Assessment of available water storage capacity of soils with restricted subsoil permeability

Abstract

The pasture production of most of the tropics is largely a function of the water regime. One of the few options for control of the water regime in these environments is choice of site. A site parameter of great importance is the available water storage capacity (AWSC) in the root zone. Although estimation of the AWSC is straightforward on freely drained soils, it is complicated on poorly drained soils by the inapplicability of the concept of field capacity. The latter soils, however, commonly have a buildup of salts at depth in the profile, presumably due to prevailing leaching conditions. The study reported herein was conducted to test the hypothesis that this salt profile can be used to indicate the depth of wetting under conditions similar to those producing maximum subsoil water recharge in this climate. Twenty sites on mainly texture contrast soils, most with poorly drained subsoils, were examined in detail. A close relationship between the observed depth of wetting and the depth to the salt bulge was found. This relationship, together with the total porosity of the top (wettest) stratum of the B horizon, provides a means of estimating the AWSC of the subsoil.