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The Changing Geography of the U.S. Water Budget: Twentieth-Century Patterns and Twenty-First-Century Projections
Abstract and Figures
Persistent changes in temperature and precipitation patterns have dramatic effects on the availability of surface water for natural vegetation, streamflow, agricultural production, and human consumption. We use a combination of historical observational climate data and water budget equations to develop time-series and maps of twentieth-century water variables within the contiguous United States and compare these with anticipated twenty-first-century patterns projected by global climate models. The results graphically demonstrate regional variation in hydroclimatic trends: areas that experienced convergent actual (AET) and potential evapotranspiration (PET) rates during the twentieth century (such as the Great Lakes and Gulf South) witnessed long-term increases in available moisture, whereas areas with divergent rates (such as the Mid-Atlantic and Great Plains) had greater water deficits. Increasing temperatures through the twenty-first century will produce higher PET across the United States; areas where AET similarly escalates will maintain average moisture levels within twentieth-century ranges, but where AET does not correspondingly increase, as in much of the South and West, average conditions will be comparable to those of extreme twentieth-century droughts. The findings highlight the importance of a regional approach to environmental change, as the impacts of climate on water in the United States will be spatially uneven.
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... Ongoing trends and future climate change scenarios for high-elevation regions of western North America foretell the predominance of warm and dry conditions as diminished snowpack retention accompanies rising temperatures and increased evaporative demand (Barnett et al., 2008;Clow, 2010;Cowell and Urban, 2010;Pederson et al., 2011). Corresponding drought-level water deficits are expected to impact an increasing proportion of ecosystem processes through the triggering of abrupt regime-shift changes (Anderson et al., 2009). ...
... This value equals half the saturation level of 150 mm typical of more well-developed soils (cf. Cowell and Urban, 2010). In the case of different AWC values between soils above and below timberline, we used the mean. ...
... Climatic water deficit data are produced in the form of a continuous monthly time series (e.g., 1900-2000), along with a suite of other climatic water budget variables (e.g., potential evapotranspiration [PET] and actual evapotranspiration [AET]) that are influenced by both current and antecedent soil moisture conditions. PET is a measure of the water that could be lost to the atmosphere, assuming the available heat energy (temperature), and AET is the actual moisture lost to the atmosphere from soil and vegetation (Cowell and Urban, 2010). An advantage of the Thornthwaitebased approach is the limited input requirements in comparison to the robust output, whereas drawbacks stem from the exclusion of local-scale data such as wind, humidity, and land cover (Stephenson, 1998). ...
Previous research collectively demonstrates the importance of taking local moisture availability and biotic threshold responses into account when seeking to reveal the ecological manifestations of climate change within upper treeline ecotones. Yet dendroecological studies that explicitly address the role of slope aspect in this context are non-existent. In this paper, we examine whether slope aspect and related temperature-precipitation interactions mediate abrupt increases in tree establishment and pulses of upper treeline advance (≥10 m) on contrasting north- and south-facing slopes during wet and dry periods of the 20th century. We used regime-shift analysis to quantify episodic changes in the rate of tree regeneration at each site (p < 0.05). We employed a climatic water deficit approach to define fine-scale moisture conditions to compare with dendroecological data from opposite aspects on eleven mountain peaks along a latitudinal gradient in the U.S. Rocky Mountains. Regime-shift analysis measured abrupt, yet asynchronous increases in tree establishment across contrasting slope aspects on 10 of 11 mountain peaks. Upper treeline advance was significantly greater (p < 0.05) during drought on north-facing slopes. On south-facing slopes, ecotonal dynamics varied more with respect to fluxes in the climatic water deficit; namely because of differences in local hydroclimate regimes. Collectively, these results underscore the importance of considering both slope aspect and temperature-moisture interactions when elucidating climate-vegetation interactions within upper treeline ecotones.
... CWD generally increased and AET decreased in the western US because western temperature increases during the study period were steeper than in the east and western precipitation trends were usually flat or declining [52-54, S1-S4 Figs]. In contrast, the eastern United States often had both higher average annual precipitation than the west during the study period and often increasing precipitation trends, which allowed for increased AET and declining CWD [53][54][55][56] (Figs 3, 5 and S1-S4). ...
