Observation on Soil Moisture of Irrigation Cropland by Cosmic-Ray Probe

The newly developed cosmic-ray probe (CRP) method for measuring area-average soil moisture at the hectometer horizontal scale was tested in the 2012 observation campaign of the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) in the irrigated cropland area of Zhangye Oasis. As compared with the traditional point observation, data analysis shows that the CRP could measure the real areal soil moisture, except when the water is frozen and during thaw. During the irrigation period, the presence of surface water can lead to an overestimation of the soil moisture. During non-irrigated periods, the CRP has a very strong correlation with the averaged soil moisture of 19 SoilNET probes in its footprint, whose R² is 0.73 and root-mean-square error is 0.0275 m³/m³. In comparison with the Polarimetric L-band Multi-beam Radiometer (PLMR) retrieved soil moisture, which has a pixel resolution of 700 m, the CRP provides much better results, with a coefficient of determination of 0.96 and 0.64, respectively. The results show that the CRP is a robust area-averaged soil moisture observation method, which can be used to obtain “true” values of field-scale soil moisture for remote sensing validation.