowcoef_sq in GSFLOW ∝ soil saturated hydraulic conductivity and land surface slope Linear gravity drainage coefficient = ssr2gw_rate in GSFLOW ∝ soil saturated hydraulic conductivity Infiltration rate for initial steady-state time step Subsurface zone Alluvium Unsaturated zone saturated hydraulic conductivity Thickness Saturated zone hydraulic conductivity Specific yield = unsaturated zone saturated water content Tertiary sedimentary rock Unsaturated zone saturated hydraulic conductivity Thickness Hydraulic conductivity Specific yield = unsaturated zone saturated water content Bedrock Unsaturated zone saturated hydraulic conductivity Thickness Hydraulic conductivity Specific yield = unsaturated zone saturated water content Specific storage Unsaturated zone Brooks-Corey exponent Unsaturated zone residual water content Evapotranspiration extinction depth = root depth

Evapotranspiration extinction water content = water content corresponding to initial steady state recharge rate Stream network Streambed hydraulic conductivity in exposed bedrock area Streambed hydraulic conductivity in exposed Tertiary sedimentary rock area Streambed hydraulic conductivity in exposed alluvium area Streambed thickness Streambed slope Unsaturated zone Brooks-Corey exponent below streambed Maximum stream channel width Streambed top elevation

Value or range 0.025–0.094 m/m2 (see Fig. 5a)

0.004 m/m2

1.0 × 10−6 /d 4.3 × 10−8–3.9 × 10−4 /md (see Fig. 5c)

3.0 × 10−8–0.051 m/d (see Fig. 5b) 3.0 × 10−5 m/d

0.02 m/d 0–50 m 10 m/d 0.2

0.004 m/d 0–200 m 0.5 m/d 0.1

0.0005 m/d 250–500 m 0.00004–0.004 m/d 0.01 1 × 10−5 m−1 1.5 0.0 Bare = 0.75 m, grass = 1.5 m, alfalfa = 2.5 m, shrubs = 4 m, trees = 10 m (see Fig. 5d) 0.0009–0.004

0.0005 m/d 0.05 m/d 0.15 m/d 0.1 m 0.5 × land surface slope 3 12 m Land surface elevation – 2 m

controlling the rate of interflow (slowcoef_sq) was larger in areas with greater land surface slope (Fig. 5c). Actual evapotranspiration depended on root depth and the availability of water in the soil zone, the unsaturated zone, and shallow groundwater. The modeled evapotranspiration extinction depth (Table 1) was specified based on the watershed vegetation distribution (Fig. 5d) determined from land use land cover information (Homer et al., 2007), the assumption that alfalfa was

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planted in sprinkler irrigated areas, and published plant root depths (Bauder, 2016b; Canadell et al., 1996; Jackson et al., 1996). Subsurface saturated and unsaturated zone properties were assigned based on hydrogeologic unit (Table 1) such that modeled groundwater recharge produced a distribution of groundwater depths that approximated observed depths and sustained baseflows that approximated late summer streamflows observed the surface water – groundwater monitoring sites. Hydraulic conductivity and specific yield of alluvium was greater than that of Tertiary sedimentary rock. Hydraulic conductivity and specific yield were the lowest in bedrock and bedrock hydraulic conductivity decreased with depth. The stream channel network (Fig. 4) was constructed by applying the flow accumulation tool of Arc Hydro (Maidment, 2002) and selecting the network of stream segments with flow accumulating from at least 20 km2 (500 grid blocks). Stream width and depth were allowed to vary with flow by using a 12-m wide stream cross-section that reproduced the range of observed changes in stream stage with streamflow. Streambed hydraulic conductivity affects the groundwater head below the streambed and thus the difference between groundwater head and stream stage. Its value was assigned according to the hydrogeologic unit underlying the stream channel and to qualitatively match the observed difference between groundwater head and stream stage at surface water – groundwater monitoring sites. 2.3.2. Watershed model stream inflow and outflow estimates The long-term record at the USGS gage was used in combination with short-term streamflow records at temporary gages to estimate

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streamflow entering (North Fork inflow) and exiting (model outflow) the modeled watershed area for the simulation period. Stream discharge measurements were available for the entire period of simulation (Fig. 6a) at the long-term USGS gage on Smith River below Newlan Creek (USGS site 06076560, Fig. 1) located downstream of the model outflow and just below the confluence of Big Birch and Newlan Creeks with the Smith River (Nilges and Caldwell, 2012; U. S. Geological Survey, 2014). Short-term streamflow records for part of water year (WY) 2010 (Nilges and Caldwell, 2012) were available at temporary gages (Fig. 1) on Big Birch Creek (USGS site 463357111031801) and Newlan Creek (USGS site 06076550). Observed Big Birch and Newlan Creek flows were subtracted from observed flow at the USGS gage to obtain observed model outflow (Fig. 6a). The summation of flow at four temporary gages in the upper North Fork watershed (Fig. 1, USGS sites 63638110460501, 463426110464801, 463438110512401, and 463340110501401) monitored during part of WY2010 (Nilges and Caldwell, 2012) was used to represent observed North Fork inflow to the model area from the upper North Fork watershed (Fig. 6b). Simple linear correlations were developed between short-term observed flows on the tributaries and concurrently observed flow at the USGS gage on the Smith River below Newlan Creek (Fig. 7). Estimates of streamflow at Big Birch Creek, Newlan Creek, and the North Fork inflow to the model area for the entire period of simulation from October 1, 2004 to October 1, 2010 were made using these correlations and the full record of streamflow at the USGS gage. Estimated observed streamflow under current irrigation practices at the model watershed

