Estimating Crop Water Use of Cotton in the Texas High Plains

Published in Agron. J. 102:1641–1651 (2010) Published online 30 Sept 2010 Available freely online through the author-supported open access option. doi:10.2134/agronj2010.0076 Copyright © 2010 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. I the semiarid Texas High Plains of the United States, the growth and yield of agricultural crops such as cotton is inexorably linked to the amount of water available from precipitation and/or irrigation. Many studies (Sammis, 1981; Hanks and Rasmussen, 1982; Howell et al., 1984; Hay and Walker, 1989) have shown a linear relationship between crop dry mass or yield and the water used by the crop over the growing season. Th e ability to quantify CWU allows researchers to study the interaction of crops and their environment, and supports practical crop production activities such as irrigation scheduling. While various defi nitions of CWU exist, we choose to defi ne it as the water used by the crop, which would exclude soil evaporation. Th us, from an agronomic sense, our defi nition of CWU and transpiration are roughly synonymous. For agricultural fi elds, measurements made with lysimeters or micrometeorological sensors (such as eddy covariance systems) typically quantify evapotranspiration (ET), the combination of crop transpiration and soil evaporation. However, ET values approach CWU when the soil evaporation component is comparatively small, as when the upper portion of the soil profi le is dry or when the crop canopy completely covers the soil surface. Numerous methods have been developed for estimating the water use of a growing crop. Perhaps the most popular is that described by Monteith (1965), which was developed by introducing aerodynamic and surface resistance terms into the surface energy balance approach of Penman (1948). While this “Penman–Monteith” method applies to evaporation from any uniform, continuous surface, it can be used to estimate crop transpiration by assuming that the evaporating surface is a uniform, continuous plant canopy that completely covers the soil surface (Van Bavel, 1966). Th is is oft en called the “Big-Leaf” form of the Penman–Monteith Equation, and can be expressed as follows (Allen et al., 1989),