Precipitation variability at the farm scale during crop phenological phases

Site-specific rainfall was recorded with four rain gauges (with gauge spaci ngs of 1.02 km, 2.22 km, and 3.04 km) on a 250-ha farm in eastern Indiana o ver four growing seasons. Precipitation was aggregated by corn and soybean phenological phases to determine and compare spatial and temporal precipita tion variability at this scale. The median absolute deviation among gauges, representing spatial precipitation variability, ranged from 0.25 to 1.73 m m/day. A reference year of phenological phases was constructed from crop gr owth literature and applied to on-farm historical precipitation data. The m edian absolute deviation of daily precipitation from the reference year, re presenting temporal (year-ro-year) precipitation variability, ranged from 0 .17 to 3.40 mm/day. Organization of yearly precipitation by phenological ph ase showed year-re-year (temporal) variability to be greater than spatial v ariability, over the same area, for most crop periods. Up to a distance of 3.04 km, median absolute deviation between gauges was less than I mm for mo st phases, which suggests that the required spacing for representing site-s pecific precipitation is 3.05 km or greater The limited time scale and Fred orientations of the gauges restrict the applicability of the results to wi despread areas. However, at least in this case, site-specific precipitation measurement is not necessary for a farm 250 ha or smaller.