Fig. 5. Maps of select parameters and information used in the upper Smith River model: (a) sat_threshold = the maximum volume of water per unit area held in the soil zone; (b) ssr2gw_rate = linear coefficient used to compute gravity drainage from the soil zone to the unsaturated zone; (c) slowcoef_sq = non-linear flow-routing coefficient for interflow; and (d) the vegetation distribution.

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outflow (Fig. 6a) was obtained by subtracting Big Birch Creek and Newlan Creek estimated streamflow from observed streamflow at the long-term USGS Smith River gage. The linear relation between observed upper North Fork inflow and the USGS gage flow was developed using the period of observed upper North Fork stream inflow not influenced by upstream reservoir releases, i.e. measured before the observed summer streamflow increase on July 26, 2010 (Fig. 6b). The WY2005 to WY 2010 estimates of North Fork inflow calculated using this relation and the USGS gage streamflow record do not include the additional North Fork inflow resulting from reservoir release for irrigation but do include the reduced North Fork inflow resulting from reservoir storage replenishment. Estimated North Fork inflow for the entire simulation period under current irrigation practices (Fig. 6b) was calculated using the flow predicted by the linear relation plus estimates of reservoir releases based on irrigation requirements (described in Section 2.3.3). Estimated natural North Fork inflow (Fig. 6b) was calculated using the streamflow predicted by the linear relation plus estimates of reservoir storage replenishment (described in Section 2.3.3). 2.3.3. Representation of current irrigation practices Irrigation practices currently implemented in the upper Smith River watershed were used to formulate the current irrigation scenario model (IrrCurrent). The exact amount and timing of irrigation, stream diversion, and groundwater withdrawal in the upper Smith River watershed are generally unknown and were simplified in the model (Fig. 4, Tables 2 and 3). Ten percent of the modeled watershed area experienced irrigation and small tributary-irrigated areas in upland regions were neglected. Areas of sprinkler and flood irrigation were first identified using Montana Department of Revenue (2014) land classification and flood irrigated areas in the river riparian zones were then expanded based on Google Earth (2015) images of the watershed. Examination of watershed images (Google Earth, 2015) also suggested that some flood and sprinkler irrigation on the north and northwest boundaries of the model watershed was supplied by canals from Newlan and Big Birch Creek outside of the model area (Fig. 4). All other irrigation water was supplied internally mainly by stream diversion with some groundwater withdrawal. In reality stream diversion for flood irrigation occurs throughout the stream network, however, modeled stream diversion was simplified and specified to occur at three locations (Fig. 4, Table 2): at the North Fork inflow, on the central reach of the South Fork, and on the Smith River below the confluence of the North Fork and South Fork. Stream diversion from the North Fork supplied both

Fig. 6. (a) Observed Smith River streamflow at the long-term Smith River USGS gage below Newlan Creek (USGS site 06076560), and observed and estimated streamflow at the model outflow; (b) observed North Fork Smith River inflow, estimated natural and IrrCurrent scenario North Fork Smith River inflow, estimated reservoir storage replenishment and reservoir release; (c) estimated IrrCurrent scenario North Fork Smith River, South Fork Smith River and Smith River stream diversions and groundwater (GW) withdrawal, and hypothetical groundwater withdrawal for the IrrGW scenario.

Fig. 7. Correlations between observed tributary streamflow at temporary gages and USGS gage streamflow at Smith River below Newlan used to estimate North Fork inflow and IrrCurrent scenario model outflow.

