Remotely Monitoring Ecosystem Water Use Efficiency of Grassland and Cropland in China’s Arid and Semi-Arid Regions with MODIS Data

Xuguang Tang,Mingguo Ma,Xibao Xu,Qing Gu,Li Yao,Zhi Ding,Lisheng Song,Xiaojuan Huang

doi:10.3390/rs9060616

Abstract

Scarce water resources are available in the arid and semi-arid areas of Northwest China, where significant water-related challenges will be faced in the coming decades. Quantitative evaluations of the spatio-temporal dynamics in ecosystem water use efficiency (WUE), as well as the underlying environmental controls, are crucial for predicting future climate change impacts on ecosystem carbon-water interactions and agricultural production. However, these questions remain poorly understood in this typical region. By means of continuous eddy covariance (EC) measurements and time-series MODIS data, this study revealed the distinct seasonal cycles in gross primary productivity (GPP), evapotranspiration (ET), and WUE for both grassland and cropland ecosystems, and the dominant climate factors performed jointly by temperature and precipitation. The MODIS WUE estimates from GPP and ET products can capture the broad trend in WUE variability of grassland, but with large biases for maize cropland, which was mainly ascribed to large uncertainties resulting from both GPP and ET algorithms. Given the excellent biophysical performance of the MODIS-derived enhanced vegetation index (EVI), a new greenness model (GR) was proposed to track the eight-day changes in ecosystem WUE. Seasonal variations and the scatterplots between EC-based WUE and the estimates from time-series EVI data (WUEGR) also certified its prediction accuracy with R2 and RMSE of both grassland and cropland ecosystems over 0.90 and less than 0.30 g kg−1, respectively. The application of the GR model to regional scales in the near future will provide accurate WUE information to support water resource management in dry regions around the world.

Highlights

Considerable attention has been given to the coupling relationship between carbon and water cycles in the context of profound influences on terrestrial ecosystems being exerted by the changing climate [1,2,3]
The changing trends in gross primary productivity (GPP), Re, and water use efficiency (WUE) at eight-day periods for both grassland and cropland are quite consistent in spite of the magnitudes
As an important linkage coupling the global carbon and water cycles, a better understanding of terrestrial ecosystem WUE will help us track the responses in water-use strategies of ecosystems to environmental stress and adopt appropriate ecosystem management, especially in the arid and semi-arid areas with limited water resources

Summary

Introduction

Considerable attention has been given to the coupling relationship between carbon and water cycles in the context of profound influences on terrestrial ecosystems being exerted by the changing climate [1,2,3]. The seasonal characteristics of WUE are essentially depending on the strength of coupled GPP and ET components, as well as individual responses to environmental controls, because climate affects carbon and water processes differently [6,7]. With the help of continuous observations of ecosystems-level carbon and water exchanges between the Earth’s biosphere and the atmosphere based on the eddy covariance (EC) technique, site-level evaluation or comparisons among multiple sites has been widely used to assess the WUE variability and its relationship with weather conditions across different time and space scales [8,9]. Using flux measurements from four grasslands [12] and temperate deciduous forests [13] in northern mid- and high latitudes, WUE was found to reach its peak during the summertime, whereas Reichstein et al [14] revealed

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