Performance of conceptual rainfall-runoff models in low-yielding ephemeral catchments

Low-yielding catchments with ephemeral streams involve highly nonlinear relationships between rainfall and runoff, and there is much less documentation and appreciation of the ability to predict stream flow in these very difficult cases than in humid catchments. The predictions of three conceptual rainfall-runoff models are assessed in three low-yielding, emphemeral streams over a 10-year period. The models are a simple conceptual model, Generalized Surface inFiltration Baseflow (GSFB; eight parameters), a hybrid metric/conceptual model, Identification of Hydrographs and Components from Rainfall, Evaporation and Streamflow data (IHACRES; six parameters), and a complex conceptual model, the Large Scale Catchment Model (LASCAM 22 parameters). The Salmon (0.82 km(2)), Stones (15 km(2)), and Canning (517 km) catchments in Western Australia were selected for their range of sizes and low runoff yields (1.6-12.2% of rainfall). Their behavior is representative of a large part of Australia and semiarid regions, where antecedent conditions are critical determinants of streamflow response to rainfall. Such catchments provide a stern test of the capability of conceptual models. Five-year calibration and validation performances were assessed with a range of statistics. The models were run daily but performance was assessed on both a daily and monthly basis by aggregating daily model streamflows and observations up to monthly. The models performed well, particularly in the monthly case where often more than 90% of the variance of observed streamflow was explained in simulation on independent periods. However, while the simple conceptual model is adequate for monthly time periods, the daily simulation results indicate that a slightly more complex model (the hybrid model or the complex conceptual model) is required for daily predictions in these dry catchments. The model simulation results extend the following notion of Jakeman and Hornberger [1993] from humid to semiarid ephemeral catchments: that a model of about six parameters, albeit in an appropriate model structure, is sufficient to characterize the information in rainfall-discharge time series over a wide range of catchment sizes. Models of such modest complexity also predict runoff with good accuracy outside calibration periods, even in ephemeral, low-yielding catchments. The simulation results highlight the critical importance of the deep groundwater and antecedent moisture conditions on stream yields in ephemeral catchments and point to the desirability of accounting for these factors in arid-zone modeling.