sprinkler and flood irrigation, while stream diversions on the South Fork and Smith River were only used for flood irrigation in adjacent riparian areas. Estimated model irrigation application rates were based on crop demand and available water supply. Transmission water losses along delivery canals were not represented in the model although the canals can be sources for local groundwater recharge. Sprinkler application rates were related to alfalfa daily water use in Montana which increases from 1.5 × 10−3 m/d in April to a maximum of 6.0 × 10−3 m/d in July followed by decrease into the fall (Bauder, 2016a). Alfalfa irrigation is generally suspended for a short period of time during mid-summer prior to the first alfalfa cutting (Caldwell and Eddy-Miller, 2013). Alfalfa water demand is partially met by natural recharge stored in the subsurface root zone and is supplemented with irrigation. For simplicity a constant sprinkler irrigation rate (Table 3) was used and the short halt in irrigation prior to harvest was neglected. Groundwater withdrawal for sprinkler irrigation (Fig. 6c) was specified to meet the irrigation requirements of sprinkler irrigated areas near existing irrigation wells (Table 3, Fig. 4). Stream diversion from the North Fork was the source of water for all other sprinkler irrigated areas (Fig. 4). North Fork streamflow was supplemented in the late summer by reservoir releases (Fig. 6b) to meet sprinkler irrigation demand. Depleted reservoir storage was replenished the following year by diversion of spring streamflow (Fig. 6b). Stream diversions for flood irrigation (Fig. 6c) were based mainly on surface water availability and flood irrigation application rates (Table 3). An iterative process was used to ensure that stream diversions did not cause drying of the stream at any time; flood irrigation and streamflow diversion were set to zero on days when irrigation resulted in zero streamflow. Stream diversions were represented in the model as negative inflow at diversion points located at the head of stream segments (Fig. 4), and irrigation applications were represented in the model as additional precipitation. Groundwater withdrawals occurred by pumping at existing irrigation well locations (Fig. 4). 2.3.4. Model scenarios Three different model scenarios (Table 2) were simulated: a natural pre-irrigation (PreIrr) scenario; a current watershed irrigation practices (IrrCurrent) scenario including flood and sprinkler irrigation supplied mainly by stream diversion with some groundwater withdrawal; and a hypothetical groundwater-sourced irrigation (IrrGW) scenario with sprinkler irrigation supplied by groundwater withdrawal. Observed streamflow and groundwater-level depths represent hydrologic conditions under current irrigation practices, therefore the IrrCurrent scenario was developed first to ensure that the simulated watershed captured observed spatial and temporal trends in peak streamflow, baseflow and

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Table 2 Model scenarios with type and source of irrigation and North Fork inflow estimate. Model scenario

Irrigated area (km2)

Irrigation type (% of area)

Source of irrigation water (% of area)

Flood

Sprinkler

Stream diversion

Groundwater withdrawal

Adjacent watersheds

0 56 North Fork12 South Fork13 Smith River 0

0 12

0 7

Natural Influenced by reservoir storage and release

100

0

Natural

PreIrr IrrCurrent

0 65.5

0 49

0 51

GWIrr

65.5

0

100

groundwater level. The IrrCurrent scenario was then used to develop the PreIrr scenario by eliminating irrigation practices and removing the influence of the upper North Fork reservoir by using estimated natural North Fork inflow. Finally, the hypothetical IrrGW scenario, in which all irrigation water was provided by groundwater withdrawal, was developed to contrast watershed response for primarily surface water-supplied (IrrCurrent) versus groundwater-supplied (IrrGW) irrigation. Hypothetical groundwater wells were distributed throughout the watershed (Fig. 4) and pumped to provide sufficient water for sprinkler irrigation of all of the irrigated area. It was assumed that no reservoir existed in the Upper North Fork watershed in the IrrGW scenario and natural North Fork inflow was used. Canopy interception and evapotranspiration extinction depths were kept the same for all scenarios to simplify comparison across scenarios; however, in reality differences between natural vegetation (grasses) and irrigated crops (hay and alfalfa) could potentially result in differences in evapotranspiration that were not represented in the models. 3. Results and discussion 3.1. Observed stream reach temperature change and estimated groundwater inflow Comparison of observed stream temperatures along the South Fork reach from SF1 to SF3 (Fig. 2c) indicates that stream temperature decreased downstream from SF1 to SF3 during the relatively warm season (May through September) and increased downstream during the cooler season (October and November). The assumption that downstream temperature changes result from groundwater discharge (Section 2.2) is supported by the fact that streambed groundwater temperatures were cooler than stream water during the warm season and warmer during the cool season (Fig. 2c). Furthermore, the measured streambed head difference (groundwater head − stream stage) was always positive at SF3 (Fig. 21 in Caldwell and Eddy-Miller (2013)) indicating that groundwater discharged to the reach throughout the observation

Table 3 Model irrigation application rates and scheduling. Irrigation type

Application rate (m3/d)

Irrigation schedule

Sprinkler supplied by North Fork diversion

4.3 × 10−3 (reduced by 75% in 2007 and 2008 because of low streamflow) 4.3 × 10−3

Mid May to early September (exact dates depend on timing of spring North Fork streamflow increase) May 16 through September 1

4.3 × 10−3

Diverted when SF1 flow N2.0 × 104 m3/d

4.0 × 10−3

Diverted when North Fork inflow N1.5 × 105 m3/d

4.0 × 10−3

Diverted when North Fork inflow N1.5 × 105 m3/d

Sprinkler supplied by groundwater withdrawal Flood supplied by South Fork diversion Flood supplied by North Fork diversion Flood supplied by Smith River diversion

