Effect of swelling and shrinkage on the calculation of water balance and water transport in clay soils

Abstract

Volume changes and resulting shrinkage cracks are of great importance for water transport in clay soils. Therefore these processes should be taken into account when applying soil water simulation models to these soils. A general procedure is outlined to model the water balance of clay soils. The main feature of this procedure is the introduction of the shrinkage characteristic into simulation models in addition to water retention and hydraulic conductivity curves. With the aid of shrinkage characteristics and additional assumptions concerning geometry of swelling and shrinkage, bypass flow, crackvolume and surface subsidence of clay soils are calculated. The effects of swelling and shrinkage on water transport are shown by comparing calculations of a model for rigid soils (FLOWEX), a model for cracking soils (FLOCR) and field measurements. Bypass flow as calculated with FLOCR amounted to 28% of precipitation, which resulted in a drier top soil, a higher groundwater table with very rapid response after precipitation events, and a higher drain outflow in the cracking-soil model. The good correspondence between results of FLOCR and field measurements of groundwater levels supports the use of this model in clay soils. The effects of cracks are unfavourable with respect to crop water availibility and transport of pollutants to subsoil, drains and surface water. However, cracks can also have favourable effects. For instance, the number of days at which the soil had insufficient bearing capacity for grazing cattle decreased from 63 to 28 when swelling and shrinkage were taken into account.