... The broad geographic patterns of AET:CWD (Fig 1) are congruent with the temperature and precipitation patterns just described, but the distribution of EPA ecoregions in our bivariate plots did not provide the separation we expected, given the many studies that have used these variables to explain vegetation distribution [9, others cited in introduction]. Higher average precipitation in the east [53][54][55][56] contributed to that region's greater AET, and warmer temperatures in the south contributed to the north-south pattern of increasing CWD (Fig 1). The failure of the EPA ecoregions to plot discretely suggest a high degree of heterogeneity at the Level I ecoregion scale. ...
A robust method for characterizing the biophysical environment of terrestrial vegetation uses the relationship between Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD). These variables are usually estimated from a water balance model rather than measured directly and are often more representative of ecologically-significant changes than temperature or precipitation. We evaluate trends and spatial patterns in AET and CWD in the Continental United States (CONUS) during 1980-2019 using a gridded water balance model. The western US had linear regression slopes indicating increasing CWD and decreasing AET (drying), while the eastern US had generally opposite trends. When limits to plant performance characterized by AET and CWD are exceeded, vegetation assemblages change. Widespread increases in aridity throughout the west portends shifts in the distribution of plants limited by available moisture. A detailed look at Sequoia National Park illustrates the high degree of fine-scale spatial variability that exists across elevation and topographical gradients. Where such topographical and climatic diversity exists, appropriate use of our gridded data will require sub-setting to an appropriate area and analyzing according to categories of interest such as vegetation communities or across obvious physical gradients. Recent studies have successfully applied similar water balance models to fire risk and forest structure in both western and eastern U.S. forests, arid-land spring discharge, amphibian colonization and persistence in wetlands, whitebark pine mortality and establishment, and the distribution of arid-land grass species and landscape scale vegetation condition. Our gridded dataset is available free for public use. Our findings illustrate how a simple water balance model can identify important trends and patterns at site to regional scales. However, at finer scales, environmental heterogeneity is driving a range of responses that may not be simply characterized by a single trend.
... Across the Western United States, heat-induced drought stress, or hotter drought, has emerged as the hallmark of climate change during the Anthropocene (Allen, Breshears, and McDowell 2015). Because diminished snowpack retention during spring accompanies sharp increases in temperature and evaporative demand during the growing season (Barnett et al. 2008;Clow 2010;Cowell and Urban 2010;Allen, Breshears, and McDowell 2015;Mote et al. 2018), hotter drought has proven lethal for most of the mountain forest belt. Since approximately 2000, for example, drought-induced mortality has spread from low-elevation pinyon-juniper woodlands up into cool and wet subalpine forests of the Rocky Mountains (Breshears et al. 2005;van Mantgem et al. 2009;Hart et al. 2014;Smith et al. 2015). ...
As we progress into the Anthropocene, rising temperatures have amplified evaporative demand and rendered heat-induced drought stress, or hotter drought, as the hallmark of climate change moving forward. It remains unknown, however, whether upper treeline environments have been affected. For this study, we grouped previously published and unpublished data from study sites within the southern Rocky Mountains by slope aspect to provide a possible baseline for what we expect for the Anthropocene. We returned to and resampled some of these study sites in the summer of 2019 after twelve years of sharply rising temperatures to measure patterns of seedling establishment. We also returned to a high-elevation site after seventeen years of warming to perform repeat photography in an attempt to capture visual evidence of threshold changes since 2002 in a location where little change had occurred during the twentieth century. Results from this research can be summarized into two main findings: (1) trends in recruitment over the past thirty years suggest that north-facing slopes are increasingly hospitable for successful seedling establishment compared to south-facing slopes at upper treeline and (2) spruce beetle–induced mortality is evident at upper treeline. Conceptually, this means that hotter drought could be progressively enveloping upper treeline along topoclimatic gradients. Unless ongoing trends in temperature deviate from expectations or unless precipitation increases considerably, there is little reason to justify the idea that upper treeline will continue to respond positively to hotter drought conditions during the Anthropocene, especially on south-facing slopes in the Northern Hemisphere.