North Fork inflow estimate

period. Comparison of observed stream temperatures along the Smith River reach from SR1 to the USGS gage (Fig. 2e) shows that downstream temperature change was less than on the South Fork reach and downstream cooling was mainly evident during June and July. The measured streambed head difference observed at SR2 (about half-way along the reach) was only positive from the beginning of June through the end of July 2008 (Fig. 26 in Caldwell and Eddy-Miller (2013)) corresponding to the period of greatest downstream cooling (Fig. 2e) suggesting that this was a period of increased groundwater discharge to the stream. This also corresponded to the period of flood irrigation in the riparian area along the reach suggesting that the increase in groundwater discharge could be a direct consequence of irrigation return flow. Estimates of the groundwater fraction of downstream flow and groundwater and surface water inflow along each reach were made using observed upstream, downstream and groundwater temperatures and Eqs. (2), (3a) and (3b) (Figs. 2d and f). Maximum groundwater inflow at the Smith River reach (2.1 × 105 m3/d) was 10 times greater than at the South Fork reach (2.0 × 104 m3/d). However, groundwater inflow was a greater fraction of total streamflow in the South Fork reach (average = 0.71) than in the Smith River reach (average = 0.12) and thus contributed to a greater change in downstream temperature in the South Fork. The South Fork groundwater fraction was relatively small at the beginning of May (Fig. 2d) but increased as the stream diversion and flood irrigation season began later in May with groundwater eventually becoming the sole source of streamflow late in the summer. Groundwater inflow was briefly suppressed on May 26, 2008 when streamflow increased and stream stage rose more rapidly than groundwater head. Groundwater inflow to the Smith River reach (Fig. 2f) was greatest during and shortly after the stream diversion and flood irrigation season (late May to early July) suggesting that its source was irrigation return flow. However, groundwater inflow to the stream also increased in late August possibly due to subsurface irrigation return flow from sprinkler irrigation. The observed warm-season downstream temperature cooling and estimates of groundwater inflow to the South Fork and Smith River stream reaches suggest that increased groundwater inflow can cool streams and some of the increase in groundwater inflow may be induced by subsurface irrigation return flow. Stream water diverted for flood irrigation infiltrates into the subsurface and cools as it passes through the ground. Subsurface irrigation return flow then discharges to the stream as groundwater resulting in downstream cooling of the reach. This groundwater inflow to a stream reach can potentially create cool temperature fish refugia as observed at SF3 (Fig. 2c). Conversely, during the cold season groundwater inflow to a stream reach can prevent the stream from freezing. Late November stream temperatures at SF3 (Fig. 2c) are above freezing due to the influx of relatively warm groundwater; however, freezing stream temperatures occur at SF1 (Fig. 2c), SR1 and the USGS gage (Fig. 2e). 3.2. Comparison of IrrCurrent scenario results with observations Results from the IrrCurrent scenario simulation are compared to observations to demonstrate that the generalized model captured the general hydrologic features of the irrigated watershed. Simulated IrrCurrent

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downstream and was greatest in the lower reaches of the Smith River below the confluence of the North and South Forks (SR2 and model outflow). Simulated IrrCurrent scenario depth to groundwater reproduced the trends in observed depth to groundwater (Fig. 9); groundwater depth was mainly controlled by topography and hydraulic properties. Simulated groundwater depth was shallowest in the relatively flat lowlands and upland riparian areas; simulated groundwater depth was greatest in the low permeability, high elevation areas. Observed and simulated groundwater depth also increased in the transition zone from low permeability bedrock to higher permeability Tertiary sedimentary rock. The qualitative correspondence between observed and simulated trends in streamflow and groundwater depth suggests that the model successfully captured the general temporal and spatial features of surface water and groundwater interaction under current irrigation practices.

3.3. Impact of irrigation practices on watershed hydrology

Fig. 8. Observed and IrrCurrent scenario simulated streamflow at the model outflow, SR2 (USGS site 06075850), NF1 (USGS site 6075700), SF3 (USGS site 06075785), SF2 (USGS site 6075780), and SF1 (USGS site 6075775).

scenario streamflow reproduced the general magnitude and timing of streamflow peaks and baseflow observed at multiple sites within the watershed (Fig. 8). Simulated and observed streamflow peaked in late spring as a result of snow melt and runoff. In general, simulated and observed streamflow along the South Fork (SF1, SF2 and SF3) was less than that of the North Fork (NF1) and late summer baseflow increased

The effect of stream diversion, groundwater withdrawal and irrigation on overall watershed hydrology can be illustrated by contrasting several components of the simulated watershed water balance including streamflow, groundwater discharge to the stream, groundwater recharge and the change in groundwater storage for the three modeled scenarios (Table 4, Fig. 10). All scenarios began with identical steadystate initial conditions corresponding to a specified recharge rate distribution and none of the simulations reached a dynamic steady state during the 6-year simulation period. Thus, the differences between the scenarios become greater and more apparent throughout the simulation period.

