Chapter 13 Watershed Storage Dynamics in the Upper Blue Nile Basin: The Anjeni Experimental Watershed, Ethiopia Temesgen Enku1*, Assefa M. Melesse2, Essayas K. Ayana1, Seifu A. Tilahun1, Gete Zeleke3, Tammo S. Steenhius4 1School of Civil and Water Resources Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Ethiopia

E-mail: temesgenku@gmail.com

Phone: +251 910489482 (corresponding author) 2Florida International University, Department of Earth and Environment, USA, Department of Earth and Environment, Florida International University, Modesto A. Maidique Campus, Miami, FL 33199, USA

E-mail: melessea@fiu.edu; Phone: 305-348-6518 3Water and Land Resource Centre, Ethiopia, University of Bern, Switzerland 4Department of Biological and Environmental Engineering, Cornell University, Ithaca, USA

Abstract

Understanding functions of a watershed is important for implementing appropriate soil and water conservation measures and for planning and development of water resources for sustainable use. Watershed storage is a significant part of a catchment water budget and its quantification provides a clue to understand the fundamental catchment hydrological processes. This study is aimed to investigate the dynamics of watershed storage of the The Anjeni (113.4ha) experimental watershed found in the upper Blue Nile basin for which a long series of rainfall and runoff data available is used to provide this investigation. The daily water balance equation was used for quantifying the watershed storage over the distinct rainy seasons. On average, 86% of the annual rainfall occurs during these distinct rainy seasons. The study showed that the watershed storage increases with the increase of cumulative effective rainfall till the watershed stores its maximum capacity. After this maximum capacity, the watershed storage remains constant, even if the rainfall continuous. The Anjeni watershed stores an average of 380 mm of water after a cumulative

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effective rainfall of 625 mm. Before the maximum storage reached, about 60% of the effective rainfall was occurred to wet up the watershed, where the remainder becomes surface runoff and interflow, during which about 40% of the flow appeared at the outlet.

Keywords: Watershed storage, Rainfall and runoff, Anjeni watershed, Blue Nile basin, Ethiopia 16.1 Introduction

Understanding the functions of watersheds helps in quantifying, planning, and development of water resources. These functions are prerequisite for implementation of best management practices. Watershed storage is defined as the quantity of water that exists within a control volume. It is quantified from the simple daily water balance of the watershed that includes the soil moisture, deep ground water percolation, surface depression storage, and the ground water leakage either to the neighboring watersheds or farther downstream from the outlet.

Watershed storage is one of the key functions of watersheds (Black 1997) and it is a significant component of a catchment water budget and its quantification provides a fundamental understanding of catchment hydrological processes. It is a primary variable for watershed rainfall-runoff modelling (Brutsaert 2005; Kirchner 2006; Sugawara and Funiyuki 1956). The commonly applied procedure in hydrologic modelling is to calibrate the model parameters based on measured watershed inputs, and measured output at the watershed outlet. However, validation of hydrologic models performance only on the basis of stream flow is misleading as models may simulate runoff while misrepresenting the hydrological processes that generate the runoff (Kirchner 2006). Thus hydrological models can provide the right answer with wrong assumptions. Taking into account the effect of watershed storage component on rainfall-runoff model accuracy and efforts towards improving storage estimates cannot be underestimated (McNamara et al. 2011). Moreover, understanding hydrological processes by which how catchments retain and release water is central to hydrologic science.

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Watershed storage also serves as a measure for catchment response comparison. A review on related literature reveals that studies on storage measurements are limited. The recent works of (Kirchner 2009; Soulsby et al. 2009; Spence 2007; Spence 2010) showed studies towards watershed storage is slowly growing. So far, the topic receives little attention that could be due to the distributed nature of watershed storage heterogeneity, difficulty of watershed characterization, and watershed scale storage measurements (McNamara et al. 2011). Despite all these importance of watershed storage, few attempts have been made to estimate the volume of subsurface water storage at the watershed scale (McDonnell 2003; McDonnell 2009; Sayama et al. 2011). In addition, subsurface heterogeneity makes the storage-discharge relationship even more complicated (Beven 2006). Quantifying the total water storage is a very challenging task and previous attempts to measure watershed storage in the subsurface are hindered by the ill-defined boundary conditions of the ground water that are very difficult to define (Sayama et al. 2011). Here, we focus on the dynamic component of total watershed storage – the amount of storage change in a system over a distinct rainy season. The main objective of study is to understand the dynamic subsurface watershed storage in a small experimental watershed at the head water of the upper Blue Nile in the rainy season. The change in the watershed storage over three categories of wetness of the rainy season is evaluated and compared for several years.

