USE OF HYDROLOGIC LANDSCAPE CLASSIFICATION TO DIAGNOSE STREAMFLOW PREDICTABILITY IN OREGON

We implement a spatially lumped hydrologic model to predict daily streamflow at 88 catchments within the state of Oregon and analyze its performance using the Oregon Hydrologic Landscape (OHL) classification. OHL is used to identify the physio-climatic conditions that favor high (or low) streamflow predictability. High prediction catchments (Nash-Sutcliffe efficiency of root Q (NS) > 0.75) are mainly classified as rain dominated with very wet climate, low aquifer permeability, and low to medium soil permeability. Most of them are located west of the Cascade Mountain Range. Conversely, most low prediction catchments (NS < 0.6) are classified as snow-dominated with high aquifer permeability and medium to high soil permeability. They are mainly located in the volcano-influenced High Cascades region. Using a subset of 36 catchments, we further test if class-specific model parameters can be developed to predict at ungauged catchments. In most catchments, OHL class-specific parameters provide predictions that are on par with individually calibrated parameters (NS decline < 10%). However, large NS declines are observed in OHL classes where predictability is not high enough. Results suggest higher uncertainty in rain-to-snow transition of precipitation phase and external gains/losses of deep groundwater are major factors for low prediction in Oregon. Moreover, regionalized estimation of model parameters is more useful in regions where conditions favor good streamflow predictability.