Combined use of optical and radar satellite data for the monitoring of irrigation and soil moisture of wheat crops

摘要

The objective of this study is to get a better understanding of radar signalover irrigated wheat fields and to assess the potentialities of radarobservations for the monitoring of soil moisture. Emphasis is put on the useof high spatial and temporal resolution satellite data (Envisat/ASAR andFormosat-2). Time series of images were collected over the Yaqui irrigatedarea (Mexico) throughout one agricultural season from December 2007 to May 2008, together with measurements of soil and vegetation characteristics andagricultural practices. The comprehensive analysis of these data indicatesthat the sensitivity of the radar signal to vegetation is masked by thevariability of soil conditions. On-going irrigated areas can be detected allover the wheat growing season. The empirical algorithm developed for theretrieval of topsoil moisture from Envisat/ASAR images takes advantage ofthe Formosat-2 instrument capabilities to monitor the seasonality of wheatcanopies. This monitoring is performed using dense time series of imagesacquired by Formosat-2 to set up the SAFY vegetation model. Topsoil moistureestimates are not reliable at the timing of plant emergence and during plantsenescence. Estimates are accurate from tillering to grain filling stageswith an absolute error about 9% (0.09 m3 m−3, 35% in relative value).This result is attractive since topsoil moisture is estimated at a highspatial resolution (i.e. over subfields of about 5 ha) for a large range ofbiomass water content (from 5 and 65 t ha−1 independently from theviewing angle of ASAR acquisition (incidence angles IS1 to IS6).