Spatial variability of hydrological processes and model structure diagnostics in a 50 km2 catchment

In this paper, we develop diagnostic methods to assess spatial variability in hydrological processes, particularly those relevant to catchment modelling. We target a range of catchment responses, including runoff volume, runoff timing, storage–discharge relationships, and threshold responses to rainfall and soil moisture. The diagnostics allow us to map the scales and patterns of process variability, to test whether climate or physical catchment characteristics can be used to predict patterns in processes, and to explore the implications for appropriate spatial variability in hydrological model structures or parameters. We apply the diagnostic tests to the mid-sized (50 km2) Mahurangi catchment in Northland, New Zealand, combining data from 28 flow gauges, 13 rain gauges, and 18 soil moisture measurement sites to build a comprehensive description of spatial variation in catchment response. The results show a complex picture: different diagnostics reveal different patterns of hydrological processes, and large variations in processes occur, even over the short length scales involved (~10 km). Catchment and climate characteristics almost all show the same pattern, i.e. that subcatchments in the far North and South of the Mahurangi are similar to each other (higher elevation, steep, forested), but contrast with central subcatchments (lower elevation, shallower slopes, pasture). Surprisingly, this pattern is not reflected in the patterns of diagnostic indices, demonstrating the difficulty of defining realistic a priori estimates of spatial variability in processes. We discuss how process variations correspond to the design of components of a simple lumped conceptual model. In the Mahurangi catchment, we find that spatial variations in multiple aspects of the hydrological response imply a need for spatial variation in both model structures and parameters. Copyright © 2013 John Wiley & Sons, Ltd.

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