Annual to Event Precipitation Undercatch at a Semi-arid Watershed in Southeastern Arizona

The observed difference in recorded precipitation between above ground and ground-surface rain gauges is referred to as ‘undercatch’ at the above ground gauge relative to the ground-surface, or pit, rain gauge, commonly attributed to wind-induced effects around the orifice of the above ground gauge.  Three pairs of above ground level and pit rain gauges were installed at the United States Department of Agriculture-Agriculture Research Service (USDA-ARS) 150 km² Walnut Gulch Experimental Watershed (WGEW) and Long Term Agroecosystems Research (LTAR) site in southeastern Arizona to evaluate undercatch at annual to event temporal scales for a 6 year period.  Much variability of undercatch is exhibited by years, seasons, days, events, and sites.  Differences of the cumulative precipitation amount between above ground and pit rain gauges at the three sites average 4% for a six year period, but vary from -1% to 12% annually, and more so during winter and summer.  Although winter has greater percent undercatch, more than 2/3 of the total amount occurs in summer.  Regression estimated daily and event undercatch ranges up to 10%, but it may be greater for individual days and events. For a subset of 167 events, combined from the three sites, undercatch ranges from 2 to 86% and averages 15%. This is equivalent to a range of 0.53 to 7.17 mm and average of 1.48 mm per event.  Undercatch of event peak intensities are greater than event total amount.  The relation of wind speed to undercatch at event and sub-event time-scales, which may have subsequent influence on watershed hydrology and erosion, are also analyzed.  Quantification of this phenomenon at WGEW will aid in understanding the water balance of this and other watersheds and provide a better insight into the inherent errors in precipitation measurement.