The vadose zone soil-water balance and transpiration rates of vegetation

A conventional geotechnical site investigation usually provides only a snapshot of soil-water conditions at a site, particularly for the vadose zone. Although it is generally recognised that soil suction and water contents will vary from season to season and from year to year, designs are almost invariably based on what is discovered during a single short-term investigation. As a result, long-term problems may, and often do, arise, which were not foreseen as a result of the site investigation. An evaluation of the annual water balance for a soil system provides a means of examining and understanding the effects of seasonal and longer-term changes in soil-water conditions, and for predicting the effects of future changes of water conditions in the soil. The paper gives an illustration of the use of the water balance technique to investigate annual changes of soil-water conditions in situ. Evapotranspiration is a major component of any soil-water balance, and as it is probably the most difficult component to estimate, special attention is given to its characteristics and estimation, and particularly to the transpiration component of total evapotranspiration and to evapotranspiration from trees.