Abstract
Irrigated croplands require large annual water inputs and are critical to global food production. Actual evapotranspiration (AET) is a main index of water use in croplands, and several remote-sensing products have been developed to quantify AET at the global scale. In this study, we estimate global trends in actual AET, potential ET (PET), and precipitation rate (PP) utilizing the MODIS Evapotranspiration product (2001–2018) within the Google Earth Engine cloud-computing environment. We then introduce a new index based on a combination of AET, PET, and PP estimates—the evapotranspiration warning index (ETWI)—which we use to evaluate the sustainability of observed AET trends. We show that while AET has not considerably changed across global natural lands, it has significantly increased across global croplands (+14% ± 5%). The average ETWI for global croplands is −0.40 ± 0.25, which is largely driven by an extreme trend in AET, exceeding both PET and PP trends. Furthermore, the trends in water and energy limited areas demonstrate, on a global scale, while AET and PET do not have significant trends in both water and energy limited areas, the increasing trend of PP in energy-limited areas is more than water-limited areas. Averaging cropland ETWI trends at the country level further revealed nonsustainable trends in cropland water consumptions in Thailand, Brazil, and China. These regions were also found to experiencing some of the largest increases in net primary production (NPP) and solar-induced fluorescence (SIF), suggesting that recent increases in food production may be dependent on unsustainable water inputs. Globally, irrigated maize was found to be associated with nonsustainable AET trends relative to other crop types. We present an online open access application designed to enable near real-time monitoring and improve the understanding of global water consumption and availability.
Highlights
Along with the increasing global temperature, evidence from the changes of precipitation [1,2], runoff [3,4], and soil moisture [5,6] suggests that the hydrological cycle has been intensified in many regions of the world during the past decades
We introduce a new index based on a combination of actual ET (AET), potential ET (PET), and PP trends to determine the sustainability of AET trend, with respect to climate condition in each pixel
In this study which has entirely done by Google Earth Engine, we developed a user-friendly evapotranspiration Trend monitoring (ETM) application that enables users to produce annual AET map for any user-selected year
Summary
Along with the increasing global temperature, evidence from the changes of precipitation [1,2], runoff [3,4], and soil moisture [5,6] suggests that the hydrological cycle has been intensified in many regions of the world during the past decades. Et al [12] used a well-validated diagnostic model to estimate the daily AET during 1981–2012. They found that, during this period, ET over land has increased significantly, which has been mainly caused by increases in transpiration from vegetation and vaporization of intercepted rainfall from vegetation. Anabalón and Sharma [13] investigated PET and AET trends of global model datasets over two time periods: from 2003 to 2012 (short term) and from 1980 to 2012 (multidecadal) and found positive significant PET trends but nonsignificant changes over the more recent short-term period
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