A SCALING BASED ESTIMATION OF SOIL UNSATURATED HYDRAULIC-PROPERTIES AT A FIELD-SCALE

This paper deals with the prediction of the soil water retention h (S) and the soil unsaturated hydraulic conductivity K(S) functions of a c lay-loam soil at a field scale (1 ha) where the variable S represents water saturation. The Van Genuchten model and the corresponding Mualem -Van Genuchten model were used to predict h (S) and K(S) functions res pectively. The field data (tensiometric and neutron probe measurements ) used in this study were provided by the soil water balance (four neu tron sites, 0.35 to 1.55 m soil layer) of a soybean crop over a 78 day s growing season. The advantages of the scaling approach for describin g the field variability of the h(S) function were confirmed. The scali ng approach accounted for 73% of the field variability of the soil mat rix potential. A simple procedure was proposed in order to predict the K(S) function using scaling theory. This was done by simultaneously a pplying a ''zero flux method'' and ''deep flux method'' to compute the soil water balance and fit the saturated hydraulic conductivity (K-sa t), the only unknown parameter in K(S).