Estimation of root zone soil moisture from gamma radiation measurements
Prof. Dr. J.A. (Sander) Huisman
Forschungszentrum Jülich | +49-2461-618607 |
Root-zone soil moisture is an essential variable in land surface models, but there is still a lack of sensing information that adequately represents this zone. The first main hypothesis is that a root-zone soil-moisture product for Europe can be derived from existing gamma radiation data from the European radiological data exchange platform (EURDEP). For this, it is important to understand how infiltration, evaporation, transpiration, and secondary cosmic radiation affect measured gamma radiation. This will be achieved with both laboratory and field experiments and the analysis of available gamma radiation time series. The insights obtained will be used to develop data processing strategies and an observation operator that relates soil moisture to gamma radiation. This operator will be coupled to a hydrological model to validate the performance for soil moisture content estimation and the improved estimation of soil hydraulic parameters.
Contribution to CRC
Project A02 will closely collaborate with A01 and A03 through joint experimental work and data analysis and a common modeling approach (SiSPAT-Isotope). The main focus of A02 is to work towards developing a new soil moisture data product at the European scale to improve the accuracy of the TerrSysMP integrated modelling system (IMS). Hence, an observation operator for gamma radiation will be developed together with C01 to enable data assimilation into TerrSysMP IMS.
A series of field experiments will provide the foundation for this project. Time series of gamma radiation will be measured with proportional gamma radiation counter tubes on different sites in Germany, Netherlands and France. These measurements will be complemented with gamma radiation spectrometer measurements. The selected sites are equipped with in-situ soil moisture sensors and meteorological stations to support data interpretation. In addition to the field experiments, laboratory experiments will be performed to investigate how the confounding factors affect the measured gamma radiation in field experiments. A data processing method will be developed based on the insights of laboratory experiments to extract the terrestrial gamma radiation component by removing unwanted gamma radiation sources. Then, an observation operator will be developed to relate the terrestrial component of the gamma radiation to soil moisture content. To obtain insights into the challenges associated with analysing the EURDEP data, 15 EURDEP stations within the TERENO Rur catchment observatory will be analysed to estimate the soil moisture content using the developed operator. In a first step, the observation operator will be coupled with the SiSPAT-Isotope model to investigate the influence of near-surface soil moisture gradients on the terrestrial gamma radiation signal and to enable the estimation of soil hydraulic parameters.
Main Results in 2022
The time series of two gamma radiation monitoring stations in Forschungszentrum Jülich (FZJ) were analyzed. The terrestrial component of gamma radiation was extracted. For this, it was assumed that the long-term contribution of secondary cosmic radiation is constant and that the variations are caused by changes in atmospheric pressure and incoming neutrons. In addition, precipitation effects creating a sudden increase in gamma radiation due to atmospheric washout of radon progenies to the ground were eliminated by excluding time periods with precipitation.
Figure 1. The ambient gamma dose rate for the FZJ test site 1 for different integration times.
Finally, multi-year terrestrial gamma radiation measurements were used to estimate weekly root-zone soil moisture and the results were compared with reference measurements. It was found that the seasonal variation of soil moisture can be reasonably well predicted with an RMSE of 7 – 9 vol.% from gamma radiation measurements.
Figure 2. Predicting soil moisture content from terrestrial dose rate measurements for two test sites in FZJ.