Spatial and temporal variability of sediment and dissolved ... - Krishikosh

Catena 76 (2008) 27–35

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Catena j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a t e n a

Spatial and temporal variability of sediment and dissolved loads from two alpine watersheds of the Lesser Himalayas Omvir Singh a,⁎, Milap C. Sharma b, A. Sarangi c, Pratap Singh d a

Division of Environmental Sciences, IARI, Pusa Campus, New Delhi 110012, India Centre for the Study of Regional Development, Jawaharlal Nehru University, New Delhi 110067, India Water Technology Centre, IARI, Pusa Campus, New Delhi 110012, India d Hydro Tasmania Consulting, 12th Floor, Eros Corporate Tower, Nehru place, New Delhi 110019, India b c

a r t i c l e

i n f o

Article history: Received 17 January 2008 Received in revised form 7 August 2008 Accepted 21 August 2008 Keywords: Sediment transport Denudation rate Weathering Himalayan Watersheds

a b s t r a c t Estimation of sediment load from Himalayan basins is of considerable importance for the planning, designing, installation and operation of hydro-power projects, including management of reservoirs. In the present study, an assessment of physical and chemical load, sediment yield and erosion rate has been undertaken at eight different locations in the Sainj and Tirthan watersheds. The analysis revealed that the maximum load was transferred during the monsoon season. Moreover, the estimated average chemical erosion rate of the Sainj (83 t km− 2 yr− 1) and Tirthan (80 t km− 2 yr− 1) watersheds were higher than that of the Indian average (69 t km− 2 yr− 1) representing all the rivers. Both watersheds were eroding physically and chemically at a faster rate than that of the world global average erosion rate (185 t km− 2 yr− 1). The flattish nature of the channels in some segments of these watersheds showed a lower transport of sediments, where as the constricted segments having steep bed slopes increased the velocity of flow and the sediment transport rate. These findings have important implications for water resource management in the context of sediments mobilization, erosion, channel management, ecological functions and operation of the hydropower projects in the Lesser Himalayan region. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Pollution from non-point sources is being recognized as a major source of surface water quality deterioration. Sediment transportation from land surfaces to oceans through rivers is one of the most important processes that affect riverbank stabilization, soil formation, upliftment rates, biogeochemical cycling of elements, crust evolution and many other earth related processes (Chakrapani, 2005). The transport dynamics of sediments in fluvial systems have been studied by engineers, hydrologists, soil scientists, geologists and process geomorphologists in different parts of the world (Holeman, 1968; Subramanian, 1979, 1993; Milliman and Meade, 1983; Jha et al., 1988; Chakrapani and Subramanian, 1990, 1993; Lu and Siew, 2006; Soler et al., 2007; Vanacker et al., 2007). Presently, the rivers alone contribute to about 95% of sediments entering the oceans in a global scale (Syvitski, 2003). It is estimated that 65% of water and 80% of the sediments are being transported to oceans each year from Southern Asian, Oceania and north-eastern South American rivers (Syvitski et al., 2003). It is also reported that the Himalayan Rivers are the major ⁎ Corresponding author. Tel.: +91 11 25841490; fax: +91 11 25848037. E-mail addresses: [email protected], [email protected] (O. Singh), [email protected] (A. Sarangi). 0341-8162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2008.08.003

contributors, which transport about 50% of the global sediment flux (Stoddart, 1969). Therefore, research interests pertaining to fluvial sediment transport in the Himalayan river systems have attracted much attention in recent times (Kumar, 1987; Rawat and Rai 1997; Singh et al., 2003, 2005; Sharma and Rai, 2004; Haritashya et al., 2006; Singh, 2007; Bhattacharya et al., 2008). The estimation of the sediment load and the transport rate governs the geomorphological, hydrological, sedimentological and ecological processes of river basins. Further, its estimation from basins is of much significance for planning, designing, installation and operation of hydro-power projects, including management of reservoirs. However, there exist hardly any studies related to the transportation and quantification of sediment fluxes from the Lesser Himalayan watersheds, which are currently being developed for various hydro-power projects. Hence, in view of this research gap related to the study of sediment fluxes in the Lesser Himalayan watersheds, the present work was undertaken with an objective to collect and analyze the primary hydrologic and water quality data. The acquired data pertaining to quantity and quality of the sediment (physical) and dissolved (chemical) loads transported by two alpine watersheds were analyzed to understand the watershed hydrology and sedimentation processes. Such type of investigations assumes importance in preparation of the water and sediment load budget at

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O. Singh et al. / Catena 76 (2008) 27–35

