Determination of Evapotranspiration from A Maize Field under Drought Conditions

Abstract

During the growing seasons of the last 5 years, we have experienced 3 summer droughts and 1 cool rainy summer. Accurate field water balance information is necessary to manage soil water for stable productivity under such varying conditions.It is well known that shortages of soil moisture in the root zone result in decreases in evapotranspiration (ET). Such conditions stimulate the root systems of many crops to develop into the deeper soil layer to prevent injury from drought. In this paper, the authors present a method to determine ET from fields of maize in consideration of root spread (depth & width) and soil moisture condition in the total root zone.Suzuki & Nakayama (1996) observed high ET rates from a maize field under drought conditions, despite soil moisture conditions in the surface to 1.0m soil layer being less than that required for optimum growth. They showed that ET under such conditions proceeded by water uptake from soil deeper than 1.0m. Those findings suggested that the evaluation of ET from soil moisture content data for only the shallow upper layer may underestimate the actual ET, and this underestimation leads to inadequate control of soil moisture for optimum crop production. Here the authors present a new formula for estimating the actual ET, taking the root system and soil moisture content in the soil mass of the total root zone into account. The accuracy of this formula depends on knowledge of the soil layer of the deepest root end in each growth stage. However, measuring this soil depth is troublesome, a new formula presented in this paper would be more practical if this depth could be estimated based on plant growth stage.