Translating above-ground cosmic-ray neutron intensity to high-frequency soil moisture profile at sub-kilometer scale

Abstract:

Aboveground cosmic-ray neutron measurements provide an opportunity to infer soil moisture at the sub-kilometer scale (with an effective area of approximately 300 meter radius). Initial efforts to assimilate those measurements have shown the ability to accurately estimate integrated soil moisture at the root zone. This study expands such analyses by investigating (1) how the information from aboveground cosmic-ray neutrons can constrain the soil moisture at distinct depths simulated by a land surface model, and (2) how changes in data availability (in terms of retrieval frequency) impact the dynamics of simulated soil moisture profiles. We employ ensemble data assimilation techniques in experiments applied at semi-arid shrubland, rainfed agricultural field, and mixed forest biomes. The performance of the land surface model is compared without and with assimilation of observations at hourly intervals and every two days. Synthetic observations of aboveground cosmic-ray neutrons better constrain the soil moisture simulated by the model in root zone soil layers (0-100 cm) despite the limited measurement depth of the sensor (estimated to be 12-20 cm). The ability of the model to reproduce a ‘true’ soil moisture profile is remarkably good regardless of the frequency of observations at the semi-arid site. However, soil moisture profiles are better constrained when assimilating synthetic cosmic-ray neutrons observations hourly rather than every two days at the cropland and mixed forest sites. This indicates potential benefits of using cosmic-ray neutron sensors for hydrometeorological modeling and in combination with satellite remote sensing. Moreover, differences in summertime meteorological forcing between the semi-arid site and the other two sites may indicate a possible controlling factor to soil moisture dynamics in addition to differences in soil and vegetation properties.