... This often leads to elevated nocturnal minimum temperature (Tmin; Yuan and Mitchell 2014), muting of daily maximum temperature (T max ; Gameda et al. 2007), and suppression of daily temperature range (DTR; Bonan 2001;McPherson, Stensrud, and Crawford 2004;Lobell, Bala, and Duffy 2006). These potential climate impacts have dependence on ample soil moisture supplies from precipitation events, an occurrence that is projected to increase in the Great Lakes and Midwest region throughout the twentyfirst century (Cowell and Urban 2010). ...
Agricultural land use changes have likely played an important role in modifying local and regional climate
factors. According to a recent study, summers in the Midwest were significantly cooler and wetter due to the
dramatic increases in production of corn and soybeans caused by an intensification of agricultural practices
(Alter et al. 2018). In this context, this study examines regional changes manifested in multiple climate
variables and directly quantifies the magnitude of potential moisture contributions from Midwest corn and
soybean agriculture. Meteorological data were collected for daily minimum, maximum, and dewpoint
temperatures over a sixty-one-year study period from fifty-nine National Weather Service first-order stations
and cooperative network stations. Regional growing season climatology for two study regions that focused on
the rain-fed Midwest Corn Belt and the southern United States extending to the Gulf Coast was analyzed.
Further, vapor pressure deficit was calculated to ascertain any regional changes. Field surveys of corn and
soybean crop transpiration were used in a multivariate model to estimate lower atmospheric moisture
contributions at midday during peak season directly from intensified rain-fed agriculture. Findings indicate an
increase in regional dewpoint, associated with elevated nocturnal minimums and suppression of both daily
maximum and vapor pressure deficit, concentrated in the Midwest Corn Belt, which was not evident within
the South. The estimation results of the atmospheric moisture contributions from the Corn Belt confirm the
intensification of Midwestern agriculture as a regional climate modifier. Key Words: agriculture, dewpoint,
energy balance, humidity, regional climate.
... R ecent International Panel on Climate Change (IPCC) models project an increase in extended drought in many semi-arid areas around the globe over the next fifty years (Wilder et al. 2010;IPCC 2014). For the Southwestern United States, IPCC models suggest a simultaneous increase of temperature of 2-5 degrees centigrade by 2100 and a decrease of precipitation of 5-8 percent (Wilder et al. 2010), an increase in actual evapotranspiration (Cowell and Urban 2011), and increasing weather and climatic variability (IPCC 2011;Stroup 2011;IPCC 2014). While water storage can mitigate the effects of short-term water shortages, few freshwater-scarce areas are equipped to deal with prolonged severe drought events. ...
This article evaluates the suitability of two supply-side drought management tools, Emergency Water Banks and Strategic Water Reserves for multi-year severe drought periods. The limited sample-size related to the implementation of either makes direct comparison impossible, but through case studies some cursory evaluation as to their efficacy and their limitations is possible. These short-term tools take different approaches to increase the amount of water available for reallocation during drought. While neither are stand-alone solutions, and may not be suitable for continual use, both hold potential to provide short-term relief during periods of severe shortage. Both however, carry drawbacks that can undermine their efficacy, or preclude adequate support for their implementation.
... The impact of changing baseline conditions coupled with increased variability can be especially complicated in regions with rapid changes in population, land development (especially urbanization), and economic disruptions. While public discussions often focus more on temperature than water availability, ecosystems and human society are highly vulnerable to water stress [5][6][7][8]. Understanding the mechanisms and geographic patterns by which anthropogenic climate change is impacting water resource variability is of critical importance to sustainable development, environmental management, and human health. ...
