Work

Accounting for biomass water equivalent variations in soil moisture retrievals from cosmic ray neutron sensor

Year: 2025

Type: article

Abstract: Cosmic ray neutron sensor (CRNS) has gained popularity in the last decade for its suitability in estimating area-averaged soil moisture (SM). The presence of fresh biomass influences the CRNS signal due to its water content, introducing bias to soil moisture estimation. Calibration and correction methods have been developed to account for this bias, but they usually require laborious sampling. We tested a novel approach to assess the biomass water equivalent (BWE) impact on CRNS soil moisture estimation in olive and cherry orchards, located in contrasting environments in Saudi Arabia and France, respectively. We monitored water-uptake rates using sap flow sensors, as well as Evapotranspiration (ET) and in-situ SM within the CRNS footprint. Concurrent environmental variables were also measured with a research-grade weather stations. We found that when vapor pressure deficit (VPD) > 1.8kPa, CRNS-derived SM (CRNS-SM) closely matched in-situ SM measurements, which indicates minimal influence from BWE. Conversely, when VPD is lower than 1.8kPa, CRNS-SM overestimates the in-situ moisture. An optimization approach was used to find a temporally-varying value of N0 parameter that minimizes the difference between soil moisture estimated with CRNS and in-situ sensors. Furthermore, we found that relative change in the optimized value of N0 (N0,opt) was well correlated with VPD in both orchards (R2 = 0.66 for olive and R2 = 0.74 for cherry orchards). Our findings suggest that integrating VPD and CRNS observations, and using the VPD-N0,opt correlation approach could be a promising way to account for the bias due to biomass dynamics on the estimation of area-averaged SM.

Source: Agricultural Water Management

DOI: 10.1016/j.agwat.2025.109493