Abstract
Soil moisture is an important hydrological parameter, which is essential for a variety of applications, thereby extending to numerous disciplines. Currently, there are three methods of estimating soil moisture: ground-based (in-situ) measurements; remote sensing based methods and land surface models. In recent years, the cosmic ray probe (CRP), which is an in-situ technique, has been implemented in several countries across the globe. The CRP provides area-averaged soil moisture at an intermediate scale and thus bridges the gap between in-situ point measurements and global satellite-based soil moisture estimates. The aim of this study was to test the suitability of the CRP to provide spatial estimates of soil moisture. The CRP was set up and calibrated in Cathedral Peak Catchment VI. An in-situ soil moisture network consisting of time-domain reflectometry and Echo probes was created in Catchment VI, and was used to validate the CRP soil moisture estimates. Once calibrated, the CRP was found to provide spatial estimates of soil moisture, which correlated well with the in-situ soil moisture network data set and yielded an R2 value of 0.845. The use of the CRP for soil moisture monitoring provided reliable, accurate and continuous soil moisture estimates over the catchment area. The wealth of current and potential applications makes the CRP very appealing for scientists and engineers in various fields.Significance:
 
 The cosmic ray probe provides spatial estimates of surface soil moisture at an intermediate scale of 18 hectares.
 A single cosmic ray probe can replace a network of conventional in-situ instruments to provide reliable soil moisture estimates.
 The cosmic ray probe is capable of estimating soil moisture in previously problematic areas (saline soil, wetlands, rocky soil).
 Cosmic ray probes can provide data for hydro-meteorologists interested in land–atmosphere interactions.
 The cosmic ray probe estimates can be promising for remote sensing scientists for product calibration and validation.
Highlights
There has been a continual need to monitor and measure the various parameters in land surface hydrology, in order to deepen the understanding of hydrological processes, their importance in the hydrological cycle and their interactions between each other.[1,2]
Soil moisture varies both spatially and temporally and fluxes in soil moisture content occur over short time periods and distances
Understanding the spatial and temporal variability of soil moisture at different scales is of great importance in many land surface disciplines, such as hydrology
Summary
There has been a continual need to monitor and measure the various parameters in land surface hydrology, in order to deepen the understanding of hydrological processes, their importance in the hydrological cycle and their interactions between each other.[1,2] Soil moisture is an important parameter in the hydrological cycle and impacts a variety of applications, including agricultural management, climate and weather applications, flood and drought forecasting and groundwater recharge. Soil moisture is a difficult parameter to continuously monitor and measure at a catchment scale because of its heterogeneous characteristics. It varies both spatially and temporally and is a dynamic resource. There are three methods of estimating soil moisture: (1) ground-based (in-situ) measurements, which are carried out using field instruments; (2) remote sensing based methods, which use specialised sensors on satellites and aircrafts and (3) land surface models, which use meteorological data as inputs, at a predefined spatial resolution.[3,4] Inherently, each of these methods possess their respective advantages and limitations, constraining their effectiveness for hydrological applications.[5]
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