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Cosmic-ray rover, an innovation in soil moisture mapping

By Thigesh Vather, PDP* PhD student, SAEON Grasslands-Forests-Wetlands Node
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Thigesh spent a month at the University of Nebraska-Lincoln last year where he gained first-hand experience with their all-terrain vehicle rover

Soil moisture is an important hydrological parameter which extends through various disciplines (hydrology, ecology, meteorology, agriculture), as knowledge of soil moisture is required for numerous applications.

Understanding the soil moisture patterns at a range of spatial scales is important. However, this is challenging due to the spatial and temporal heterogeneity of soil moisture.

Soil moisture is currently estimated by three broad methods - in-situ (ground-based) techniques, remote sensing and modelling. Each method possesses its own advantages and subsequent limitations, which hinder its use for critical hydrological applications.

Cosmic-ray technology

In recent years the development and implementation of cosmic-ray technology has emerged across the globe. Cosmic-ray technology currently consists of the cosmic-ray probe, which is a stationary instrument that is used for continuous soil moisture monitoring and the cosmic-ray rover, which is an instrument that is placed in a vehicle and driven around to map soil moisture.

Cosmic-ray neutrons originate in space. These high-energy particles penetrate the earth’s surface and interact with atmospheric nuclei, which leads to the generation of fast neutrons.

These fast neutrons travel towards the earth’s surface and are moderated primarily by hydrogen. This is due to hydrogen’s scattering probability, the logarithmic decrement of energy per collision and the number of atoms of an element per unit mass of material. Since hydrogen at the land surface is mostly in the form of soil water, the fast neutron intensity above the land surface is inversely correlated to the soil water.

The appeal of this technology is its capability to provide area-averaged soil moisture estimates at an intermediate scale, which bridges the measurement gap between conventional in-situ point measurements and satellite-based methods. The instrument works by measuring the neutron intensity above the soil surface. Since these neutrons are predominately moderated by hydrogen in the soil, the soil moisture can be determined once all the other hydrogen sources (lattice water, water in soil organic carbon, biomass and atmospheric water vapour) are accounted for.

The cosmic-ray probe was developed first and is implemented across the globe. The cosmic-ray rover was developed because of the capabilities of the cosmic-ray probe.

There are only a handful of cosmic-ray rovers worldwide. The cosmic-ray rover uses the technology of the cosmic-ray probe; hence it has the same horizontal footprint (≈240 m radius) and a measurement depth range of 12-72 cm.

The footprint of the cosmic-ray rover is the swath with its width equal to the footprint of the stationary cosmic-ray probe, and its length equal to the distance travelled during the counting interval. Due to the novelty of the technology, its applications and limitations are still being investigated.

Investigating the Vazi area

Cosmic-ray rover surveys were conducted in the Vazi area located in the Maputaland Coastal Plain (north-eastern KwaZulu-Natal). The aim of the surveys was to evaluate the suitability of the cosmic-ray rover to provide spatial estimates of soil moisture. Hydro-sense (portable TDR) surveys were also conducted during the cosmic-ray rover surveys to calibrate and validate the surveys.

The Maputaland Coastal Plain is of great interest as it is a groundwater-driven system, rich in wetlands, where the ecology of the systems is strongly influenced by the water table. The region is also home to the largest freshwater lake in South Africa, Lake Sibyai.

Freshwater pans, wetlands and the lake provide valuable ecosystem services. However, over the past decade or more, the groundwater table has been dropping significantly, causing the degradation of wetland systems now dry and vulnerable to fire and agricultural invasion. There has also been a concerning drop in Lake Sibayi’s water level to below previously recorded low levels.

These negative effects on the hydrology of the area are in part due to forestry, increased abstraction and development in the area, combined with an extended period of below-average rainfall, although the relative contribution of these impacts is still under investigation. This study will hopefully provide an additional tool for assessing land-use impacts on the groundwater table in this vulnerable area.

Cosmic-ray rover surveys were conducted at two sites in Vazi - a moist grassland and a pine forest (Figures 1 and 2).

