A NATIONAL MONTHLY WATER BALANCE MODEL

A monthly water balance (WB) model that partitions water among various components of the hydrologic system is being used to aid the development of a national hydrologic model (NHM). The WB model includes the concepts of climatic water supply and demand, snow accumulation and melt, and soil-moisture storage. Using monthly temperature and precipitation data from the Precipitation-elevation Regression on Independent Slopes Model as inputs to the WB model, monthly time series of runoff have been generated for the conterminous US on a 4 kilometer by 4 kilometer grid for the period 1895 through 2009. These time series of estimated monthly runoff are aggregated to estimate runoff for river basins of various sizes across the conterminous US for comparison with time series of measured runoff. These comparisons, and other analyses, provide direction for improving the WB model and a starting point to understanding the hydrologic model detail needed for a daily time step NHM. Some of the issues to address include accounting for groundwater contributions to surface water runoff, improving estimates of evapotranspiration, and accounting for anthropogenic influences such as water use, reservoirs, and diversions.