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Georgia Institute of Technology (1978)

Hydrologic simulation in a semi-arid region

Saad, Adnan Ahmad

Titre : Hydrologic simulation in a semi-arid region

Auteur : Saad, Adnan Ahmad

Université de soutenance : Georgia Institute of Technology

Grade : Doctor of Philosophy (PhD) 1978

Résumé
The objective of this research was to develop a watershed simulation model which would reproduce the essential features of the hydrologic regime of a semi-arid region such as Jordan. The Jordan Watershed Model is intended to replace the empirical methods presently being used in simulating streamflow records for wadis where water resources development projects are planned.
The procedure of conducting the research was to collect all available information on soils, geology, topographic maps, rainfall, streamflow, evaporation and previous studies by various agencies and consultants.. The rainfall and the streamflow data was screened and analyzed to get an insight into the major hydrologic and the seasonal charactaristics. A continuous streamflow simulation model was formulated based on the findings of the above analyses and the availability of data on the meteorological and the physical charactaristics of the region. The model was designed to accept daily rainfall and daily pan evaporation. Weighted rainfall was computed by utilizing the constructed isohyetal maps to account for the variability of rainfall pattern.
The model performs a daily moisture accounting on a system composed of infiltration, upper and lower soil moisture storages, drainage, groundwater recharge and evaporation, components which are intended to represent the significant hydrologic processes in a rational manner. The hydrologic processes components which represent the evaporation and the base flow distinguish the Jordan model from others. Evaporation from the depression storage occurs at the potential rate. Upper soil moisture evaporation takes place at a rapid rate due to the shallowness of the soil. Progressive evaporation dries the upper soil and forms a hard layer causing the moisture in the lower soil to evaporate at a reduced rate. The location of the water table at a greater depth restricts further evaporation from the groundwater storage. The variability of base flow recession led to a development of a model component to estimate the base flow recession constant as a function of the groundwater storage.
There are 20 parameters and constants in the model. Ten parameters were selected to be optimized utilizing the Pattern Search technique. The parameters which govern a sequence of model components and those which determine the curvature of the various model component functions are the most sensitive parameters. The domination of the low flows in the streamflow records suggests utilizing the sum of the absolute values of the errors rather than the sum of the squared errors as the objective function in the optimization runs.
For the two basins for which results are presented in this study, the model was successful, on the average, in simulating daily flows, except where the observed streamflow values are questionable. The model gave better results in reproducing low ,flows than flood flows. Streamflow simulation was more successful on a monthly basis than a daily basis. The errors in simulation resulted both from the approximity of the hydrologic processes representation and from the errors in rainfall and streamflow data. The streamflow records which are characterized by low flows suggest utilizing the average absolute value of simulation error rather than the standard error of prediction as a statistical tool for measuring the level of accuracy of the simulation results.
For the five years of record (1969-1973) for the Zerqa River basin, the average value of each element of the basic hydrologic equation, expressed in a percentage of the average annual rainfall, was estimated as two percent surface runoff, three percent groundwater recharge, 92 percent evaporation, two percent losses to seeps, spring, and deep aquifers and one’ percent increase in the soil moisture storage. The importance of this research, being an initial attempt to study the hydrological cycle of Jordan based on approximate mathematical representations of the major hydrologic processes, is that this model offers a significant improvement to streamflow simulation methods presently being used. This research provides a study of the* hydrolpgy of a region that has received very little prior study and is useful in understanding the major hydrologic processes in such a region.

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Page publiée le 25 octobre 2013, mise à jour le 28 août 2017