Hydrology of the Nile River basin has been studied by various researchers. These studies encompass various areas including stream flow modeling, sediment dynamics, teleconnections and river flow, land use dynamics, climate change impact, groundwater flow modeling, hydrodynamics of Lake Tana, water allocation and demand analysis (Melesse et al. 2009 a, b, c; Abtew et al. 2009a, b; Melesse et al. 2011; Dessu and Melesse 2012, 2013; Dessu et al. 2014; Yitayew and Melesse 2011; Chebud and Melesse 2009a, b, 2013 ; Setegn et al. 2009a, b, 2010; Abtew and Melesse 2014a, b, c; Melesse 2011; Melesse et al. 2014).

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16.2 Study Area

Under the Soil Conservation Research Program (SCRP), seven experimental watersheds were established in Ethiopia in the 1980’s. The main objective of the SCRP were to help understand land degradation processes and to combat land degradation. The program was administered jointly by Ministry of Agriculture of Ethiopia and Centre for Development and Environment of Bern University (Bosshart 1997). The watershed for this study, Anjeni watershed (10o40’N and 37o 31’E, Figure 13.1) is located at the head water of the Blue Nile basin southwest of the Choke Mountain (4100 m amsl). Meteorological, rainfall, steam flow, and sediment data has been collected since the establishment of this station in 1984, as one of the seven experimental watersheds. It is located at 365 km from Addis Ababa on the main road to Bahir Dar, with 15 km off road in the north direction at Dembecha town.

Anjeni watershed with a catchment area of 113.4 ha is in the moist temperate type agro–climatic zone locally named (Weyna Dega). It has an extended uni-modal type of rainfall. The 17 years 1985 to 2004, (except incomplete data years of 1991, 1999 & 2001) long term mean annual rainfall is 1716 mm with standard deviation of 128 mm. The minimum, median, and maximum rainfall over the study area was 1372, 1739, and 1907 mm, respectively. About 86% of the annual rainfall occurs during the wet seasons from mid-May to mid-October. The long term mean monthly rainfall in the watershed is shown in Figure 13.2. The standard deviations of the 17 years monthly rainfall in June, July, and August was 65 mm whereas in September it was 50 mm. The annual mean air temperature for the period between 1995 and 2006 was 9.2 oC. The elevation of the Anjeni watershed ranges from 2405 to 2500 m. Anjeni is a perennial stream that drains north-south to Chereka stream that joins the Birr River, which finally drains to the Blue Nile River. The watershed is among the most productive areas with cereals, beans and oil seed as dominant crop types.

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Fig. 13.1 Location map of the Anjeni watershed

Fig. 13.2 Long term mean monthly rainfall of the Anjeni watershed

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The soils of Anjeni are developed on the basalt and volcanic ash of

the plateau. More than 90% of the watershed consists of four types of soils. The southern valley bottoms of the watershed are predominantly covered with deep, well weathered Humic Alisols (21%); most of the northern part of the watershed is covered with the shallower Haplic Alisols (21%). Haplic Nitosols (17%) is distributed mostly in the south western gently sloping part and some in the east steep upper slopes and the central northern watershed is covered with Humic Nitosols (6.6%). Regosols and Leptosols (12%) scattered in the northern steep slopes and in southern boundaries of the watershed with very shallow depth. The middle area covered with moderately deep, young Dystric Cambisols (19%). Other soils like Luvisols, Leptosols and Acrisols that adds to (3%) are also found in small pockets in the watershed (Legesse 2009; Zeleke 2000). 16.3 Methods

On the basis of the long term (17 years) mean rainy seasons rainfall three categories of wetness are identified: dry, normal and wet. A ‘dry’ rainy season refers to a rainy season with total rainfall 5% less to that of the long term mean. The ‘normal’ season falls within 5% of long term mean. Season with total rainfall of 5% plus the long term mean is categorized ‘wet’. Nine rainy seasons (three in each category) were selected. We used a simple water balance technique to estimate the water storage in the watershed. Watershed storage in distinct rainy seasons is computed as the difference between the input precipitation (P) and the outputs of stream flow and evapotranspiration (Q & ET) of the topographically defined watershed on daily basis over a distinct rainy season. Applications of equation (16.1) should take into account how the individual components are measured or estimated, and the spatial scales over which such measurements/estimations are applicable. Rainfall measurements are basically local. Rainfall rates vary in space and time. Single rain gauge measurements cannot represent areal distribution accurately. Similarly evapotranspiration estimates from a single station data are not representative of the actual areal watershed evapotranspiration. Added to this is the need to account for the accuracy of the method used to estimate ET.