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Figure 1. Cosmic-ray rover survey sites

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Figure 2. Grassland (a) and pine forest (b)

The following are preliminary soil moisture maps generated from the surveys. The moist grassland survey (Figure 3) shows the survey maps of the hydro-sense soil moisture measurements (Figure 3a) and the corrected neutron counts (Figure 3b).

The inverse relationship between soil moisture and neutron intensity can be seen, as the areas of higher soil moisture subsequently have the lowest neutron intensities. The cosmic-ray rover soil moisture map (Figure 3c) correlates well with the hydro-sense soil moisture map with regards to the soil moisture patterns. The cosmic-ray rover mapped high soil moisture areas very well.

The cosmic-ray rover and hydro-sense maps, when compared to the landcover image (Figure 3d), show that the areas of high soil moisture correspond to the areas of wetland vegetation, which occur in the depressions of the landscape.

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Figure 3. Maps of the hydro-sense measurements (a), corrected neutron counts (b), cosmic-ray rover estimates (c) and landcover (d)

The pine site survey (Figure 4) shows the hydro-sense soil moisture map (Figure 4a), which indicates that the soil moisture is lower in the south-western side of the survey site and increases towards the north-eastern side of the site. The corrected neutron count map (Figure 4b) agrees with the inverse relationship between soil moisture and neutron intensity, as there is a decrease in neutron intensity in the north-eastern side of the site, which correlates to the higher soil moisture area of the site.

The cosmic-ray rover soil moisture estimates map (Figure 4c) correlates well with the hydro-sense soil moisture map, as the general soil moisture patterns are similar. Both maps show an increase in soil moisture from the south-west to the north-east of the survey site.

The landcover image (Figure 4d), when compared with the hydro-sense and cosmic-ray rover soil moisture maps, shows that the area of high soil moisture corresponds to the area of bracken in the catchment. There is a slight difference in soil moisture ranges between hydro-sense and the cosmic-ray rover.

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Figure 4. Maps of the hydro-sense measurements (a), corrected neutron counts (b), cosmic-ray rover estimates (c) and landcover (d)

The use of the cosmic-ray rover to map soil moisture is a promising technique, as the soil moisture maps produced by the cosmic-ray rover correlate well with the soil moisture maps produced with the hydro-sense data. Overall, the spatial patterns were adequately captured, and the cosmic-ray rover could identify changes in soil moisture over the landscape and effectively illustrate the soil moisture gradients.

Although the cosmic-ray rover correlated well with the hydro-sense with regards to the spatial soil moisture patterns, there were discrepancies between the soil moisture ranges. This is due to the hydro-sense measuring at a point scale, while the cosmic-ray rover was measuring at an area-average scale.

Getting to grips with the technology

The first survey is always the most challenging, as the cosmic-ray rover needs to be calibrated at each site, which requires in-situ estimates of soil moisture (hydro-sense data). Due to the cosmic-ray rover technique being new and innovative, there is currently no “correct” method in conducting the surveys. Coupled with each site being different, the surveys need to be planned beforehand. Thus, each survey is useful in better understanding the technology and its capabilities.

The results of the survey show the capability of the cosmic-ray rover to provide spatial estimates of soil moisture. The soil moisture map shows that the higher soil moisture values occur in the depressions of the landscape and highlights peat’s ability to hold water.

With the occurrence of droughts, the importance of peatlands increases. This is problematic in the Vazi region as the drop in the groundwater table results in more peatland being destroyed.

In conclusion, successful cosmic-ray rover surveys were conducted at two different sites in Vazi. The cosmic-ray rover has shown promise as an innovative technique to map soil moisture and could potentially be a tool to delineate wetlands and riparian areas, as the changes in soil moisture is spatially captured and represented.

Its applications can also be extended to calibrating and validating satellite-based and modelled soil moisture products, as well as creating soil moisture maps for precision irrigation in large-scale agriculture.

* The Professional Development Programme of the Department of Science and Technology and the National Research Foundation aims to accelerate the development of scientists and research professionals in key research areas.

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