Hydrological processes in a small arid catchment

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It is usually assumed that runoff and erosion increase as slope length and angle increase. ... effective rainfall control the spatial structures of surface properties that further enhance the effects of temporal .... channel scales are supported by long-term data collect- .... rain showers is sufficient to cause a flow discontinuity.
Geomorphology 61 (2004) 155 – 169 www.elsevier.com/locate/geomorph

Hydrological processes in a small arid catchment: scale effects of rainfall and slope length Aaron Yair *, Naama Raz-Yassif Department of Geography, The Hebrew University of Jerusalem, Mount Scopus Campus, Jerusalem 91905, Israel Received 1 July 2003; received in revised form 1 December 2003; accepted 3 December 2003 Available online 4 February 2004

Abstract Many studies have been concerned with the scale issue in geomorphology and hydrology. Most studies focus on the possibility of data transfer from small to large watersheds. Less attention has been given to the scale problem within very small watersheds where differences in slope length may seriously affect the spatial distribution and extent of runoff-contributing areas to the channel. It is usually assumed that runoff and erosion increase as slope length and angle increase. However, field observations in the Negev Highlands show positive relationships between slope length and deposition rates, regardless of slope angles. Short hillslopes are devoid of colluvial mantle, while thick colluvial deposits, with well-developed soil profiles indicative of long-term stability, are found at the base of long hillslopes. The hypothesis advanced is that temporal variations in effective rainfall control the spatial structures of surface properties that further enhance the effects of temporal variations in rainfall. Long-term monitoring of rainfall and runoff (1982 – 1998), conducted at spatial scales varying from few hundreds of m2 up to 0.3 km2, show a decrease in runoff at the hillslope scale. The low efficiency of runoff and erosion processes on long hillslopes is because the concentration time required for continuous flow along such slopes is longer than the duration of most effective rain showers prevailing in the area. Field data lead to the notion that runoff and erosion models in which a positive relationship between slope length and angle and runoff and erosion rates is assumed should not be applied to arid and semiarid areas. More attention should be given in latter areas to the complex hydrological relationships between rainfall scales and spatial scales. The implications of data obtained regarding the spatial distribution of areas contributing to channel flow, at a geological time scale and under changing climatic conditions, are discussed. Published by Elsevier B.V. Keywords: Hillslope hydrology; Rainfall scales; Hillslope scale; Erosion and deposition processes; Contributing area; Geological time scale

1. Introduction There are many studies on the issue of the scale problem in geomorphology and hydrology (e.g., Amerman and McGuiness, 1967; Pilgrim, 1983; * Corresponding author. Fax: +972-2-5324284. E-mail address: [email protected] (A. Yair). 0169-555X/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2003.12.003

Klemes, 1983; De Boer, 1992; Seyfried and Wilcox, 1995; Kirkby et al., 1996; Fiorentino and Iacobellis, 2001). It is well known that specific runoff yield decreases with increasing area, and the reasons for this trend are not independent of scale. For large watersheds, covering hundreds or thousands of square kilometers, runoff decreases with increasing area is usually attributed to such factors as size of rain-cells,

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duration and intensity of the rain at a given area, lateral changes in lithology and channel width. Some of these factors are especially important in arid and semiarid areas, where rainfall is often localized and convective. When small watersheds, covering ten up to a few hundred hectares, completely covered by the same rain-cell, are considered, one would expect a uniform hydrological response and therefore a positive relationship between runoff and scale. However, hydrological monitoring carried out within humid regions has revealed that runoff generation is spatially nonuniform, even within small watersheds in homogeneous lithology. Runoff generation responsible for storm channel flow was found to be limited to the channel itself and a riparian belt where saturated conditions eliminate or inhibit infiltration (Kirkby and Chorley, 1967; Betson and Marius, 1969; Dunne and Black, 1970; Anderson and Burt, 1987; Seyfried and Wilcox, 1995). The above findings led to the formulation of the concept of Partial Area Contribution and in its dynamic form to the concept of Variable Source Area. A different situation exists in arid and semiarid areas where the paucity of rainstorms, the limited storm rainfall amounts, the shallow and patchy soils, together with the low-density vegetal cover inhibit significant contribution to storm channel flow by groundwater flow, saturated overland flow or shallow subsurface flow. In such areas the classic process of Hortonian overland flow is predominant and represents the only contributor to channel flow. Under such conditions, one would expect small drainage basins in arid and semiarid areas to respond quite uniformly to rainfall. However, many studies show that this is not the case. Significant differences in runoff have been obtained for small watersheds covering a few hectares in various environments such as badlands (Yair et al., 1980; De Boer, 1992) and rocky terrains (Yair, 1992; Yair and Lavee, 1985; Bergkamp and Imeson, 1999; Shanan, 2000). A sharp decrease in the specific runoff yield over short distance is observed at the hillslope scale and channel scale of first-order drainage basins (Yair, 1992; Yair and Kossovsky, 2002). The term ‘‘Response Units’’ is often used to describe the nonuniform spatial hydrological response over short distances. We assume that different surface units, such as rocky, colluvial or alluvial surfaces, respond differently to