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Stream flow is assumed as an aggregated measurement from the entire watershed, but yet the entire watershed does not contribute to stream flow. Bearing these uncertainties in mind, the point measurements of rainfall and estimates of ET from a single station data are assumed to represent the areal watershed values. It was also assumed that the entire watershed is contributing to the measured stream flow at the outlet. With all these assumptions, the analysis presented here explores the watershed storage change over the distinct rainy seasons. The storage change was computed from the daily water balance equation (Eq. 13.1) as:

푺 = (퐏(퐭)−푸(풕) −푬푻(풕))푻풕 ퟏ 13.1

where, t (day) is the days from the beginning of the rainy season (t = 1, is the first day of the rainy season and t = T, is the last day of the rainy season, S (t) represents the dynamic storage increase or decrease from t = 1 to t = T., P (mm) is the average daily rainfall over the watershed, Q (mm) is the average stream flow from the entire watershed, and ET (mm) is the areal average actual evapotranspiration of the watershed. Since the absolute volume of total watershed storage cannot be quantified, using water balance method, the analysis focused exclusively on how dynamic storage changes over the distinct wet season (from the beginning to the end of the rainy season).

Rainfall was measured using pluviograph where the rain is collected in a bucket supported on a spring balance; a mechanical lever arm of the spring is connected with a pen which touches a clock mounted drum with a graph paper. The record shows the accumulation of precipitation over time. The precipitation can be computed from the required time step from the printed graph. Stream flow discharge was measured with automatic float which the ink pen draws graph on a pluviograph. The automatic float readings were combined with the manual gauge readings. More detail information on the data collection and processing is found in (Hurni 1984) and ( Bosshart 1997). During high flood events, smaller time steps (as low as 5 minutes) flow data records was computed and changed to volume. This smaller time step volumetric flow was summed up for the daily volumetric flow. The total daily volumetric flow was changed to depth units, assuming the entire watershed contributes to runoff. The daily evapotranspiration in the wet season

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was estimated using Enku’s temperature method (Enku and Melesse 2014) Equation 13.2. 퐸푇 =

. 13.2

where, ET is the daily evapotranspiration (mm/day), Tmax is the daily maximum temperature (oC), k is estimated as 38*Tmm – 63, where Tmm is the long term mean maximum daily temperature (oC). During rainy seasons, the watershed surface is sufficiently wet, so that evaporation is assumed to be at the potential rate. In analysing the watershed storage dynamics as rainfall progresses during the rainy seasons; effective rainfall that is rainfall minus evapotranspiration (P-ET), is used instead of rainfall alone since the combined value is a more accurate estimate of the water available for movement or storage in the soil (Liu et al. 2008; Steenhuis et al. 2009). Cumulative effective rainfall during each rainy season was also calculated.

Since the starting and ending dates of the rainy season varies from year to year in the highlands of Ethiopia, a simple and consistent method was used to delineate the rainy season of each of the years. The start of the rainy season was determined when a cumulative rainfall exceeds cumulative evaporation during a seven day period (P – ET > 0) and a rainfall event day with rainfall larger than evaporation, afterwards rainfall continues. If none of the days in the subsequent ten days period had rainfall in excess of evapotranspiration (P - ET > 0), then the rainy season was stopped.

16.4 Results

The total water storage change in the Anjeni experimental watershed during the distinct rainy seasons was evaluated in three wetness categories which were divided based on their wetness. On the basis of the classification threshold the wet seasons of the years 1985, 1986, and 1996 were categorized as ‘dry’, that of 1987, 1990, and 1998 as ‘normal’ and 1988, 1997 and 2000 as ‘wet’. Table 16.1a-c summarizes the characteristic annual rainfall, rainy season duration, non-rainy days and number of rainy days. A stream flow (mm day-1) vs cumulative change in storage (mm) plot helps to visualize the relationship in the each of the three categories. The plot portrays how stream flow responds to rainfall after an extended dry season where watershed storage is at the minimum.

