Development of Synthetic Unit Hydrographs for Watersheds In The Upper Awash and Upper Tekeze Basins

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Addis Ababa University


Hydro-meteorological data are very indispensable for the assessment and development of water resources. However, most catchments in Ethiopia are ungauged and direct streamflow observations are not available at most sites for which rainfall–runoff relationships are required. Therefore, if there are no flow data from a catchment, a technique is needed for estimating the required design parameters, which do not require the availability of hydrological records. One option is to develop models for gauged catchments and link their parameters to physical characteristics, so that the approach can be applied to ungauged basins in the region, whose physical characteristics can be determined. Usually, one resorts to Synthetic Unit Hydrographs (SUHs) if there are no observed discharge hydrographs. This requires that the catchment characteristics be obtained or determined which are then used to adapt the SUH to suit a particular catchment. In this research work coefficients required for the construction of synthetic unit hydrographs using Snyder’s method for watersheds, which have areas in the range from 59.8km2 to 2449km2 in the upper Awash and Tekeze basins, have been determined. Moreover equations for estimating the lag time coefficient (Ct) and peak discharge (qp) from watershed characteristics were also developed. Thirty concurrent rainfall-runoff events from six gauaged watersheds, four in the upper Tekeze and two in upper Awash, were used to estimate the coefficients and develop the equations. 27 rainfall-runoff events were used for calibration and three were used for verification. Due to the lack of getting watersheds with adequate rainfall and runoff records in Tekeze basin two watersheds that have almost similar hydrological characteristics were used to produce a reasonable and largely reliable estimate. Lag time coefficient (Ct), peak discharge coefficient (Cp), unit hydrograph widths coefficients (Cw) at 50% and 75% of the peak and base time coefficient (Cb) were determined by calibrating Snyder’s equations with the available rainfall- runoff data. The mathematical relations for lag time coefficient (Ct) and peak discharge (qp) with watershed characteristics were established by using regression analyses. xiii Lag time coefficient (Ct) for the gauged watersheds range from 0.362 to 0.736 with mean value of 0.542 and a standard deviation of 0.157. The peak coefficients of the unit hydrographs of the gauged watershed range from 0.064 to 0.346 with mean value of 0.180 and a standard deviation of 0.112. Coefficients for the base time (CB) and for the widths of unit hydrograph (Cw) at 50% and 75% of the peak discharge of the gauged catchments are found to be 1.006, 0.20 and 0.108 respectively these values are recommended to construct UH for an ungauged watershed in upper Awash and upper Tekeze basins. Lag time is an essential input for Snyder’s synthetic unit hydrograph model. Strong correlation with R2 = 0.90 is seen between Lag time coefficient (Ct) and slope of a watershed. And hence the equation Ct = 0.032*S-0.597 is recommended to estimate lag time coefficient (Ct) for an ungauged watershed in the upper Awash and Upper Tekeze basins. In addition to this lag time can be estimated directly from the physical characteristics of the watershed by using the equation tp = 0.127*(LLc/S0.5) 0.352. Strong correlation with R2= 0.92 is seen also between qp (in m3/s) and the variable Z = A/tl and hence the formula qp = 8.71*10-3 *(Z) 1.78 Can be used to estimate the peak discharge. Therefore, applying the equations and the coefficients give an estimate to the required UH characteristics, which might serve the intended purpose, as long as the hydrometeorological and physical characteristics of the watershed under consideration are with in the range of those characteristics for the watersheds in this study.



Upper; Tekeze; Basins