MODELING YIELD CHANGES AFTER STRIP THINNING IN A MOUNTAIN ASH CATCHMENT - AN EXERCISE IN CATCHMENT MODEL VALIDATION

Lumped catchment models spatially average hydrologic processes at the hillslope scale or larger. Typically they can only be calibrated to st reamflow data, and judgements on the reliability of their predictions must be based upon the results of a split-sample test in which part of the streamflow record is used for calibration and the remainder reser ved for an independent test of the model. However, it has been argued that such a split-sample test is not an adequate test of model structu re or of the hypotheses upon which the model is based because the inde pendent record is typically hydrologically similar to the calibration record, rendering the test an exercise in model interpolation. A stron ger test of model structure would force the model to extrapolate beyon d the range of conditions encountered in the calibration record. One w ay to achieve this is to select a catchment which undergoes a major la nd use change during the independent test period. The small-scale expe riments of forest hydrology are particularly important in this regard because they typically provide high-quality data on the effects of rad ical land use changes. This study illustrates application of this modi fied split-sample approach for the Crotty Creek experimental catchment which underwent a 6 year strip thinning treatment that removed 42% of its forest cover. Two lumped catchment models, the SDI and CATPRO mod els, were compared. To make use of the full rainfall and streamflow re cord it was necessary to modify these models to differentiate between the interception and evapotranspiration fluxes of the forest and the s crub communities which developed in the cleared strips. Both models we re calibrated to the pre-treatment record during which there was 100% forest cover. Only the scrub parameters were calibrated to the post-tr eatment record. It was found that the SDI model, even though it had be en previously extensively tested, failed to predict adequately the hig h monthly flows that followed the strip thinning treatment, whereas th e CATPRO model, using a different conceptualization of quickflow, prov ided a reasonably good description of monthly water yields before and after the strip thinning treatment.