Browsing by Subject "Drought"
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Item Open Access An Analysis of Water Management Strategies in Drought Prone Areas(2018-04-26) Vogel, SarahThere is an old adage in the West: “whiskey is for drinking; water is for fighting”. In the American West, as well as locales all over the globe, water scarcity is a subject rife with conflicts and emotion. Human beings approach drought in reactionary ways. Rather than plan for the eventuality of drought, societies enact drought policy or regulations well into, and not before experiencing drought conditions. Researchers have predicted that more than half of humanity will live in water-stressed areas in the near future. Understanding the significant role drought plays in water management and the costs of reactionary decision making can help stakeholders create proactive approaches to water allocation. This paper seeks to understand how drought affects water management strategies; how regulation is affected by drought conditions; how local agencies and state authorities interact to manage water resources in California; and how California water management compares to water management strategies employed in other drought prone areas of the globe.Item Open Access Climate change induced changes in moisture availability in eastern Wyoming ranchlands with management recommendations for adaptation(2008-04-25T20:24:54Z) Fox, RobIn the future there is an expectation for climate change to have impacts on both natural systems and agricultural enterprises. A number of studies have been conducted for the purpose of determining the effects of a changing climate on agricultural enterprises, but most of these studies are large scale in their scope and give non-specific recommendations for adaptation. In the United States much of agriculture, including ranching, requires large capital shifts to change their products and as such they need to have more specific advice as to how to respond. Having more specific advice today also means that individuals in agriculture can start planning to adapt today, rather than being surprised a few decades from now. This project utilizes historical climate information and projections of future temperature and precipitation based on IPCC regional expectations and local climate variability. These projected values were used in two versions of the Thornwaite moisture balance model to calculate a range of possible changes for moisture availability from 2009 to the year 2100. The estimated changes in available moisture (potential evapotranspiration, soil moisture, atmospheric moisture deficit, etc.) were compared to the baseline values to determine the decrease from normal values. The literature was searched to determine the amount of decrease in moisture availability that would likely result in ecological drought and hinder production. The evidence indicates that there will be varying degrees of diminishing of available moisture dependent upon the amount of temperature increase. Because of the range of possible impacts, a variety of management practice recommendations are included, as well as mechanisms to monitor the climate more carefully to better spot droughts as they begin. For scenarios with severe shifts in the climate, recommendations are made to make strong changes in their production methods or the uses of the land.Item Open Access Drained coastal peatlands: A potential nitrogen source to marine ecosystems under prolonged drought and heavy storm events-A microcosm experiment.(Sci Total Environ, 2016-10-01) Wang, Hongjun; Richardson, Curtis J; Ho, Mengchi; Flanagan, NealOver the past several decades there has been a massive increase in coastal eutrophication, which is often caused by increased runoff input of nitrogen from landscape alterations. Peatlands, covering 3% of land area, have stored about 12-21% of global soil organic nitrogen (12-20Pg N) around rivers, lakes and coasts over millennia and are now often drained and farmed. Their huge nitrogen pools may be released by intensified climate driven hydrologic events-prolonged droughts followed by heavy storms-and later transported to marine ecosystems. In this study, we collected peat monoliths from drained, natural, and restored coastal peatlands in the Southeastern U.S., and conducted a microcosm experiment simulating coupled prolonged-drought and storm events to (1) test whether storms could trigger a pulse of nitrogen export from drought-stressed peatlands and (2) assess how differentially hydrologic managements through shifting plant communities affect nitrogen export by combining an experiment of nitrogen release from litter. During the drought phase, we observed a significant temporal variation in net nitrogen mineralization rate (NMR). NMR spiked in the third month and then decreased rapidly. This pattern indicates that drought duration significantly affects nitrogen mineralization in peat. NMR in the drained site reached up to 490±110kgha(-1)year(-1), about 5 times higher than in the restored site. After the 14-month drought phase, we simulated a heavy storm by bringing peat monoliths to saturation. In the discharge waters, concentrations of total dissolved nitrogen in the monoliths from the drained site (72.7±16.3mgL(-1)) was about ten times as high as from the restored site. Our results indicate that previously drained peatlands under prolonged drought are a potent source of nitrogen export. Moreover, drought-induced plant community shifts to herbaceous plants substantially raise nitrogen release with lasting effects by altering litter quality in peatlands.Item Open Access Ecosystem Response to a Changing Climate: Vulnerability, Impacts and Monitoring(2017) Seyednasrollah, BijanRising temperatures with increased drought pose three challenges for management of future biodiversity. First, are the species expected to be vulnerable concentrated in specific regions and habitats? Second, are the impacts of drought and warming varying across regions? Third, could recent advances in remote sensing techniques help us in monitoring the impacts in real-time? This dissertation is an effort to address the above questions in the three chapters.
