1995 Volume 72 Pages 47-53
The soil-crop-atmosphere continuum is a dynamic system in which water is recycling. It is convenient to use models for analyzing dynamic systems like this. There are many models to predict soil water contents profile and crop yields. In this paper, I introduce models which describe water balance and grass production, and discuss some study subjects in order to construct more accurate model. The subjects are summarized as follows : 1) The root zone of grassland has been considered to be shallow. But grass roots in subsoils can absorb nutrients and water as well as roots in shallow layers. We have to know how deep roots can absorb nutrients and water. 2) Unsaturated conductivity is essential to calculate water flux in soil. But it is not so easy to measure unsaturated conductivity, so there are few reports concering it. It is necessary to increase data on unsaturated conductivity. 3) Transpiration rates change with soil water content and weather conditions. There are some empirical equations for estimating transpiration rates. But it is not clear which equation is best for a certain condition. 4) Water requirement is necessary to convert the water use of grass into dry matter production. Examples of water requirement, measured in Hokkaido, have wide ranges of values. More measurements are needed.
