Field methods for studying soil moisture regimes and irrigation practices in clay soils

Characterization of water flow through swelling clay soils with macropores (''cracks'') requires special techniques because these soils are not isotropic and homogeneous as required by standard flow theory. The techniques should preferably be rapid and inexpensive to allow applications in the field. Three experimental techniques, which were recently developed at the Netherlands Soil Survey Institute, are discussed. The measure: (i) vertical and horizontal Ksub(sat) in a gypsum-covered cube of soil which is carved out in situ (the cube method); (ii) the Ksub(unsat) near saturation down to pressure heads of about -15cm by determining fluxes through a series of crusts and the associated negative pressure heads below the crusts (the crusts test); and (iii) short-circuiting, which is the preferential movement of free water along vertical macropores in unsaturated soil, by applying sprinkling irrigation to large, undisturbed cores. In addition, three examples are discussed whereby soil morphological field data are used for simulation models which characterize soil moisture regimes of clay soils. These examples cover: (i) the effect of horizontal cracks on upward unsaturated flow; (ii) infiltration of sprinkling irrigation in a cracked clay soil; and (iii) ponded infiltration of water in a clay soil with worm channels. (author)