Self-calibrated evaporation-based disaggregation of SMOS soil moisture: An evaluation study at 3 km and 100 m resolution in Catalunya, Spain

Abstract

A disaggregation algorithm is applied to 40km resolution SMOS (Soil Moisture and Ocean Salinity) surface soil moisture using 1km resolution MODIS (MODerature resolution Imaging Spectroradiometer), 90m resolution ASTER (Advanced Spaceborne Thermal Emission and Reflection radiometer), and 60m resolution Landsat-7 data. DISPATCH (DISaggregation based on Physical And Theoretical scale CHange) distributes high-resolution soil moisture around the low-resolution observed mean value using the instantaneous spatial link between optical-derived soil evaporative efficiency (ratio of actual to potential evaporation) and near-surface soil moisture. The objective is three-fold: (i) evaluating DISPATCH at a range of spatial resolutions using readily available multi-sensor thermal data, (ii) deriving a robust calibration procedure solely based on remotely sensed data, and (iii) testing the linear or nonlinear behavior of soil evaporative efficiency. Disaggregated soil moisture is compared with the 0–5cm in situ measurements collected each month from April to October 2011 in a 20km square spanning an irrigated and dry land area in Catalunya, Spain. The target downscaling resolution is set to 3km using MODIS data and to 100m using ASTER and Landsat data. When comparing 40km SMOS, 3km disaggregated and 100m disaggregated data with the in situ measurements aggregated at corresponding resolution, results indicate that DISPATCH improves the spatio-temporal correlation with in situ measurements at both 3km and 100m resolutions. A yearly calibration of DISPATCH is more efficient than a daily calibration. Assuming a linear soil evaporative efficiency model is adequate at kilometric resolution. At 100m resolution, the very high spatial variability in the irrigated area makes the linear approximation poorer. By accounting for non-linearity effects, the slope of the linear regression between disaggregated and in situ measurements is increased from 0.2 to 0.5. Such a multi-sensor remote sensing approach has potential for operational multi-resolution monitoring of surface soil moisture and is likely to help parameterize soil evaporation at integrated spatial scales.