Resolving discrepancies between laboratory-determined field capacity values and field water content observations: implications for irrigation management

Authors: Evett, Steven - Steve; Stone, Kenneth - Ken; Schwartz, Robert; O`Shaughnessy, Susan; Colaizzi, Paul; ANDERSON, SCOTT - Acclima, Inc; ANDERSON, DAVID - Acclima, Inc

Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2019
Publication Date: 7/9/2019
Citation: Evett, S.R., Stone, K.C., Schwartz, R.C., O'Shaughnessy, S.A., Colaizzi, P.D., Anderson, S.K., Anderson, D.J. 2019. Resolving discrepancies between laboratory-determined field capacity values and field water content observations: Implications for irrigation management. Irrigation Science. 37:751-759. https://doi.org/10.1007/s00271-019-00644-4.
DOI: https://doi.org/10.1007/s00271-019-00644-4

Interpretive Summary: The decreasing supply of water from the Ogallala and High Plains aquifers for irrigation is a threat to agricultural productivity and sustainability in the Southern High Plains of Colorado, Kansas, Oklahoma and Texas. In the eastern US, summertime flash droughts often decrease yields despite normally sufficient rainfall. Agricultural production practices, including irrigation, must become more efficient and better managed to sustain productivity in the face of declining and increasingly expensive resources. Careful irrigation management may be aided by accurate soil water sensors. Scientists with USDA ARS at Bushland, TX, and Florence, SC, and Acclima cooperated in integrating a new soil water sensor into a center pivot control system for variable rate irrigation, but found that soil water contents reported by the sensor were larger than the so-called “field capacity”. The problem was resolved in two steps. First soil samples were taken in the exact locations where sensors were installed in both sandy and clayey soils, which proved that the sensor data were accurate. Second, the concept of field capacity was discarded as being inapplicable in layered soils, and emphasis was placed on the soil profile water content and soil water depletion, which were calculated from the accurate sensor data. By focusing on soil water depletion, irrigators were able to quickly see how much water had been taken up by the crop and thus how much needed to be added by irrigation. Used in a precision irrigation system based on weather, soil and plant sensing, this enables irrigation response to crop water needs as needs vary in space and time. By responding to stress periods and critical crop growth periods, yields and profitability are maintained in the face of declining water availability and short term summer flash droughts.

Technical Abstract: The concept of soil water contents at field capacity (FC at 0.33 MPa) and at wilting point (WP at 15 MPa) is often used to explain plant water availability and as maximal and minimal limits on observed soil water content, yet field observations often differ from laboratory determined FC and WP water content values. However, as more capable sensors have become available and graphical plots of soil water dynamics have become common, plotting of FC and WP lines on such graphs often engenders confusion rather than enlightenment. Resolving this confusion has been greatly eased by the introduction of soil water sensors that encapsulate an entire time domain reflectometry (TDR) system in individual sensor heads and the recent availability of a reader for capturing georeferenced values of the TDR waveform and estimated values of soil volumetric water content (VWC), permittivity, temperature and bulk electrical conductivity. The present study illustrates the typical confusion with season-long graphs of soil water content that greatly exceed the FC and WP values for individual soil horizons, and it resolves the confusion with concurrent and co-located TDR sensor readings and volumetric soil sampling to ascertain sensor accuracy. It was found that sensor readings were reasonably accurate (RMSE = 0.01 m**3 m**-3) across a range of textures from fine sandy loam to clay even though up to 0.19 m**3 m**-3 larger than FC values. Water contents in a sandy eluviated horizon above a dense clay were larger than FC due to the clay layer impeding water flow and perching water in the sand, augmented by the capillary fringe in the fine sand. Confusion was in part created by plotting water content for four different depths of different textures but plotting the FC and WP values for only one soil texture. Misperception of water available for crops was greatly reduced by converting the water content values to equivalent water depth values for the four soil layers and plotting only the soil water storage depth for the entire profile depth covered by the sensing network. The ambiguity was further reduced by determining the maximum value of soil water storage for the season and calculating soil water depletion by subtracting the maximum value from the soil water storage throughout the season. When this was done it was easy to see depths of water removed from the soil and needing replacement, and the extra soil water depletion that occurred when a plot was not irrigated compared with irrigated plots.