First, I used foliar chemistry as a proxy for drought vulnerability. I used soil and moisture gradients to quantify habitat variation that could be critical for alleviating drought. I used a large dataset of forest plots covering the eastern united states to understand how community weighted mean foliar nitrogen and phosphorus vary across climate and soil gradients. I exploited trends in these variables between species, traits, and habitats to evaluate sensitivity. Critical to our approach is the capacity to jointly model trait responses. Our data showed that nutrient demanding species strongly respond to environmental gradients. I identified a wide range of sites across low to high latitudes threatened by drought. The sensitivity of species to high temperatures is largely explained by soil variations. Drought vulnerability of nutrient and moisture demanding species could be amplified depending on local soil and moisture gradients. Although local soil moisture may dampen drought-induced stress for species with large leaves and high water use, nutrient demanding species remain vulnerable in wet regions during droughts. Phosphorus demanding species adapted to dry sites are drought resilient compared to communities in wet sites. This research is consistent with the studies that supports declining nutrient demanding species with increasing temperature and decreasing moisture. I also detected strong soil effects on shaping community weighted traits across a large geographical and environmental range. Our data showed that soil effects on controlling foliar traits strongly vary across different climates. The findings are critical for conservations and maintaining the biodiversity.
Next, I used space-borne remotely sensed vegetation indices to monitor the process of leaf development across climate gradients and ecoregions in the southeastern United States. A hierarchical state-space Bayesian model was developed to quantify how air temperature, drought severity, and canopy thermal stress contribute to changes in leaf opening from mountainous to coastal regions. I synthesized daily field climate data with daily remotely sensed vegetation indices and canopy surface temperature during spring green-up season. The study was focused on observation of leaf phenology at 59 sites in the southeast United States between 2001 to 2012. Our results suggest strong interaction effects between ecosystem properties and climate variables across ecoregions. The findings showed that despite the much faster spring green-up in the mountains, coastal forests express a larger sensitivity to inter-annual anomaly in temperature than mountain sites. In spite of the decreasing trend in sensitivity to warming with temperature in all regions, there is an ecosystem interaction: Deciduous-dominated forests are less sensitive to warming than are those with few deciduous trees, possibly due to the presence of developed leaves in evergreen species throughout the season. The findings revealed mountainous forests are more susceptible to intensifying drought and moisture deficit, while coastal areas are relatively resilient. I found that increasing canopy thermal stress, defined as canopy-air temperature difference, slows the leaf-development following a dry year, accelerates it after a wet year.
Finally, I demonstrate how space-borne canopy “thermal stress”, i.e. surface-air temperature difference, could be used as a surrogate for drought-induced stress to estimate forest transpiration. Using physics-based relationships that accommodates uncertainties, I showed how changes in canopy water flux may be reflected in surface energy balance and in remotely-sensed thermal stress. Validating with field measurements of canopy transpiration in the southeastern US, I quantified sensitivity of transpiration to thermal stress in a range of atmospheric and climate conditions. I found that a 1 mm change in daily transpiration may cause 3 to 4 °C of thermal stress, depending on site conditions. The cooling effect is large when solar radiation is high or wind speed is low. The effect has the highest control on water-use during warm and dry seasons, when monitoring drought is essential. I applied our model to available satellite and metrological data to detect patterns of drought. Using only air and surface temperatures, I predicted anomaly in water-use across the contiguous United States over the past 15 years, and then compared with anomaly in soil water content and conventional drought indices. Our simple model showed a reliable accuracy in compare to the state-of-the-art general circulation models. The technique can be used in varying time-scales to monitor surface water-use and drought in large scales.
