DEVELOPMENT OF RAINFALL-RUN OFF ESTIMATION COMPUTER-AIDED MODELS FOR DESIGN OF WATER HARVESTING IN WADI SYSTEMS: CASE STUDY, WADI ABU FARGA - GEDAREF AREA - EASTERN SUDAN

Abstract

Water is one of the limiting factors to food production and socioeconomic development in arid and semi-arid areas. One method to improve livelihoods of farmers in areas of mechanized rain fed agriculture is to adopt water harvesting techniques. Usually , there is in adequate data to design water harvesting projects in engaged wadis. In designing water harvesting or a drainage project, it is essential to quantify the peak runoff rate, for designing the cross-sections of main drainage canals, culverts, and siphons or the capacity of pumping stations. Consequently, this study reviews all constraints hindering design and implementation of water harvesting system with examples. It gives particular attention to determining aspects of hydrological design parameters such as expected rain fall and rain off rates from a watershed. Hence, this study, discuses the rainfall frequency approaches and presents a software package that involves performing a statistical analysis of the recorded rainfall data using different forms of probability of rain fall distributions (Normal, Log-Normal, Weibul, California, Gumbel, Exponential) and then making an estimate for the desired design return period. Using certain rainfall-runoff relationships (Rational method, Curve Number method and Soil Moisture Balance), then this design rainfall is converted into a design runoff; the runoff is thus considered indirectly. x These models were applied to the Abu Farga watersheds in Gedaref State. The effectiveness of the estimation methods for ungauged basins was tested, verified, and validated by comparing the model simulations to observed data using descriptive performance model statistics. The statistics showed that for small areas use of Rational method is more preferred while for large areas it is advised to use the empirical method and SCS – Curve Number method. It is found that there is uncertainty in hydrologic estimates by the various methods used. It is thus, advisable to produce estimate by more than one method and take the most conservative result. Application of the procedure in Wadi Abu Farga reveals the possibility of constructing small scale water harvesting reservoir to store water to be used for supplementary irrigation. Keywords: frequency, analysis, rainfall, distribution, probability, Runoff farming; rain fed farming; water harvesting