Characterizing hydrologic change through catchment classification

There has been an intensive search in recent years for suitable strategies to organize and classify the very heterogeneous group of catchments that characterize our landscape. One strand of this work has focused on testing the value of hydrological signatures derived from widely available hydro-meteorological observations for this catchment classification effort. Here we extend this effort by organizing 314 catchments across the contiguous US into 12 distinct clusters using six signature characteristics for a baseline decade (1948–1958, period 1). We subsequently develop a regression tree and utilize it to classify these catchments for three subsequent decades (periods 2–4). This analysis allows us to assess the movement of catchments between clusters over time, and therefore to assess whether their hydrologic similarity/dissimilarity changes. We find examples in which catchments initially assigned to a single class diverge into multiple classes (e.g., midwestern catchments between periods 1 and 2), but also cases where catchments from different classes would converge into a single class (e.g., midwestern catchments between periods 2 and 3). We attempt to interpret the observed changes for causes of this temporal variability in hydrologic behavior. Generally, the changes in both directions were most strongly controlled by changes in the water balance of catchments characterized by an aridity index close to one. Changes to climate characteristics of catchments – mean annual precipitation, length of cold season or the seasonality of precipitation throughout the year – seem to explain most of the observed class transitions between slightly water-limited and slightly energy-limited states. Inadequate temporal information on other time-varying aspects, such as land use change, limits our ability to further disentangle causes for change.