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Cosmic-Ray probe at Cathedral Peak provides important validation data for flood forecasting model

By Colin Everson, Hydro-meteorologist, SAEON
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The vulnerability of Southern African countries such as South Africa to climate and environmental change is likely to increase as demands on resources continue to rise in conjunction with rapidly growing populations.

With South Africa currently gripped in the worst drought in a century, disaster management agencies have to adapt to an increasing number of droughts and floods.

In addition, water resources management, crop modelling and irrigation scheduling all require accurate daily estimates of soil moisture and evaporation from catchment to national scale. This is only feasible through remote-sensing technologies and it is therefore essential to further the development and integration of space-based technologies within already existing national disaster management plans.

In South Africa, evapotranspiration and soil moisture have only been available at isolated sites until recently, when researchers at the University of KwaZulu-Natal (UKZN) developed a detailed spatial product of real-time estimates of soil moisture and evapotranspiration . These variables are now routinely calculated in real time and made available as up-to-date images on the website of the UKZN Satellite Applications and Hydrology Group (SAHG).

The model has shown promise, but still requires further development, as errors in the input data streams are hampering the quality of the product. The key focus of this product is to provide a proof of concept for operational use by the South African Weather Service (SAWS) in their national Flash Flood Guidance system.

Until the development of the Cosmic-Ray probe, there was no suitable technology that could measure soil water at the appropriate scales to validate the models. The Cosmic-Ray probe is a new technology that has not been used by researchers in Southern Africa before.

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The cosmic-ray probe at Cathedral Peak

Global Cosmos server network

Measurements using the Cosmic-Ray probe at area scales of up to 34 ha have the potential to provide hydro-meteorologists with an entirely new way of evaluating surface soil water at spatial scales never achieved with ground-based techniques. There are proposed plans for a global Cosmos server network, so a South African network operated by SAEON would fit nicely into that theme. A COSMIC array network could provide a powerful new addition to the flood forecasting ability of SAWS.

The need to provide an independent validation of the HYLARSMET (Hydrologically Consistent Land Surface Model for Soil Moisture and Evapotranspiration Modelling Over Southern Africa Using Remote Sensing and Meteorological Data) model was recognised and a Water Research Commission (WRC) project was initiated to provide a spatially explicit validation procedure for the 1 km grid of soil moisture and evapotranspiration produced by the SAHG at UKZN and other global climate models.

Automatically tracking the current soil moisture state is a core function that allows the SA government's Flash Flood Guidance System to provide alerts based on current and predicted rainfall. In addition, South Africa’s current Flash Flood Guidance system uses a relatively crude evapotranspiration model. Therefore, validating evapotranspiration and soil moisture estimates with better temporal and spatial resolution obtained from SAEON sites will make improvements to South Africa’s Flash Flood Guidance system.

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Figure 1. Time series analysis of SAHG and Cosmic-Ray probe soil moisture estimates for the Cathedral Peak Catchment VI site

Measurements using the Cosmic-Ray probe at area scales of up to 34 ha have the potential to provide hydro-meteorologists with an entirely new way of evaluating surface soil water at spatial scales never achieved with ground-based techniques. This will provide water resource managers, engineers and agriculturalists with an invaluable but economical new tool to monitor the critical interface between the ground and atmosphere.

Water demand forecasting

This new technology can be employed in water demand forecasting and promises to improve the utilisation of irrigation water, especially in water-scarce regions like South Africa. The probe can also be used for predictive weather and climate models by measuring soil water content.

Spatially distributed field-based measurements of soil moisture will also be used to verify the Cosmic-Ray probe estimates. The aim here is to assess how spatially determined soil moisture measurements compare with the point measurements of soil moisture and soil moisture measured using Cosmic-Ray probe.

A graph of the SAHG and Cosmic-Ray probe soil moisture estimates is presented in Figure 1. The SAHG soil moisture estimates followed the same seasonal trend as the CRP estimates, with good correlation between the two datasets despite measuring at different horizontal and vertical scales, demonstrating the usefulness for model validation of the Cosmic-Ray probe set up by SAEON and UKZN at Cathedral Peak.

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