Fig. 1. Delineated map of the Sainj and Tirthan watersheds with natural drainage network and sampling locations.

regional scale, which will assist in the development of a global sediment flow budget. 2. Materials and methods 2.1. Description of the study area The study area is delineated into two distinct watersheds named as Sainj (741 km2) and Tirthan (687 km2). Both watersheds fall under the left bank of Upper Beas river system in the Lesser Himalayan alpine zone. The area extends between latitudes 31°30′28″ and 31°55′02″ north and longitudes 77°13′02″ and 77°45′57″ east as shown in Fig. 1. The watersheds present a typical mosaic of moderate to high rugged topography with numerous mountain peaks over 4000 m above mean Table 1 Relationship between different land use classes and erosion rates in the Sainj and Tirthan watersheds Station

Erosion Different land use rates Snow Agricultural Degraded Forest Glaciers Hilly open (t km− 2 land forest scrub/rocky bound −1 yr ) outcrop

sea level (amsl). The average slope of the Sainj and Tirthan watersheds are 38.12° and 40.04° along with the mean elevation of 3510 and 2826 m amsl, respectively. The rock types were mainly of colluvium, alluvium, glacial deposits, phyllite, slate, quartzites, dolomites, sandstone, schist and granites. The soil texture varies from sandy loam to loam with average organic matter content of around 70%. Soils vary from very shallow to moderately deep in depth and pale yellow, yellowish brown and dark brown in colour. The climate of the watersheds is mostly warm temperate and receive an average annual rainfall of 1000 mm and more than 50% of which is received during the south-west monsoon (June–September). Average annual snowfall in the region is about 345 mm, which is confined to upper reaches and occurs only during the winter season Table 2 Distribution and subsidiary information of water sampling points in the Sainj and Tirthan watersheds Longitude

Sampling Area (km2) site elevation (m)

Major Name of watershed location

ID Latitude no.

Sainj

Niharni

1

31° 46′ 29″ 77° 22′ 90″ 1586

Suind Sainj Talara

2 3 4

31° 47′ 08″ 77° 19′ 92″ 1419 31° 46′ 19″ 77° 18′ 56″ 1251 31° 45′ 79″ 77° 15′ 95″ 1149

Larji 5 Bhatkanda 6 (Jiwa Nal) Khrongcha 7

31° 43′ 64″ 77° 13′ 60″ 997 31° 47′ 24″ 77° 19′ 81″ 1388

Larji

31° 43′ 48″ 77° 13′ 36″ 989

(%) Sainj at Niharni Suind Sainj Talara Larji Jiwa Nal

608.00 164.47 446.32 733.80 266.86 258.26

0.38 2.02 2.07 4.25 4.49 2.23

1.25 1.59 1.55 1.57 1.54 1.43

27.30 36.20 36.46 38.11 38.21 37.25

12.48 10.35 10.61 9.61 9.44 11.40

29.04 25.28 23.97 23.52 23.77 19.97

29.55 24.56 25.35 22.95 22.55 27.74 Tirthan

Tirthan at Khrongcha Larji

574.54 196.56

2.38 16.88

0.60 1.19

42.83 56.16

3.50 1.14

28.93 17.57

21.76 7.07

8

31° 39′ 55″ 77° 27′ 33″ 1786

Dominant vegetation type

410.65 Deodar and pine mixed 495.01 Pine 658.41 Pine 727.26 Scattered pine 740.02 Pine 163.40 Scattered pine 191.13 Mixed vegetation 686.36 Scattered pine

O. Singh et al. / Catena 76 (2008) 27–35 Table 3 Maximum and minimum daily discharge rates at different gauging sites during 2004 Watershed

Station

Maximum discharge rate (m3 s− 1)

Date

Minimum discharge rate (m3 s− 1)

Date

Sainj

Niharni Suind Sainj Talara Bhatkanda (Jiwa Nal) Larji Khrongcha Larji

71.9 63.8 41.1 101.3 106.4

June 29 August 8 August 8 August 8 August 8

6.3 8.4 3.6 12.3 14.5

February 4 January 14 January 13 January 13 December 29

117.8 38.5 141.6

August 8 September 11 August 8

15.3 2.1 7.4

December 29 January 28 January 16

Tirthan

29

Tirthan watersheds, respectively. Moreover, the land use information at different sampling sites corresponding to the delineated watershed area between two consecutive gauging sites is presented in Table 1. Besides, there was a growth of 27.2% in human and 47.4% in livestock population envisaged during a period from 1981 to 2001 in these watersheds. Also, 25–30% encroachment of agricultural activities in the forest land coupled with enhanced deforestation led to the removal of 119,438.4 m3 of timber during a period from 1987 to 2005. The construction of 55 km of roads during 1994 to 2002 in the watersheds resulted in excavation works amounting to 2.2 to 4.4 Mm3 of debris accentuating the sediment loads in the channels (Singh, 2007). 2.3. Collection and analysis of water samples

