Water Requirements of Cotton (Gossypium Hirsutum) Sown on Different Dates in a Monsoon Climate

Abstract

The consumptive use of water by cotton (G. hirsutum L.) variety 149F, sown at three dates, was calculated using Penman Et x crop factor method. The earlier the sowing date the more water evapotranspired, mainly due to differences in the length of the growing season and the amount by which it coincided with the monsoonal depression of potential evapotranspiration. An example is given of calculating the irrigation requirements of a mid-June sown cotton, accounting for average rainfall, showing that the water requirements of cotton during the early stages may be adequately met by the establishment irrigation and rains during July. Examples are cited to show that a charged profile at the beginning of the season may sustain good growth and produce satisfactory yield.The actual water use by crops varies greatly between seasons and locations, depending mainly on the evaporative condition of the atmosphere and the crop characteristics that determine ground cover and roughness of the canopy. Several direct and indirect methods for determining crop water requirements are reported in the literature (Israelson and Hansen, 1962;Hudson, 1965;Chang, 1968). The indirect methods include the use of different evaporation pans and empirical formulae to estimate evaporation from an open water surface, or potential evapotranspiration from a reference crop. These estimates are then related to actual water use by various constants, in a general relation represented by:where ET= Actual evapotranspiration or consumptive use, E = Potential evapotranspiration (evaporation from a short green crop, actively growing, completely covering the ground and adequately supplied with water), E o = Evaporation from an open water surface, and K = Crop or pan factor.The Penman method (Penman, 1948) which estimates E o , and its modification (Penman, 1956), which estimates E t , are widely used to calculate crop water requirements in both temperate and tropical climates (Chang, 1968;Farbrother, 1970). The Penman formula, combining a radiation energy term and an aerodynamic component, requires the following meteorological data; t This work has been carried out under UNDP/FAO Project PAK/73/026.