Global Trends in Evapotranspiration Dominated by Increases Across Cropland Regions: Insights Into Sustainability in Food Production

Abstract:

Irrigated croplands require large annual water for irrigation 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. Here, we utilize the Google Earth Engine cloud-computing environment to estimate global trends in AET and potential ET (PET) from the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD16 product (2001-2018), and precipitation rate (PP) from the TerraClimate product (2001-2018). 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 AET has remained relatively constant across global natural lands, while it has significantly increased across global croplands (+14% +/- 5%). The average ETWI for global croplands 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 consumption in Thailand, Brazil, and China. These regions were also found to experience 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.

https://doi.org/10.3390/rs12071221