Fig. 9. IrrCurrent scenario simulated June 30, 2008 depth to groundwater (GW) below land surface with average groundwater depth observed in wells.

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Table 4 Select model water balance components for the 6-year simulation period and the percent change in the component relative to natural PreIrr conditions (GW = groundwater). Water balance component

PreIrr scenario m3

Water applied at land surface (precipitation + irrigation) Evapotranspiration Watershed stream outflowa Streamflow generated in the watershed (outflow − inflow) GW discharge to stream GW recharge Change in watershed GW volume a

1.65 1.55 3.52 7.34 7.10 1.02 2.41

× × × × × × ×

109 109 108 107 107 108 107

IrrCurrent scenario m3 1.79 1.58 2.84 1.06 9.65 1.92 5.27

× × × × × × ×

IrrGW scenario % change

109 109 108 108 107 108 107

8.2% 2.5% −19% 44% 36% 89% 120%

m3 1.82 × 1.59 × 3.19 × 4.09 × 5.70 × 1.90 × −2.15

109 109 108 107 107 108 × 107

% change 10% 3.1% −9.2% −44% −20% 87% −190%

Includes stream inflow from Upper North Fork watershed.

Comparison of total simulation period water balances for the three scenarios illustrates the overall hydrologic changes resulting from irrigation practices. Relative to the natural PreIrr scenario the IrrCurrent and IrrGW irrigation scenarios both resulted in an increase in water applied at the land surface (precipitation + irrigation), an increase in evapotranspiration, an increase in groundwater recharge and a decrease in streamflow out of the watershed for the total period of simulation (Table 4). The IrrCurrent scenario exhibited an increase in the amount of streamflow generated within the modeled watershed (North Fork inflow − watershed outflow), an increase in groundwater discharge to the stream, and a greater increase in groundwater stored in the watershed during the period of simulation relative to the natural PreIrr scenario. In contrast, the IrrGW scenario resulted in a decrease in streamflow generated within the modeled watershed, a decrease in groundwater discharge to the stream, and a decrease in groundwater storage during the period of simulation relative to the natural PreIrr scenario (Table 4). Comparison of time series of water balance components for the three scenarios illustrates the seasonal aspects of hydrologic changes resulting from irrigation practices. Simulated streamflow (Fig. 10a) peaked in the spring and early summer in all scenarios, however, the peaks were lowest for the IrrCurrent scenario because of the large stream diversions for flood and sprinkler irrigation. In all scenarios streamflow was lowest in the late summer and fall when flow was mainly sustained by groundwater discharge to the stream. The IrrGW scenario resulted in the lowest summer/fall streamflow reaching critically low values during drier years (e.g. summer 2007 and 2008 in Fig. 10a) because groundwater discharge to the stream decreased (Fig. 10b) due to pumping-induced lowering of the water table and capture of natural groundwater discharge. Conversely, fall and winter groundwater discharge to streams in the IrrCurrent scenario was greater than under PreIrr conditions (Fig. 10b) due to increased groundwater recharge (Fig. 10c) and subsurface irrigation return flow. Spring and early summer IrrCurrent scenario groundwater discharge to the streams was also greater than PreIrr conditions (Fig. 10b) due to the irrigationinduced increase in groundwater recharge and subsurface irrigation return flow as well as the increase in head difference between groundwater and surface water caused by stream stage lowering due to stream diversion. The majority of irrigation causing increased groundwater recharge in the IrrCurrent scenario came from the application of diverted streamflow with only a small fraction coming from groundwater withdrawal. This resulted in an increase in watershed groundwater storage compared to the natural PreIrr scenario (Fig. 10d). In the IrrGW scenario the irrigation-induced increase in groundwater recharge (Fig. 10c) was not sufficient to offset the water table drawdown due to groundwater withdrawal for irrigation and the net result was a decrease in groundwater storage relative to the natural PreIrr and IrrCurrent scenarios (Fig. 10d). The decrease was greatest during the irrigation season with partial recovery of lost groundwater storage by post-irrigation groundwater recharge. The impact of irrigation practices on spatial distributions of streamflow within the watershed is illustrated with June 30, 2008 snapshot-in-time images of simulated PreIrr streamflow (Fig. 11a) and

