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Drought can affect both the quantity and quality of water in lakes and reservoirs, yet larger, highly managed waterbodies, such as Lake Mead, may be somewhat buffered from drought effects. From 2000 to present, Lake Mead has experienced a 71% decline in volume; however, influent water quality has remained high and consistent outflow volumes through Hoover Dam have been maintained. Furthermore, management activities, such as increased removal of phosphorus by wastewater dischargers and legacy contamination cleanup efforts, have been initiated since the drought began. These efforts have led to small improvements in values of water quality parameters, such as phosphorus, nitrogen, and perchlorate, despite loss of volume for dilution of constituents, and consequently, decreased residence time. As the drought continues, Lake Mead is projected to continue declining in volume, inflows are projected to become warmer, and the population of Las Vegas is projected to grow, potentially adding additional stress to the hydrologic system. Maintenance of outflow may mitigate some potentially negative consequences, and understanding the drivers behind continued high water quality despite prolonged drought is important to continue to maintain the health and vitality of the entire Lower Colorado River Basin. [ABSTRACT FROM AUTHOR]

Non‐native ungulate grazing has negatively impacted native species across the globe, leading to massive loss of biodiversity and ecosystem services. Despite their pervasiveness, interactions between non‐native grazers and native species are not fully understood. We often observe declines in demography or survival of these native species, but lack understanding about the mechanisms underlying these declines. Physiological stress represents one mechanism of (mal)adaptation, but data are sparse. We investigated glucocorticoid levels in a native avian herbivore exposed to different intensities of non‐native grazing in the cold desert Great Basin ecosystem, USA. We measured corticosterone, a glucocorticoid in feathers for a large sample (n = 280) of female greater sage‐grouse (Centrocercus urophasianus) from three study areas in Northern Nevada and Southern Oregon with different grazing regimes of livestock and feral horses. We found that greater feral horse density was associated with higher corticosterone levels, and this effect was exacerbated by drought conditions. Livestock grazing produced similar results; however, there was more model uncertainty about the livestock effect. Subsequent nesting success was lower with increased feather corticosterone, but corticosterone levels were not predictive of other vital rates. Our results indicate a physiological response by sage‐grouse to grazing pressure from non‐native grazers. We found substantial among‐individual variation in the strength of the response. These adverse effects were intensified during unfavorable weather events, highlighting the need to reevaluate management strategies in the face of climate change. [ABSTRACT FROM AUTHOR]

The Colorado River Basin is a hydrologic river network that directs runoff from rain and snow falling on mountains, primarily in Colorado and Wyoming, to the Colorado River Delta in Mexico. Over the last century, in response to basin‐wide water shortages, legal agreements between stakeholders in seven U.S. states and Mexico, hydrologic flows from users on the main stem of the river have been reallocated to junior water rights holders. Municipalities, businesses, farmers, and households utilize the Colorado River water to produce and trade valuable, water‐derived goods and services, which effectively reallocates water through a continually adapting, boundary‐free economic river network providing indirect access to "virtual" Colorado River water. We conceptualize the Colorado River Basin as a multiplex network comprised of interdependent natural flow networks, direct (infrastructural) flow networks, and indirect (virtual) flow networks. Using this reframing, we quantify the total hydrosocial impact of the Drought Contingency Plan (DCP) on Lower Basin states. For each Mm3 of water reduced through the DCP, Arizona, Nevada, and California lose an additional 0.42–0.43 Mm3, 0.33–0.51 Mm3, and 1.06–1.10 Mm3 of virtual water flow, respectively. Hence, the DCP will require Arizona, Nevada, and Southern California to restructure how they use water, relying less on direct and indirect consumption of the Colorado River water and finding more indirect water sources outside that basin. [ABSTRACT FROM AUTHOR]

Recent high impact wildfires and droughts in California and Nevada have been linked to extremes in the Evaporative Demand Drought Index (EDDI) and Standardized Precipitation Evapotranspiration Index (SPEI), respectively. Both indices are dependent on reference evapotranspiration (ET0). Future changes in ET0 for California and Nevada are examined, calculated from global climate model simulations downscaled by Localized Constructed Analogs (LOCA). ET0 increases of 13–18% at seasonal timescales are projected by late century (2070–2099), with greatest relative increases in winter and spring. Seasonal ET0 increases are most strongly driven by warmer temperatures, with increasing specific humidity having a smaller, but noteworthy, counter tendency. Extreme (95th percentile) EDDI values on the 2‐week timescale have coincided with recent large wildfires in the area. Two‐week EDDI extremes are projected to increase by 6–10 times during summer and 4–6 times during autumn by the end of the century. On multiyear timescales, the occurrence of extreme droughts based on 3‐year SPEI below the historical fifth percentile, similar to that experienced during the 2012–2016 drought across the region, is projected to increase 3–15 times by late century. Positive trends in extreme multiyear droughts will further increase seasonal fire potential through degraded forests and increased fuel loads and flammability. Understanding how these drought metrics change on various climate timescales at the local level can provide fundamental information to support the development of long‐term adaptation strategies for wildland fire and water resource management. Plain Language Summary: Since the start of the 21st century, California and Nevada have observed extreme wildland fires and droughts that have caused devastating impacts to ecosystems and society. A common feature of these events has been very high atmospheric evaporative demand—the "thirst" of the atmosphere—which has largely been driven by increased air temperatures caused by anthropogenic climate change. This study examines projected changes in evaporative demand, which of the input variables are causing those changes and how the frequency of extreme wildfire potential and multiyear droughts will change. Evaporative demand is found to increase during all seasons, and increased temperatures drive most of that change. The likelihood of extreme wildfire potential based on 2‐week periods of elevated evaporative demand during summer and autumn increases substantially. A climatic water balance based on precipitation and evaporative demand indicates extreme 3‐year droughts that hold potential to deplete regional‐scale water supply also become much more likely. Future adaptation planning efforts for wildfire management agencies, forest management, and water resource managers should account for a greater likelihood of more extreme events. Key Points: All climate models show increasing reference evapotranspiration (ET0) through the end of the centuryIncreased air temperature has the greatest contribution to projected ET0 increasesExtreme ET0‐based wildfire potential and 3‐year droughts based on precipitation minus ET0 become much more frequent in the future [ABSTRACT FROM AUTHOR]

