Estimation of regional evapotranspiration over the Hebei Plain using geostationary satellite data

Fengyun-2C (FY-2C) is the first Chinese operational geostationary meteorological satellite with one visible and four infrared channels of optical imaging radiometer. Image acquisition temporal frequencies of FY-2C are 1 h and 30 min for dry and rainy seasons, respectively. This allows for mapping diurnal variations in land surface temperature (Ts) at a relatively high temporal scale. Data from FY-2C satellite were applied in combination with Moderate Resolution Imaging Spectroradiometer (MODIS) satellite products to assess regional evapotranspiration (ET) over the Hebei Plain. The approach was based on the improved triangle method, where the temperature-vegetation index space included thermal inertia. As the method requires information on temperature change, the emergence of geostationary meteorological satellite data capable of determining land Ts at high temporal scale had made it possible to estimate daily Ts change from optimal observations. Two thermal infrared channels (IR1: 10.3~11.3 μm and IR2: 11.5~12.5 μm) from FY-2C were used to estimate Ts via a split window algorithm originally proposed for the MSG-SEVIRI sensor. As it compared reasonably well with observations of Ts, the FY-2C high temporal resolution data were exploited to determine morning rise in Ts. The chosen time windows were 8:00 and 12:00 local time. Combined with 16-day composite MODIS vegetation indices product (MOD13) at a spatial resolution of 5 km and Ts difference (ΔTs) from FY-2C, evaporative fraction (EF) was interpolated in ΔTs-NDVI triangular-shaped scatter space based on the improved triangle method. Using energy balance, regional daily actual ET was eventually derived from the EF and available energy estimated from MODIS surface albedo products MCD43. Spatial variations in the estimated surface variables (Ts, EF and ET) corresponded well to land cover patterns and farmland management practices. Estimates based on this method were validated with field observations from Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in Hebei Province (37o53′N, 114o41′E). The estimated ET and EF also compared well with lysimeter data collected for the period from June 2005 to September 2007, with RMSE of 0.92 mm for ET and 0.14 for EF and corresponding R² of 0.74 and 0.55. Of particular interest was the high temporal frequency of FY-2C acquired images, increasing the likelihood of acquiring cloud-free images with the potential for plotting estimated daily ET maps. The study demonstrated that FY-2C was highly valuable in water resources and agricultural management applications in China.