RUNOFF AND SEDIMENT YIELD ANALYSIS OF ARIQUA WATERSHED, TEKEZE BASIN, ETHIOPIA.

Abstract

Land and water resources degradation are the major problems in Ethiopia. Poor land use practices and improper management systems have been playing a significant role in causing high soil erosion rates, sediment transport and loss of soil nutrients. So far limited measures have been taken in the study area to combat the problems. In this study a physically based watershed model, SWAT2012 was applied to the Ariqua watershed for runoff and sediment yield analysis. The objectives were to estimate runoff and sediment yield analysis of Ariqua watershed, Tigray region with respect to different land use practice factors, soil and slope, in order to Priotize the most erodible sub watershed of Ariqua watershed. To set up the model for simulation a 30 x 30 m DEM (Digital Elevation Model), land use map and soil map were used. The daily recorded weather data from 1991 to 2015 were used as input to the model. Daily stream flow and sediment data from 2001 to 2015 were applied for SWAT simulation. The model was calibrated using both automated and manual calibration for monthly and daily stream flows and sediment yield. The first six years (i.e 2006 to 2011) were used for calibration and the rest (i.e 2012 to 2015) were used for validation. The homogeneity and consistency tests for the collected data were homogeneous and the consistency of the rainfall was linear. Sensitivity analysis, model calibration and validation were also performed to evaluate the model performance. Thirteen sensitive parameters were identified for flow of which SCS runoff curve number moisture condition (CN2) was the most sensitive one and ten sensitive parameters were identified for sediment of which USLE soil erodibility factor (USLE_K) was the most sensitive. The coefficient of determination (R2) and Nash-Sutcliffe (ENS) were used to evaluate model calibration and validation. The results found for flow and sediment were satisfactory for the gauging station (R2 = 0.89 and ENS = 0.61) for calibration,(R2 = 0.79 and ENS = 0.67) for validation and (R2 = 0.87 and ENS = 0.72) for calibration, (R2 = 0.81 and ENS = 0.59) for validation respectively. The model prediction result indicated that the 15 years annual average measured and simulated stream flow generated were 137 m3/s and 124 m3/s, whereas for suspended sediment yield were 12.54 ton/ha/yr and 13.92 ton/ha/yr respectively. Out of 23 sub watersheds, sub watershed (SWT-23) and (SWT-20) generated high stream flow with 383.5 m3/s and 287.3 m3/s. Similarly, five of the sub watersheds, (SWT-17),(SWT-22) and (SWT-15) were found to be critical contributing high sediment yield with an average annual sediment load ranging from 20 to 30 ton/ha/yr and (SWT-23) and (SWT-16) contributed very high sediment yield, 33.1 ton/ha/yr and 31.1 ton/ha/yr respectively. Based on the results of the study, different soil and water conservation measures were recommended and designed. Because of heavy runoff in the study watershed it is generally recommended to apply graded structures (graded soil bund, cutoff drains and waterways), in order to drain excess surface runoff and to conserve soil within the watershed. Structural soil and water conservation practices should be laid out and designed based on estimated peak runoff rate in the watershed and with proper integration of measures and maintain soil and water conservation measures every year in order to sustain their benefits.