Evaluation of Evapotranspiration for Exorheic Catchments of China during the GRACE Era: From a Water Balance Perspective

Yulong Zhong,Yuna Mao,Bing Ji,Min Zhong

doi:10.3390/rs12030511

Abstract

Evapotranspiration (ET) is usually difficult to estimate at the regional scale due to scarce direct measurements. This study uses the water balance equation to calculate the regional ET with observations of precipitation, runoff, and terrestrial water storage changes (TWSC) in nine exorheic catchments of China. We compared the regional ET estimates from a water balance perspective with and without considering TWSC (ETWB: ET estimates with considering TWSC, and ETPQ: ET estimates from precipitation minus runoff without considering TWSC). Results show that the regional annual ET ranges from 417.7 mm/yr to 831.5 mm/yr in the nine exorheic catchments based on the water balance equation. The impact of ignoring TWSC on calculating ET is notable, as the root mean square errors (RMSEs) of annual ET between ETWB and ETPQ range from 12.0–105.8 mm/yr (2.6–12.7% in corresponding annual ET) among the exorheic catchments. We also compared the estimated regional ET with other ET products. Different precipitation products are assessed to explain the inconsistency between different ET products and regional ET from a water balance perspective. The RMSEs between ET estimates from Gravity Recovery and Climate Experiment (GRACE) and ET from land surface models can be reduced if the deviation of precipitation forcing data is considered. ET estimates from Global Land Evaporation Amsterdam Model (GLEAM) can be improved by reducing the uncertainty of precipitation forcing data in three semiarid catchments. This study emphasizes the importance of considering TWSC when calculating the regional ET using a water balance equation and provides more accurate ET estimates to help improve modeled ET results.

Highlights

Evapotranspiration (ET) is one of the most important components of the climate system connecting the water, energy, and carbon cycle [1,2]
The deviations between ETWB and ETPQ range from 6.7 to 37.2 mm/month for twelve months in the Middle Yangtze River Basin (MYRB), and its root mean square errors (RMSEs) accounts for 37.1% of variations of ETWB
The large uncertainties of terrestrial water storage changes (TWSC) may result from the small study area (HuRB and Minjiang River Basin (MRB)) and large variations of TWSC caused by abundant precipitation (PRB and MRB)

Summary

Introduction

Evapotranspiration (ET) is one of the most important components of the climate system connecting the water, energy, and carbon cycle [1,2]. ET changes can be used as an indicator of climate change, especially in areas where the water cycle is accelerated [3,4]. Regional ET is often difficult to estimate. Remote sensing provides an opportunity to monitor spatial-temporal changes in ET [6,7], but regional calibration and uncertainty from vegetation cover data will lead to large uncertainty in ET [8]. Land surface models (LSMs) can provide grid-to-regional scale ET estimates, such as the multiple LSMs simulations using the global land data assimilation system (GLDAS) issued by NASA [9]. Regional ET can be derived from the terrestrial water budget, namely the residual between precipitation (P) and the sum of runoff (Q) and terrestrial water storage change (ds/dt), which have been regarded as benchmark estimates for validating ET products or estimates on a regional scale [10,11]

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Conclusion