Selected Hydrographs and Statistical Analysis Characterizing the Water Resources of the Arkansas

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Precipitation Stations where precipitation data have been collected were selected based on the length of record and geographic location. The 30 precipitation stations chosen are shown on plate 1 and listed in table 1. The record available since 1920 and mean annual precipitation at each station are shown in table 2. Hydrographs of annual precipitation for these 30 stations are shown in figures 2 -31. Tables of the monthly data are presented in the "Supplemental Information" section (tables 13 -42) at the end of the report. It is readily apparent from the hydrographs that precipitation is highly variable from year to year and from station to station. Five stations were selected to characterize the precipitation at their respective physiographic locations: Mountainous areas, mountain valleys, and the plains. In the mountainous areas, orographic effects dominate precipita- tion; the higher the elevation, the greater the precipitation. Station 4750 (Lake Moraine) is the highest of the 30 stations and has the greatest mean annual precipitation. It exhibits a frequency distribution typical of annual precipitation (fig. 32) for the mountainous areas. The distribution of monthly precipitation for station 4750 is shown on figure 33. Precipitation in the mountainous areas during the fall and winter is predominantly snowfall. The monthly means and year -to -year variations in precipitation (as defined by one standard deviation on either side of the mean) are less in the fall and winter than during the spring and summer. Although July and August average the most precipitation, the maximum monthly extreme of 11.70 in. occurred in April, and as much as 9.63 in. has occurred in May. Zero precipitation has been recorded at station 4750 for an entire month only in December and January. The mountain valleys are the driest areas in the basin, even though they are at high elevations. The Arkansas River valley in the upper basin is pro- tected by high mountains on all sides but the south. Most of the moisture in the predominantly eastward - moving clouds precipitates as the air masses rise over the mountains, which leaves generally warmer, drier air entering the valley. Station 7370, Salida, exhibits a frequency distribution typical of annual precipitation in the mountain valleys (fig. 32), with some years of precipitation as low as 4 in. The monthly precipitation for station 7370 (fig. 33) shows a relatively even distribution throughout the year. The year - to -year variations for each month are also reasonably small and uniform. Zero precipitation has been recorded at station 7370 for all but the summer months. On the plains of eastern Colorado, precipitation exhibits considerable year -to -year variation, as shown by the frequency distribution of annual precipitation (fig. 32) for stations 6740 and 6741 (Pueblo WSO AP and Pueblo); station 4834, Las Animas (1N); and station 4076, Holly. Annual precipitation has ranged from 5 to 30 in. At these stations, precipitation on the plains generally increases from west to east. This increase is characterized in the monthly distributions for the four stations (fig. 33); both the monthly means and year -to -year variations in precipitation for the summer months increase from stations 6740 and 6741 to station 4834, and on east to station 4076. Months of zero precipitation have been recorded at one of the four stations for every month except July. At the other extreme, 18.8 in. was recorded during June 1965, at station 4076. 4