Delineating hydrologic response units in large upland catchments and its evaluation using soil moisture simulations

We present here a basis for delineating Hydrologic Response Units (HRUs) to capture heterogeneity in the catchment's topography, landforms and geomorphologic attributes. To delineate topologically connected HRUs, the catchment is divided into four landforms and sub-basins. These four major landforms represent macroscopic changes in the catchment landscapes, using thresholds derived from a range of terrain analysis techniques - the Cumulative Area Distribution (CAD) curve, average local slope, curvature, Compound Topographic Index (CTI) and the MultiResolution Valley Bottom Flatness (MRVBF) index. The adequacy of the HRUs delineation approach is ascertained by soil moisture movement modelling in the unsaturated zone based on a two-dimensional solution of Richards' equation, across multiple cross-sections of the catchment. The modelling results of the four landform delineated cross-sections are compared with those from the simplest case of a single landform delineated cross-section and with the most complex case of cross-sections divided on a pixel basis. The modelling results indicate gain in accuracy when using the four landform formulation compared to the use of a single landform, and little loss of accuracy compared to simulations on a pixel basis. This study investigates the stability of this HRUs delineation methodology using the data for the Maclaughlin, Bombala and Delegate catchments of the Snowy River at Burnt Hut, New South Wales, Australia.