Abstract: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 0Ð60 cm 3 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 0Ð60 cm 3 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.

Intensive grazing has been responsible for environmental degradation of moorland landscapes over the last 50 years. Anecdotal evidence on Dartmoor, UK, for example, has suggested that it is particularly associated with a greater frequency of large floods, lower baseflow and a greater occurrence of erosion. A study was therefore conducted at Holne Moor, a small catchment on east Dartmoor, to investigate the impacts of grazing animals on hillslope hydrology and stream discharge. The vegetation in the study area ranged from heather (Calluna vulgaris), in areas with low grazing intensity, to short grass with occasional erosion scars visible associated with larger sheep numbers. The physical characteristics of the topsoil beneath heather species were very different compared with short grass. This was not due so much to the direct physical impact of grazing animals, but indirectly to the chemical and biological differences brought about by changes in vegetation response and soil structure. The experiment at Holne Moor indicated that soil water content on the hillslope was linked directly to the stream's rainfall-runoff response via the soil water pathways. In relatively dry conditions, the soil water content is mainly determined by local factors such as vegetation. In wet periods, non-local influences such as slope and contributing area are more important. A soil water content threshold of around field capacity separated the two conditions. A comparison between soil physical properties and soil moisture under different vegetation types indicated that this 'runoff' threshold between dry and wet states is lower in heavily grazed areas. It is argued that more intense grazing causes conditions suited to the increased delivery of soil water to rapid flowpaths such as sheep tracks and other rapid routes which connect the source of this hillslope water to the contributing areas during large floods. Any alteration of vegetation and soil properties due to management causes a substantial increase in 'active' source areas and therefore discharge during large storm events even before any erosion occurs.

Abstract:The paper reviews a number of possible fast and slow hydrological flow mechanisms to account for rapid runoff generation within a catchment. A new interpretation of the kinematic wave process is proposed which develops some of these concepts to explain rapid subsurface flow from a watershed. Evidence for the process is provided by the results from a laboratory soil core experiment and an investigation of the hydrology of a Dartmoor hillslope. A tension response was monitored in the soil core in which pressure waves were propagated downwards and expelled water from the base. The transmission of the wave down the core was considerably faster than the movement of a chloride tracer. The concept of this kinematic wave process and associated water flux was then extended to the Dartmoor watershed. Raindrops reaching the wet soil surface caused pressure waves to travel laterally downslope. During large rainstorms, the hillslope became hydrologically highly connected and the pressure waves forced existing water from seepage faces into the saturated area adjacent to the stream, contributing substantially to the stream discharge. A kinematic contributing area was defined, as determined by both rainfall-runoff ratios and geostatistical analyses of hillslope soil moisture contents, which extended over at least 65% of the catchment area. This kinematic wave theory is consistent with results of translatory flow and macropore flow models, and stable isotope field studies of 'old/new' water.