Event to multidecadal persistence in rainfall and runoff in southeast Arizona

Abstract

[1] Spatial and temporal rainfall variability over watersheds directly impacts the hydrologic response over virtually all watershed scales. Changes in the precipitation regime over decades due to some combination of inherent local variability and climate change may contribute to changes in vegetation, water supply, and, over longer timescales, landscape evolution. Daily, seasonal, and annual precipitation volumes and intensities from the dense network of rain gauges on the Agricultural Research Service, U. S. Department of Agriculture Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona are evaluated for multidecadal trends in amount and intensity over a range of watershed scales (1.5 ha to 149 km 2 ) using observations from 1956 to 2006. Rainfall and runoff volume and rate variability are compared over the same spatial scales over a 40 year period . The major findings of this study are that spatial variability of cumulative precipitation decreases exponentially with time, and, on average, became spatially uniform after 20 years of precipitation accumulation. The spatial variability of high-intensity, runoff-producing precipitation also decreased exponentially, but the variability was still well above the measurement error after 51 years. There were no significant temporal trends in basin scale precipitation. A long-term decrease in runoff from 1966 to 1998 from ephemeral tributaries like the WGEW may be a critical factor in decreasing summer flows in the larger San Pedro due to changes in higher-intensity, runoff-producing rainfall.

Influence of the hydrologic regime on resource availability in a semi‐arid stream‐riparian corridor

“…Although decreased precipitation has not been observed in the San Pedro Basin, decreased run-off in ephemeral channels has likely contributed to reduced flows in the mainstem (Thomas and Pool, 2006;Goodrich et al, 2008). During the period of our study, annual contrasts in precipitation were strongly influenced by variability in the timing and abundance of monsoon precipitation.…”

“…The summer monsoon season is characterized by intense storms and flash floods, and is preceded by a period of seasonal drought (May to July). No directional changes in precipitation abundance have occurred approximately over the past 40 years, although significant variation occurs among years (Goodrich et al, 2008).…”

“…Precipitation was monitored in the Walnut Gulch research watershed, a subcatchment within the San Pedro Basin, located ¾13 km downstream from the study site (Goodrich et al, 2008). Median daily precipitation was calculated from 92 precipitation gages distributed across the catchment (data obtained from http://www.tucson.ars.ag.gov/unit/gis/wg.html, 2/08).…”

Evapotranspiration Estimates Derived Using Multi-Platform Remote Sensing in a Semiarid Region

“…An additional source of spatial variability in ET between land cover classes may be due to the convective and episodic nature of summer precipitation in the region and the subsequent role moisture pulses have in the ET flux over the region [50,95,96]. Regions such as southern Arizona exhibit significant spatial trends and variability in precipitation at the annual scale [97], thus contributing to large variability in ET. A histogram of annual ET for the three specified land use/land cover classes shows large spatial variability (Figure 3).…”

Predicting Coarse Sediment Transport from Patchy Beds in Ephemeral Channels

“…Mean temperatures in Walnut Gulch range from 22˚C in January to 33˚C in July. The mean annual precipitation at Lucky Hills is approximately 300 mm, with the majority occurring during the summer monsoon season, July to September (Goodrich et al 2008). Surface runoff is predominately a result of convective summer thunderstorms that are characterized by brief periods, on the order of a few hours, of intense precipitation.…”