Primary Drivers of Meso-Scale Soil Moisture Variability: A Cosmic-Ray Neutron Rover Study.

Understanding the primary drivers of the spatial structure of soil moisture at the meso-scale (~1-100 km) has been hindered due to the scale gap between satellite remote sensing and in situ point measurements. A cosmic-ray neutron rover, with a footprint of 400-m diameter, makes it possible to perceive spatial patterns in soil moisture at this scale. A rover was repeatedly transported by vehicle along a 150-km transect recording fast neutron counts in order to observe mesoscale soil moisture patterns over 13 months. Spatial structures of soil moisture were characterized using autocorrelation functions and fitted with exponential models. The estimated ranges of the model parameter (correlation lengths) of soil moisture, sand content, and antecedent precipitation index (API) were 12-32 km, 13-20 km, and 14-45 km respectively. Relative importance of land surface characteristics and atmospheric processes were compared using correlation coefficients between soil moisture and sand content/API. Stronger and more stable correlations were observed between soil moisture and sand content. These results suggested that soil texture exerted a stronger influence on the mesoscale spatial structure of soil moisture than did API.