Item Open Access Effects of Vegetation and Infiltration Feedbacks on Hydrologic Partitioning and Droughts(2017) Wilson, Tiffany GaleThis dissertation addresses feedbacks between vegetation dynamics and land surface response to rainfall events, particularly in Mediterranean climates. Specifically, we ask how a saturated hydraulic conductivity value (ks) that is tied to vegetation biomass affects how water is divided into infiltration and runoff under a range of conditions. First, a field campaign in Sardinia was conducted in which a 4 m by 4 m rainfall simulator was constructed and deployed on a number of dates. Measurements of surface runoff from the plot and soil moisture within the plot informed estimates of the effective ks for each experimental run, and a comparison between ks and vegetation height measurements revealed a monotonically increasing relationship between the two. We then fit a logistic equation to this relationship and incorporated it into the calculations of a coupled vegetation dynamics and land surface model. Using the model, which is calibrated for the Sardinia field site, we investigated the effect of the variable ks by comparing the model results of biomass, saturation, and runoff to results using a static ks. We then used the same model to investigate the effects of a variable ks on drought recovery by simulating drought severity through a range of biomass levels relative to a no-drought condition. Our modeling results revealed that the primary result of a variable ks is modification of the quantity and mechanism of surface runoff; specifically, runoff increased over the constant ks case and shifted from saturation excess runoff to infiltration excess runoff. These effects are more pronounced in drier conditions and when rainfall intensities are in a critical region similar to the ks value. We conclude that a dynamic ks value is relevant for prediction of surface runoff and may improve the performance of land surface models.
Item Open Access Hydrological and ecological responses of ecosystems to extreme precipitation regimes: A test of empirical-based hypotheses with an ecosystem model(Perspectives in Plant Ecology, Evolution and Systematics, 2016-10-01) Ye, JS; Reynolds, JF; Maestre, FT; Li, FMMany uncertainties exist in our quest to understand and predict how terrestrial ecosystems will respond to climate change. A particularly challenging issue is how increases in extreme precipitation regimes, which are characterized by larger but fewer individual precipitation events, will impact ecosystems. Based on a wide-ranging review of empirical studies of both hydrological and ecological processes, Knapp et al. (2008) generated a suite of hypotheses positing how these processes would respond to an increase in extreme precipitation regimes and, from this, concluded that mesic ecosystems would be more detrimentally impacted than xeric ones. In this study we present the first thorough test of these hypotheses by examining how forest, shrubland, grassland and desert ecosystems of the Tibetan Plateau, having very different vegetation and climate characteristics, respond to more extreme rainfall regimes. We accomplished this by using a simulation model (Biome-BGC) to examine the integrated behavior of these ecosystems based on the simultaneous responses and interactions of 10 hydrological and ecological processes: runoff, canopy evaporation, soil evaporation, soil water storage, transpiration, net primary productivity, soil respiration, net ecosystem exchange, nitrogen [N] mineralization, and N leaching. We ran forty-year simulations (1986–2008) where we manipulated mean growing season precipitation to create more extreme intra-annual precipitation regimes characterized by lower precipitation frequencies, longer dry periods, and larger individual (daily) precipitation events. When compared to ambient conditions, our simulations showed that increases in extreme rainfall regimes (1) impacted all hydrological processes in mesic ecosystems, resulting in a reduction of soil mineral N due to increased leaching; and (2) enhanced plant growth in xeric ecosystems, leading to larger and denser canopies and higher light interception. The responses of hydrological processes tended to follow Knapp et al.’s hypotheses more so than ecological responses. Overall, responses of mesic ecosystems closely followed the hypotheses but xeric ecosystems were highly variable and only weakly consistent with them. Our findings provide new insights as to how more extreme rainfall regimes may potentially affect the functioning of terrestrial ecosystems.Item Open Access Influence of Increased Human Presence in the Mills River Basin on Water Availability and Drought(2016) Hodes, JaredPeriods of drought and low streamflow can have profound impacts on both human and natural systems. People depend on a reliable source of water for numerous reasons including potable water supply and to produce economic value through agriculture or energy production. Aquatic ecosystems depend on water in addition to the economic benefits they provide to society through ecosystem services. Given that periods of low streamflow may become more extreme and frequent in the future, it is important to study the factors that control water availability during these