Groundwater recharge and discharge response to rainfall on a hillslope

Abstract

Groundwater recharge was investigated within a representative hillslope segment of a small forested catchment, where the depth to the water table increased when progressing upslope from a free water outlet. Catchment soils varied with progression upslope from grey through yellow to red earths, which were underlain by low hydraulic conductivity B horizons and permeable substrata. The catchment was equipped for measuring stream discharge, rainfall characteristics, water table position, soil water content and soil hydraulic properties. Measurements commenced when soil water was severely depleted by drought, and were continued to monitor infiltration and redistribution with depth after more than 1000 mm of rainfall. Water movement occurred under approximately unit hydraulic gradients to the layer of restricted hydraulic conductivity; with movement through this layer proceeding under gradients considerably in excess of unity. Between rainfall events water movement in the soil profiles with deep water tables occurred by redistribution, with the capillary flux exceeding the gravitational flux. Where water tables were shallow (<m), profile recharge occurred within 7 weeks, after which sustained recharge to the groundwater body occurred at rates of the order of 3 mm day-1. Where water tables were deep (>7 m) it took many months for the soil water deficit to be replenished by rainfall. Recharge to groundwater in this case commenced with a flux of about 0.5 mm day-1 and decreased to an estimated value of 0.3 mm day-1 some 5 months later. Following winter rain more than 30% of the annual groundwater discharge from the catchment originated from a relatively small but expanding area near the free water outlet.