change in streamflow relative to the PreIrr scenario for the IrrCurrent (Fig. 11b) and IrrGW (Fig. 11c) scenarios. Simulated PreIrr streamflow (Fig. 11a) indicates that the North Fork contributes more than the South Fork to the model watershed outflow. North Fork flow was mainly generated in the higher elevation upper North Fork watershed entering the model area as North Fork inflow. PreIrr streamflow in the South Fork increased steadily downstream, and by the end of June only tributaries tapping the higher elevations within the model watershed had substantial streamflow. IrrCurrent and IrrGW irrigation resulted in a substantial change in streamflow relative to PreIrr (i.e. N 100 m3/d) along the lower reach of the South Fork and all of the North Fork and Smith River reaches within the model area (Fig. 11b and c). There was also a small impact on some lower tributary reaches near their confluence with the main stream channel. In the IrrCurrent scenario June 30, 2008 represents a time shortly after riparian flood irrigation by stream diversion has ended and sprinkler irrigation (mainly from stream diversion) has begun but before any supplemental reservoir releases have been made. IrrCurrent scenario North Fork and Smith River streamflow decreased relative to PreIrr streamflow (Fig. 11b) because of substantial stream diversion for irrigation. In contrast, IrrCurrent scenario streamflow on the South Fork increased relative to PreIrr streamflow due to irrigation return flow. A few tributaries also showed an increase in streamflow relative to PreIrr near their confluence with the main stem of the South Fork and Smith River. In the IrrGW scenario June 30,

Fig. 10. Simulated PreIrr, IrrCurrent and IrrGW scenario (a) streamflow at the model outflow, (b) groundwater (GW) discharge to the stream, (c) groundwater recharge, and (d) groundwater storage change relative to the PreIrr scenario (simulated scenario storage change minus PreIrr scenario storage change) for the upper Smith River modeled watershed area.

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Fig. 11. Simulated June 30, 2008 (a) PreIrr streamflow and (b) IrrCurrent and (c) IrrGW change in streamflow relative to PreIrr conditions; simulated June 30, 2008 (d) PreIrr groundwater discharge to streams and (e) IrrCurrent and (f) IrrGW change in groundwater discharge to streams relative to PreIrr conditions.

2008 represents a time shortly after sprinkler irrigation supplied by groundwater withdrawal has started. IrrGW scenario streamflow decreased relative to PreIrr conditions in all stream reaches impacted by irrigation (Fig. 11c) but on the North Fork and Smith River the decrease was less than in the IrrCurrent scenario because there was no stream diversion. The IrrGW scenario decrease in streamflow resulted from general lowering of the water table caused by groundwater withdrawal.

The impact of irrigation practices on the spatial distribution of surface water-groundwater interaction within the watershed is illustrated with June 30, 2008 snapshot-in-time images of simulated PreIrr groundwater discharge to the stream (Fig. 11d) and change in groundwater discharge relative to the PreIrr scenario for the IrrCurrent (Fig. 11e) and IrrGW (Fig. 11f) scenarios. Total watershed PreIrr groundwater discharge to the stream on June 30, 2008 was positive (Fig. 10b) indicating gaining stream conditions at the watershed scale. However, the

H.I. Essaid, R.R. CaldwellScience of the Total Environment 599–600 (2017) 581–596

spatial distribution of groundwater discharge (Fig. 11d) shows that some reaches of the stream network were gaining (positive groundwater discharge) and some were losing (negative groundwater discharge). The main stem of the South Fork was generally gaining and this groundwater discharge contributed to the downstream increase in streamflow along the South Fork (Fig. 11a). The North Fork and some tributaries lost water as they flowed out of the uplands with the North Fork and some tributaries gaining water near their confluence with the South Fork and Smith River in the lowlands. The Smith River reach below the confluence of the North and South Forks was gaining in its upper reaches and losing in its lower reaches where alluvial sediments thin. IrrCurrent and IrrGW irrigation resulted in a substantial change in groundwater discharge relative to PreIrr (i.e. N10 m3/d) along the lower reach of the South Fork, all of the North Fork and Smith River reaches within the model area, and some tributary reaches near their confluence with the main stream channel (Fig. 11e and f). Groundwater discharge to the stream increased relative to PreIrr conditions in almost all of the irrigation influenced stream reaches of the IrrCurrent scenario (Fig. 11e); there was a decrease in a few localized reaches influenced by irrigation wells. The increase in groundwater discharge was due to subsurface irrigation return flow and reduced stream stage resulting from stream diversion. Groundwater discharge to the stream in the IrrGW scenario decreased relative to PreIrr conditions in almost all of the irrigation influenced stream reaches (Fig. 11f). However, there were a few reaches where groundwater discharge increased because subsurface irrigation return flow offset water table drawdown caused by groundwaterwithdrawal. Although the models developed in this study were based on an abstraction of the upper Smith River watershed these model results suggest that current irrigation practices in the watershed have led to increased evapotranspiration, decreased streamflow, increased groundwater discharge to streams, and increased groundwater recharge and storage. Furthermore, model results suggest that increased development of groundwater resources in the watershed could potentially lead to decreased groundwater discharge to the stream and depletion of groundwater storage. This is because stream diversion and relatively inefficient riparian flood irrigation increase groundwater inflow to a stream reach. However, extensive groundwater withdrawal for more efficient sprinkler irrigation may reduce groundwater discharge to a stream reach. This impact is generally greatest when groundwater wells are located near the stream and decreases as wells are placed farther away from the stream (Kendy and Bredehoeft, 2006). 3.4. Model-predicted groundwater inflow and downstream temperature change Model-predicted groundwater discharges to the stream were used to calculate downstream temperature change resulting from groundwater inflow along the South Fork from SF1 to SF3 and the Smith River from SR1 to the model outflow for the PreIrr, IrrCurrent and IrrGW scenarios (Fig. 12). Model results indicated that tributary flow into the stream reaches was generated mainly by groundwater inflow into the tributaries near their confluence with the main channel (see Fig. 11d, e and f). Thus, these tributary inflows were added to modelpredicted groundwater discharges into the main channel of the reach to calculate total groundwater discharge into the reach. The IrrCurrent model-predicted groundwater flow fractions (model groundwater discharge to reach/model streamflow out of reach) for the South Fork (average = 0.63) and Smith River (average = 0.11) stream reaches (Fig. 12c and d) were smaller than the estimated groundwater flow fractions calculated from temperature data (average = 0.71 and 0.12, respectively) although they displayed similar temporal trends (compare Fig. 2d and f with Fig. 12c and d). This may indicate that diversion and irrigation applied in the IrrCurrent model under-estimated that of the real system. Furthermore, the Smith River temperature data estimate was for the reach from SR1 to the USGS gage whereas the model-predicted estimate

