Determination of Soil Water Content at Intermediate Spatial Scale Using Cosmic-Ray Neutrons: Field Application

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A major limitation to field-based hydrologic investigations is a lack of means of obtaining soil water content across spatial scales. In particular, there is an absence of methods operating at the intermediate scale (100-1000 sq. m), and considerable uncertainty in scaling measurements at coarser or finer resolution to the scale required for hydrologic models. In order to fill that gap, we have developed an intermediate-scale method that utilizes neutrons produced naturally in soil by cosmic rays. In this project we will apply the cosmic-ray neutron method to the problem of upscaling soil moisture measurements. In order to do this we will first need to characterize how the detector sample volume depends on soil water content and detector positioning and how the neutron signal averages heterogeneities in water content at various scales. We will accomplish this by extensive neutron flux modeling and by performing measurements at field sites where soil water content can be controlled and carefully monitored with an extensive network of time-domain reflectometry probes.The work will be performed at three field sites in the Tucson area. Controlled flooding experiments will be performed at the Central Avra Valley Storage and Recovery basin operated by the city of Tucson. We will utilize the center-pivot irrigation system at the University of Arizona experimental farm as another means of conducting controlled wetting experiments. We will investigate the dependence of soil water content on scale by placing neutron detectors at different heights above the ground at the Mt. Lemmon Cosmic Ray Laboratory and measuring soil conditions.