Estimation of the moisture supply capacity of some swelling clay soils in The Netherlands

Abstract

The growing season in The Netherlands is characterized by a precipitation deficit (evapotranspiration surplus) in most years. Optimal crop development requires therefore moisture supply by the soil in addition to natural rainfall. Moisture can either be supplied as available water from the root zone or as an upward flux from the water table towards the root zone. A computer model was used to simulate soil moisture regimes of a swelling clay soil. The model uses thickness of the root zone, hydraulic conductivity and moisture retention data for the soil horizons and needs depth to water table and climatic data as boundary conditions. Existing flow theory does not describe flow of water in swelling clay soils where dessiccation is associated with the formation of both vertical and horizontal cracks in the soil. Cracks strongly influence the flow regime by reducing upward fluxes to the root zone (horizontal cracks) and by inducing rapid downward flow of water along vertical cracks (short-circuiting). A field technique is used which allows an estimate of the effective reduction of the hydraulic conductivity due to horizontal crack formation. This reduction is a function of the pressure head. The amount of short-circuiting was estimated by analysing the occurrence of heavy and very heavy rains over a 30-year period. Since these rains are likely to partly bypass the root zone, short-circuiting effectively increases the precipitation deficit. The simulation model was experimentally validated using field data for the growing season of 1979. Calculated moisture conditions in the soil agreed only with those that were measured in situ, when the effects of both horizontal and vertical cracking were considered. Additional runs with the validated model were made using statistically expressed climatic data and selected water-table levels. Results allow an estimate of the amount of water which needs to be supplied to the soils, so as to achieve potential evapotranspiration.