Submitted to: Climate and Weather Research Workshop Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/19/1995
Publication Date: N/A
Technical Abstract: Rainfall on a given day is a critical part of the decision making process in farm operations and application of chemicals in a manner that would reduce potential environmental impact from runoff. Rainfall in convective storms is considered to be highly variable; however, there is little direct evidence of this variation within small watersheds. This study was designed to evaluate the spatial variation of rainfall and the consistency of these patterns among years for the Walnut Creek watershed in central Iowa. Nineteen raingages are located throughout a 5600 ha watershed that is characterized by gently rolling topography; however, only 15 were used in this study because of the length of record. Rainfall is measured with tipping bucket raingages with an accuracy of 0.25 mm and is recorded as five minute and daily totals. Each raingage is placed through a quality assurance/quality control criteria to examine the response of each gage. Calibration is conducted monthly on all raingages with weekly checks and cleaning. There was no consistent spatial pattern within the watershed, although individual storms exhibited a large variation. The coefficient of variation on individual storms ranged from 5 to 450 percent, however, on the annual total it was less than 10 percent. Spatial relative variability ranged from 36 to 106 percent with only the small rainfall year of 1994 showing the highest spatial variation. Even within this year there was no consistent pattern among raingages. Decision making purposes within a field and comparisons among practices across a watershed will require direct measurement of the rainfall at that site.