Investigating Water Stress in Drylands

Abstract

Drylands are defined as tropical and temperate areas with a ratio of precipitation to potential evapotranspiration of less than 0.65. They cover 40% of the Earth's surface and they are home to one-third of the world’s population. Water Stress is by definition characteristic of dryland systems and only 8% of global renewable water supply is in dryland regions while 2 billion people live there. This dissertation consists of four studies that address water stress in these complex areas. All span multiple components of the water stress in drylands on site, regional, and global scales.In the first study, we studied water stress impacts on dust storms severity and predictability in drylands utilizing the relation between drought events in Iraq and dust storms in Southwest Iran (SWI) between 2003 and 2018. Dust storms are common meteorological events in arid and semi-arid regions, particularly in SWI. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model showed central and southern Iraq are the main dust sources for SWI. Mean annual aerosol optical depth (AOD) analysis demonstrated 2008 and 2009 were the dustiest years since 2003 and there is an increased frequency of summertime extreme dust events in the years 2008 and 2009. The Standardized Precipitation Evapotranspiration Index (SPEI) revealed drought in Iraq significantly affects dust storms in Iran. Similarly, dramatic desiccation of Iraq wetlands has contributed to increasing fall dust events in SWI. AOD in SWI was highly correlated (−0.76) with previous-month vapor pressure deficit (VPD) over Iraq, demonstrating the potential of VPD for dust event forecasting. In the second study, we focused on global evapotranspiration products as an index of water stress. Evapotranspiration (ET) is one of the main components of water and energy balance. In this study, we compared two ET products, suitable for regional analysis at high spatial resolution: The recent WaPOR product developed by the Food and Agriculture Organization and METRIC algorithm. WaPOR is based on ETLook, which is a two-source model and relies on microwave images. WaPOR is unique as it has no limitation under cloudy days, but METRIC is limited by clouds. METRIC and WaPOR are more sensitive to land surface temperature and soil moisture, respectively. We showed that in most regions, ET from METRIC is higher than WaPOR and the deference has an ascending trend with the elevation. The ET of lysimeter station was fairly consistent with METRIC based on a single observation. Our analysis using NDVI and land cover suggested the histogram of ET from WaPOR might be more realistic than METRIC, but not its amount. The fraction of ET to precipitation in rainfed agriculture areas showed WaPOR is more accurate than METRIC, mainly because in the absence of other water resources such as groundwater annual ET cannot exceed annual precipitation. In contrast, METRIC produced a more realistic estimate than WaPOR over irrigated farms. The results suggested the two products can complement each other and making a product which use METRIC in low-elevation and WaPOR in high-elevation areas would be helpful. In the third study, we studied the global evapotranspiration trends as a proxy of vegetation water stress. Irrigated croplands require large annual water inputs and are critical to global food production. Actual evapotranspiration (AET) is the main index of water use in croplands, and several remote-sensing products have been developed to quantify AET at a 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 introduced 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 consumption 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 also presented an online open-access application designed to enable near real-time monitoring and improve the understanding of global water consumption and availability. In the fourth study, we explored diurnal vegetation water stress using canopy temperature. Plant canopy temperature (Tc) is partly regulated by evaporation and transpiration from the canopy surface and can be used to infer changes in stomatal regulation and vegetation water stress. In this study, we used a thermal Unmanned Aircraft Systems (UAS) in conjunction with eddy covariance, sap flow, and spectral reflectance data to assess the diurnal characteristics of Tc and water stress status over a semiarid mixed conifer forest in Arizona, USA. Diurnal Tc dynamics were closely related to tree sap flow and changes in spectral reflectance associated with stomatal regulation. Consistent with previously reported deviations, we found that on average Tc was 1.8 °C lower than the above canopy air temperature (Ta). However, the relationship between Tc and Ta varied significantly according to tree density and tree height classes, with taller and denser trees exhibiting relatively high |Tc-Ta| (2.4 and 2.1 °C cooler canopies, respectively) compared to shorter and less-dense tree stands (1.7 and 1.5 °C cooler canopies, respectively). We used these data to evaluate space-borne diurnal measurements of Tc and water stress from the ECOSTRESS mission. We found that ECOSTRESS observations of Tc accurately tracked seasonal shifts in diurnal surface temperatures and vegetation water stress, and that site-level observations of heterogeneity in forest composition and structure could be applied to separate the processes of canopy transpiration and soil evaporation within the ECOSTRESS footprint. This study demonstrates how proximal and satellite remote sensing approaches can be combined to reveal the diurnal and seasonally dynamic nature of Tc and water stress.