Estimating Crop Water Use of Cotton in the Texas High Plains

The growth and yield of cotton (Gossypium hirsutum L.) in the semiarid Texas High Plains is driven by the amount of water available to the crop through rainfall and irrigation. Various methods have been developed for quantifying the crop water use (CWU) of agricultural crops. In this study, we described a method for estimating CWU that uses a modified version of the Penman-Monteith Equation. In this method, CWU is equal to the transpiration of a well-watered crop with complete ground cover (determined from ambient environmental conditions) multiplied by the amount of plant canopy present (quantified by crop ground cover) and a parameter (F-s) related to the effects of stomata! closure. For irrigated and dryland cotton that are acclimated to their respective environments, F-s approximate to 1. This suggests that control of canopy leaf area is a primary mechanism for acclimating to the surrounding environment. When F-s < 1, the plant is not acclimated with its environment and must rely on stomatal closure to conserve available water in the root zone. The method was developed using surface energy balance data and remotely sensed crop ground cover for three fields near Lubbock, TX. This method could be used in irrigation scheduling where irrigation is used to replace the daily CWU of a crop. This approach might be superior to the standard crop coefficient approach because it could use remotely sensed crop ground cover as a "spectral crop coefficient" that would make the resulting estimates of CWU specific to individual fields.