Soil Moisture and Runoff Processes at Tarrawarra

It has been recognised for at least two to three decades that saturation excess is the dominant surface runoff process operating in most landscapes with a humid climate (Betson, 1964; Dunne et al., 1975). This mechanism is associated with soil moisture patterns characterised by high moisture zones in depressions and drainage lines (Anderson and Burt, 1978a,b,c; Anderson and Kneale, 1980, 1982; Burt and Butcher, 1985, 1986; Dunne and Black, 1970a,b; Moore et al., 1988a). It has also been known that event based hydrologic models are sensitive to initial conditions (Stephenson and Freeze, 1974). Blöschl et al. (1993) and Grayson et al. (1995) showed that event hydrographs simulated by spatially distributed hydrologic models were sensitive to the way in which the antecedent moisture was arranged spatially (see Chapter 1, pp. 12–13 for more detail). However, until recently there has been little spatial soil moisture data available that could be used to determine the characteristics of soil moisture patterns in natural catchments. Most of the data that were available were not sufficiently detailed to provide spatial soil moisture patterns without significant ad hoc interpretation of the data. It is important to recognise that catchment runoff does not give us much insight into internal catchment processes (Grayson et al., 1992a,b). Given that we want to understand runoff processes in catchments, it is necessary to measure internal information, i.e. soil moisture patterns in this case. The apparent importance of spatial soil moisture patterns, combined with the limitations of existing data, motivated us to perform a series of experiments in which we measured actual soil moisture patterns and used these patterns for testing and developing distributed models. In addition to the soil moisture patterns, we measured other key variables that would enable us to interpret the observed patterns in terms of the controlling hydrologic processes. Our aims centred on understanding the spatial variability of soil moisture, its representation in geostatistical and hydrological models and its importance from the