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16.4.1 Dry Rainy Seasons

The rainy season in this category lasts with average of 135 days and average 14 days of non-rainy days. The watershed storages for the ‘dry’ years were 335 mm, 260 mm, and 350 mm at cumulative effective rainfall of 540 mm, 400 mm, and 670 mm, respectively (Table 13.1a and Figure 13.3a). With the occurrence of 540 mm of average cumulative effective rainfall (i.e. 58% of the total effective rainfall), the average total watershed storage was 315 mm (Table 13.1a). (Figure 13.3a) shows the cumulative change in storage over the rainy period vs time. The total watershed storage was reached maximum after about 55% of the total rainy season rainfall had occurred.

Table 13.1a Description of rainfall, runoff, and storage in a dry rainy season category in Anjeni watershed

Year Dry rainy season

1985 1986 1996 Avg. Annual Rainfall (mm) 1556 1372 1697 1542 Rainy season rainfall (mm) 1272 1205 1372 1283 Rainy season rainfall (%) 82 88 81 83 Rainy season duration (days) 125 129 152 135 No. of non- rainy days 9 13 21 14 No. of days with rainfall <= 5mm 50 44 72 55

Cum. ΔS at threshold 335 260 350 315 Cum. excess RF at storage threshold 540 400 670 537 Cum. rainfall at storage threshold 687 553 865 702 Rainfall before threshold (%) 54 46 63 54 Total effective rainfall 945 833 984 921 Effective rainfall at threshold (%) 57 48 68 58

Q (mm) 572 510 637 573 Q at threshold (mm) 207 123 294 208 Q after threshold (mm) 365 387 343 365

Q before threshold (%) 36 24 46 35 Q after threshold (%) 64 76 54 65

Rainy season C 0.45 0.42 0.46 0.45 C before threshold 0.3 0.22 0.34 0.29 C after threshold 0.62 0.59 0.68 0.63

Note: Q is runoff; C is runoff coefficient; Cum. is cumulative; No. is number; ΔS is change in storage; RF is rainfall

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Table 13.1b Description of rainfall, runoff, and storage in a normal wet season category in Anjeni watershed

Year Normal rainy season

1987 1990 1998 Avg.

Annual Rainfall (mm) 1811 1668 1773 1751 Rainy season rainfall (mm) 1539 1381 1504 1475 Rainy season rainfall (%) 85 83 85 84 Rainy season duration (days) 152 130 151 144 No. of non- rainy days 11 8 8 9

No. of days with rainfall <= 5mm 58 53 52 54

Cum. ΔS at threshold 380 350 416 382 Cum. effective RF at storage threshold 660 672 730 687

Cum. RF at storage threshold 937 886 942 922 Rainfall before threshold (%) 61 64 63 63 Total excess rainfall 1100 1032 1101 1078

Effective RF at threshold (%) 60 65 66 64

Q (mm) 607 598 612 606 Q at threshold (mm) 273 316 301 297

Q after threshold (mm) 334 282 311 309

Q before threshold (%) 45 53 49 49

Q after threshold (%) 55 47 51 51

Rainy season C 0.39 0.43 0.41 0.41 C before threshold 0.29 0.36 0.32 0.32

C after threshold 0.55 0.57 0.55 0.56

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Table 13.1c Description of rainfall, runoff, and storage in a wet rainy season category and overall average for the three categories for Anjeni watershed

Year Wet rainy season Table a, b, c

1988 1997 2000 Avg. overall average Annual Rainfall (mm) 1855 1708 1907 1823 1705

Wet season rainfall (mm) 1649 1545 1710 1635 1464 Wet season rainfall (%) 89 90 90 90 86 Wet season duration (days) 140 158 177 158 146 No. of non- rainy days 10 12 6 9 11

No. of days with rainfall <= 5mm 53 64 75 64 58

Cum. ΔS at threshold 430 456 415 434 377

Cum. excess RF at storage threshold 590 650 707 649 624 Cum. rainfall at storage threshold 715 900 980 865 829 Rainfall before threshold (%) 43 58 57 53 57 Total excess rainfall 1323 1090 1210 1208 1069

Rainfallexcess at threshold (%) 45 60 58 54 59

Q (mm) 631 503 678 604 594 Q at threshold (mm) 160 195 292 216 240

Q after threshold (mm) 471 308 386 388 354

Q before threshold (%) 25 39 43 36 40

Q after threshold (%) 75 61 57 64 60

Wet season C 0.38 0.33 0.4 0.37 0.41 C before threshold 0.22 0.22 0.3 0.25 0.29

C after threshold 0.5 0.48 0.53 0.5 0.56

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Fig. 13.3 (a) Cumulative change in storage vs time (b) Daily stream flow (mm/day) vs cumulative change in storage (mm) for the dry rainy season

