Temporal-spatial analysis of farmland evapotranspiration based on complementary relationship model and IKONOS data

Abstract: Mapping high spatial-temporal resolution evapotranspiration (ET) over large areas is important for water resources planning, precision irrigation and monitoring water use efficiency. Recently accurate estimation of ET is becoming available via a number of methods using surface meteorological and sounding observations, which are used to represent only local processes, meet insuperably difficulty to mapping ET in large areas due to land surface heterogeneity and the dynamic nature of the heat transfer processes. Satellite remote sensing is a promising tool for this purpose. Nevertheless, most of the existing techniques of ET estimation from satellite remote sensing are not satisfactory, because satellite monitoring of ET has not been feasible at high pixel resolution. Therefore, using traditional measurements and high resolution image data to generate high spatial-temporal resolution ET is becoming an important research direction. In this paper, the complementary relationship model (CR) was employed together with meteorological data to estimate actual ET, and the results were validated by lysimeter observation. Furthermore, CR model was combined with high resolution image, IKONOS data, to estimate instantaneous field scale ET and they also were transferred into daily ET. The cumulative evapotranspiration (ET) of winter wheat during the reproductive phase from March to June of 2011 was 469.12 mm, essentially corresponding to the annual precipitation in the Beijing area. The most high accuracy of estimated ET by CR model is also on May(R2=0.863，RMSE=0.103 mm). The daytime ET accounted for 86% of the total ET for the four-month period, while the nighttime ET constituted the remaining 14% of the total. Therefore, the nighttime ET must also be considered. The transferred daily ET by self-preservation of evaporative fraction(EF) method were consistent with lysimeter measurements for all four months(R2=0.937，RMSE=0.668 mm). The estimated daily ET by the EF method was consistent with lysimeter measurement for each of the four months. The IKONOS image-based instantaneous and daily ET over vegetation-covered area increased with increment of leaf area index (LAI) and decreased with increment of albedo. It was proved in this study that CR model can be used to estimate precision field scale ET with meteorological data and high resolution remote sensing data together in a region with limited ground data availability, e.g. without soil moisture and surface temperature .