Accuracy and precision of the cosmic‐ray neutron sensor for soil moisture estimation at humid environments

Abstract

AbstractThe accuracy and precision of the cosmic‐ray neutron sensor (CRNS) neutron count and soil moisture estimate are affected differently by distinct neutron moderating factors. Moreover, whereas the accuracy can be improved by correcting for different hydrogen pools, the precision can only be improved by increasing the surface area of the CRNS sensors or by increasing the integration time. To date, the effects of different neutron moderating factors on CRNS accuracy and precision are not completely understood. We used data from three agricultural and low biomass sites located within a few kilometres distance from each other in South England. We developed an extended version of the COsmic‐ray Soil Moisture Interaction Code (COSMIC), which included the effects of relevant neutron moderating factors on the neutron counts. With sensitivity analysis, we found atmospheric pressure and soil moisture content to be most influential on neutron count accuracy and precision. These two factors were, respectively, seven and four times more important than soil bulk density, lattice water and soil organic matter. Above ground biomass was substantially less influential compared to these variables. However, because the three sites had similar soil organic matter and meteorology, calibration results showed that differences between sites could be mostly explained by differences in above ground biomass and to a lesser extent intercepted water. The neutron count differences due to above ground biomass corresponded with substantial soil moisture estimate differences up to ; a significant effect on soil moisture accuracy. The precision of the soil moisture estimate was 10 times more sensitive to soil moisture than other factors and was therefore mostly a function of soil moisture content itself. The neutron count integration times required to obtain a soil moisture precision were typically less than a day. Our results are indicative for other humid, temperate, agricultural sites.