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Soil Moisture Dynamics throughout a Semiarid Valley Ecotone Using Quasi-3D Time-Lapse Electrical Resistivity Imaging

Benjamin J. Bass

This time-lapse ER profile
This time-lapse ER profile captures soil moisture dynamics in the spring (March 15th, 2011) for Transect 2. Warmer colors in the time-lapse electrical resistivity profile reflect a decrease in soil moisture (increase in resistivity). Due to the drought conditions during the time of this survey, both desert perennials, juniper and creosote, demonstrate drought-responsive water uptake strategies by utilizing soil moisture down to their maximum rooting depths.

This study investigates the spatial distribution and seasonal variation of soil moisture conditions throughout a first-order drainage basin characterized by opposing hillslopes with distinct vegetation types and soil properties. The study site, located 75 km south of Albuquerque, NM in the Sevilleta National Wildlife Refuge, hosts a pronounced ecotone (ecosystem boundary), but is representative of the hillslopes in the region. The north-facing slope of the valley is characterized by deep rooted juniper trees and finer soils, while desert-adapted creosote bushes are the dominant vegetation on the south-facing slope as a result of the roughly 20% greater annual solar radiation received on this slope in comparison to the north-facing slope. A series of multiple time-lapse, quasi-3D electrical resistivity (ER) measurements were made using 56 electrodes with a 1.5 m electrode spacing for four seasons from November 26th, 2010 to August 11th, 2011 to capture soil moisture changes to a maximum depth of 21 m throughout the 300 ´ 125 m study valley site.

An extensive prolonged drought related to La Niña southern oscillation resulted in information from the time-lapse ER profiles that demonstrates the spatial and temporal water uptake strategies of the desert-perennials found at the study site. Results indicate that both juniper and creosote bushes utilize uptake water stored at their maximum rooting depths during prolonged drought conditions when shallower soil moisture is not available. Furthermore, results suggest that no water is capable of draining past the root zone of either type of vegetation to result in recharge, explained hydrologically by evapotranspiration rates on both hillslopes that are greater than the mean annual precipitation. Findings from this project show that the desert perennial vegetation types at the study site affect surface-atmosphere fluxes as well as the potential for recharge even during the severe La Nina drought when soil moisture conditions in the vadose zone are expected to be static. The soil moisture fluxes observed throughout the seasonal time-scale at the semiarid study valley site provide critical information for predictions associated with how climate and land-use change may affect hydrologic conditions in semiarid basins in the semiarid southwestern U.S.A.

Advisor: M. Bayani Cardenas