|Goodrich, David - Dave|
|Scott, Russell - Russ|
Submitted to: American Geophysical Union
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/4/2004
Publication Date: 10/13/2004
Citation: Goodrich, D.C., Williams, D.G., Unkrich, C.L., Scott, R.L., Hultine, K., Pool, D.R., Coes, A.L., Hogan, J.F., Miller, S.N. 2004. Comparison of Methods to Estimate Ephemeral Channel Recharge, Walnut Gulch, San Pedro River Basin, Arizona. In Recharge and Vadose Zone Processes: Alluvial Basins of the Southwestern United States, ed. by F.M. Phillips, J.F. Hogan, and B. Scanlon, Water Science and Application 9, Washington, DC. American Geophysical Union. p. 77-99. 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. An important part of the water supply for many semi-arid regions is groundwater. Understanding how rainfall soaks into the ground to become groundwater is not well understood. Observations from the Walnut Gulch Experimental Watershed, operated by the U.S. Dept. of Agriculture, Agricultural Research Service were used to estimate the amount of runoff water resulting from monsoon thunderstorms that becomes groundwater as it travels down dry channels. It was found that for the wet monsoon years of 1999 and 2000 runoff water soaking into the channels caused substantial changes in groundwater levels. When estimates of the volume of this runoff water are scaled up to the basin level it was concluded that groundwater recharge from runoff soaking into dry channels is an important groundwater source that can be subsequently pumped for agricultural and municipal water supplies.
Technical Abstract: Ephemeral channel transmission losses play an important role in ground water - surface water dynamics in numerous arid and semiarid regions and they are potentially a significant source of recharge at the basin scale. However, identification of the processes driving these dynamics is difficult. Specifically, data on the proportion of runoff transmission losses that escape from near-channel evapotranspiration (ET) and wetted channel evaporation to become deep ground water recharge are difficult to obtain. This issue was addressed via coordinated field research and modeling within the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) located in southeastern Arizona. Groundwater, surface water, chemical, isotopic, tree sap flux, micrometeorological techniques, and changes in microgravity were used to independently estimate ephemeral channel recharge. It was found that during the relatively wet 1999 and 2000 monsoon seasons the ephemeral channel recharge estimated from these methods all differed by a factor of less than 2.9. A rough scaling of these rates to the entire basin, these estimates of ephemeral channel recharge would constitute roughly 15% at the low end of the range and 45% at the high end, respectively, of all water recharged annually into the regional aquifer as derived from a calibrated groundwater model estimates. However, in 2001 and 2002 no discernable ephemeral channel recharge in the intensely studied reach was detected due to weak monsoon seasons.