VARIABILITY OF UNSATURATED ZONE WATER TRANSPORT PARAMETERS - IMPLICATIONS FOR HYDROLOGICAL MODELING .1. IN-SITU MEASUREMENTS

Physically based hydrological models require values of saturated and unsaturated hydraulic conductivity to solve the equations used to describe subsurface flow. Variability of these parameters within and between various land uses and soil types is an important consideration if hydrological behaviour is to be described realistically. A catchment in south-west England has been chosen to validate the European Hydrological System (Systeme Hydrologique Europeen - SHE). The catchment includes three land use domains - arable (including temporary grass), permanent grassland and woodland. For the unsaturated zone component of the SHE both saturated hydraulic conductivity, K(s), and unsaturated hydraulic conductivity as a function of matric suction, K(psi), are required. The aim of this investigation was to measure the parameters for input to the model, to assess the variability of these parameters and to determine whether or not parameters should be used for each domain independently. A Guelph permeameter was used to obtain K(s) and alpha, which describes the slope of the logarithmic unsaturated hydraulic conductivity function (K(psi) = K(s) exp(alphapsi)). Measurements at four depths were carried out. Domains were chosen according to land use - arable land (including temporary grass), permanent grassland and woodland. Both K(s) and alpha were found to be log-normally distributed. The K(s) values were highest in the grassland and lowest in arable land. The difference between means was significant for K(s) but not for alpha in the three domains. High values of K(s) were attributed to preferential flow along slate faces and root-induced channels in the grassland and woodland, respectively, as indicated by dye tests. Finally, it was concluded that, as K(s) and K(psi) differ significantly between domains, different hydrological behaviour would be expected. It is therefore recommended that different parameter sets be used for modelling the individual domains.