Hydrological links between cosmic-ray soil moisture retrievals and remotely sensed evapotranspiration across a semi-arid pasture site

Soil moisture and land surface evaporation, which includes soil evaporation, canopy interception and transpiration processes, are key factors affecting water cycle behavior and feedbacks between the land surface and the atmosphere at a range of scales. Measurement of soil moisture at intermediate resolutions is a challenge that has been addressed to a certain extent by the Cosmic Ray Soil Moisture Observing System (COSMOS). We present here a study to examine the link between the COSMOS soil moisture retrievals and evapotranspiration (ET) estimates obtained from remote sensing at a semi-arid pasture site near Baldry, in the central-west of New South Wales, Australia. COSMOS soil moisture was compared to ET estimates obtained by applying the PT-JPL method to remote sensing products of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. Ancillary data included air temperature, humidity, and radiation components. Use of Quartile-Quartile (Q-Q) plots and Analysis of Variance (ANOVA) with box plots provide a graphical estimate for the similarity of the distributions of the two quantities. The relationships were tested across the entire period of record, as well as across shorter periods in order to analyze local scale discrepancies. Results show that the COSMOS soil moisture, which is representative of much of the root zone of the pasture field, is well correlated with the modeled ET under most conditions. It was also noted that under high soil moisture conditions with low temperatures, the PTJPL method produced ET values inconsistent with measurements from a local eddy covariance tower, and also with the COSMOS soil moisture. This leads to further investigations regarding appropriate models for particular conditions.