Applying Cosmic-Ray Neutron Sensing in Highly Heterogeneous Conditions: Monitoring Snow Water Equivalent in Periods with Partial Snow Coverage

Cosmic-Ray Neutron Sensing (CRNS) constitutes an emerging method for monitoring soil moisture and snow dynamics at intermediate spatial scales of several hectares. In complex environments such as mountain regions, however, the presence of areas with a high contrast of hydrogen content was found to cause a hysteresis in the relationship between neutron counts and water equivalent. A simulation study using the newly developed hierarchical scenario tool YULIA (Your URANOS Layer Integration Assistant) for the Monte-Carlo neutron simulation model URANOS was conducted to quantify the effect of snow-free areas on above-ground neutron sensing of the snow water equivalent (SWE). It was found that the size and distance of the snow free patches have the largest impact on the neutron flux. The simulations also showed a sensitivity of the signal towards soil moisture and SWE. Correction functions were developed and validated with observed CRNS measurements and LiDAR based distributed SWE maps. The main aim of the correction procedure is to estimate SWE under partly snow-covered conditions. Furthermore, also the soil moisture of the snow-free areas can be inferred if the SWE distribution is known. The latter can be used for other high-contrast CRNS applications like monitoring soil moisture in the presence of ponding water.