the same rainfall. Most studies dealing with the scale problem focus therefore on the effects of spatial variability of surface properties (e.g., infiltration, antecedent moisture, stoniness and vegetation cover) on runoff generation and flow continuity. The decrease in specific runoff with increasing area is usually explained by the increasing importance of infiltration losses in the downslope or downstream direction. However, as geomorphologists, we have to answer the question: why is it that long-term differential spatial deposition or erosion processes occur within small to very small watersheds, often carved in uniform lithologies? Such differential processes exist at the hillslope scale (development of a colluvial mantle) as well as along valley bottoms of first- to third-order drainage basins. The hypothesis advanced here is that short-term temporal variations in rain intensity and duration during a storm control the spatial distribution of the hillslope and channel hydrological response units. Once developed, these units further enhance the effects of temporal variations in rainfall. We therefore contend that little attention was given, at least in semiarid and arid areas, to how the scale of effective rainfall during a storm affects runoff generation, flow continuity and deposition processes and that there is a need to adapt the analysis of the scale effect to the appropriate scale of both rainfall and surface response units. Our approach follows that of Klemes (1983) who stated: ‘‘we cannot impose scale but have to search for those which exist and try to understand their relationships and patterns’’. Such scales may be completely different in humid and arid areas. The main aim of this work is to analyse the factors that control the decrease in the frequency and magnitude of the specific runoff yield and flow continuity with increasing area within a small arid watershed extending over 0.3 km2.

2. Description of research area The study was conducted at the Sede Boqer Experimental Watershed, located in the Negev Highlands. Mean annual rainfall is 93 mm, with a high annual variability (34 –167 mm). Rainfall is limited to the winter season. The watershed is in a limestone

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terrain of Turonian age and represents a third-order basin that covers 0.3 km2. The watershed is equipped with devices for automatic and simultaneous measurements of rainfall and runoff. The location of hillslope and channel hydrometric stations is shown on Fig. 1. Channel runoff is measured at three locations. The first (named Upper Channel) is located at the transition from the headwater rocky area into the alluvial reach of a first-order drainage basin and drains 0.8 ha. The convergent slopes are short, and very extensive rocky outcrops, almost devoid of soil and vegetation cover, form most of the surface. The second (Lower Channel) is located at the mouth of the same drainage basin and drains 2.1 ha. The

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valley side slopes are long and soil cover is more extensive. It is characterized by soil strips at the base of rock steps at the upper rocky slope sections and a colluvial mantle at the base of long slopes (Fig. 2). The alluvial fill in the narrow channel extends from the rocky headwater area down to the mouth of the valley and is narrow, less than 50 cm thick, discontinuous, with some rocky outcrops. The third station (Main Channel) represents a third-order basin that drains 0.3 km2. The alluvial fill is wide and up to 8 m thick. Hillslope runoff is measured at plots that extend over 181 m2 (plot A), 465 m2 (plot B) and 607 m2 (plot C). Plot A is 27 m long and represents a rocky surface. Plots B and C are rocky at their

Fig. 1. The Sede Boqer Experimental Site.

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Fig. 2. The relationship between slope length and the length of the colluvial section.

upper part and colluvial at their base and their lengths are 63 and 72 m, respectively.