(October–March). The mean monthly temperatures at Larji (the out let of watersheds) ranged from a minimum of 8.7 °C during January to the maximum of 26.3 °C during June. The minimum and maximum relative humidity is recorded in the months of May (63.3%) and August (78.7%) respectively. Evaporation was observed to be minimum in the months of December (36.1 mm) and January (38.7 mm), the coldest months of the year and maximum during June (165.0 mm), the warmest month of the year. 2.2. Watersheds delineation and preparation of land use map The study watersheds (Sainj and Tirthan) were delineated from the topological information of the watersheds using the Watershed Morphology Estimation Tool (WMET) interface (Sarangi et al., 2004). The land use maps of these watersheds were generated using the topographical maps and False Colour Composite (FCC) images of December 5,1999 captured by Indian Remote Sensing (IRS-1C) satellite. While generating the land use maps, the ground truth information and visual interpretation techniques were also considered. Further, the FCC generated land use map of the watersheds were scanned, digitized and projected to real world co-ordinate systems using the Auto Cad and Arc Info GIS softwares and their areal extent under different land use conditions were estimated at different sampling points. It was observed from the land use maps of the Sainj and Tirthan watersheds that the forested area was maximum with 38.2 (282.8 km2) and 56.2% (385.5 km2), respectively. The second major land use type was identified as rocky outcrops, which covered an area of 23.8 (175.9 km2) in the Sainj and 17.6% (120.6 km2) in the Tirthan watershed. Snow bound area was 22.6 (166.9 km2) and 7.1% (48.5 km2) in the Sainj and Tirthan watershed, respectively. The agricultural land was predominant in the Tirthan watershed with 16.9% (115.8 km2) as compared to 4.5% (33.2 km2) in the Sainj watershed. The area covered by glaciers were 9.4 (69.9 km2) and 1.1% (7.8 km2) for the Sainj and

The water samples from the drainage channels of the watersheds were collected form both the main channel and its tributaries joining the main stream at the gauging site thrice a day during morning, noon and evening. This sequence was followed once in each season during summer (April–May), monsoon (June–September) and winter (October–March) from eight different gauging sites (Fig. 1). The samples were collected seasonally (May 1–8, for summer season; August 15–22, during monsoon season and December 20–27, 2004 in winter season). This resulted in acquisition of 24 samples for the year 2004. The depth integrating sampling technique was used for collection of 1000 ml sample in plastic bottle from reasonable turbulent river flow (Ostrem, 1964; Bhutiyani, 2000). Further, a global positioning system (GPS) was used to record the latitude, longitude and the altitudinal location of these water sampling points (Table 2). The collected samples were analyzed in the laboratory following a standard procedure to estimate the suspended sediment concentration (SSC). Suspended sediments were separated from the water samples in the laboratory by using 0.45 µm millipore membrane filters of 47 mm diameter. To facilitate faster filtration, a vacuum pump was attached with the filtration assembly. The weight of the sediment was measured by subtracting the weight of the filter paper from the weight of the dried filter paper along with the filtered sediment after filtration. Further, before taking the weight of the sediments on the filter paper, it was kept in a desiccator to remove moisture from the sediment. The SSC was calculated for 1 l of the collected water sample. Further, the pH and EC of the collected samples were estimated using the Jackson (1967) and Richards (1954) methods. These parameters were measured in the laboratory by using the collected water samples before the filtration process. Further, information pertaining to human interventions (human and livestock growth, road construction, deforestation and encroachments) were collected through personal interaction with the local Table 4 Suspended sediment concentration, TDS and alkalinity at various stations in the Sainj and Tirthan watershed (2004) Location

Summer (Maya) TDS

SSC

Monsoon (Augusta) pH

(mg l− 1) (mg l− 1) Sainj at Niharni Suind Sainj Talara Larji Bhatkanda (Jiwa Nal)

Fig. 2. Gravel bedded nature of channels in the study watersheds.

