A Revised Temporal Scaling Method to Yield Better ET Estimates at a Regional Scale

Yi Song,Mingguo Ma,Long Jin,Xufeng Wang

doi:10.3390/rs70506433

Abstract

This study presents a revised temporal scaling method based on a detection algorithm for the temporal stability of the evaporative fraction (EF) to estimate total daytime evapotranspiration (ET) at a regional scale. The study area is located in the Heihe River Basin, which is the second largest inland river basin in China. The remote sensing data and field observations used in this study were obtained from the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) project. The half-hourly EF values (EFEC) calculated using meteorological observations from an eddy covariance (EC) system and an automatic meteorological station (AMS) represented the diurnal pattern of the EF across the majority of the study area. The remotely sensed instantaneous midday EF (EFASTER), which indicates the spatial distribution of the midday EF over the entire study area, was calculated from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image. The temporal stability of EFEC was examined using a detection algorithm. Intervals with inconsistent EFEC values were distinguished from those with consistent EFEC values; the total daytime ET (from 9:00 to 19:00) within these interval types was integrated separately. Validation of the total daytime ET at the satellite pixel scale was conducted using measurements from17 EC towers. Using the detection algorithm for the temporal stability of the EF and dynamic adjustment, the revised temporal scaling method resulted in a root-mean-square error (RMSE) of 0.54 (mm·d−1), a mean relative error (MRE) of 7.26% and a correlation coefficient (Corr.) of 0.81; all of these values were superior to those of the two other methods (i.e., the constant EF and variable EF methods). The revised method easily extends to other areas and exhibits a superior performance in flat and regularly-irrigated farmlands at the regional scale.

Highlights

Because the hydrological cycle is strongly affected by crop water consumption in semiarid agricultural regions, precise ET estimates are important for saving water through efficient irrigation practices [4,5]
The evaporative fraction (EF) values remained relatively stable from 10:00 to 15:00 with some variability during the early morning and late afternoon based on the surface and atmospheric conditions
The Variable EF Method (vEF) method did not simulate the significant increase in the EF after 15:00

Summary

Introduction

Sensed data can provide instantaneous two-dimensional spatial information on ground surface properties, such as albedo, vegetation fraction, land surface temperature and net radiation (Rn). These properties are important variables for remotely sensed ET estimates [6,7,8]. ET-related disciplines, total daily, weekly and monthly ET readings, rather than instantaneous values, are required at multiple scales. To convert instantaneous spatial ET values to time-integrated values, temporal scaling is required. Sine function [13] and constant evaporative fraction (EF) [14] methods are commonly used for scaling instantaneous λE to total integrated ET [3,15]

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Conclusion