times. In the absence of precipitation the slower hydrological response of groundwater systems will play an amplified role in water supply. Understanding the variability of the fraction of streamflow contribution from baseflow or groundwater during periods of drought provides insight into what future water availability may look like and how it can best be managed. The Mills River Basin in North Carolina is chosen as a case-study to test this understanding. First, obtaining a physically meaningful estimation of baseflow from USGS streamflow data via computerized hydrograph analysis techniques is carried out. Then applying a method of time series analysis including wavelet analysis can highlight signals of non-stationarity and evaluate the changes in variance required to better understand the natural variability of baseflow and low flows. In addition to natural variability, human influence must be taken into account in order to accurately assess how the combined system reacts to periods of low flow. Defining a combined demand that consists of both natural and human demand allows us to be more rigorous in assessing the level of sustainable use of a shared resource, in this case water. The analysis of baseflow variability can differ based on regional location and local hydrogeology, but it was found that baseflow varies from multiyear scales such as those associated with ENSO (3.5, 7 years) up to multi decadal time scales, but with most of the contributing variance coming from decadal or multiyear scales. It was also found that the behavior of baseflow and subsequently water availability depends a great deal on overall precipitation, the tracks of hurricanes or tropical storms and associated climate indices, as well as physiography and hydrogeology. Evaluating and utilizing the Duke Combined Hydrology Model (DCHM), reasonably accurate estimates of streamflow during periods of low flow were obtained in part due to the model’s ability to capture subsurface processes. Being able to accurately simulate streamflow levels and subsurface interactions during periods of drought can be very valuable to water suppliers, decision makers, and ultimately impact citizens. Knowledge of future droughts and periods of low flow in addition to tracking customer demand will allow for better management practices on the part of water suppliers such as knowing when they should withdraw more water during a surplus so that the level of stress on the system is minimized when there is not ample water supply.
Item Open Access Integrated Water Finance Solutions to Drought in the Yakima Basin: Recommendations for the Yakima Drought Relief Pumping Plant (YDRPP)(2017-04-28) Bowler, Catherine; Brennan, Jennifer; Kuzma, SamanthaLocated within the United States Department of the Interior (DOI), the Natural Resource Investment Center (NRIC) was created under the Obama Administration to facilitate resource conservation through innovative partnerships and market-based strategies. One of the current projects at the NRIC involves developing the financial strategy for an emergency drought-relief pumping plant in central Washington’s Yakima Basin. The Yakima Drought Relief Pumping Plant (YDRPP) is a project conceived by one of the Basin’s junior users, Roza Irrigation District (Roza), whose annual water supply has been depleted by recent drought. The spatial distribution of junior and senior water rights combined with the various competing uses for water has created an opportunity for YDRPP water to yield synergistic benefits across the Basin. Roza will finance the YDRPP and has multiple strategies at its disposal to do so. The NRIC is tasked with creating an affordable project that serves the water needs of Roza. As student consultants for the NRIC, we are evaluating the hydrologic, ecological, and financial implications of the project for Roza and other major stakeholders in the Basin, and recommending financial strategies to actualize the many potential benefits of the YDRPP.Item Open Access Plant water transport and photosynthesis in water-limited environments(2020) Mrad, AssaadTerrestrial ecosystems depend on vegetation for many indispensable services including carbon fixation from the atmosphere, food production, and the maintenance of the global water and carbon cycles. As the climate changes, temperature and precipitation patterns shift and extreme climatic events become more frequent. In many areas, droughts are increasing in intensity and frequency, posing a challenge to ecosystem health and food security. Plants depend on water for physiological functioning including photosynthesis. The ability of plants to continue supplying water to the leaves from the soil during droughts depends on the anatomy and structure of its vascular network, the xylem. Droughts cause gas bubbles, or embolisms, to spread within the xylem, blocking water movement.
A combination of modeling water flow in xylem of flowering plants and theoretical considerations derived from graph theory is used to explain the response of different xylem functional types to droughts. An open-source model of plant xylem hydraulics was developed with which it was shown how 'network' effects, such as the spatial distribution of anatomy throughout growth rings, alter the response of Maples to drought.
