A Multidataset Assessment of Climatic Drivers and Uncertainties of Recent Trends in Evaporative Demand across the Continental United States

Abstract

Increased atmospheric evaporative demand has important implications for humans and ecosystems in water-scarce lands. While temperature plays a significant role in driving evaporative demand and its trend, other climate variables are also influential and their contributions to recent trends in evaporative demand are unknown. We address this gap with an assessment of recent (1980-2020) trends in annual reference evapotranspiration (ETo) and its drivers across the continental US based on five gridded datasets. In doing so, we characterize the structural uncertainty of ETo trends and decompose the relative influences of temperature, wind speed, solar radiation, and humidity. Results highlight large and robust changes in ETo across much of the western US, centered on the Rio Grande region where ETo increased 135-235 mm during 1980-2020. The largest uncertainties in ETo trends are in the central and eastern US and surrounding the Upper Colorado River. Trend decomposition highlights the strong and widespread influence of temperature, which contributes to 57% of observed ETo trends, on average. ETo increases are mitigated by increases in specific humidity in non-water limited regions, while small decreases in specific humidity and increases in wind speed and solar radiation magnify ETo increases across the West. Our results show increases in ETo across the West that are already emerging outside the range of variability observed 20-40 years ago. Our results suggest that 21st century land and water managers need to plan for an already increasing influence of evaporative demand on water availability and wildfire risks.

The Severity of the 2014–2015 Snow Drought in the Oregon Cascades in a Multicentury Context

“…The 1888–1889 event in the Oregon Cascades was most likely caused by a deficit of snow, rather than warm temperatures and precipitation falling as rain. Nonetheless, this further emphasizes the widespread effects that warming temperatures will have on a low‐to‐no snow future (Rhoades et al., 2022; Siirila‐Woodburn et al., 2021), compounded by increased evaporative demand (Albano et al., 2022). In the McKenzie River Basin, a major tributary to the Willamette River Basin, the 2014 and 2015 snow drought years serve as an analog for projected snow conditions under +1°C and +2°C warming, respectively (E. A. Sproles et al., 2017).…”

The vulnerability of springs and phreatophyte communities to groundwater level declines in Oregon and Nevada, 2002–2021

“…Long-term groundwater level trends are an indicator of sustainability and reflect changes in aquifer storage, which can occur as a result of decreased recharge and increased extraction, or both (Currell 2016). Groundwater is over-appropriated compared to available groundwater in aquifers of both states (e.g., Garcia et al, 2022;Saito et al, 2022), and pressure to develop groundwater will increase as crop evapotranspiration rates require more water to sustain current yields (Huntington et al, 2015;Albano et al, 2022). Projected increases to frequency and extent of drought (Ahmadalipour et al, 2016;Saito et al, 2022) could lead to more groundwater development as surface water availability becomes unreliable.…”

Field experiments have enhanced our understanding of drought impacts on terrestrial ecosystems—But where do we go from here?

“…Severe water limitation of ecosystem function has historically been framed through the lens of insufficient precipitation and low soil moisture, but drought can also develop from high levels of atmospheric evaporative demand (VPD, Figure 1). There is abundant evidence that VPD has already been altered by climate change—not only increasing across much of the world but also decreasing in some regions (Albano et al., 2022; Ficklin & Novick, 2017; Seager et al., 2015; Yuan et al., 2019). Trends in VPD are not simply due to altered temperatures, as changes in specific humidity can also contribute (Albano et al., 2022).…”

“…There is abundant evidence that VPD has already been altered by climate change—not only increasing across much of the world but also decreasing in some regions (Albano et al., 2022; Ficklin & Novick, 2017; Seager et al., 2015; Yuan et al., 2019). Trends in VPD are not simply due to altered temperatures, as changes in specific humidity can also contribute (Albano et al., 2022). Reduced precipitation inputs and high VPD can both reduce ecosystem function by reducing soil moisture (Zhao et al., 2022), but rising VPD also directly affects stomatal conductance and carbon uptake (Grossiord et al., 2020, Figure 5a) potentially increasing ecosystem water loss and the probability of drought even when precipitation inputs are not abnormally reduced.…”

“…Over the last 30 years, many ecosystems have experienced increases in temperatures and VPD at the global scale (Yuan et al., 2019). The interactive effect of increased VPD (driven by warming and alterations in specific humidity, Albano et al., 2022) and temporal trends in precipitation on drought frequency/severity can be visualized by comparing the time course of two standardized climate indices—the Standardized Precipitation Index (SPI, based only on precipitation inputs) and the Standardized Precipitation Evapotranspiration Index (SPEI, based on precipitation inputs and estimated evaporative demand). This comparison, which highlights the role of VPD in determining drought, is shown for four central United States ecosystems (see map) with mean annual precipitation varying from ~850 mm in the mesic grassland to ~270 mm in the desert grassland.…”