The atmospheric branch of the hydrologic cycle is investigated to determine the wintertime accumulation of water over the Upper Colorado River Basin. The parameter precipitation minus evaporation is computed as a residual from the atmospheric water balance equation. The study covers the seven winter seasons 1957 through 1963. The results show that the periods of evaporation as well as the periods of heavy precipitation determine the seasonal water balance of the basin. The seasonal course of daily evaporation rate is determined. The evaporation rate varies by a factor of two over the winter season. Further, a strong decay with time of evaporation rate is observed during the early and mid-winter months. A less pronounced decay is obtained during March and April. The basin precipitation data obtained from the atmospheric water balance computation are compared to a basin precipitation estimate independently obtained using data from fourteen rain gauges. The conclusion is reached that the gauge data underestimate the basin precipitation by about fifty per cent. Much of this bias is shown to be due to the lack of sampling over the high elevation regions where the precipitation is greatest. The wintertime accumulation of water over the basin is shown to be highly related to the April through March runoff from the basin. The relationship shows that the accumulated water is apportioned by a ratio of one to four between runoff and evaporation respectively. Finally the application of the atmospheric water balance computation to the problem of runoff forecasting is discussed.