Prediction of Runoff and Sediment Yield Using AnnAGNPS Model: Case of Erer-Guda Catchment, East Hararghe, Ethiopia

Thesis Overview

<p> 1. INTRODUCTION&nbsp;</p><p>Rainfall-runoff-sediment yield is the most complex hydrological phenomenon to comprehend due to tremendous spatial variability of watershed characteristics and precipitation patterns, making the physical modeling quite complex and involved. The quantity of runoff and sediment yield resulting from a given rainfall depends mainly on rainfall intensity, duration, and distribution besides others, such as initial soil moisture, land use, slope, etc. The runoff is critical to many water resources activities, for example, design of flood protection works, protection of agricultural lands, planning of water storage, etc. The erosion in the watershed may be occurred due to rainfall and runoff, and degrades its land. The sediment transport caused the reduction of storage capacity of rivers and reservoirs [2]. <br></p><p> In Ethiopia, soil and water are the most critical natural resources. Nearly 85% of the population depends on subsistence agriculture. One process that threatens the resource base is soil erosion. Studies have shown that billions of tons of soil are lost annually [48]. Due to greater population pressure and consequently more intensive cultivation, erosion losses have been increasing to an annual areal average of 7 ton/ha equivalent to depth 0.5 mm [22]. Local erosion rates are highly spatially variable ranging from less than 1 to over 400 tons/ha/year [26, 28, 31 and 45]. The high variation in soil loss might be due to variations in slope, rainfall, soil type, land use, plot size and method of estimations. Beside soil losses, rainwater loss in the form of runoff is an important production constraint. The poor land use practices, improper management systems and lack of appropriate soil conservation measures have been major causes of soil erosion and land degradation problems in Ethiopia. Because of the rugged terrain, the rates of soil erosion and land degradation in Ethiopia are high. In addition to this, the combined effects of deforestation, overgrazing, expansion of cropland and unsustainable use of natural resources have contributed to land degradation [14]. <br></p><p> Soil erosion has a serious ecological impact that costs a nation due to both on-site and offsite damages. Three major universal impacts of erosion are reduction in productivity of crops, flood hazards, and the decrease of the life expectancy of water storage structures [15]. Sediment degrades water quality, and carries soil adsorbed polluting chemicals. Sediment deposition in irrigation canals, stream channels, reservoirs, water conveyance structures, reduces their capacity and would require costly operation for removal [21]. In order to formulate management options, soil erosion must be considered. Soil loss from a watershed can be estimated based on an understanding of the underlying hydrological process in a watershed, climatic conditions, landforms and soil factors. One option for formulating management options is to use models to elucidate processes controlling the hydrologic and sediment fluxes. Assessing and mitigating soil erosion at the watershed level is complex both spatially and temporally. Soil type, depth, and location, land cover type and management, topology and other factors make the watershed a complex system where hydrologic and erosive process may differ greatly over a small spatial scale. Knowledge of rainfall, runoff, and soil loss, and their relationships as well as variation in time and space are very important for soil and water management such as designing soil and water conservation and water harvesting structures [42].There are many event based that continuously simulate stream flow, erosion/sedimentation, or nutrient loss from a watershed. However, few models have been developed or tested in the monsoonal climates of Ethiopia.&nbsp;</p><p>To estimate soil erosion and develop optimal soil erosion management plans, many erosion models such as: Universal Soil Loss Equation (USLE) [49], Water Erosion Prediction Project (WEPP) [20], Soil and Water Assessment Tool (SWAT) [3]. European Soil Erosion Model (EUROSEM) [33] and AnnAGNPS [5] have been developed and used over many years. Among these models, the USLE has remained the most practical method of estimating soil erosion potential and to estimate the effects of different erosion factors on soil erosion. USLE has been used for more than 40 years [12; 29] whereas other process-based erosion models developed afterward have limitations in applicability due to intensive data and computation requirements. However, studies using the USLE do not consider the sediment delivery ratio to estimate the sediment delivered to the downstream point of interest [30]. As a result, more recent physicalmodels for soil erosion estimation have been developed that consider the sediment delivery process. Therefore, physical-based hydrologic models are required for studying hydrological process and hydrological responses to land use and climatic changes. AnnAGNPS is a continuous-simulation, physical-based, watershed-scale model intended to be used as a tool to evaluate non-point source pollution from agricultural watersheds ranging in size up to 300,000 ha [5]. The physically based distributed watershed models have higher accuracy in analyzing the impact of land management practices on water and sediment yields in large complex watersheds. The selection of AGNPS for the project was based on the capability of the watershed approach to assess the impact of conservation planning, including BMPs, to reduce sediment loadings to ErerGuda catchment. It incorporates the most current methodologies used by NRCS such as the Revised Universal Soil Loss Equation (RUSLE) [38] and Soil Conservation Service (SCS, now NRCS) hydrologic procedures [44]. In addition, AnnAGNPS provides the ability to aid in the identification and evaluation of sources of water and sediment production within the watershed. There is a knowledge gap with respect to the interdependence between the runoff and sediment yield and watershed on different temporal and spatial scale in Erer-Guda river catchment. The magnitude of sediment transported by the upper Erer-Guda catchment has become a serious concern for planning, design and implementation of numerous national development projects in the area. Furthermore natural vegetation has been almost cleared for agricultural crop production. Shrubs covers small area in the catchment and classified as deciduous and dry forest with medium and small trees including bushes, and some scattered trees showing evidence of former natural forest. Reduction in the soil production capacity, change in river bank and flooding due to sediment deposition are problems calling for estimation of annual runoff and sediment yield in upper Erer-Guda river catchment. Although a number of researchers have conducted erosion studies in Ethiopia, the lack of compelling tool or method has hindered adoption and implementation of their findings [36; 46]. Both mathematical and parametric methods require a lot of information, which is a major constraint in many developing countries [46]. These countries have no appropriate and accurate soil erosion prediction models although universal soil lose equation (USLE) is used in different tropical countries [34].&nbsp;</p><p>Assessment of Annualized Agricultural Nonpoint Source (AnnAGNPS) Pollution model for predicting runoff and sediment yield in Upper Erer-Guda river catchment is imperative. The general objective of this study is to provide a basis for future scenario analysis of water resource management of Erer-Guda River. The specific objectives of this study were: to evaluate the AnnAGNPS model capability to predict the runoff and sediment yield in Erer-Guda river catchment, and to assess the sediment yield and runoff generation with respect to different land use practice and to identify the most erodible sub-catchment of Erer-Guda river catchment. <br></p>