Seasonal evapotranspiration of irrigated cotton in a low-latitude environment

Abstract

Evapotranspiration (E_t) from irrigated cotton was determined on a sequence of plantings covering a whole year. Averaged over the year, evapotranspiration directly after irrigation (E_t1) was greater than the Penman estimate of potential evaporation (E) but less than the evaporation from a standard Australian tank (E_tank). The mean annual ratios were 1.2 for E_t1/E and 0.9 for E_t1/E_tank. The rate of evapotranspiration was a function of soil moisture, declining rapidly as the available moisture fell below 60 %. Because of the high variability in the estimates of evapotranspiration it was not possible to evaluate precisely the influence of growth stage on evapotranspiration. Although there was evidence that evapotranspiration varied with the stage of growth, meteorological factors were a dominant influence because of the high watering regime. Overall, a crop planted in the wet season used little more water than a crop planted in the dry season. After the maximum leaf area index (LAI) had been reached, evapotranspiration in the wet season crop declined more rapidly and fell to a lower value than evapotranspiration during the corresponding period in the dry season crop. Water was used more efficiently by the wet season crop than by the dry season crop. The conclusion was reached that in this environment advection of energy persisted for most of the year, and that during periods of rapid height increases, particularly when ground cover was incomplete, crop surface roughness enhanced evapotranspiration. The possible interactions of some of the factors influencing evapotranspiration in a crop in the field are discussed.