Flood Frequency Analysis For Lower Awash Subbasin [Tributaries From Northern Wollo High Lands] Using Swat 2005 Model

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Addis Ababa University


Flood frequency analysis provides vital information for the planning and design of many hydraulic structures and risk assessment in flood plain use. The objective of flood frequency analysis is to estimate a flood magnitude corresponds to any required recurrence interval. The resulting relation between flood magnitudes and return period is referred to as Q-T relation (Flood Frequency curve). This study is based on peak discharge, in particular the annual maximum flood Flood frequency Analysis, the determination of flood flows at different recurrence interval, is a common problem in hydrology. The standard procedure to determine probabilities of flood flows consists of fitting the observed stream flow record to specific probability distributions. However, this procedure only works for basins: That have ‘long enough ‘stream flow records to warrant statistical analysis (gauged catchments) because a reliable estimates of the Q-T relationship cannot be obtained from small samples of at-site data because of the high variability involved and where flood flows are not appreciably altered by reservoir regulation, channel improvements (levees) or land use change. The majority section of this study contains the procedure of generating stream flow data for Mille and logiya watershed using rainfall –runoff model approach for use in flood frequency analysis. This thesis describes a spatially distributed watershed model of Lower Awash basin (Mille and Logiya watersheds) that has been developed using SWAT 2005 to describe stream flow generation for Mille and Logiya watershed at Mille station (Gauge033021) and Logiya, (Gauge033027). The Soil and Water Assessment Tool 2005(SWAT2005) was developed by the Agricultural Research Service of the United States Department of Agriculture and distributed by the US Environmental Protection Agency for watershed management. SWAT2005 Simulates through time the daily soil water balance. XII The spatially distributed nature of SWAT 2005 means that the hydrological surface runoff processes are represented independently in different regions across the basin. There are 52 and 29 different regions called HRU (hydrological response units) for Mille and Logiya river watershed respectively in this modeling application (shown at Appendix G and H for Mille and Logiya watershed respectively). A set of SWAT2005 input representative of the basin has been developed from a wide array of data and the flow simulation is successfully tested against measured flow data. For the Mille and Logiya river basin, SWAT2005 model was calibrated and validated over a four year on a daily time base for daily peak surface runoff from January 1990 to December 1994 for calibration and January 1995 to December 1998 for validation period. Sensitive model parameters were adjusted within their feasible ranges during calibration to minimize model predictive error for daily peak flows. Model performance testes were evaluated to test the model accuracy during the validation and calibration period .These measures included percent differences, coefficient of correlation (R2) and Nash-sutcliffe measures (ENs). A summery of the statistical results for hydrological calibration and validation of Mille and Logiya watershed at Mille and Logiya stations is summarized in table5.9 and table5.11 for Mille and table5.10 and table5.12 for Logiya. The daily ENs , R2, and the percentage difference values range from 0.69 to 0.79 , 0.71 to 0.83 and 15.8% to 27.5% respectively for both watersheds at the gauging stations during the calibration and validation period. These hydrology performance test results ranges indicate the model is effectively simulating in the watershed. The results fulfilled the requirements suggested by Santhi et al. (2001) for R² >0.6 and ENS > 0.5. Continuous hydrologic simulation is a valuable tool to determine flood frequencies in Ungauged watershed and in gauged watersheds that have short stream flow records or are heavily regulated. Since hydrologic simulation models the rainfall-runoff relationship in the basin, it can also be used to check the validity of the probabilistic distribution selected for gauged unregulated watersheds with long stream flow records. XIII Stream flow records, if available at all, are often much shorter and most of the sub watersheds are unguaged. The SWAT2005 continuous hydrologic simulation was used the monthly metrological data as an input for weather generator input file (.wgn file) that contains the statistical data needed to generate representative daily climate data for the sub basins and soil, land cover and DEM data layers for simulation and extend the existing short stream flow records form a few years to 50 years data for the watersheds. The objective of the research is to use the watershed (Mille and Logiya) SWAT2005 model generated Annual maximum flow data for determining the best fit distribution for each watershed and Comparative analysis of flood frequency results. The extended records were fitted to a probabilistic distribution using “Easyfit” Statistical application Software and the Gamma three parameter distribution was the best fit distribution for both Mille and Logiya watersheds among many distributions presented within the softwar



Wollo ;High Lands