Estimating dynamic groundwater recharge rates for the field scale with soil moisture from cosmic-ray neutron sensing

Lena Scheiffele1, Gabriele Baroni2, Sascha Oswald1, Matthias Munz1
1 Wasser- und Stofftransport in Landschaften, Universität Potsdam
2 Dipartimento di Scienze e Tecnologie Agro-Alimentari, Università di Bologna

P 13.6 in Artificial and natural groundwater recharge (co-organized by IAH)

The use of soil moisture measurements together with soil hydrological modelling is one of the methods to estimate groundwater recharge with high temporal resolution. Soil moisture measurements are usually collected at the point scale, and therefore are very sensitive to soil spatial heterogeneity and often may not be representative of average field conditions. In recent years, cosmic-ray neutron sensing (CRNS) has been established as a non-invasive method to measure an integral soil moisture on field scale (support volume with a radius of up to 240 m, integration depth of up to 50 cm), overcoming the limitations of traditional point measurement techniques. It utilizes naturally occurring fluctuations in background radiation, and the detected number of neutrons are directly proportional to the soil moisture within the support volume. CRNS covers the most dynamic part of the vadose zone and is thus promising to improve estimations of potential groundwater recharge for the field scale.

Within this study, the soil moisture from CRNS is compared to soil moisture from an extensive distributed point-scale sensor network. Both soil moisture data sets are used in HYDRUS 1D for the inverse calibration of soil hydraulic parameters. Resulting groundwater recharge represents an average for the field scale because of the nature of the soil moisture products, even though the model is calculating percolation fluxes for a profile. First results show a promising performance of CRNS soil moisture for estimating daily recharge rates and compares well to results based on the sensor network.

CRNS is, however, sensitive to the vertical distribution of water content and this behavior should be explicitly considered. Two approaches are assessed further to account for that. On the one hand, a correction of CRNS based on measured soil moisture profiles is tested (Baroni et al., 2018, Scheiffele et al., 2020) and CRNS soil moisture is directly used within HYDRUS. On the other hand, the COSMIC-Operator, as implemented within HYDRUS (Brunetti et al., 2019), is used for calibrating the model by directly comparing neutron count rates from simulated soil moisture. Both approaches will be assessed in respect to their ability to estimate natural groundwater recharge rates.



Baroni, G., Scheiffele, L. M., Schrön, M., Ingwersen, J., and Oswald, S. E. (2018). Uncertainty, sensitivity and improvements in soil moisture estimation with cosmic-ray neutron sensing. J. Hydrol. 564, 873–887. doi:10.1016/j.jhydrol.2018.07.053.

Brunetti, G., Šimůnek, J., Bogena, H., Baatz, R., Huisman, J. A., Dahlke, H., et al. (2019). On the Information Content of Cosmic-Ray Neutron Data in the Inverse Estimation of Soil Hydraulic Properties. Vadose Zone J. 18. doi:10.2136/vzj2018.06.0123.

Scheiffele, Lena M., Gabriele Baroni, Trenton E. Franz, Jannis Jakobi, and Sascha E. Oswald. 2020. A Profile Shape Correction to Reduce the Vertical Sensitivity of Cosmic-Ray Neutron Sensing of Soil Moisture.” Vadose Zone J. 19. doi:10.1002/vzj2.20083.