On the feasibility of temporally upscaling instantaneous evapotranspiration using weather forecast information

Abstract

Quantification of land-surface evapotranspiration (ET) is highly significant in water resources management, climate change studies, and numerical weather prediction. The constant reference evaporative fraction method (EF_r, the ratio of the actual to reference ET), which assumes that the daily EF_r is equal to that at the satellite overpass time, is a scheme that has been widely applied to upscale remotely sensed instantaneous ET to daily ET. To overcome the difficulties encountered in the acquisition of tower-based meteorological variables, this study investigates the feasibility of using publicly available weather forecast information to estimate the daily reference ET using the constant EF_r method. A two-source energy balance model is adopted to compute the instantaneous ET using Moderate-Resolution Imaging Spectroradiometer (MODIS) remote-sensing data acquired between January 2011 and October 2012 at the Yucheng Comprehensive Experimental Station in the North China Plain. The results show that the daily maximum and minimum air temperatures from weather forecast information are consistent with the corresponding ground-based measurements, with a bias of 0.8 K and a root mean square error (RMSE) of <2.0 K. The daily global solar radiation and daily wind speed were poorly forecast when compared with the ground-based measurements. Using the meteorological variables from the daily weather forecast information produced a small bias of 0.1 mm day^–1 and an RMSE of 0.6 mm day^–1 when the estimated daily reference ET was compared with that derived using the ground-based meteorological measurements. When the remotely sensed instantaneous ET and half-hourly reference ET were as accurate as the ground-based measurements, the upscaling method produced the daily ET, using the meteorological variables from the weather forecast information, with a bias of 0.1 mm day^–1 and an RMSE of 0.7 mm day^–1.

Acknowledgements

The staff members at the Yucheng site are acknowledged for their hard work on the setting up and maintenance of the ground-based instruments and data collection. Associate Professor Yuanyuan Jia and Professor Chuanrong Li at the Academy of Opto-Electronics, Chinese Academy of Sciences, are thanked for their cooperation in providing the surface measurements.

Additional information

Funding

This work was partly supported by the National Natural Science Foundation of China [grant number 41101332], [grant number 41201366]; the Institute of Geographic Sciences and Natural Resources Research-funded outstanding young talent project [grant number 2013RC201]; the high-resolution Earth observing-water application demonstration [grant number 08-Y30B07-9001-13/15-01].