30.1 43.4 27.3 34.3 34.3 34.3

346.85 268.70 540.13 435.74 108.70 149.00

Tirthan at Khrongcha 27.3 Larji 42.0

75.42 59.80

a

TDS

SSC

pH

(mg l− 1) (mg l− 1) 8.52 8.32 7.44 8.03 8.52 8.20

Winter (Decembera) TDS

SSC

pH

(mg l− 1) (mg l− 1)

72.1 68.6 68.6 71.4 72.1 63.7

466.83 89.46 350.86 749.33 201.83 85.44

7.87 7.51 7.88 8.00 8.10 8.04

58.1 59.5 44.8 55.3 57.4 58.8

18.55 6.79 5.21 3.40 7.62 13.81

8.73 8.53 8.31 8.27 8.11 8.05

7.50 60.2 8.26 78.4

369.74 156.00

7.82 51.8 8.05 70.7

5.16 4.74

8.54 8.26

Month of the season during which the samples were collected and analyzed.

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Table 5 Comparison of the alkalinity levels in the Indian rivers and the rivers of study watershed (Subramanian, 1979; Kumar, 1987) Rivers

pH value

Major Indian rivers Cauvery Gandak Godavari Krishna Mahanadi Narmada Tungabadra

7.8 7.3 7.1 7.5 8.0 7.2 7.5

Himalayan rivers Brahmaputra Chenab Dhauli Ganga Ganga Jhelum

7.1 7.7 8.0 7.6 7.5

Study watersheds Sainj Tirthan

8.2 7.6

population of the study watersheds and from various governmental organization in the state of Himachal Pradesh, India.

2.4.2. Bed load The bed load, which is primarily composed of coarser particles, plays a major role in fluvial process, even though its quantity in most cases remains less than that of the suspended load (Xiaoping, 2003 ). In the present study, the bed load is being estimated because it was not possible for authors to measure the bed load in the river section. Some studies have suggested the crude empirical relationship between suspended load and bed load (Colby and Hembree, 1955; Fergusson, 1984; Bhutiyani, 2000; Sitaula et al., 2007). It has been reported that the ratio of bed load to the suspended sediment load based on annual loads amounts to about 0.05–0.15 (5–15%) with a possible estimation error of ±5%. However, for rivers with relatively stable boundary and inflow conditions, the range of variation may not be so large (Zhang and Long, 1998). Further, this ratio also varies with the compositions of channel bed and concentrations of suspended sediment load (Vanoni, 1975). In general, the channels of the study watersheds were observed to be gravel bedded due to mass wasting processes (Fig. 2). Therefore, the bed load was heuristically considered as 10% of the suspended sediment load in the present study. 2.4.3. Dissolved load The estimation of total dissolved solids (TDS) was carried out by following the methodology as proposed by Gregory and Walling (1973). In this method, the TDS was estimated by using the equation:

2.4. Computation of load

TDS ¼ AK

2.4.1. Suspended sediment load The suspended sediment concentration (SSC) for all days of the season were derived by multiplying the daily discharge rate at the gauging station with the ratio of suspended sediment concentration (SSC) and discharge rate recorded on the day of sample collection. This procedure of proportional changes in the SSC and the discharge rate was considered to account for the changes in SSC with the varying discharge rates at the gauging sites with an assumption that the fluvial energy, sediment availability for transport, rainfall intensity–duration–frequency, droplet size etc. does not change significantly during the data acquisition periods. Moreover, the daily discharge rate values were obtained from the hydrologic monitoring stations maintained by Bhakra Beas Management Board (BBMB), in which the flow depths acquired from stage level recorders were converted to the discharge rate using the developed stage discharge rating curves for the respective gauging sites (BBMB, 1997 ). The daily discharge rates acquired from the gauging sites were analyzed to obtain the maximum and minimum daily flows and are shown in Table 3. The above discussed proportionality method was used to estimate the daily sediment flow rate for all the seasons using the data of eight gauging sites. Further the seasonal and annual river flow volumes and the total suspended sediment load was estimated by using the standard accounting procedures spanning through the days, months, seasons and years.

Where, K = electrical Conductivity (EC) in dS m− 1; A = conversion factor. In general, the conversion factor varied from 0.55 to 0.75 according to the types of solutes (Gregory and Walling, 1973). The conversion factor of 0.64 pertaining to Indian river systems was used for estimation of TDS in this study (Singh et al., 1999). The total dissolved load (TDL) was calculated in the same manner as that of total suspended sediment load. Subsequently, the total sediment load (TSL) was estimated by adding the total suspended sediment load and the bed load. Finally, the total load (TL) was calculated by adding the estimated TSL and TDL.