593

only extended from SR1 to the model outflow and did not include groundwater contributions from the Big Birch and Newlan Creek drainages. Despite the differences in estimated magnitude of groundwater fraction the IrrCurrent model estimates, like the observed temperature data estimates, indicate that groundwater inflow was a larger fraction of streamflow in the South Fork reach than the Smith River reach and increased during and shortly after the flood irrigation season (late May to mid-July). The natural PreIrr model-predicted groundwater flow fractions for the South Fork (average = 0.51) and Smith River (average = 0.08) stream reaches (Fig. 12a and b) were smaller than in the IrrCurrent model indicating that irrigation has resulted in an increase in the fraction of groundwater discharged to the stream especially during the flood irrigation season. The IrrGW model-predicted groundwater flow fractions for the South Fork (average = 0.10) and Smith River (average = 0.03) stream reaches (Fig. 12e and f) were smaller than in the natural PreIrr model indicating that groundwater withdrawal for irrigation resulted in an decrease in the fraction of groundwater that discharged to the stream and by late summer the stream reaches lost water to subsurface infiltration (fGW =0). Downstream temperatures at SF3 and the model outflow (Fig. 12g and h) were estimated for the three model scenarios using model-estimated reach groundwater flow fractions (Fig. 12a, b, c, d, e, f), average daily observed stream temperatures at SF1 and SR1 (Fig. 2c and e), and daily average groundwater temperatures at SF3 and SR1 (Fig. 2c and e) in Eq. (1). The differences between IrrCurrent and observed downstream temperatures reflects differences in estimated (Fig. 2d and f) and modeled (Fig. 12c and d) groundwater flow fractions. The natural PreIrr scenario predicted most of the observed late summer downstream cooling and fall downstream warming along the South Fork reach (Fig. 12g), but little of the early summer downstream cooling along both the South Fork and Smith River reaches (Fig. 12h). The observed early summer downstream cooling was reproduced in the stream temperature estimates from the IrrCurrent scenario suggesting that the cooling resulted from irrigation practices. The IrrCurrent maximum increase in downstream cooling relative to the PreIrr scenario was 7.1 °C on June 21, 2008 in the South Fork reach (Fig. 12g) and 1.9 °C on July 7, 2008 in the Smith River reach (Fig. 12h). Predicted downstream temperatures for the IrrGW scenario in the summer were generally warmer than under natural PreIrr conditions with a maximum warming of 7.5 °C on July 26, 2008 in the South Fork reach (Fig. 12g) and 1.1 °C on August 18, 2008 in the Smith River reach (Fig. 12h) caused by the reduction in groundwater inflow to the stream. During the cool season, starting October 6, 2008, IrrGW scenario downstream temperatures became generally cooler than under natural conditions with a maximum cooling of 4.3 °C on October 12, 2008 in the South Fork reach and 0.3 °C on October 12, 2008 in the Smith River reach. Upstream observed daily average temperatures at SF1 in the South Fork reach dropped to near or below freezing on November 21, 2008. Model results predict that inflow of warmer groundwater in the PreIrr and IrrCurrent scenarios kept the downstream end of the reach (SF3) above freezing, however, the lack of groundwater inflow in the IrrGW scenario resulted in freezing along the whole reach. 3.5. Implications of irrigation practices for instream fish habitat Sufficient streamflow and suitable stream water temperatures are among the necessities for a healthy fish habitat. This analysis has characterized the changes in both of these factors in response to stream diversion for flood and sprinkler irrigation and groundwater withdrawal for sprinkler irrigation. The IrrCurrent scenario, representing ongoing irrigation practices in the upper Smith River watershed, involved substantial stream diversion that reduced peak spring instream flows (Fig. 10a) but reservoir releases supported stream diversion during the summer and sustained summer streamflow. Model results suggest that these practices have generally maintained relatively healthy levels of streamflow for fish. The hypothetical IrrGW scenario that included

