Prediction of soil water deficits and optimum irrigation regimes in arid and semi-arid regions

Oginni, Felix Adeniran (1982) Prediction of soil water deficits and optimum irrigation regimes in arid and semi-arid regions. University of Southampton, Doctoral Thesis.

Abstract

The implication of limited availability of water as a constraint to the level of agricultural development in arid and semi-arid regions is that allocation of irrigation water should be optimised with a view of maximising water productivity. To arrive at this end, new approaches were developed for simulating soil water availability and response to yield due to water deficits. An approach for estimating the characteristic costs of irrigation water supply was also presented. Two forms of soil water stress models were proposed and used in-predicting irrigation water deficits for conditions of moderate and higher evaporative demands. Published data were used to test and compare the models with an existing model giving considerations to variabilities in evaporative demands and crop types. Favourable results were obtained. Further analyses indicated that a model's parameters could be determined from knowledge of the evaporative demand. This suggests the possibility of developing a generalised approach. Illustrative examples were presented to show how these models can be used in scheduling irrigation operations. A method for evaluating yield sensitivity during any irrigation interval was developed. It is sensitive to the soil water deficit levels in previous growth stages. The method was exemplified by data on maize, grain sorghum and cotton for longest and shortest growing seasons.Information on water demand in the field and the crop response were combined with characteristic cost of unit water supply and revenue from unit yield. This was included in a computer simulation model developed to predict the optimum soil water regimes in the entire growth cycle.The regional, field and crop data used for testing the computer simulation model characterise an irrigation station at latitude 12° N and south of Lake Chad in Nigeria. Using maize for this program, results generally indicate that the flowering stage should be placed on highest priority for allocating irrigation water. This can be followed by the yield formation and the vegetative stages in that order. The allocated optimum soil water regimes were generally influenced by variabilities in unit cost of water supply.

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