In this article, we present the results of a study investigating the influence of vulnerability to climate change as a function of physical vulnerability, sensitivity, and adaptive capacity on climate change risk perception. In 2008/2009, we surveyed Nevada ranchers and farmers to assess their climate change-related beliefs, and risk perceptions, political orientations, and socioeconomic characteristics. Ranchers' and farmers' sensitivity to climate change was measured through estimating the proportion of their household income originating from highly scarce water-dependent agriculture to the total income. Adaptive capacity was measured as a combination of the Social Status Index and the Poverty Index. Utilizing water availability and use, and population distribution GIS databases; we assessed water resource vulnerability in Nevada by zip code as an indicator of physical vulnerability to climate change. We performed correlation tests and multiple regression analyses to examine the impact of vulnerability and its three distinct components on risk perception. We find that vulnerability is not a significant determinant of risk perception. Physical vulnerability alone also does not impact risk perception. Both sensitivity and adaptive capacity increase risk perception. While age is not a significant determinant of it, gender plays an important role in shaping risk perception. Yet, general beliefs such as political orientations and climate change-specific beliefs such as believing in the anthropogenic causes of climate change and connecting the locally observed impacts (in this case drought) to climate change are the most prominent determinants of risk perception. [ABSTRACT FROM AUTHOR]

The goal of this study was to understand better the role of interannual and interdecadal climatic variation on local pre-EuroAmerican settlement fire regimes in fire-prone Jeffrey pine (Pinus jeffreyiGrev.&Balf.) dominated forests in the northern Sierra Nevada Mountains.Our study was conducted in a 6000-ha area of contiguous mixed Jeffrey pine-white fir (Abies concolorGordon&Glend.) forest on the western slope of the Carson Range on the eastern shore of Lake Tahoe, Nevada.Pre-EuroAmerican settlement fire regimes (i.e. frequency, return interval, extent, season) were reconstructed in eight contiguous watersheds for a 200-year period (1650–1850) from fire scars preserved in the annual growth rings of nineteenth century cut stumps and recently dead pre-settlement Jeffrey pine trees. Superposed epoch analysis (SEA) and correlation analysis were used to examine relationships between tree ring-based reconstructions of the Palmer Drought Severity Index (PDSI), Southern Oscillation Index (SOI), Pacific Decadal Oscillation (PDO) and pre-EuroAmerican fire regimes in order to assess the influence of drought and equatorial and north Pacific teleconnections on fire occurrence and fire extent.For the entire period of record (1650–1850), wet conditions were characteristic of years without fires. In contrast, fire years were associated with drought. Drought intensity also influenced fire extent and the most widespread fires occurred in the driest years. Years with widespread fires were also preceded by wet conditions 3 years before the fire. Widespread fires were also associated with phase changes of the PDO, with the most widespread burns occurring when the phase changed from warm (positive) to cold (negative) conditions. Annual SOI and fire frequency or extent were not associated in our study. At decadal time scales, burning was more widespread during decades that were dryer and characterized by La Niña and negative PDO conditions. Interannual and interdecadal fire–climate relationships were not stable over time. From 1700 to 1775 there was no interannual relationship between drought, PDO, and fire frequency or extent. However, from 1775 to 1850, widespread fires were associated with dry years preceded by wet years. This period also had the strongest association between fire extent and the PDO. In contrast, fire–climate associations at interdecadal time scales were stronger in the earlier period than in the later period. The change from strong interdecadal to strong interannual climate influence was associated with a breakdown in decadal scale constructive relationships between PDO and SOI.Climate strongly influenced pre-settlement pine forest fire regimes in northern Sierra Nevada. Both interannual and interdecadal climatic variation regulated conditions conducive to fire activity, and longer term changes in fire frequency and extent correspond with climate-mediated changes observed in both the northern and southern hemispheres. The sensitivity of fire regimes to shifts in modes of climatic variability suggests that climate was a key regulator of pine forest ecosystem structure and dynamics before EuroAmerican settlement. An understanding of pre-EuroAmerican fire–climate relationships may provide useful insights into how fire activity in contemporary forests may respond to future climatic variation. [ABSTRACT FROM AUTHOR]