Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2011
Publication Date: January 12, 2012
Citation: Evett, S.R., Schwartz, R.C., Casanova, J.J., Heng, L.K. 2012. Soil water sensing for water balance, ET, and WUE. Agricultural Water Management. 104:1-9. Interpretive Summary: Electronic soil water content sensors are increasingly appearing on the market. They are used in irrigation scheduling, determining crop water use, and the efficiency of water use under different management schemes. The nation's water resources are increasingly stretched to cover agricultural, municipal, and industrial needs. It is critically important to achieve irrigated crop production with the least water use and greatest efficiency possible. Farmers, irrigation equipment manufacturers, and agricultural scientists are working to develop more effective irrigation management and to develop crop varieties that use water more efficiently. To do so, they need the most accurate soil water sensors possible. The USDA Agricultural Research Service at Bushland, Texas, worked with the International Atomic Energy Agency and led a team to investigate newer electronic soil water sensors. The team compared sensors with direct measurements and assessed their usefulness for irrigation scheduling and determination of crop water use and water use efficiency. They found that the most common class of electronic sensor, the capacitance sensors, was highly influenced by soil variability and changes in soil temperature and electrical conductivity (common in irrigated soils). These sensors were too inaccurate for use in agricultural science and were suspect for use in irrigation scheduling.Recommendations were made for which sensors were practical for use and how a new class of sensors could be developed.
Technical Abstract: The soil water balance can be solved for evapotranspiration (ET) using data from either weighing lysimetry or soil water sensing and measurement. Weighing lysimeters are expensive and, although accurate, are difficult to manage and afford little replication. Direct soil water measurement by coring is accurate enough, but this is plagued by spatial variability that induces unwanted variability in the change in soil water storage between dates, and is also destructive and time consuming. Here we focus on soil water sensing using the neutron probe and various electromagnetic (EM) sensors (capacitance and quasi-time domain reflectometer(TDR)) with respect to the relative levels of uncertainty in profile water content, change in soil water storage, and estimates of deep flux; and their impact on estimated ET and water use efficiency (WUE). Studies consistently showed errors up to and greater than 0.05 m**3 m**-3 for capacitance sensors used in access tubes, which implied errors in soil water flux estimation of up to 50 mm d**-1, and calibrations that were so sensitive to soil bulk electrical conductivity (BEC) and temperature that water content and change in storage estimates were rendered unreliable. Also, larger spatial variability of water contents reported by capacitance sensors was tied to the EM field penetration in structured soils around access tubes being non-uniform and influenced by the random arrangement of soil micro-scale water content, BEC, and bulk density distribution. Thus, we recommend that profiling sensor systems based on capacitance technology should not be used for studies of water balance, ET and WUE, nor for irrigation scheduling. Recommended methods include the neutron probe, direct volumetric soil sampling and, in some cases, conventional time domain reflectometry with waveform capture and analysis. New sensor development efforts should focus on waveguide approaches using TDR technology.