Abstract
A simple method of predicting soil water content changes is described and used to predict soil water content changes in irrigated and non-irrigated Pangola grass (Digitaria decumbens stent.) pastures on Piarco fine sand soil series (Aquoxic Tropudults). Soil water content changes were determined by adding the effective rainfall and the amount of irrigation applied to the initial or previously calculated soil water content and subtracting the soil water depletion (SWD) calculated from the regression of SWD and total open pan evaporation. Soil water content changes over 3-, 4-, 5- and 7-day intervals were predicted from data measured over the corresponding time intervals. All predictions were good but predictions over 4-day intervals gave the highest correlation coefficients and the best degree of coincidence with measured values. Soil water content changes were also accurately estimated over 7-day intervals using total open pan evaporation from a location about 9km away and using evaporation from an open water surface calculated by the method of Penman. The method and equations proposed can therefore be used to schedule irrigation of Pangola grass on this soil type. The method has the following advantages: (1) it is simple· (2) it is accurate over time intervals from 3 to 7 days; (3) it allows progressive computation to be made; and (4) it requires a limited number of data inputs.
