5.1  The Water Balance

The water balance is computed based on the method proposed by Thornthwaite and Mather (1955, 1957), Fenn et al. (1975), and Dass et al. (1977), and used by Whelan et al. (1987).  The method involves a monthly water-balance calculation using meteorologic and site information, including monthly estimates of precipitation, potential evapotranspiration, snowmelt, temperature, and runoff.  Long-term meteorological data needed for the water balance can be obtained from the Local Climatic Data (LCD) annual summaries published by the Environmental Data Service, National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Center, U.S. Department of Commerce.  Information for areas of the United States pertaining to each month is contained in the LCDs.  The monthly tabulated data for 30 years of record are under "Normals, Means, and Extremes" in the LCDs.