Hydroclimatic Differences in Precipitation Measured by Two Dense Rain Gauge Networks

Abstract

Two large, dense recording rain gauge networks of identical size and gauge density, the Central Illinois Network (CIN) and the Southern Illinois Network (SIN), operating over the same 10-yr period, offered unique data to assess causes for the climatological differences in their precipitation and to identify results relevant to hydrologic design and operations. Long-term averages show SIN has 32% more precipitation in the colder half year and has slightly higher heavy rainfall frequency values, but both networks experience the same average number of days with precipitation and thunderstorms, with nearly identical precipitation amounts in the warmer half year. The 10-yr data sample (1958?67) had near-normal precipitation conditions, but likely is representative not of long-term conditions but only a 10-yr period. Storms, discrete periods of precipitation in a network, had durations in SIN that were 2.5 h longer than in CIN in the colder half year. The primary direction of precipitation movement was from the northwest for CIN and from the west-southwest for SIN, revealing important differences related to the predominance of stationary-frontal rain conditions in the south as compared with cold-frontal conditions being primary in the central area. Both networks measured the same annual frequency of storms producing measurable precipitation, but SIN had 23% more storms producing average network amounts in excess of 1.27 cm. The SIN network had 2 times the number of storms with 2-yr frequency values in winter?spring. Precipitation from these additional storms in the south represented 46% of the total annual difference between the two networks. In summary, many cold-season rain-producing conditions in SIN tended to last longer, have stronger convection, and produce heavier, more-widespread rainfalls than in CIN. Further, many cold-season rains were tied to the higher frequency of stationary-frontal conditions in the south. Warm-season network rainfall amounts were similar, but many rainfall characteristics differed. The southern area exhibited more rainfall variability and more airmass-type storms, whereas the central area had longer summer storms. Data from these dense rain gauge networks were essential in ascertaining the causes for the regional hydroclimatic differences. Furthermore, several results reveal the magnitude within these networks of point?area and point?point differences likely to be sampled in a 10-yr period. The number of 2-yr events at some gauges was 4 times greater than at others.