Runoff index values for frozen soil areas of the Pacific Northwest

Several operational hydrology and erosion models use the NRCS Runoff Curve Number procedure to estimate runoff. However, runoff Curve Numbers are poorly defined and perhaps inappropriate for areas such as the Pacific Northwest where the annual hydrology is dominated by winter processes of snowfall and soil freezing and thawing. Hence, these models do not perform well under these conditions. This has hampered successful use of models such as EPIC, CREAMS, and AGNPS in the region. Runoff index values have been calculated using runoff curve number relationships from runoff plot data from the Palouse Conservation Field Station near Pullman, Washington. These runoff index values range considerably higher than Curve Numbers commonly used in hydrologic planning and can be used to improve peformance of Curve Number based models under winter conditions of the Pacific Northwest.