The xylem of similar flowering plants was further investigated through the model in addition to the the physics of percolation. This was the first instance percolation theory has ever been applied to embolism spread inside xylem. It was shown how embolism spread inside the xylem can be represented by an edge percolation process. The results indicate that an increased connectivity among the conduits in the xylem is a necessary feature in plant organs that are resistant to droughts.
The detrimental effects of droughts on plant water translocation cascade to inhibit photosynthesis. Soil-to-leaf resistance to drought is represented by a vulnerability to embolism curve (VC) that plots the percent loss in plant hydraulic conductivity as water potential declines. The whole-plant VC affects plant CO2 fixation under drought. The results show how different VC shapes give rise to typical isohydric and anisohydric plant responses to drought. To arrive at this conclusion, the calculus of variations is used to integrate plant hydraulics into the trade-off between CO2 fixation and transpiration during a drought.
Item Open Access Predicting Forest Responses to Changing Environmental Conditions(2016) Berdanier, Aaron BairdForests change with changes in their environment based on the physiological responses of individual trees. These short-term reactions have cumulative impacts on long-term demographic performance. For a tree in a forest community, success depends on biomass growth to capture above- and belowground resources and reproductive output to establish future generations. Here we examine aspects of how forests respond to changes in moisture and light availability and how these responses are related to tree demography and physiology.
First we address the long-term pattern of tree decline before death and its connection with drought. Increasing drought stress and chronic morbidity could have pervasive impacts on forest composition in many regions. We use long-term, whole-stand inventory data from southeastern U.S. forests to show that trees exposed to drought experience multiyear declines in growth prior to mortality. Following a severe, multiyear drought, 72% of trees that did not recover their pre-drought growth rates died within 10 years. This pattern was mediated by local moisture availability. As an index of morbidity prior to death, we calculated the difference in cumulative growth after drought relative to surviving conspecifics. The strength of drought-induced morbidity varied among species and was correlated with species drought tolerance.
Next, we investigate differences among tree species in reproductive output relative to biomass growth with changes in light availability. Previous studies reach conflicting conclusions about the constraints on reproductive allocation relative to growth and how they vary through time, across species, and between environments. We test the hypothesis that canopy exposure to light, a critical resource, limits reproductive allocation by comparing long-term relationships between reproduction and growth for trees from 21 species in forests throughout the southeastern U.S. We found that species had divergent responses to light availability, with shade-intolerant species experiencing an alleviation of trade-offs between growth and reproduction at high light. Shade-tolerant species showed no changes in reproductive output across light environments.
Given that the above patterns depend on the maintenance of transpiration, we next developed an approach for predicting whole-tree water use from sap flux observations. Accurately scaling these observations to tree- or stand-levels requires accounting for variation in sap flux between wood types and with depth into the tree. We compared different models with sap flux data to test the hypotheses that radial sap flux profiles differ by wood type and tree size. We show that radial variation in sap flux is dependent on wood type but independent of tree size for a range of temperate trees. The best-fitting model predicted out-of-sample sap flux observations and independent estimates of sapwood area with small errors, suggesting robustness in new settings. We outline a method for predicting whole-tree water use with this model and include computer code for simple implementation in other studies.
Finally, we estimated tree water balances during drought with a statistical time-series analysis. Moisture limitation in forest stands comes predominantly from water use by the trees themselves, a drought-stand feedback. We show that drought impacts on tree fitness and forest composition can be predicted by tracking the moisture reservoir available to each tree in a mass balance. We apply this model to multiple seasonal droughts in a temperate forest with measurements of tree water use to demonstrate how species and size differences modulate moisture availability across landscapes. As trees deplete their soil moisture reservoir during droughts, a transpiration deficit develops, leading to reduced biomass growth and reproductive output.
This dissertation draws connections between the physiological condition of individual trees and their behavior in crowded, diverse, and continually-changing forest stands. The analyses take advantage of growing data sets on both the physiology and demography of trees as well as novel statistical techniques that allow us to link these observations to realistic quantitative models. The results can be used to scale up tree measurements to entire stands and address questions about the future composition of forests and the land’s balance of water and carbon.