A significant challenge posed by changing climates is how water cycling and surficial and subsurface water availability will be affected at global and regional scales. Such alterations are critical as they often lead to increased vulnerability in ecosystems and human society. Understanding specifically how climate change affects water resource variability in different locations is of critical importance to sustainable development in different parts of the world. The papers included in this special issue focus on three broad perspectives associated with water resource variability and climate change. Six papers employ remote sensing, meteorological station-based observational data, and tree-ring records to empirically determine how water resources have been changing over historical time periods. Eight of the contributions focus on modeling approaches to determine how known processes are likely to manifest themselves as climate shifts over time. Two others focus on human perceptions and adaptation strategies in the midst of unstable or unsettled water availability. The findings and methods presented in this collection of papers provide important contributions to the increased study and awareness of climate change on water resources.
... Climate change in Badlands National Park, which is in the center of our study area, is projected to result in warmer (3-58C) weather by the end of the century, and this temperature increase is likely to increase evapotranspiration (Amberg et al. 2012). Thus, although precipitation is actually projected to increase slightly in the region, the temperature increase will likely offset the precipitation increase and result in drier weather in summer months (Cowell and Urban 2010). Declines in response to extreme droughts are thus of particular concern because the grassland bird community has experienced the steepest population declines of any regional avian community within the U.S., primarily because of the loss of grassland habitat as agricultural land use has expanded (Vickery and Herkert 2001). ...
Avian populations can respond dramatically to extreme weather such as droughts and heat waves, yet patterns of response to weather at broad scales remain largely unknown. Our goal was to evaluate annual variation in abundance of 14 grassland bird species breeding in the northern mixed-grass prairie in relation to annual variation in precipitation and temperature. We modeled avian abundance during the breeding season using North American Breeding Bird Survey (BBS) data for the U.S. Badlands and Prairies Bird Conservation Region (BCR 17) from 1980 to 2012. We used hierarchical Bayesian methods to fit models and estimate the candidate weather parameters standardized precipitation index (SPI) and standardized temperature index (STI) for the same year and the previous year. Upland Sandpiper (Bartramia longicauda) responded positively to within-year STI (β = 0.101), and Baird's Sparrow (Ammodramus bairdii) responded negatively to within-year STI (β =-0.161) and positively to within-year SPI (β = 0.195). The parameter estimates were superficially similar (STI β =-0.075, SPI β = 0.11) for Grasshopper Sparrow (Ammodramus savannarum), but the bestselected model included an interaction between SPI and STI. The best model for both Eastern Kingbird (Tyrannus tyrannus) and Vesper Sparrow (Pooecetes gramineus) included the additive effects of within-year SPI (β=-0.032 and β= -0.054, respectively) and the previous-year's SPI (β=-0.057 and-0.02, respectively), although for Vesper Sparrow the lag effect was insignificant. With projected warmer, drier weather during summer in the Badlands and Prairies BCR, Baird's and Grasshopper sparrows may be especially threatened by future climate change.
... Western North Carolina receives 40-50 inches of rain per year. Global climate change models have predicted a wide range of future scenarios in North Carolina, including both increases and decreases in precipitation (Cowell and Urban, 2010). While it is uncertain how much drought frequency and intensity will increase, they are not expected to decrease (State Climate Office of North Carolina, 2012). ...