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The stream flow (mm day-1) vs cumulative change in storage (mm)

plot (Figure 13.3b) for the ‘dry’ season shows that at the start of the rainy season, when the watershed was dry, stream flow is minimum in the order of 1 mm day-1) due to extended dry season. But as the rainfall progresses the watershed storage started filling up and when the cumulative watershed storage reaches 100 mm, the daily stream flow started increasing almost linearly with increase in cumulative change in watershed storage. After the watershed storage reached maximum threshold value, although the change in watershed storage stays almost constant, the daily stream flow continuous to increase as effective rainfall increases (Figure 13.3b). The daily stream flow reached as high as 23 mm day-1 in 1996 due to high rainfall event after storage reached maximum (Figure 13.3b). The average maximum watershed storage (threshold) was 315 mm in the dry distinct wet season. The runoff coefficient (C) increased from an average of 0.29 before the watershed storage threshold to an average of 0.63 after the watershed storage threshold and 65% of the stream flow during the rainy seasons occurred after the storage reached maximum (Table 16.1a). The average runoff coefficient during the whole rainy season was 0.45 (Table 13.1a). 16.4.2 Normal Rainy Seasons

The rainy season in this category lasts an average of 144 days with an average 9 days of non-rainy days. It was found that, the total watershed storage in these seasons are 380 mm, 350 mm, and 416 mm at a cumulative effective rainfall of 660 mm, 670 mm, and 730 mm, for the respective years (Table13.1b and Figure 13.4a). The cumulative watershed storage change in the normal wetness years varies from a minimum of 350 mm in 1990 to a maximum of 416 mm in 1998. This difference could be due to the ill-defined ground water boundary condition and rainfall properties in the wet season (Table13.1b). The average total watershed storage was 380 mm with a cumulative effective rainfall (sum of RF – ET in the wet season) of 690 mm, 64% of the total effective rainfall during the rainy season Table (13.1b). Figure 13.4a shows the cumulative change in storage (logarithm scale) over the wet period vs time linear scale. The total watershed storage was reached maximum after about 63% of the total rainy season rainfall had occurred.

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Fig. 13.4(a) Cumulative change in storage vs time (b) Daily stream flow vs cumulative change in storage (mm) in the normal rainy seasons

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The stream flow (mm day-1) vs cumulative change in storage (mm)

plot (Figure 13.4b) for the ‘Normal’ rainy season shows that at the start of the rainy season, when the watershed was dry, stream flow is minimum in the order of 1 mm day-1) due to extended dry season. When the cumulative watershed storage reaches 100 mm, the daily stream flow started increasing almost linearly with increase of cumulative change in watershed storage. After the water shed, storage reached maximum threshold value of 380mm, although the change in watershed storage keeps almost constant, the daily stream flow continuous to increase as effective rainfall increases (Figure 13.4b). The daily stream flow reached as high as 28 mm day-1 in 1990 due to high rainfall event after watershed storage threshold (Figure 13.4b). The runoff coefficient increased from an average of 0.32 before the watershed storage threshold to an average of 0.56 after the watershed satisfies its storage capacity. The average runoff coefficient during the normal wet season was 0.41 (Table 13.1b).

16.4.3 Wet Rainy Seasons

The rainy season period in the wet category is generally prolonged, lasting on average of 158 days with 9 non-rainy days. In this category change in storage was analysed for three rainy seasons; 1988, 1997, and 2000. The cumulative change in storage in this category is generally larger. On average, the watershed storage is 435 mm after a cumulative effective rainfall of 650 mm (Table 13.1c). Moreover the watershed storage steadily keeps on increasing with increase in cumulative effective rainfall even after the watershed stores its maximum capacity (Figure 16.5a). A possible explanation here will be groundwater leakage to an adjacent watersheds and farther ground water flow downstream of the gauging station and/or error of closure due to the ill-defined boundary condition of the ground water.

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Fig. 13.5 (a) Cumulative change in storage vs time (b) Daily stream flow vs cumulative change in storage in the wet rainy season

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The stream flow (mm day-1) vs cumulative change in storage (mm) plot (Figure 13.5b) for the ‘wet’ category show similar to the ‘dry’ and ‘normal’ rainy seasons: when the watershed was dry, stream flow is minimum in the order of 1 mm day-1. Similarly, when the cumulative watershed storage reaches 100 mm stream flow start to increase linearly with cumulative change in storage. When watershed storage reached its maximum threshold value, stream flow kept on increasing as effective rainfall increased (Figure 13.5b). The runoff coefficient increased from an average of 0.25 before the watershed storage threshold to an average of 0.50. Sixty four percent of the steam flow had occurred during after the watershed storage reached maximum (Table 13.1c). The average runoff coefficient during the wettest season was 0.37 (Table 13.1c).

