Groundwater-surface water interaction and the climatic spatial patterns ofhillslope hydrological response

A transient, mixed analytical-numerical model of hillslope hydrological beh aviour is used to study the patterns of infiltration, evapotranspiration, r echarge and lateral flow across hillslopes. Computational efficiency is ach ieved by treating infiltration and phreatic surface movement analytically. The influence of dynamic coupling of the saturated and unsaturated zones on the division of hillslopes into units of distinct hydrological behaviour i s analyzed. The results indicate the importance of downhill groundwater flo w on the lateral distribution of soil moisture and hydrological fluxes; uns aturated lateral flow is shown to be of relatively minor importance. For mo st conditions, the hillslope organizes itself into three distinct regions; an uphill recharge and a downhill discharge zone separated by a midline zon e over which there is, on average, no recharge or discharge. A temporal per turbation analysis of the phreatic surface, made to quantify the deviations between the equivalent-steady water table derived by Salvucci and Entekhab i (1995) and the long-term mean water table, shows that the equivalent-stea dy water table effectively couples the unsaturated and saturated zone dynam ics across storm and interstorm periods and divides the hillslope into dist inct hydrological regions. The second order closure terms in the perturbati on analysis, expressed as the gradient of water table variance, quantify th e deviations and tend to make the hydrological zones relatively less distin ct.