Runoff generation in relation to soil moisture patterns in a small Dartmoor catchment, Southwest England

A hydrological experiment was conducted in a small headwater catchment in southeast Dartmoor, UK, to monitor the temporal and spatial variations in soil moisture content at the hillslope scale in order to determine how the spatial organization affected runoff generation. Two distinctly different types of rainfall response were found. During the dry state the soil moisture pattern was very patchy and the increase in stream discharge was relatively small for most rainstorms. The catchment response was limited to about 10% of the area, a figure that is similar in extent to the saturated area identified in the valley bottom. During the wet state, however, modest to large storms resulted in significantly higher discharge rates. The area generating the runoff increased up to 65% of the area. The division between the two ‘preferred’ states occurred at a catchment wetness of about 0·60 cm3 cm−3. This figure was based firstly on the exceptional increase in range, as determined by geostatistical analyses, for the soil moisture content measured associated with very high stream discharges. Secondly, it was consistent with a steep rise in gradient noted for the soil moisture characteristic curves at about 0·60 cm3 cm−3. The greater catchment responses were therefore dependent on the pore size distribution plus other soil characteristics and the connectivity between the wet areas. Copyright © 2003 John Wiley & Sons, Ltd.