3. Field observations relevant to the scale problem 3.1. Hillslope scale Numerous studies, conducted mainly in agricultural areas, using erosion plots, contend that erosion increases as slope length and slope angle increase (e.g., Wischmeier and Smith, 1978; Desmet and Govers, 1996; Renard et al., 1997; Millward and Mersey, 1999). Such studies assume flow continuity and efficient sediment removal along whole slopes. The situation seems different when natural hillslopes in arid and semiarid areas are considered. Here a positive relationship is observed between slope length and deposition rates. Short valley side slopes (15 – 20 m long) are rocky from top to bottom and completely devoid of colluvium, but longer and

steeper slopes display a colluvial mantle at their base. The length and thickness of the colluvial mantle increase with slope length, regardless of slope angle (Fig. 2). Colluvium thickness at the base of slopes 100 m long is of the order of 2 –3 m, with well-developed soil profiles indicative of a high stability (Yair, 1990). These observations clearly indicate that over a long time scale erosion rates are far more effective on short than on long and steep valley side slopes, where deposition processes prevail. As hillslopes in the study area are steep (Figs. 1 and 2), a possible explanation of the above observations is to assume continuous flow and efficient sediment removal along short slopes but a high frequency of flow discontinuity at long slopes. Flow discontinuity is held responsible for the deposition of the material, removed from the upper slope section, at the base of steep and long slopes. The process described above is observed even when the longer slopes are steeper than the shorter slopes (Fig. 2). Therefore, slope angle and slope length cannot be

A. Yair, N. Raz-Yassif / Geomorphology 61 (2004) 155–169 Table 1 Sede Boqer Experimental Site: rainfall – runoff relationships (1982 – 1998) Runoff response unit

Upper channel Rocky Lower Main and short colluvial channel channel slope (27 m) slope (63 m)

No. of 47 storm flows recorded Mean specific 11 runoff (l/m2) Lower threshold 2 (mm) Upper threshold 4.5 (mm)

42

20

12

4.3

5

4

6

15

18

37

6.5

3.2

regarded as predominant factors affecting runoff, erosion and deposition processes in the study area. The important water losses at the hillslope and channel scales are supported by long-term data collected at the Sede Boqer Research Site (Table 1). Annual rainfall, during the period considered, varied in the range 41 –164 mm, with a mean of 86.3 mm (Fig. 3). Data analysis shows the following: (1) A sharp decrease in flow frequency occurs on passing from the hillslope scale (short slopes at the headwater area and long valley side slopes) to the channel scale. A further increase is

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obtained on passing from the first- to the thirdorder drainage basin. (2) The most important water losses occur already at the hillslope scale, on passing from short to long slopes. (3) The difference between the ‘‘lower’’ and the ‘‘upper’’ threshold values increase with increasing area. The larger the area considered, the larger the difference between the two values. The ‘‘lower threshold’’ is the minimum rain amount required for runoff generation when antecedent soil moisture is high or when high rain intensities exceed infiltration rates, even under dry surface conditions. The ‘‘upper threshold’’ is recorded when rain intensities are very low and rainfall is highly intermittent. Similar, more pronounced, differences in runoff generation at the hillslope scale were also obtained at a semiarid experimental site located north of Sede Boqer (Yair and Kossovsky, 2002)). Scales considered in this case were 1.4, 36 and 200 m2. Runoff frequency and especially runoff magnitude decrease with increasing plot area. Losses are especially sharp on passing from small to medium plots, indicating important losses on hillslopes, within very short distances of 8 m (Fig. 4). It is interesting to note that runoff decrease with slope length is not unique to the study

Fig. 3. Annual rainfall—Sede Boqer 1982 – 1998.

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Fig. 4. Lehavim site—runoff coefficients in (A) 1990 – 1991 and (B) 1991 – 1992.

area. Similar results have been obtained in Nigeria by Lal (1997) who stated that ‘‘runoff from no-till treatment significantly increased with a decrease in slope length.’’ 3.2. Channel scales 3.2.1. First- and second-order channels First- and second-order drainage basins in the study area are characterized by narrow (< 50 cm)

active channels. Some channel stretches are devoid of alluvium and vegetation. Where alluvium exists, its thickness varies from 10 to 30 cm. The thickest alluvium, with a high-density vegetation cover, is found at the bottom of valley heads. With narrow channels, and a shallow discontinuous alluvial fill, one would expect the concentrated channel flow to be continuous along short distances, with a downstream increase in discharge. However, long-term hydrological data collected at Sede Boqer (Table 1; Figs. 5 and

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Fig. 5. The storm of 25 – 27 December 1988.