ð1Þ

2.5. Estimation of erosion rates The rates of denudation were estimated by using the relationship given by (Gregory and Walling, 1973): Rate of denudation ¼

Total load ðtonnesÞ Area of watershed specific gravity ð2Þ

Denudation rates are expressed in mm/1000 yr or in m3/km2/yr. In this study, the specific gravity of the rock material was considered to be 2.65 (Valdiya, 1987).

Table 6 Sediment and dissolved load transfer in the Sainj and Tirthan watersheds Station/river Sainj at Niharni Suind Sainj Talara Larji Bhatkanda (Jiwa Nal) Tirthan at Khrongcha Larji

Sediment/physical (P) load (103 t yr− 1)

Dissolved/chemical (C) load (103t yr− 1)

C/P ratio

Total load (103t yr− 1)

35.64 31.46 48.62 57.51 61.18 16.93

0.17 0.63 0.20 0.12 0.45 0.67

249.67 81.41 293.86 533.66 197.47 42.19

17.67 54.97

0.19 0.69

109.81 134.90

Summer

Monsoon

Winter

Total sediment load

Summer

Monsoon

Winter

Total dissolved load

37.59 23.30 74.68 66.44 17.50 7.38

173.50 25.34 168.97 408.57 116.15 16.43

2.92 1.30 1.57 1.13 2.62 1.44

214.02 49.94 245.23 476.14 136.29 25.26

2.96 3.42 3.43 4.75 5.02 1.54

24.36 17.66 30.03 35.39 37.72 11.13

8.31 10.38 15.16 17.36 18.44 4.25

4.39 6.93

87.38 71.68

0.36 1.31

92.13 79.93

1.44 4.42

12.93 32.75

3.29 17.79


31

3. Results and discussion 3.1. Suspended sediment concentration, TDS and alkalinity at different stations Suspended sediment concentration (SSC), total dissolved solids (TDS) and pH values at various locations during summer, monsoon and winter seasons are summarized in Table 4. The spatial and temporal variations in SSC and TDS had a significant effect on the annual sediment transport behaviour from both the watershed systems. Majority of the sediment load was in the form of suspended sediment and was transported during the monsoon months at all locations. The amount of TDS increased towards the downstream during all the seasons in Tirthan watershed due to joining of various spring fed streams to the main channel. Conversely, changes in TDS values were not adequately pronounced over the Sainj watershed in different seasons. Moreover, the winter season observed higher TDS amounts than the summer due to prolonged dry season spanning from October to December in both the watersheds. Because, during this period, there was occurrence of base flow in which the solute content increased due to low discharge contributed mainly by the water stored in the aquifer. Further, TDS content was comparatively lower during summer season than the other seasons and it was presumably due to the contribution of increased snowmelt runoff in the watersheds, which diluted the total flow. The TDS content was on the higher side during the monsoon, which could be attributed to high seasonal flows. Overall, the dry season could be important for chemical processes and the wet season may be critical for sediment mass transfer due to high intensity of rainfall and mass wasting phenomenon in the watersheds. It was observed that, like other major rivers of the world, the Sainj and Tirthan rivers were also alkaline in nature. The comparison of the alkalinity levels of the river system in the study watersheds and some other Indian rivers are also presented in Table 5. It was observed from the Table 5 that the alkalinity level of the Sainj watershed was slightly higher than the majority of Indian rivers. Where as, the alkalinity of the Tirthan watershed was almost in line with other Indian rivers.

Fig. 4. Alluviation and widening of channel at Suind in Sainj watershed resulting in the reduced sediment transport.

Based on the values of SSC, bed load, TDS and the discharge at various stations, the annual sediment transport as a function of space and time was estimated and are summarized in Table 6. The total sediment and total dissolved load from the Lesser Himalayan alpine watersheds indicated the pronounced effect of the seasonality on

sediment transport. The total load (TL) from the Sainj and Tirthan watersheds was 197.47 and 134.90 t3 during 2004. Moreover, about 78% of the TL was transported during the monsoon season from both watersheds. The TL of these watersheds showed alternatively rising and falling trends during summer and monsoon seasons respectively. Moreover, the TL not only revealed the temporal variation in different seasons but also showed the spatial variation during the same season. The dissolved load contributed about 31 and 40% of the TL in the Sainj and Tirthan watershed, respectively. The elevated transport of total sediment load (TSL) during the monsoon season was due to higher discharge rates coupled with non-point sources of SSC in surface runoff. The TSL from the Sainj and Tirthan watersheds constituted about 60–90%. Also, the total dissolved load (TDL) constituted about 10–40% of the TL depending upon the site characteristics. High altitude of both the watersheds resulted in generation of more TSL than the TDL. The highest TL for all seasons were observed to be at Talara bridge (533.66 t3) falling in the Sainj watershed. The reason for this exceptionally high load could be attributed to the widening of roads, hydro-power construction activities, mining and agricultural activities as shown in Fig. 3. Further, very high load in Sainj watershed was observed throughout the year and could be attributed to the joining of glacial fed tributaries, steep sidewalls and frequent mass wasting phenomenon. However, the Sainj river carried lowest amount of TSL at Suind (81.41 t3), located between Niharni and Sainj, because of more flattish nature of river course, low gradient, maximum channel alluviation and higher channel widening activities (Fig. 4).

