BLOSM: Boron-based Large scale Observation of Soil Moisture
Measuring soil moisture in the field at a scale comparable to that of satellite pixels or model raster cells is a central hydrological problem for calibration and validation. Using the standard gravimetric method, this involves taking many (100-1000) samples, which can only be achieved through expensive field campaigns. There is one way to measure soil moisture at a scale of about 30 hectares and that is, somewhat surprisingly, by measuring fast and slow neutrons at a point.
Our atmosphere is constantly being bombarded by cosmic particles, which cause showers of sub-atomic particles to hit the Earth’s service. Among these particles are fast moving neutrons (20,000km/s). As these neutrons bounce into atoms, they come into thermal equilibrium moving at a speed of about 200km/s. Water is especially good at slowing down neutrons. The ratio of slow and fast neutrons is a robust measure of the amount of water present within a radius of about 300m.This has been known and used for some time by the Helium-3 (3He) based COSMOS sensor. Unfortunately, 3He and COSMOS are relatively expensive and also subject to export limitations due to their nuclear applications.
TU Delft and Microstep MIS have developed a new neutron detector that is based on natural boron. Natural boron consists for 20% of 10B, which captures thermal neutrons, after which it splits into a lithium atom and an alpha-particle. The alpha-particle is then detected with a ZnS‑Ag pigment, which produced thousands of photons. These photons are captured by wavelength-shifting optical fibers that guide them to a silicon photon-multiplier. The upshot is that we can build a slow and a fast neutron detector for a total of Euro 500 worth of materials. As these are not subject to export limitations, they will be installed soon in Africa for final testing.
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The main application of this sensor will be the continuous and dynamic calibration of satellite based soil moisture estimates that feed the insurance services. By placing, say, 100 BLOSMs at strategic places along moisture gradients in Africa, such calibration would make the satellite product very reliable and increase trust. They can readily be plugged into existing TAHMO stations.