6) clearly show a significant decrease in all hydrological variables considered (flow frequency and magnitude, flow duration and peak flows) over the short distance of 200 m that separates the upper and lower channel hydrometric stations. The storm of December 1988 (Fig. 5) represents a medium rain event. Five individual flows have been recorded at the upper channel station but only one at the lower channel station. Similar trends were obtained on the most extreme storm recorded in the area (in terms of rain amount, more than 50% of mean annual rainfall) for the last 50 years (Fig. 6). In both cases, the down-

stream decrease in the magnitude of hydrological variables is due to a high frequency of flow discontinuities between the two stations. In the latter case, flow discontinuities have even been recorded at the very end of the extreme storm, when antecedent soil moisture must have been very high. When antecedent moisture is high, spatial differences in infiltration rates are minimized, thus limiting the importance of surface properties in the process of runoff generation and flow continuity. Under such conditions, flow discontinuities observed over short distances in narrow channels with shallow alluvium separated by rocky reaches,

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Fig. 6. Sede Boqer site. Storm of January 1992.

should not be attributed to surface properties. They point at the importance of the other factor affecting runoff processes, namely rainfall properties. 3.2.2. Third-order channels Whereas first- and second-order channels are narrow, with a shallow and patchy alluvial fill, a completely different depositional regime exists on passing from first- or second-order streams to third-order

streams. Channels of latter streams are wider (20 – 50 m), and the valley fill is thick and composed mainly of fine-grained silty clay of eolian material. Boreholes and trenches dug in the area indicate a thickness of 5 –10 m. Many of the small flat valley bottoms of such drainage basins were under cultivation (Fig. 7) by water harvesting methods during Nabatean and Byzantine times (Evenari et al., 1982; Yair, 1983; Shanan, 2000). Flow frequency and mag-

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Fig. 7. Negev Highlands—cultivated fields in a third-order channel.

nitude are much lower than those obtained for a firstorder stream, with much higher thresholds for runoff generation (Table 1).

4. Discussion Flow continuity, along a hillslope or the channel of a small drainage basin, requires that the duration of effective rainfall is at least as long as the concentration time. This is especially the case when infiltration losses increase downslope or downstream, where colluvial and alluvial deposits are widespread. Concentration time will always be shorter for shorter slopes, and for first-order than for higher order streams. Under such conditions, the chances for flow continuity and efficient sediment removal can be expected to decrease with increasing area. However, the relevant scale is highly dependent on the duration of effective rain. Long and continuous rain will increase antecedent soil moisture, reduce infiltration rates, increase runoff rates and consequently the chances for flow continuity. The critical importance of rainfall duration is well demonstrated in Fig. 6. Storm rain amount (53.2 mm) was the highest recorded during the period 1982 –1998. Rainfall intensity during most of the storm seldom exceeded 8 mm/h, allowing for good infiltration over the colluvial and alluvial deposits. By the end of this extreme storm, one would expect soil moisture to be

high and infiltration losses low, allowing for flow continuity between the upper and lower channel hydrometric stations. Despite such favourable conditions, flow duration, peak flow and specific runoff, yield were higher at the upper station than at the lower station. The fact that flow discontinuities have been recorded at the end of the storm, when soil moisture must have been very high, draws attention to the aspect of effective rainfall duration. It appears that the subdivision of the storm into many individual showers, each lasting for durations shorter than the concentration time required to allow a continuous flow between the two stations, is the main reason for flow discontinuities in the study area. Long-term data collected at the Sede Boqer Station show that a time interval of 10– 15 min between two consecutive rain showers is sufficient to cause a flow discontinuity at the hillslope and channel scales. Flow discontinuities result in the deposition of the material eroded upslope or upstream. The material deposited may enhance infiltration losses, thus further limiting chances for flow continuity. The negative feedback process described above may therefore be attributed to rainfall characteristics in the study area. The discussion will therefore focus on the scale of the effective rain and its effects on hydrological responses at the spatial scales considered. Table 2 and Fig. 8 display the rainfall characteristics recorded in the area.

