Application of Geochemical Tracers to Determine the Sources Contributing to Runoff at the Semi-arid Walnut Gulch Experimental Watershed, Arizona

Abstract

Prior observations by Koch (2005) suggest the presence of soil water mixed with current rainfall in runoff samples at the Kendall subwatershed of the USDA-Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW). However, previous observations elsewhere in the WGEW have shown runoff to consist mainly of current precipitation (Goodrich et al., 2004). The disparity between the two studies demonstrates a need for continued research; therefore, this study uses isotope and solute geochemical tracers to determine the sources contributing to runoff at the Lucky Hills subwatershed of the WGEW during the 2015 monsoon season. A comparison of the stable isotopes of water (2H and 18O) in runoff to those in precipitation and soil water, shows that runoff and precipitation are distinct (p < 0.05), while soil water cannot be differentiated from runoff (p > 0.05). However, graphical representations of chloride and sulfate concentrations in the three water sources (precipitation, soil water and runoff), conclusively reveal that there is negligible soil water present in the runoff at Lucky Hills. Therefore, it is likely that there is no soil water mixing into the runoff at the Lucky Hills subwatershed, but more research is necessary to confirm these results. Because no runoff or soil water data was generated at the Kendall subwatershed during the 2015 monsoon season, continued research is necessary to draw conclusions about the sources contributing to runoff in the Kendall subwatershed, and in other portions of the Walnut Gulch Experimental Watershed.