Abstract
The regression tree method is used to upscale evapotranspiration (ET) measurements at eddy-covariance (EC) towers to the grassland ecosystems over the Dryland East Asia (DEA). The regression tree model was driven by satellite and meteorology datasets, and explained 82% and 76% of the variations of ET observations in the calibration and validation datasets, respectively. The annual ET estimates ranged from 222.6 to 269.1 mm yr−1 over the DEA region with an average of 245.8 mm yr−1 from 1982 through 2009. Ecosystem ET showed decreased trends over 61% of the DEA region during this period, especially in most regions of Mongolia and eastern Inner Mongolia due to decreased precipitation. The increased ET occurred primarily in the western and southern DEA region. Over the entire study area, water balance (the difference between precipitation and ecosystem ET) decreased substantially during the summer and growing season. Precipitation reduction was an important cause for the severe water deficits. The drying trend occurring in the grassland ecosystems of the DEA region can exert profound impacts on a variety of terrestrial ecosystem processes and functions.
Highlights
Hydrological and ecological processes are tightly coupled in arid and semi-arid regions
Solar radiation of Modern-Era Retrospective Analysis for Research and Applications (MERRA) showed a positive trend of 3.36 MJ m22 yr21 (p,0.01), which is consistent with that of observed sunshine hours (Figure 2B)
Northern Xinjiang, and northeast of Inner Mongolia experienced a positive trend of total solar radiation during 1982–2009, which accounts for 77% of the grassland area over the Dryland East Asia (DEA) region
Summary
Hydrological and ecological processes are tightly coupled in arid and semi-arid regions. Over the last few decades, hydrological processes over the Dryland East Asia (DEA) regions have shown substantial changes, including precipitation, river discharge, soil moisture content, and associated changes in lakes area [1,2,3,4]. Precipitation in Mongolia had an average of 7.5% decrease in summer over the past half century [5]. A growing number of evidences indicate that the changes in the regional water cycle are altering ecosystem processes and functions in this region. Occurrences of widespread and persistent drought have increased across northern Mongolia recently, resulting in a general decrease in vegetation productivity for grassland ecosystems [6,7,8]. It is necessary to understand the changes in the spatial and temporal distribution of major hydrological variables and their dominant driving variables
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