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Table 2 Sede Boqer Experimental Site: frequency distribution of rain events (1982 – 1998) Rain amount

No. of events

(A) From 0 to 55 mm 0–5 5 – 10 10 – 15 15 – 20 20 – 30 30 – 40 40 – 55

183 38 21 10 5 5 3

(B) From 0 to 5 mm 0–1 1–2 2–3 3–4 4–5

76 38 28 23 18

4.1. Rainfall characteristics Table 2 displays the precipitation frequency distribution for the 265 rain events recorded at Sede Boqer in 1982– 1998. Rain events up to 5 mm represent some 70% of the rain recorded in the area, and 77.6% of these had rain amounts up to 3 mm. Rain events up to 10 mm represent 83 % of recorded falls. The analysis of the distribution of rain intensities and duration of individual rain showers is presented in Figs. 8 and 9. Fig. 8 shows the duration of individual rain showers in 1994 –1995. Annual rain amount (131 mm) was higher than the long-term (1982 – 1998) mean of 93 mm, with one storm of 20 mm and another of 34 mm. Some 62% of the individual rain showers lasted up to 10 min and 80% up to 20 min. Similar trends were obtained for the two most extreme rainstorms (in terms of amount, intensity and duration) recorded in the area for the period considered (Fig. 9). High rain intensities, above 20 mm/h, lasted for no more than a few minutes. Intensities in the range of 5 – 10 mm/h last for more than 100 min. Such intensities are high enough to generate runoff over the rocky surfaces but far below the final infiltration rates of the colluvial and alluvial units (Yair and Lavee, 1985). Under such conditions any time rain intensity drops below the value of about 10 mm/h, a flow discontinuity may be expected on passing from the upper rocky slope sections and rocky headwater area

into the colluvial or alluvial units. In addition, the short duration of most individual rain showers is not sufficient to allow flow continuity over long distances. Detailed analysis of the time lag between the beginning of flow at the upper and lower channel stations shows that the time interval during the extreme storms of March 1991 and January 1997 varied between 8 and 37 min. Previous studies in the same watershed show that the cessation of the rain for 6 min is sufficient to cause a flow discontinuity over the hillslopes (Lavee, 1986).

5. Implications for landscape development Data presented raise two important theoretical issues relative to landscape development in the study area. The first relates to temporal changes, at the geologic time scale, in the spatial location of the area contributing to channel flow. The second relates to the effects of climatic changes on the structure and function of small arid watersheds. 5.1. Temporal changes in contributing areas 5.1.1. Stage 1 At the initial stage of the development of firstand second-order channels, the degree of channel incision is limited and all hillslopes are short. Concentration times are short, allowing for continuous flow from the whole watershed area down to the channel. At this stage, runoff generation may be expected to be quite uniform within the watershed, with a positive relationship between contributing area and runoff yield. Flow continuity, coupled with an increase in runoff energy in the downslope and downstream directions, will result in efficient sediment removal through the whole system. However, by the end of this stage, a differentiation in slope length will develop between the valley side slopes and headwater slopes. Due to the deeper incision in the downslope direction, latter slopes will become shorter than the former. 5.1.2. Stage 2 Assuming no change in rainfall conditions, especially in the duration of individual rain showers, at some time the length of valley side slopes will reach a

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Fig. 8. Frequency distribution of rain showers in 1994 – 1995 (131 mm).

point where at many storms, especially small to medium rainstorms, the duration of effective individual rain showers will be shorter than the time required for continuous flow along the valley side slopes from top to bottom. At this stage, deposition will start at the base of valley side slopes. The gradual development of a colluvial mantle at the base of the long slopes will further increase the chances of flow discontinuities caused by the high absorption and infiltration rate of

the noncohesive colluvial material. A similar depositional process will not take place at the hillslopes within the headwater area. Due to limited incision in the headwater area, hillslopes will remain short and concentration times sufficiently long enough for continuous flow along these short slopes. Sediment removal will remain high, preserving the rocky surfaces with their low infiltration rate (Table 1) and high runoff.

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Fig. 9. Rainfall characteristics of two extreme rainstorms.

5.1.3. Stage 3 Once a colluvial mantle develops at the base of valley side slopes, their contribution to channel flow may decrease significantly, both in frequency and magnitude, being limited to extreme rainstorm conditions when the duration of effective rainfall is long

enough to allow flow continuity along whole long slopes. However, due to infiltration losses within the colluvial slope section, runoff per unit area will always be lower than that of hillslopes forming the headwater area. Under such conditions, most runoff recorded at the outlet of a small watershed must have

