Water use by irrigated cotton in the Sudan. 4. Water use, potential evaporation and yield.
Estimates of water used by irrigated cotton crops, obtained by micrometeorological methods, soil water sampling and lysimetry, were related to Penman's estimate of potential evaporation and to crop growth , development and yield. The most serious limitations of the methods used were: errors in the micrometeorological estimates due to large vertical and horizontal variations; imprecision in estimates of soil water content; unrepresentative crop growth inside the lysimeter; and inaccuracies in the measurement of irrigation water applied. Waterlogging decreased crop growth, soon after some irrigations, but insufficient water was available for optimum crop growth in the latter part of most irrigation cycles. Shortage of water was most pronounced near the leading edge so that yield was smaller and was produced later at the upwind end. Advective energy increased evaporation up to 140 m from the leading edge, and up to 200 m on windy days. There was no obvious seasonal pattern of water use by the crop because meteorological and soil factors were overriding. The amount of water required for satisfactory crop growth could be estimated using Penman's formula. The leading half of the field would require about 1.2 ET and the other half about 1.0 ET. Modifications of irrigation practice are suggested to increase yield near the leading edge.