Soil water and bulk electrical conductivity sensor technologies for irrigation and salinity management

Authors: Schwartz, Robert; Evett, Steven - Steve; DOMINGUEZ, ALFONSO - University Of Castilla-La Mancha(UCLM); LELLIS, BRUNO - University Of Castilla-La Mancha(UCLM); PARDO, JOSE - University Of Castilla-La Mancha(UCLM)

Submitted to: IAEA-FAO Research Coordination Meeting for CRP
Publication Type: Abstract Only
Publication Acceptance Date: 7/9/2018
Publication Date: 7/12/2018
Citation: Schwartz, R.C., Evett, S.R., Dominguez, A., Lellis, B.C., Pardo, J.J. 2018. Soil water and bulk electrical conductivity sensor technologies for irrigation and salinity management. Final Report for IAEA-FAO Research Coordination Meeting - CRP D1.20.13 on Landscape Salinity and Water Management for Improving Agricultural Water Productivity. July 9-12, 2018. Vienna, Austria.

Interpretive Summary:

Technical Abstract: Assessment of the soil water content, bulk electrical conductivity, and pore water electrical conductivity is critical in the evaluation and management of irrigation and leaching regimes for crop production in salt-affected soils. Soil water content sensing systems based on electromagnetic (EM) properties of water are problematic in salt-affected soils due to severe interference from the bulk electrical conductivity, which can vary greatly throughout an irrigation season and across a field. In these soils, capacitance-based EM measurements have been shown to be nearly useless whereas the neutron probe can provide accurate soil water sensing but is labor intensive and subject to regulations. We investigated several new technologies suitable for evaluation of field soil water contents including (i) time domain reflectometry (TDR) probes with pulse generation directly coupled to the electrodes (ii) prototypes of profiling soil water sensors based on directly-coupled TDR and (iii) a cosmic ray neutron probe (CRNP) that senses background fast neutrons generated by cosmic rays as well as background thermal neutrons. Laboratory investigations of the directly-coupled TDR probes demonstrated that they can estimate soil water contents with accuracies of less than 0.03 m3 m-3 at bulk electrical conductivities up to 2.5 to 3.0 dS m-1 in fine textured soils (clay less than 400 g kg-1) with a soil specific calibration. In coarser textured soils, accurate estimation of soil water content using these sensors is feasible up to a bulk electrical conductivity of ~5 dS m-1. The corresponding electrical conductivity of the pore water at saturation would range from 5 to 10 dS m-1. Several prototypes of the profiling soil water sensor were designed, evaluated, and found to be satisfactory. However, the principal difficulty was achieving a structural design that could withstand the forces of installation while keeping manufacturing costs reasonable. The CRNP was evaluated during two seasons and found to respond positively to elevated near-surface atmospheric humidity and be sensitive only to water within a shallow soil depth (less than 0.3 m). Consequently, the CRNP data would have little relevance for irrigation management for the field crops commonly grown in the Southern Great Plains. Field evaluations of arrays of EM sensors deployed at multiple depths demonstrated that sensed changes in stored profile water could resolve daily changes in evapotranspiration (ET) and correlated strongly with changes in storage determined with the neutron probe. This indicates the possibility that properly deployed EM sensors of the types used here could provide change in storage data accurate enough to determine ET from the soil water balance.