Fig. 3. Development of Hydro-power projects generates huge quantity of muck and increases the sediment load in Sainj watersheds.

Fig. 5. Encroachment of agricultural activity on marginal sloping lands of the study watersheds.

3.2. Sediment and dissolved load transport behaviour of watersheds

32


Fig. 6. Monthly variations of discharge rate, dissolved, suspended and total load at different locations in study watersheds.


Similarly, in the Tirthan watershed, Khrongcha site yielded very high TSL (92.13 t3). This could be attributed to the presence of glacial fed streams in upper course, steep sidewalls and more frequent landslides along the channel in upstream section. Moreover, the enhanced TSL was due to the presence of cultivated land and encroachments of agricultural activities in the forestlands and pastures (Fig. 5). The amount of TSL decreased at Larji site due to the flattish nature, channel widening and sedimentation at the downstream site of Khrongcha. Additionally, the tributaries joining downstream site of Khrongcha have smaller contributions of SSC due to various spring fed streams. Analysis of data form the gauging stations in the Sainj and Tirthan watersheds also indicated monthly variations in discharge rate, TSL, TDL and TL (Fig. 6). TSL and TDL were maximum during the month of August for all stations, which may be attributed to the weathering of materials during the dry season and maximum discharge during the month. Further, TDL was observed to be uniform from October to March, which may be attributable to the ground water contribution in the watershed systems. 3.3. Comparison of physical and chemical erosion The ratio of chemical (C) to physical (P) loads (C/P) for all the gauging sites in the Sainj and Tirthan watersheds was observed to be less than one, indicating overall dominance of TSL over the TDL (Table 6). However, higher C/P ratio of individual location on the mainstream and tributary suggested that they are chemically more active. The differences in C/P ratio at different locations were mainly due to the differences in lithological pathways through which the water flows. Generally, the C/P ratio decreases with the increase of watershed area. However, in the present study, the C/P ratio of the Sainj watershed does not reflect a specific trend but an increasing trend from Khrongcha to Larji in the Tirthan watershed was observed. Such variations may be attributed to the changes in human activity,

33

vegetation cover and spatial and temporal variability in the intensity of precipitation throughout and within the sub-watersheds. Further, the scattered diagram indicated an increasing trend between physical and chemical erosion rates for the selected locations in the Sainj and Tirthan watersheds. However, similar relationship between these two processes has been observed in other Indian, Himalayan and world rivers (Fig. 7). 3.4. Erosion rates and lowering of the watersheds Rates of erosion have been estimated based on the total sediment and total dissolved load discharged by the watersheds at various locations. The physical, chemical and total denudation rates were calculated at various points in Sainj and Tirthan watersheds and their tributary sub-basins (Table 7). The total rates of erosion in Sainj watershed varied from a minimum value of 164.47 t km− 2 yr− 1 for the Suind site to a high value of 734 t km− 2 yr− 1 at Talara site. The erosion rates for entire Sainj and Tirthan watersheds during 2004 sampling period were calculated to be 267 t km− 2 yr− 1 and 197 t km− 2 yr− 1 respectively. These erosion rates of the Sainj and Tirthan watersheds were very high as compared to other Indian, Himalayan and some major world rivers (Table 8). Moreover, based on the TL at different sites, the lowering rates of the watersheds were estimated and are given in Table 7. Physical weathering seems to be an effective mechanism of erosion leading to the lowering of the watersheds. The lowering rates for the entire Sainj and Tirthan watersheds based on the observations during 2004 were estimated to be 0.10 mm yr− 1 (0.07 mm yr− 1 due to physical erosion and 0.3 mm yr− 1 due to chemical erosion) and 0.07 mm yr− 1 (0.04 mm yr− 1 due to physical erosion and 0.03 mm yr− 1 due to chemical erosion), respectively. These findings were in agreement with the denudation rate of the watersheds in the Central Himalayan region of India (Rawat and Rawat, 1994).

