Novel Cosmic Ray Neutron Sensor Accurately Captures Field-Scale Soil Moisture Trends under Heterogeneous Soil Textures

Kade D Flynn,Briana M Wyatt,Kevin J Mcinnes

doi:10.3390/w13213038

Abstract

Soil moisture is a critical variable influencing plant water uptake, rainfall-runoff partitioning, and near-surface atmospheric conditions. Soil moisture measurements are typically made using either in-situ sensors or by collecting samples, both methods which have a small spatial footprint or, in recent years, by remote sensing satellites with large spatial footprints. The cosmic ray neutron sensor (CRNS) is a proximal technology which provides estimates of field-averaged soil moisture within a radius of up to 240 m from the sensor, offering a much larger sensing footprint than point measurements and providing field-scale information that satellite soil moisture observations cannot capture. Here we compare volumetric soil moisture estimates derived from a novel, less expensive lithium (Li) foil-based CRNS to those from a more expensive commercially available 3He-based CRNS, to measurements from in-situ sensors, and to four intensive surveys of soil moisture in a field with highly variable soil texture. Our results indicate that the accuracy of the Li foil CRNS is comparable to that of the commercially available sensors (MAD = 0.020 m3 m−3), as are the detection radius and depth. Additionally, both sensors capture the influence of soil textural variability on field-average soil moisture. Because novel Li foil-based CRNSs are comparable in accuracy to and much less expensive than current commercially available CRNSs, there is strong potential for future adoption by land and water managers and increased adoption by researchers interested in obtaining field-scale estimates of soil moisture to improve water conservation and sustainability.

Highlights

Soil cores collected at radial distances of 0 m, 50 m, 100 m, and 150 m, in each cardinal direction from the cosmic ray neutron sensor (CRNS) were used to characterize the soil properties of the field
Soil textures at 150 m from the CRNS included loamy sand (LS), sandy clay loam (SCL) and silt loam (SiL). These soil textural classes are indicative of the high level of soil texture variability of the field; measured sand contents ranged from 80% and clay contents ranging from 5% to nearly 40% (Table 1)
3 He CRNS at 0 m from the sensors and at 150 m from the sensors. These results indicate that the HydroSense, which had 12 cm rods, did not provide measurements to the full depth of influence for CRNS readings

Summary

Introduction

Soil moisture is an essential climate variable which influences a number of important hydrological processes, including rainfall infiltration and surface runoff, plant water use, and groundwater recharge, among others [1,2]. A large number of local, regional, and national in-situ soil moisture monitoring networks have been developed around the world in recent years, and the advent of soil moisture remote sensing satellites such as the. Missions have further increased the availability of soil moisture information [5]. Despite the increasing availability of soil moisture observations, there remains a gap in the spatial scale between in-situ sensors, which typically measure conditions only within a few centimeters of the sensor itself, and remote sensing observations, which provide a single mean soil moisture value over areas of multiple square kilometers

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