FIELD-TEST OF A WATER-BALANCE MODEL OF CRACKING CLAY SOILS

This paper describes a field test of a water balance model of cracking clay soils. An important feature of the model is the dynamic treatment of both soil structure (which varies as a function of soil water content) and the crack water balance which is solved as a dynamic equilibrium between input at the soil surface, storage in cracks and uptake into aggregates. Model predictions are compared with measurements of soil water content made by neutron probe for two access tube groups located about 40 m apart in a heavy clay soil (50% clay content) in southern England in two years with contrasting weather (one dry, one wet). The results clearly demonstrated the importance of bypassing flow, with soil water recharge following dry periods occurring nearly simultaneously at all depths in the profile. Root water uptake was also affected by soil structure. For example, very low values of the critical soil air content were inferred (0.5%) and this was thought to reflect preferential root growth in the well-aerated structural porosity. Model predictions and measurements were generally in excellent agreement, although there was an apparent tendency to overestimate the amount of water stored in the surface layers (0.1-0.2 m depth), particularly during autumn soil water recharge periods. This may have been due to simplifications and assumptions in the model, in particular those related to the treatment of rainfall pattern and interception loss, and also the neglect of soil water redistribution in the matrix. © 1990.