An analysis of the ET/E_p-curve and its application irrigation scheduling

Abstract

The well known sigmoidel ET/E_p -curve often quoted in the literature for a variety of crops, merely depicts a smoothed curve drawn through points representing the average ET/E_p ratio for each depletion cycle co-ordinated with the middle day of the cycle. In this form it is of little use for monitoring the water balance of a crop during periods shorter than a normal irrigation cycle. The seasonal curve can be broken down into a number of separate curves based on the actual ET/E_p ratios (f_i) within each depletion cycle. From these f_i-curves, hypothetical curves were derived from which cumulative soil moisture depletion up to a given day within an irrigation cycle can be deduced with ease. This approach provides a way in which the efficiency of rain precipitation can be estimated to a higher degree of accuracy so that the irrigation programme can be adjusted accordingly.