Technical Abstract: Efficient water use is crucial in agricultural water management, and in some crop species, stress-control is an effective approach in the improvement of the product quality. Therefore, it is essential to estimate how much water could be and/or is lost by a crop under natural conditions. The major objective of this study was to develop and verify a simple method for estimating daily crop water loss based on readily available information. We found that the water loss of a soybean crop could be determined based on measurements of incoming solar radiation, air temperature, crop reflectance and crop temperature. Analysis of water loss from irrigated soybean fields showed that this simple approach was an effective method for estimating crop water loss. This technique could provide a basis for monitoring water use efficiency and crop water stress for crop management, and monitoring loss from naturally vegetated surfaces as well.