HYDROLOGIC PROCESSES, SCALE, CLIMATE VARIABILITY, AND WATER RESOURCES FOR SEMIARID WATERSHED MANAGEMENT
Location: Southwest Watershed Research
Title: Event to multidecadal persistence in rainfall and runoff in southeast Arizona 1908
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 19, 2008
Publication Date: May 19, 2008
Citation: Goodrich, D.C., Unkrich, C.L., Keefer, T.O., Nichols, M.H., Stone, J.J., Levick, L., Scott, R.L. 2008. Event to multidecadal persistence in rainfall and runoff in southeast Arizona. Water Resour. Res., 44, W05S14, doi:10.1029/2007WR006222.
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 water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions. This paper analyzed the highly detailed long-term rainfall and runoff records from the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona. The major findings of this study are that the spatial variability of precipitation decreases rapidly (exponentially) as the time over which it is summed up increases; that long-term precipitation trends can be captured with a smaller number of rain gauges (roughly six) then the 85 gauges deploy in the WGEW; and, that an observed decrease in runoff from 1966 to 1998 from the 150 square kilometer (58 square miles) watershed is not significantly affected by factors other than precipitation such as land cover change.
Spatial and temporal rainfall variability over watershed scales impacts the hydrologic response which in turn affects runoff, agricultural production, and soil water availability. Changes in the precipitation regime over decades may contribute to changes in vegetation, water supply and, over longer time scales, landscape evolution and geomorphology. In the southwestern US, increasing human population is changing the landscape from rangeland to urban areas, placing pressure on limited water supplies. Daily, seasonal and annual precipitation volumes and intensities from the densely gaged network of raingages on the USDA-ARS Walnut Gulch Experimental Watershed are evaluated for multi-decadal trends in amount and intensity over a range of watershed scales (1.5 ha to 150 km2). The influence of teleconnections is tested for local-scale rainfall variability. Rainfall and runoff volume and rate variability are compared over the same spatial scales and over a 40 year period of high-quality runoff observations. The major findings of this study are that spatial variability of precipitation decreases exponentially with time scale; that long-term precipitation trends can be captured with a low spatial resolution; and that runoff from the 150 km2 watershed is not significantly affected by factors other than precipitation.