Summary of precipitation data for the Black Hills area of South Dakota, water years 1931-98

Long-term precipitation records are sum-marized for the Black Hills area of South Dakota. Precipitation data are available for numerous gaging locations; however, few gages have continuous, long-term records, and periods of missing record are common. Thus, a geographic information system (GIS) utilizing an inverse-distance weighting method was developed to generate spatial precipitation distributions from point precipitation data for the Black Hills area, based on available monthly records. The spatial distributions were used to estimate periods of missing record for all 94 gages considered. The resulting monthly records of measured and estimated precipitation are tabulated for water years 1931-98. Average values for water years 1961-90, which is the period used for calculation of climatic normals, were used to develop an isohyetal map of normal annual precipitation for the Black Hills area.

Temporal trends in precipitation for the Black Hills area also were examined. Sustained periods of deficit precipitation occurred during 1931-40 and 1948-61. Sustained periods of surplus precipitation occurred during 1941-47, 1962-68, and 1991-98, with the late 1990's identified as the wettest period since 1931. The driest 30-year period was 1931-60, when annual precipitation averaged 17.17 inches for the study area. The wettest 30-year period was 1969-98, when annual precipitation averaged 19.61 inches. Normal annual precipitation (1961-90) for the study area is 19.06 inches, compared with the long-term (1931-98) annual average of 18.61 inches. Annual extremes for the study area have ranged from 10.22 inches in water year 1936 to 27.39 inches in water year 1995.