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Fig. 12. Model-based estimates of downstream flow, groundwater (GW) fraction of downstream flow and surface water (SW) and groundwater inflow along (a) the PreIrr South Fork Smith River reach from SF1 to SF3, (b) the PreIrr Smith River reach from SR1 to the model outflow, (c) the IrrCurrent South Fork Smith River reach, (d) the IrrCurrent Smith River reach, (e) the IrrGW South Fork Smith River reach, and the (f) the IrrGW Smith River reach; and observed upstream and mixing analysis predicted downstream temperatures for the PreIrr, IrrCurrent and IrrGW scenarios on the (g) South Fork Smith River reach and (h) Smith River reach.

groundwater withdrawal for sprinkler irrigation and no reservoir development did not reduce spring streamflow as much as the IrrCurrent scenario, however, it did result in very low streamflow during the late summer and fall (Fig. 10a, most notably in 2007 and 2008) that could lead to critical conditions for fish, especially those that spawn in the fall such as the brown and brook trout. Model results indicate that IrrCurrent scenario stream diversion and flood and sprinkler irrigation increased groundwater discharge to the stream (Fig. 10b) relative to natural PreIrr conditions, especially during the spring and early summer, and resulted in spring and early summer stream cooling (Fig. 12g and h) improving fish habitat for spawning and hatching. This spring/summer cooling was most pronounced in the South Fork reach and would be especially favorable for the health of brook and cutthroat trout that prefer upland higher elevation streams. On the other hand, groundwater withdrawal for sprinkler irrigation (IrrGW scenario) reduced groundwater discharge to the stream relative to natural PreIrr conditions (Figs. 10b, 12e and f) and resulted in spring and summer warming of the stream (Fig. 12g and h) causing a general degradation of fish habitat. Model results and stream temperature estimates also indicated that natural PreIrr and IrrCurrent scenario relatively warm groundwater discharge in the late fall moderated climatic cooling of the stream preventing freezing. In contrast, reduction in groundwater discharge in the IrrGW scenario led to freezing along the entire South Fork stream reach, a potentially critical degradation of

fish habitat especially for species such as brook and cutthroat trout that prefer higher elevation streams. 4. Conclusions Agricultural irrigation practices evolve with time in a watershed and surface water and groundwater resources are developed and managed in new ways. It can be difficult to predict the impacts of these changes on water availability and water quality. Coupled groundwater – surface water modeling was used in this study to investigate the change in streamflow, groundwater discharge to the stream and stream temperature in response to irrigation practices. Stream temperature fluctuations are moderated by groundwater discharge to streams, and therefore changes in groundwater discharge to the stream resulting from irrigation practices can impact stream temperature and consequently fish habitat. Hydrologic and stream temperature changes for an irrigation scenario dominated by diversion of stream water for sprinkler irrigation and riparian flood irrigation were contrasted with changes in an irrigation scenario restricted to only groundwater withdrawal for sprinkler irrigation. Results illustrate that relatively inefficient irrigation such as flood irrigation by stream water diversion can have beneficial outcomes such as increased groundwater discharge to the stream, resulting from subsurface irrigation return flow, and cooling of spring/summer stream temperatures resulting in improved fish habitat. However, more

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efficient irrigation practices that generate less subsurface irrigation return flow and/or lower water table levels due to groundwater withdrawal will result in habitat degradation because of reduced streamflow and increased stream temperature. The irrigation scenarios presented in this study do not represent the full range of potential irrigation management schemes or river conditions. Negative impacts of groundwater withdrawals can be mitigated by adjusting the timing and location of groundwater pumping (Bredehoeft, 2011; Bredehoeft and Kendy, 2008; Kendy and Bredehoeft, 2006). Wells can be located further from the stream or completed in deeper, confined aquifer units. If streamflow is dominated by large reservoir releases the temperature of water released from the dam can have a large impact on stream temperature (Risley et al., 2010). Agricultural activities that result in denudation of riparian vegetation and reduction in stream shading can lead to stream warming and habitat degradation regardless of changes in streamflow (Kurylyk et al., 2015). Likewise, climate change can impact stream temperature (Arora et al., 2016). Thus, effective watershed and ecosystem management in irrigated areas requires understanding the relative impacts of changes in surface water - groundwater interaction, climate, hydrography and vegetation.

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