Methods for downhole soil water sensor calibration - complications of bulk density and water content variations

Authors: Evett, Steven - Steve; Marek, Gary; Colaizzi, Paul; Copeland, Karen; Ruthardt, Brice

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2022
Publication Date: 11/3/2022
Citation: Evett, S.R., Marek, G.W., Colaizzi, P.D., Copeland, K.S., Ruthardt, B.B. 2022. Methods for downhole soil water sensor calibration - complications of bulk density and water content variations. Vadose Zone Journal. Article e20235. https://doi.org/10.1002/vzj2.20235.
DOI: https://doi.org/10.1002/vzj2.20235

Interpretive Summary: The scarcity of water resources in the U.S. Southern High Plains is of regional, national, and even international concern due to the fact that the region acts as a breadbasket for the nation and world. The majority of agricultural production in this region depends on irrigation, primarily pumping from the Ogallala or High Plains Aquifer, which is yielding less water every year. The use of soil water sensors has been demonstrated to improve irrigation management and crop yield per unit of water applied. However, most soil water sensors are not accurate for specific soils and use of manufacturers’ calibrations may mislead irrigation managers, hurting crop yields and quality. Scientists at the USDA ARS Conservation & Production Research Laboratory at Bushland, Texas, developed methods for calibrating soil water content sensors so that irrigation needs could be accurately determined. By using the developed methods, irrigation managers can prevent both over-irrigation and under-irrigation so that crop yield per unit of water used can be maximized.

Technical Abstract: Downhole soil water content sensors are used in metallic or plastic access tubes to assess the soil water content at multiple depths in the soil profile. If sensor readings are spaced closely enough vertically and are accurate enough, then accurate soil profile water content storage and change in storage can be determined over the depth range of readings. If such readings cover the soil profile from the surface to well below the bottom of the root zone, then accurate estimates of evapotranspiration (ET) may be calculated using the soil water balance equation. Even if sensing only covers the active root zone, soil water depletion may be determined well enough to inform irrigation scheduling. While sensor accuracy is dependent on many factors, including the sensor’s physical principle of operation, soil-specific calibration is typically required for good accuracy; in soils with multiple horizons (layers) of different texture, bulk density, or chemical composition, horizon-specific calibrations may be necessary. We describe methods and equipment used for downhole sensor calibration to typical accuracy of <0.01 m3 m-3 with specific reference to calibration of ten meters using the neutron scattering method in a soil that required three different horizon-specific calibrations.