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its origin within the headwater area, where the frequency and magnitude of runoff generation over the short and rocky hillslopes remain high. This assumption is supported by Yair (1992) as well as by data presented in Table 1 and Fig. 7. The implications of this conceptual model are that landscape development in arid and semiarid areas will follow two basic stages. The first stage, when all hillslopes are still short, will be characterized by a positive relationship among watershed area, runoff generation and sediment removal. Such a process will lead to channel incision and lengthening of valley side slopes. The second stage will begin when the short duration of prevailing rain showers will not allow flow continuity along the long valley side slopes, resulting in the development of colluvial mantles. The contribution of these slopes to channel flow will decrease, while that of the headwater area will remain unchanged, thus increasing significantly its relative importance (Yair, 1992). At this stage, the conceptual model of Partial Area Contribution and in its dynamic form that of Variable Source Area may be applied to arid watersheds. However, the spatial location of the areas contributing to channel runoff is completely different from that found in small humid watersheds, being mainly limited to the headwater areas.

processes, the rocky slope sections must have been far more extensive that at present. In view of the very low rainfall threshold required for runoff generation over rocky areas (Table 1), runoff frequency and magnitude, over more extensive rocky areas, must have been significantly higher than at present, allowing for a better connectivity along hillslopes and adjoining channels, thus limiting deposition rates. The decrease in flow continuity along long hillslopes and small watersheds observed in the area must have been enhanced with loess penetration into the area (Yair, 1987). The increase in the loess cover over the hillslopes increased infiltration rates and decreased the frequency and magnitude of runoff, thus reducing the interconnectivity of the system. Laboratory experiments (Yair and Bryan, 2000) show that slight changes in the surficial material (deposition of a thin loess or sand layer) markedly affected rainfall –runoff relationships. The decrease in connectivity following loess penetration into the area may have been enhanced by parallel changes in rainfall characteristics, such as decreases in rain intensity and the duration of individual rain showers.

5.2. Effects of climatic changes

This work deals with a basic geomorphic issue: the effect of scale (hillslope and watershed scales) on runoff and erosion processes. Most studies and models contend that runoff and erosion rates increase as slope length and angle increase. The fieldwork has been conducted in a very small arid watershed in quite uniform lithology, where slope length and angle differ over short distances. Hydrological and sedimentological data presented reveal a negative relationship between the frequency and magnitude of runoff and slope length, regardless of slope angle. The longer the hilllslopes the higher the chances of flow discontinuity along the hillslope. Such discontinuities result in deposition processes at the base of long slopes that further increase infiltration losses downslope and therefore the frequency of flow discontinuity. The limited connectivity over long hillslopes is attributed to the short duration of effective rain showers that is most often shorter than the concentration time necessary for a continuous flow along long valley side slopes. Under such conditions long valley side slopes

The hydrological processes and their geomorphic implications described above did not always prevail to the same degree in the study area. Prior to the development of both the deep alluvial fill in the main channel and the colluvial mantle, a better connectivity along the whole system must have existed, allowing for the deep channel incision. The alluviation –colluviation process is therefore indicative of a change in geomorphic processes. This change is well indicated by the stratigraphy of many sections exposed along channel banks and along the hillslope trench shown in Fig. 2. Most of the alluvial fill is composed of a thick eolian loess deposit underlain by a conglomerate. On hillslopes, angular gravels mixed in a fine-grained matrix appear at the lower part of the slopes and these gravels are overlain by 2– 3 m of loess. The loess unit extends upslope where it directly covers the local bedrock. These stratigraphic sections suggest that prior to the change in the hydrologic –geomorphic

6. Conclusions

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are quite often hydrologically disconnected from the channel that receives most or all of its runoff from the headwater areas where short and rocky hillslopes, devoid of a soil cover, are responsible for high rates of runoff generation. The process of flow discontinuity over long hillslopes has been enhanced following a climatic change that resulted in the deposition of eolian loess over the hillslopes. This deposition increased infiltration rates, decreased runoff frequency and magnitude, thus reducing the chances for flow continuity. The significance of such results casts doubt on the classical runoff and erosion models that assume a positive relationship between runoff and erosion rates and slope length and angle. They show that when dealing with the scale problem, in very small arid and semiarid watersheds, there is a need to adapt the scale of runoff and erosion processes to the duration of effective rainfall. More information is needed for the latter aspect in order to properly understand landscape development.

Acknowledgements The study conducted at the Sede Boqer Research Site was partly supported by the Arid Ecosystem Research Centre of the Hebrew University. We are grateful to Mr. E. Sachs for his help in data collection and processing. The comments of the reviewers (Drs. M.A. Fullen and J. Poesen) are greatly appreciated. Thanks are also due to Mrs. M. Kidron of the Department of Geography for drawing the illustrations.

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