Catchment Similarity of Hydrologic Partitioning Along Climate Gradients

Abstract

Climate variability and landscape characteristics interact to define specific catchment hydrological response. Catchments are considered fundamental landscape units to study the water cycle, since all aspects of the land surface component of the hydrological cycle come together in a defined area, which enables scientific research through mass, momentum and energy budgets. The role of climate-landscape interactions in defining hydrologic partitioning, particularly at the catchment scale, however, is still poorly understood. In this study, a catchment scale process-based hydrologic model (hillslope storage Boussinesq- soil moisture model, hsB-SM) was developed to investigate such interactions. The model was applied to 12 catchments across a climate gradient. Dominant time scales (T.S.) of catchment response and their dimensionless ratios were analyzed with respect to climate and landscape features to identify similarities in catchment response. A limited number of model parameters could be related to observable landscape features. Several T.S. and dimensionless numbers show scaling relationships with respect to the investigated hydrological signatures (runoff coefficient, baseflow index, and slope of the flow duration curve). Some dimensionless numbers vary systematically across the climate gradient, pointing to the possibility that this might be the result of systematic co-variation of climate, vegetation and soil related T.S. Each of 12 behavioral hsB-SM models were subsequently subjected to each of 12 different climate forcings. Mean deviations from Budyko's hypothesis controlling long-term hydrologic partitioning (represented by the evaporation index, E/P, dependence on the aridity index, PET/P) were computed per catchment and per climate. The trend observed per catchment could be explained by the dimensionless ratio of perched aquifer storage-release T.S. and mean storm duration T.S. The trend observed per climate could be explained by an empirical relationship between fraction of rainy days and average daily temperature during those days. Catchments that produce more E/P have developed in climates that produce less E/P, when compared to Budyko's hypothesis. Also, climates that give rise to more E/P are associated with catchments that have vegetation with less efficient water use parameters. These results suggest the possibility of vegetation and soil co-evolution in response to local climate leading to (catchment scale) predictable hydrologic partitioning.