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