Item Open Access Response of Southern Shrub Peatland Phenolics and Carbon Dioxide Flux to Drought and Nitrogen Additions(2013-04-26) Burke, MeaghanPeat forms under wetland conditions where flooding obstructs flows of oxygen from the atmosphere and reduces the decomposition rate of plant litter. Peatlands only cover three percent of land area worldwide, yet they store one third of all terrestrial carbon due to thwarted decay. Wetlands are currently threatened by increasingly severe and frequent drought as well as nitrogen loading from agriculture and atmospheric deposition. Furthermore, the length of exposure to these inputs may produce varying outcomes. The degradation of critical wetland ecosystems amplifies carbon dioxide emissions and dissolved organic carbon release. Existing research focuses on sphagnum or grassland peat while this study examines shrub peatland soil from the Pocosin Lakes region of Eastern North Carolina. This project utilizes chemical and statistical analyses to determine the impacts of drought and nitrogen on the biogeochemical processes that occur within a shrub peatland.Item Open Access The Impact of Drought on Electricity Supply in North Carolina(2009-04-24T20:47:54Z) Milazi, DominicNorth Carolina is located within the Southeastern region of The United States, an area which has experienced varying degrees of drought over the last two decades. Water withdrawal and consumption are integral and unavoidable components of the electricity generation process whenever it is fueled by non-renewable sources of primary energy. It follows from these two statements, that North Carolina - which largely depends on base-load coal and nuclear thermoelectric power generation - will be vulnerable to electricity supply disruptions in times of severe drought. In this report, an attempt is made to quantify the financial or economic impact on privately-owned utility companies as well as the state economy. The report will cover various issues en-route to quantifying the impact of drought including: identifying areas in the state historically prone to drought, locations of major power plants in relation to these drier areas, electricity generation costs of different powers plants within the state as well as changes in aggregate generation costs under different scenarios when countering the adverse effects of drought. Major findings include that roughly 5000 MW of baseload generation capacity is situated in especially drought prone regions and that droughts severe enough to result in plant shutdowns occur once in three decades. Economic impact on the state would run well into the hundreds of millions while individual utilities will see lower cost impacts in absolute terms potentially leading to underinvestment in drought-mitigating measures. These potential economic and financial losses are then used as a basis to explore the possibilities for making investments in less water-intensive technologies for electricity supply. The alternative investment options are then compared to find the most cost effective. Such investments would alleviate the pressure on the state’s finite fresh surface water resources while simultaneously mitigating against any drought-induced electricity supply disruptions.Item Embargo Urban Tree Stress in Socio-Ecological Systems(2024) Poulton Kamakura, RenataUrban trees provide crucial ecosystem services for cities, and their health impacts how effectively they provide those ecosystem services. Urban areas present stressful conditions for trees that depend on social as well as biophysical conditions. Despite their importance, patterns of non-lethal tree stress are rarely studied in cities. Although some tree care practices are effective, the influence of the underlying socio-ecological context is poorly understood. In this dissertation, I use a literature review, field data collection, and remote sensing to assess: 1) which tree care practices are effective in mediating tree stress in urban areas and what elements of the socio-ecological context impact their effectiveness 2) how tree-level characteristics, site conditions, and tree care practices correlate to street tree stress, 3) what socio-demographic characteristics and patterns of construction are associated with street tree stress across neighborhoods, and 4) how added climatic stress from drought impacts urban trees. Analysis includes two cities (Chicago and Durham), which offer contrasts in city layout, density, climate, and historical development. When examined across neighborhoods with field data, the few tree care practices examined (mulching, pruning, landscaping) do not reduce stress for urban street trees. Despite the differences between the cities studied, urban tolerant trees consistently have lower stress. In both cities planting site type and land use can influence tree stress levels. Scaled up to a regional level, tree stress is higher in areas with more new building construction and fewer renovations or additions. Drought does not appear to differentially impact trees depending on land use or nearby construction. The relationship between tree stress and urban growing conditions is visible across scales and contexts, but some of the specific relationships, including the effectiveness of tree care practices, are more context specific.