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Seasonal precipitation and soil moisture dynamics of a hyperarid wash in the Sonoran Desert, U.S.A.




Howe, Susan R., author
Wohl, Ellen E., advisor
Laituri, Melinda J., committee member
Kelly, Eugene F., committee member
Rathburn, Sara L., committee member

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Precipitation and runoff in arid and hyperarid landscapes is infrequent and both spatially and temporally variable, and the relationship between these hydrologic components and vegetation, soils, and geomorphology in these environments is complex and not well understood. In this study, precipitation and soil moisture were monitored beneath three cover types in three locations across two geomorphic surfaces in the Yuma Wash watershed, located in the Lower Colorado River Valley of the Sonoran Desert, on the US Army Yuma Proving Grounds in Yuma, Arizona. Monitoring, sampling, and characterization occurred from July 2006 to February 2010. Six tipping bucket rain gages and sixty time domain reflectometry soil moisture sensors recorded moisture inputs and storage on a middle to late Pleistocene age alluvial terrace, and a younger, Holocene age alluvial wash. Sensors were spatially distributed in the lower, middle and upper locations of the watershed, beneath bare ground at 2.5, 25, 50, and 100 cm, and beneath the dripline radius of Olneya tesota and Parkinsonia microphylla, at 25, 50, and 100 cm depths. These data suggest that precipitation is highly variable in space and time, and is generally greater than the surrounding valley bottoms of Yuma Proving Grounds. Findings also suggest that soils beneath the dripline radius of these plant species on terraces are wetted more frequently and to greater depths in response to smaller magnitude and lower intensity storm events relative to soils beneath the same species on washes, and relative to bare ground soils. Threshold precipitation conditions necessary to generate changes in soil moisture were compared across surfaces, and illustrate that the vesicular structure in the A (Av) horizons beneath desert pavement plays a key role in redistribution of moisture as runon to O. tesota and P. microphylla on terraces, and that soils beneath the dripline radius of both species on washes receive moisture only during rainfall events exceeding 30 mm. There is also some evidence to suggest precipitation and near surface soil moisture may be greater in the upper basin relative to the mid- and lower basin on both surfaces, but at depths of 25-100 cm, soil moisture responses were difficult to interpret due to local soil properties not quantified in this study. The influence of soil temperature on the imaginary permittivity component of soil moisture readings due to high soluble salt content, the presence of enriched clay layers, soil compaction and induration is discussed. Findings highlight the need to quantify these age-dependent soil pedogenic and hydrologic properties when assessing soil moisture response to spatially variable precipitation in these water-limited environments. Implications for management of military lands are discussed.


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desert pavement
arid lands geomorphology
desert hydrology
arid lands hydrology


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