Dynamic catchment water storage-discharge partitioning across water- and energy-limited catchments

Abstract:

A catchment water balance is a tool typically applied at time scales long enough that its residual is negligible, which facilitates catchment classification in terms of long-term hydroclimatic averages. However, doing so subsumes intra-annual variability in its components and corresponding variability in its residual. We suggest that, as a result, assumed runoff response distinctions between catchments based on water- or energy- limitation might be much less pronounced when intra-annual changes in the water balance are explored. To investigate this, we calculated daily changes in relative storage during average and wet water years, across four catchments in different hydroclimates, and compared changes in storage to runoff responses. In catchments in both the humid subtropics and semi-arid temperate climate, we identified three distinct stages of catchment hydrologic partitioning: (1) a wet-up period, where most precipitation is partitioned towards storage; (2) a wet period plateau, where storage is at a maximum, and runoff increases proportionally to each new precipitation event; and, (3) a dry-down period, where evapotranspiration losses play an outsize role in the water balance. Similarities between catchments on either side of the water-/energy- limitation threshold is highlighted by their differences with two wetter catchments, one temperate and one tropical, where runoff remains a more consistent proportion of precipitation, and changes in storage are a proportionally smaller component of the water balance. Although evapotranspiration was observed to increase during the dry-down period, its magnitude alone was insufficient to explain marked decreases in runoff generation during this period, suggesting its further role in redistributing catchment storage within the subsurface.