Importance of soil moisture measurements for inferring parameters in hydrologic models of low-yielding ephemeral catchments

Low-yielding catchments with ephemeral streams provide a stem test of the capability of conceptual catchment models for predicting the hydrologic response of the natural landscape. Sustained periods of little or no flow mean that the information content of the streamflow time-series for parameter estimation is limited. During periods with no streamflow, such ephemeral catchments also offer no information on a catchment's soil moisture status. As a result, parameters estimated solely from streamflow data are often poorly identified and span a wide range of the feasible parameter space. These general observations were confirmed by an application of the conceptual VIC model in a 6 ha experimental catchment in eastern Australia. Using a Monte Carlo style assessment of parameter uncertainty, it was shown that the simple three-parameter model was ill-posed when calibrated solely to the strearnflow response. Failure of the calibration procedure to distinguish unique antecedent moisture storage conditions prior to large rainfall events meant that the observed strearnflow response could be replicated from a large envelope of potential parameter combinations. The inclusion of an estimated time-series index of areal soil moisture status into the calibration procedure, however, significantly reduced the number of feasible parameter combinations, and resulted in predictions that confirmed Bowen ratio measurements of actual evapotranspiration. Attempts to further reduce parameter uncertainty by including the measured evapotranspiration data into the joint calibration procedure were unsuccessful. The shortness of the measurement record was seen as a major factor inhibiting improvement. The results of this study highlight the critical importance of antecedent moisture conditions on strearnflow yields in ephemeral catchments and point to the desirability of spatio-temporal soil moisture accounting. Future research efforts are discussed in terms of establishing the appropriate spatial and temporal resolution of soil moisture measurements needed to extend the results observed for this small experimental study to larger catchments. (C) 2002 Elsevier Science Ltd. All rights reserved.