An exponential root-water-uptake model

Macroscopic root-water-extraction models often do not adequately account fo r the non-uniform distribution of roots in the soil profile. We developed a n exponential root-water-uptake model, which was derived from a measured ro ot density distribution function. The model, incorporated in the Soil-Water -Atmosphere-Plant (SWAP) simulation model, was tested on a clay loam soil c ropped to soybeans and on a sandy loam soil cropped to corn, near Ottawa. C omparisons of measured and simulated soil water contents with the exponenti al model, a linear depth-dependent model and a constant-extraction-rate mod el were also made. The exponential model performed satisfactorily (average relative errors <20%) when used to simulate measured field soil water conte nts at various depths. The constant-extraction-rate model overestimated the soil water contents in the upper part of the soil profile (maximum error 0 .24 cm(3) cm(-3)) and underestimated them (maximum error -0.09 cm(3) cm(-3) ) in the lower part. The exponential model and the linear model performed f airly similarly at the lower depths, but the exponential model gave better results in the nearsurface horizons. The exponential model was sensitive to the root distribution coefficient and to the rooting depth, when the latte r was approximately less than 40 cm. The results of this study suggest that the exponential root-water-uptake model as incorporated in SWAP is an impr ovement over those models, which do not account for the root distribution i n the soil.