Fig. 7. Relationship between chemical and physical erosion rates at different watershed scales.

34


Table 7 Average erosion rates in the Sainj and Tirthan watersheds Station/ river

Sainj at Niharni Suind Sainj Talara Larji Bhatkanda (Jiwa Nal)

Drainage Total Lowering rate of the basin Physical Chemical area (km2) erosion erosion erosion Physical Chemical Total (t km− 2 yr− 1) (mm yr− 1) 410.65 495.01 658.41 727.26 740.02 163.40

521.20 100.90 372.46 654.72 184.17 154.61

86.81 63.57 73.86 79.08 82.69 103.65

608.00 164.47 446.32 733.80 266.86 258.26

0.20 0.04 0.14 0.25 0.07 0.06

0.03 0.02 0.03 0.03 0.03 0.04

0.23 0.06 0.17 0.28 0.10 0.10

Tirthan at Khrongcha 191.13 Larji 686.36

482.07 116.46

92.47 80.09

574.54 196.56

0.18 0.04

0.03 0.03

0.22 0.07

3.5. Effect of watershed elevation and land use on erosion rate The results of the present study did not show any clear relationship between rates of erosion and watershed elevation. The Sainj watershed having higher mean elevation of 3510 m amsl and basin area of 741 km2 transported sediments at the rate of 267 t km− 2 yr− 1, whereas Tirthan discharged 197 t km− 2 yr− 1 with a mean elevation of 2826 m amsl and an area of 687 km2. Further, Jiwa Nal sub-basin with mean elevation of 3684 m amsl and basin area of 164 km2 discharged a total sediment load of 258 t km− 2 yr− 1, whereas Tirthan in its upper

course transported about 575 t km− 2 yr− 1 sediments with a mean elevation of 3585 m amsl and basin area of 191 km2. Land use and agricultural practices in a watershed system have drastic effects on erosion and sediment yield. In general, the land covered with natural vegetation retards the rate of erosion (Cox et al., 2006). However, in this study the data of different locations in Sainj watershed revealed that the erosion rates are not directly related with the area under the forest cover. Moreover, there exists a direct relationship between forest cover and erosion rates in Tirthan watershed (Table 1). Nonetheless, it was observed during periodic field visits that various anthropogenic factors were responsible for high level of sediment transport besides the natural factors. The anthropogenic interventions include large-scale encroachment in forest lands for agriculture, deforestation, cultivation of steep slopes and indiscriminate grazing in the watershed by local and migratory animals. Also, road construction, mining, forest fires and other developmental projects have accelerated the process of soil erosion. Amongst all these, the natural factors which could have accentuated the erosion process are the steep topographic gradient, weak geology, glacial erosion, erosive storms, erosion by streams, less infiltrability of soils and multitude of mass wasting phenomena. 4. Limitations of study Hydrological investigations in inaccessible mountainous terrains and subsequent data analysis are based on some specific assumptions. The present study carried out in the Lesser Himalayan watersheds is

Table 8 Comparative erosion rates for the Sainj and Tirthan rivers and some major world rivers River

Country

Erosion rate (t km− 2 yr− 1) Physical

Indian rivers Ganges (Calcutta) Bhramputra Yamuna (Allhabad) Chambal Betwa Ken Gomti Ghaghra Son Gandak Krishna Godavari Cauvery Mahanadi India average Himalayan rivers Ganga (Hardwar Yamuna (Tajewala) Ramganga Dhauliganga Nana Kosi Sainj Jiwa Nal Tirthan World rivers Huang Ho Yangtze Irrawady Amazon Magdalena Mississippi Orinoco Mekong Congo World average

India India India India India India India India India India India India India India

438 865 186 129 211 219 225 978 709 383 16 555 0.5 13.3 327

Chemical

Total

111 88 122 154 1817 230 40 130 96 140 41 55 40 67.6 69

549 953 308 283 2088 449 265 1108 805 523 57 610 40.5 80.9 396

Physical/chemical ratio

Source

3.90 9.80 1.52 0.83 0.12 0.95 5.62 7.52 7.38 2.73 0.39 10.00 0.01 0.19 4.74

Abbas and Subramanian (1984) Subramanian 1983 Jha et al. (1988) Jha et al. (1988) Jha et al. (1988) Jha et al. (1988) Abbas and Subramanian (1984) Abbas and Subramanian (1984) Abbas and Subramanian (1984) Abbas and Subramanian (1984) Ramesh and Subramanian (1988) Bikshamaiah and Subramanian (1988) Subramanian et al. (1985) Chakrapani and Subramanian (1990) Jha et al. (1988)

