Location: Southwest Watershed Research
Title: Spatiotemporal Variability of Summer Precipitation in Southeastern Arizona Authors
Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2013
Publication Date: December 15, 2013
Citation: Stillman, S., Zeng, X., Shuttleworth, W., Goodrich, D.C., Unkrich, C.L., Zerda, M. 2013. Spatiotemporal Variability of Summer Precipitation in Southeastern Arizona. Journal of Hydrometeorology. 14:1944-1951. DOI http://dx.doi.org/10.1175/JHM-D-13-017.1. Interpretive Summary: Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. Detailed study of precipitation and storm patterns is important if we are to continue to populate and use these regions. This paper analyzed the highly detailed long-term rainfall records from the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona during July and August when the majority of rainfall occurs from monsoon thunderstorms. The study found that there were no significant trends in rainfall total, intensity or frequency of storms from 1956 to 2011. However, the multi-decadal variation in storm intensity and spatial coverage of storms was correlated with the Pacific Decadal Oscillation (PDO) and the Atlantic Multi-decadal Oscillation (AMO) indices. The rainfall patterns were found to have a pronounced diurnal cycle with the highest storm totals and frequency occurring between 3:00 and 10:00 p.m. It was also found that 5-8 rain gauges (of the 88 present in the watershed) can provide a reasonable estimate of the area-averaged monthly total precipitation during July and August. This has important implications for resources needed to adequately monitor and model rainfall in this environment.
Technical Abstract: The Walnut Gulch Experimental Watershed (WGEW) in Southeastern Arizona covers ~150 km2 and receives the majority of its annual precipitation from highly variable and intermittent summer storms during the North American Monsoon. In this study the patterns of precipitation in the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) 88 rain gauge network are analyzed for July and August from 1956 to 2011. Because small-scale convective systems generate most of this summer rainfall precipitation, the total (T), intensity (I), and frequency (F), exhibit high spatial and temporal variability. Although subsidiary periods may have apparent trends, no significant trends in T, I, and F were found for the study period as a whole. Observed trends in the spatial coverage of storms change sign in late 1970s, and the multi-decadal variation in I and spatial coverage of storms have statistically significant correlation with the Pacific Decadal Oscillation (PDO) and the Atlantic Multi-decadal Oscillation (AMO) indices. Precipitation has a pronounced diurnal cycle with the highest T and F occurring between 3:00 and 10:00 p.m., and its average fractional coverage over 2 and 12 hour periods is less than 40% and 60% of the gauges, respectively. Although more gauges are needed to estimate area-average daily precipitation, 5-8 gauges can provide a reasonable estimate of the area-averaged monthly total precipitation during July and August.