Western North Carolina is water rich, with high annual rainfall and historically low population. Therefore, water management has traditionally not been a significant policy issue. Recent droughts and high population growth, however, have stressed many water supply systems. To deal effectively with these stresses, new policies and management practices have been initiated, prompted by both state mandates and local pressure. As pressures are likely to continue, there is a need to understand what motivates policy development and what processes decision makers use when creating water management policies and programs. Previous research finds that decision makers are apt to base decisions on perceptions, personal beliefs and historical practice rather than on relevant water data. In this study, survey results are used to understand how decision-maker perceptions about water availability, growth, and environmental concerns correlate with water allocation and conservation policies. Results indicate that respondents are only moderately concerned about water availability and drought is the primary concern, rather than population growth. Few of these decision makers have implemented water education programs, but many have implemented drought-related conservation programs. Environmental concerns related to water management are quite low among all respondents.Environmental Practice 16: 94–101 (2014)
The New England and Mid‐Atlantic regions of the Northeast United States have experienced climate‐induced increases in both the magnitude and frequency of floods. However, a detailed understanding of flood seasonality across these regions, and how flood seasonality may have changed over the instrumental record, has not been established. The annual timing of river floods reflects the flood‐generating mechanisms operating in a basin and many aquatic and riparian organisms are adapted to flood seasonality, as are human uses of river channels and floodplains. Changes in flood seasonality may indicate changes in flood‐generating mechanisms, and their interactions, with important implications for habitats, floodplain infrastructure, and human communities. I applied a probabilistic method for identifying flood seasons at a monthly resolution for 90 Northeast U.S. watersheds with natural, or near‐natural, flood‐generating conditions. Historical trends in flood seasonality were also investigated. Analyses were based on peaks‐over‐threshold (POT) flood records that have, on average, 85 years of data and 3 peaks per year—thus providing more information about flood seasonality than annual maximums. The results show rich detail about annual flood timing across the region with each site having a unique pattern of monthly flood occurrence. However, a much smaller number of dominant seasonal patterns emerged when contiguous flood‐rich months were classified into commonly recognized seasons (e.g., Mar‐May, spring). The dominant seasonal patterns identified by manual classification were corroborated by unsupervised classification methods (i.e., cluster analyses). Trend analyses indicated that the annual timing of flood‐rich seasons has generally not shifted over the period of record, but 65 sites with data from 1941‐2013 revealed increased numbers of June‐October floods—a trend driving previously documented increases in Northeast U.S. flood counts per year. These months have been historically flood‐poor at the sites examined, so warm‐season flood potential has increased with possible implications for aquatic and riparian organisms.
Hydrology is a key variable in the structure and function of a wetland; it is a primary determinant of wetland type, and it drives many of the functions a wetland performs and in turn the services it provides. However, wetland hydrology has been understudied. Efforts by scientists from Riparia, a wetland and aquatic systems research center at Penn State University, have advanced the understanding of wetland hydrology in the Mid-Atlantic Region over the past two decades primarily through a series of studies at a set of long-term monitoring sites. This work contributed to four primary issues in wetland hydrology: validation of regional hydrogeomorphic classification schemes, establishment of reference criteria for monitoring and assessment, identification of targets for restoration or mitigation, and evaluation of the hydrologic behavior of created vs. non-created wetlands. This chapter (1) summarizes some of the key findings of hydrologic studies of wetlands from the published and non-published research of wetland scientists associated with Riparia and secondarily, (2) describes general, seasonal, and inter-annual hydrologic patterns of the water level data that has been collected at some of the long-term monitoring sites or reference sites. © 2013 Springer Science+Business Media New York. All rights are reserved.
Available at:
http://www.ipcc.ch/pdf/technical-papers/climate-change-water-en.pdf
Changes of variability with climate change are likely to have a substantial impact on vegetation and society, rivaling the importance of changes in the mean values themselves. A variety of paleoclimate and future climate simulations performed with the GISS global climate model is used to assess how the variabilities of temperature and precipitation are altered as climate warms or cools. In general, as climate warms, temperature variability decreases due to reductions in the latitudinal temperature gradient and precipitation variability increases together with the intensity of the hydrologic cycle. If future climate projections are accurate, the reduction in temperature variability will be minimized by the rapid change in mean temperatures, but the hydrologic variability will be amplified by increased evapotranspiration. Greater hydrologic variability would appear to pose a potentially severe problem for the next century.
Recent improvements in worldwide climatic data sets and analytical techniques have made possible more accurate estimates of precipitation, evapotranspiration, runoff, and water deficit by latitude and by continent. These new results are compared with ones from previous studies. The difficulties inherent in using precipitation measurements corrected for wind, wetting, and evaporation losses in any water budget originally developed for use with gauge-measured precipitation are analyzed. -Authors
Open access to an unprecedented, comprehensive coordinated set of global coupled climate model experiments for twentieth and twenty-first century climate and other experiments is changing the way researchers and students analyze and learn about climate. The history of climate change modeling was first characterized in the 1980s by a number of distinct groups developing, running, and analyzing model output from their own models with little opportunity for anyone outside of those groups to have access to the model data. This was partly a consequence of relatively primitive computer networking and data transfer capabilities, along with the daunting task of collecting and storing such large amounts