India India India India India India India India

152 1889 337 392 96 184 155 116

26 289 87 278 100 83 104 80

178 2178 424 670 196 267 258 197

5.84 6.54 3.87 1.41 0.96 2.21 1.49 1.45

China China Burma Brazil Colombia U.S.A Venezuela Vietnam Zaire

1402 246 662 146 916 64 212 200 13 150

30 116 211 46 117 40 52 74 10 35

1432 362 873 192 1033 104 264 274 23 185

46.70 2.10 3.10 3.10 7.80 1.60 4.00 2.70 1.30 4.28

Abbas and Subramanian (1984) Jha et al. (1988) Abbas and Subramanian (1984) Kumar (1987) Rawat and Rawat (1994) Present study Present study Present study

Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Milliman and Meade Jha et al. (1988)

(1983) (1983) (1983) (1983) (1983) (1983) (1983) (1983) (1983)


not an exception to that. Uncertainty in data collection and analysis of present investigations are highlighted in this section. In this study, the sampling of water from channel was done using very simple method. Hence, uncertainties pertain to the measurement of width, depth, velocity of water in channel and the efficiency of the suspended sediment sampler. However, the laboratory analysis of SSC, EC, pH etc. was carried out as per the international standards. Authors feel that the sample collection procedure did not represent the distribution of SSC in total channel width, depth and velocity. Moreover, it is understood that a number of sections along the width and at different depths (near the bed, mid depth and at water surface) and an increase in the frequency of observations could have provided a better representation of the sediment load. It is expected that because of errors in sample collection and assumptions made in the analysis might have resulted in an error of about 10% in the results. 5. Conclusions The watersheds exhibited a dominance of total sediment load (TSL) over the total dissolved load (TDL) reflecting that the physical weathering (mass wasting, slumping of heavily sediment laden portal ice in the streams, occurrence of rainstorms etc.) were more pronounced in the watersheds. The bulk of sediment load transported by glaciers in the upper reaches gets transformed into temporary storage within the alpine basins and is removed during the periods of high discharges. The uniform values of TDS during the lean runoff period (October–March) reflected the contributions of base flow in the watersheds. Further, human interventions in the form of developmental activities played a significant role in the spatial and temporal variability of sediment load. Estimated sediment yield in these watersheds ranged between 164 to 734 t km− 2 yr− 1 at different gauging locations. These estimates were very high as compared to the estimates available for the drainage basins around the globe. Above all, the periodic monitoring of water quality in the streams within the watershed systems would assist in estimating its spatial and temporal variability. In addition, this would also help in understanding the hydrologic responses of the Lesser Himalayan alpine watershed systems for taking up appropriate soil and water conservation measures leading to integrated watershed management. Acknowledgements We are grateful to the BBMB authorities for providing daily discharge rate values for respective gauging sites. Additionally, the authors would also like to thank the editor of the Catena journal and the two anonymous reviewers for their critical, but highly constructive comments that have considerably improved the quality of the paper. References Abbas, N., Subramanian, V., 1984. Erosional and sediment transport in the Ganges River Basin, India. Journal of Hydrology 69, 173–182. BBMB (Bhakra Beas Management Board), 1997. Sedimentation Survey Report. BBMB, Bhakra Dam Circle, Nangal, India. Bhattacharya, P., Bhatt, V.K., Mandal, D., 2008. Soil loss tolerance limits for planning of soil conservation measures in Shivalik–Himalayan region of India. Catena 73, 117–124. Bhutiyani, M.R., 2000. Sediment load characteristics of a proglacial stream of Siachen Glacier and the erosion rate in Nubra valley in the Karakoram Himalayas, India. Journal of Hydrology 227, 84–92. Bikshamaiah, G., Subramanian, V., 1988. Sediment transport of the Godavari river basin and its controlling factors. Journal of Hydrology 101, 275–290. Chakrapani, G.J., 2005. Factors controlling variations in river sediment loads. Current Science 88, 569–575. Chakrapani, G.J., Subramanian, V., 1990. Prelimnary studies on the geochemistry of the Mahanadi River basin, India. Chemical Geology 81, 241–253. Chakrapani, G.J., Subramanian, V., 1993. Rates of erosion and sedimentation in the Mahanadi river basin, India. Journal of Hydrology 149, 39–48. Colby, B.R., Hembree, C.H., 1955. Computations of total sediment discharge of Niobrara River near Cody, Nebraska. US Geological Survey Water Supply Paper No. 1357. USGS, Washington, DC.

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