Hydrogeodesy - Going deep: combining CRNS with other non-invasive soil moisture measurements with varying integration depths

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The main goal of the module Hydrogeodesy (HG) within the RU Cosmic Sense is to extrapolate the CRNS (Cosmic Ray Neutron Sensing) soil moisture beyond its near-surface sensing depth to the entire unsaturated zone. A unique data set of depth-resolved soil moisture at the CRNS footprint and at the catchment scale will be generated to develop and test depth-scaling approaches, assuming that CRNS observations provide valuable information for assessing the water content in the deeper vadose zone. We combine CRNS with two other non-invasive monitoring techniques with a similar horizontal footprint as CRNS (in the order of 100 m) but with different vertical integration depths: GNSS reflectometry (GNSS-R) with a depth of a few centimeters, and terrestrial gravimetry which integrates over the entire vadose zone. At field sites with collocated sensors of the three techniques (CNRS, GNSS-R and gravimeters) and with in-situ sensor profiles and/or lysimeters, we will merge the observations and derive time-variable functional relationships between the soil moisture variations at different depths. Main objectives of this project include: 1) Increasing the CRNS observational window by vertical extrapolation at the local footprint and at the regional scale based on depth-scaling approaches. The application and evaluation of the depth-scaling approaches at the regional scale will be enabled by the campaign-based combination of stationary and roving CRNS with gravimetry on a spatially distributed network of support points (during RU joint field campaigns). This will also facilitate extrapolating remotely sensed soil moisture to greater depths.2) Comparing and correlating soil moisture dynamics of the different monitoring techniques from a twofold perspective: a) understanding in which way the respective time series are related to each other with regard to correlation, time lags and signal damping between observations of different integration depths and b) assessing the respective data quality and uncertainties. 3) Advancing terrestrial gravimetry as a complementary technique to CRNS, adding information on water storage dynamics at larger depths. 4) Advancing the CRNS method by investigating the CRNS signal response to horizontal and vertical soil moisture heterogeneities within the footprint. This includes a comparative response analysis of neutrons with different energy levels at sites with either a natural or artificially induced pronounced heterogeneity in moisture conditions, and exploring the value of both CRNS and gravimeter observations at different positions below ground for better resolving soil moisture heterogeneity with depth.This work will ultimately contribute to a better understanding of water fluxes between atmosphere, soil and groundwater as a function of soil water content, and to quantifying water balances beyond the point or profile scale, including deeper storage variations.