A new procedure to determine soil water availability

The integral water capacity is first introduced as a flexible method to quantify various soil physical limitations when calculating available water in non-swelling soils. 'Weighting' functions that account for hydraulic conductivity, aeration, and soil resistance to penetration are applied to the wet and dry ends of the differential water capacity, and then integration is performed. The concept is extended to swelling soils by applying the theory of Groenevelt and Bolt (1972), which enables overburden pressures to be taken into account. A set of shrinkage lines measured by Talsma (1977) is analysed using this theory, which enables precise values of overburden potentials to be calculated as a function of the moisture ratio for different load pressures. The addition of the overburden potential to the unloaded matric potential causes minor shifts in the classical limits of plant-available water (viz. -1/3 bar and -15 bar). However, when other soil physical restrictions are taken into account (such as in the concept of the least limiting water range), the consequence for available water deeper in the root-zone (due to an overburden pressure) is far more serious. This is primarily because the matric potential at which aeration begins to be satisfied shifts to a considerably lower value, making a large quantity of water at the wet end no longer available. Examples of weighting functions derived from the literature are applied and their implications for available water in swelling soils are discussed.