16.5 Discussions and Conclusions Understanding functions of a watershed is important for appropriate soil and water conservation measures and for planning and development of water resources for sustainable use. Watershed storage is a significant part of a catchment water budget and its quantification helps to know the fundamental catchment hydrological processes. The study is aimed to understand the dynamics of watershed storages in distinct wet seasons. The daily observed rainfall and runoff data and modelled evapotranspiration (ET) estimates were used in quantifying watershed storage dynamics over distinct wet seasons using simple water balance approach. The dynamics of watershed storage was evaluated in three wetness categories of the distinct wet season of the Anjeni watershed. During the dry wet season the watershed storage reached a maximum threshold average value of 315 mm after a cumulative effective rainfall of 540 mm. In the normal rainy season, the average total watershed storage was 380 mm after a cumulative effective rainfall of 690 mm. In the wet rainy season, the average watershed storage is 435 mm after a cumulative effective rainfall of 650 mm. The watershed maximum storage threshold point is generally lower in the dry category when compared with the normal and wet categories. The water storage of the Anjeni watershed generally ranges from 315 mm in the dry rainy season to 435 mm in the wet rainy season. The difference in the watershed storage among the rainy seasons could be due to the rainfall volume, rainfall characteristics, and the number of rainy and non-rainy days. For example, the ‘dry’, ‘normal’, and ‘wet’ rainy seasons’ on average last 135, 144, and 158 days, respectively (Table 13.1a, b, c). The dry, normal, and wet rainy season non-rainy days were 14, 9, and 9, respectively (Table 13.1a, b, c). The dry rainy seasons were generally lasts

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in short periods but relatively longer non-rainy days than the normal and wet rainy seasons (Table 13.1a, b, c). For instance, the 1986 dry category rainy season lasted 129 days with 13 non-rainy days, while the 2000 wet category lasted an extended 177 days with only 6 non-rainy days. It is this difference that brought the difference in the watershed storage dynamics among the wetness categories.

The general increase of the watershed storage volume in the wet rainy seasons and the slow increase of the storage volume as rainfall progresses is most probably due to ground water flow leakage to the neighbouring watersheds and ground water flow further downstream of the gauging station. The loss of water from one watershed to another through deep groundwater systems can be important in such small watersheds like Anjeni. Quantifying this ground water flux was not possible due to ill-defined boundary conditions that could over estimate the total change in storage of the watershed especially in the wet category. Irrespective of the wetness of the rainy season category, the watershed responds very similarly till the watershed stores 100 mm. After 100 mm of storage, stream flow increases linearly with increase in watershed storage. But once, the watershed storage reached its maximum threshold value, stream flow kept on increasing as rainfall continues, while storage was almost constant at the maximum storage (Figs 16.3, 16.4, and 16.5). As rainfall continues to accumulate during a rainy season, the watershed eventually reaches a threshold point where it cannot store any more water. The watershed runoff response can be predicted by a linear relationship with effective rainfall indicating that the proportion of the rainfall that became runoff was constant during the remainder of the rainy season. The watershed runoff coefficient before the storage threshold was 0.29. This increased to 0.56 after the watershed storage threshold is reached. On the average, 40% of the flow occurred before the watershed storage threshold point even though 60% of the wet season’s rain had occurred. While the remaining 60% the flow occurred after the storage threshold point with the remaining 40% of the wet season rainfall (Table 16.1a, b, c). The majority of the stream flow occurred after a storage threshold value with less rainfall volume. This showed that stream flow is a function of watershed storage threshold. Since the majority of the stream flow occurred after watershed storage threshold point, this helps to conclude that saturation excess runoff is the dominant runoff generation mechanism in the Anjeni watershed. This is in line with the previous studies of (Collick et al. 2009; Liu et al. 2008; Steenhuis et al. 2009; Tilahun et al. 2013) which concluded that the dominant runoff generation mechanism is saturation excess. So, hydrology models which consider the watershed storage

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dynamics in their input could model better the rainfall and runoff relations in such watersheds in the monsoonal climates of the Upper Blue Nile. Further studies with watersheds having different sizes and data collected in sub daily time step could help better understand the functions of the watersheds in the Upper Blue Nile basin.

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