Electromagnetic and nuclear soil water sensing methods: Comparisons and newer technologies

Authors: Evett, Steven - Steve; Schwartz, Robert; Schomberg, Harry

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 4/16/2018
Publication Date: 4/20/2018
Citation: Evett, S.R., Schwartz, R.C., Schomberg, H.H. 2018. Electromagnetic and nuclear soil water sensing methods: Comparisons and newer technologies [abstract]. In: Report of the Second Research Coordination Meeting of the CRP D1.50.17 "Nuclear Techniques for a Better Understanding of the Impact of Climate Change on Soil Erosion in Upland Agro-ecosystems (D1.50.17)". April 16-20, 2018. Rabat, Morocco.

Interpretive Summary:

Technical Abstract: The Cosmic Ray Neutron Probe (CRNP) was compared with a suite of 96 electromagnetic (EM) soil water sensors for 180 days at Bushland, Texas in 2017. The EM sensors were installed at 16 locations within approximately 50 m of the CRNP at depths of 0.06, 0.20, 0.35, 0.50, 0.70 and 1.00 m. Under fallow conditions and with a soil profile containing water at near field capacity, the CRNP was sensitive to soil water content mainly in the shallow (0.02 and 0.06 m depths) soil, and nearly insensitive to soil water content at depths of 0.20 m and greater. Correlation of CRNP data with absolute humidity was greater (r2 = 0.52) than correlations of CRNP data with soil profile water contents for individual depths greater than 0.06 m depth. For this data set, CRNP data would have little relevance for irrigation management for the crops grown commonly in the Southern Great Plains (alfalfa, corn, cotton, sorghum, soybean, sunflower, winter wheat). However, soil water content changed <0.10 m3 m-3 at depths greater than 0.06 m during the study period because there was no crop growing (crop was destroyed by hail). Because of the strong correlations with soil water content near the surface (0.02 and 0.06 m depths) and with absolute humidity of the air at 2-m elevation, the CRNP would have applications in mesoscale weather forecasting. The EM sensors consisted of 48 model TDR-315L (Acclima Inc., Meridian, ID) and 48 model CS655 (Campbell Scientific, Inc., Logan, UT) sensors. The two kinds of EM sensor produced quite similar results after the apparent permittivity from the CS655 sensors was corrected to correspond to that determined by conventional time domain reflectometry (TDR), followed by use of soil-specific TDR calibrations for the A and B soil horizons. The EM sensors produced profile water contents that were well correlated with field-calibrated neutron probe data over the same depth range (0 to 1.15 m depth range). Importantly, profile water content changes in storage calculated from the EM sensor data were even more strongly correlated (r**2 = 0.94) with change in storage determined with the neutron probe over eight measurement periods. 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. This question will be the subject of further investigation. The CRNP data were not well correlated with the NP data over the 1.15-m depth range and so would not be useful for ET determination under these circumstances. In a related project run by Dr. Schomberg, a wireless node and gateway system was studied for datalogging of soil water sensor data and transmission of the data using the LoRa radio transmission scheme from nodes in the field to a gateway at the edge of the field. The inexpensive (~$150 US), solar powered node collected data from up to eight sensors using the SDI-12 wired data protocol and transmitted the data over distances exceeding 300 m to the gateway (~$150 US) where the data were stored prior to upload to a smart phone, tablet or other device equipped with Bluetooth. Both model TDR-315L and model CS655 sensors were used successfully with this system in four states in the southeast US.