Matching Items (20)
Description
The North American Monsoon System (NAMS) contributes ~55% of the annual rainfall in the Chihuahuan Desert during the summer months. Relatively frequent, intense storms during the NAMS increase soil moisture, reduce surface temperature and lead to runoff in ephemeral channels. Quantifying these processes, however, is difficult due to the sparse nature of coordinated observations. In this study, I present results from a field network of rain gauges (n = 5), soil probes (n = 48), channel flumes (n = 4), and meteorological equipment in a small desert shrubland watershed (~0.05 km2) in the Jornada Experimental. Using this high-resolution network, I characterize the temporal and spatial variability of rainfall, soil conditions and channel runoff within the watershed from June 2010 to September 2011, covering two NAMS periods. In addition, CO2, water and energy measurements at an eddy covariance tower quantify seasonal, monthly and event-scale changes in land-atmosphere states and fluxes. Results from this study indicate a strong seasonality in water and energy fluxes, with a reduction in Bowen ratio (B, the ratio of sensible to latent heat fluxes) from winter (B = 14) to summer (B = 3.3). This reduction is tied to shallow soil moisture availability during the summer (s = 0.040 m3/m3) as compared to the winter (s = 0.004 m3/m3). During the NAMS, I analyzed four consecutive rainfall-runoff events to quantify the soil moisture and channel flow responses and how water availability impacted the land-atmosphere fluxes. Spatial hydrologic variations during events occur over distances as short as ~15 m. The field network also allowed comparisons of several approaches to estimate evapotranspiration (ET). I found a more accurate ET estimate (a reduction of mean absolute error by 38%) when using distributed soil moisture data, as compared to a standard water balance approach based on the tower site. In addition, use of spatially-varied soil moisture data yielded a more reasonable relationship between ET and soil moisture, an important parameterization in many hydrologic models. The analyses illustrates the value of high-resolution sampling for quantifying seasonal fluxes in desert shrublands and their improvements in closing the water balance in small watersheds.
ContributorsTempleton, Ryan (Author) / Vivoni, Enrique R (Thesis advisor) / Mays, Larry (Committee member) / Fox, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
Woody plant encroachment is a worldwide phenomenon linked to water availability in semiarid systems. Nevertheless, the implications of woody plant encroachment on the hydrologic cycle are poorly understood, especially at the catchment scale. This study takes place in a pair of small semiarid rangeland undergoing the encroachment of Prosopis velutina Woot., or velvet mesquite tree. The similarly-sized basins are in close proximity, leading to equivalent meteorological and soil conditions. One basin was treated for mesquite in 1974, while the other represents the encroachment process. A sensor network was installed to measure ecohydrological states and fluxes, including precipitation, runoff, soil moisture and evapotranspiration. Observations from June 1, 2011 through September 30, 2012 are presented to describe the seasonality and spatial variability of ecohydrological conditions during the North American Monsoon (NAM). Runoff observations are linked to historical changes in runoff production in each watershed. Observations indicate that the mesquite-treated basin generates more runoff pulses and greater runoff volume for small rainfall events, while the mesquite-encroached basin generates more runoff volume for large rainfall events. A distributed hydrologic model is applied to both basins to investigate the runoff threshold processes experienced during the NAM. Vegetation in the two basins is classified into grass, mesquite, or bare soil using high-resolution imagery. Model predictions are used to investigate the vegetation controls on soil moisture, evapotranspiration, and runoff generation. The distributed model shows that grass and mesquite sites retain the highest levels of soil moisture. The model also captures the runoff generation differences between the two watersheds that have been observed over the past decade. Generally, grass sites in the mesquite-treated basin have less plant interception and evapotranspiration, leading to higher soil moisture that supports greater runoff for small rainfall events. For large rainfall events, the mesquite-encroached basin produces greater runoff due to its higher fraction of bare soil. The results of this study show that a distributed hydrologic model can be used to explain runoff threshold processes linked to woody plant encroachment at the catchment-scale and provides useful interpretations for rangeland management in semiarid areas.
ContributorsPierini, Nicole A (Author) / Vivoni, Enrique R (Thesis advisor) / Wang, Zhi-Hua (Committee member) / Mays, Larry W. (Committee member) / Arizona State University (Publisher)
Created2013
Description
Land-atmosphere interactions of semiarid shrublands have garnered significant scientific interest. One of the main tools used for this research is the eddy covariance (EC) method, which measures fluxes of energy, water vapor, and carbon dioxide. EC fluxes can be difficult to interpret due to complexities within the EC footprint (i.e. the surface conditions that contribute to the flux measurements). Most EC studies use a small number of soil probes to estimate the land surface states underlying the measured fluxes, which likely undersamples the footprint-scale conditions, especially in semiarid shrublands which are characterized by high spatial and temporal variability. In this study, I installed a dense network of soil moisture and temperature probe profiles in the footprint region of an EC tower at two semiarid sites: a woody savanna in southern Arizona and a mixed shrubland in southern New Mexico. For data from May to September 2013, I link land surface states to EC fluxes through daily footprints estimated using an analytical model. Novel approaches are utilized to partition evapotranspiration, estimate EC footprint soil states, connect differences in fluxes to footprint composition, and assess key drivers behind soil state variability. I verify the hypothesis that a small number of soil probes poorly estimates the footprint conditions for soil moisture, due to its high spatial variability. Soil temperature, however, behaves more consistently in time and space. As such, distributed surface measurements within the EC footprint allow for stronger ties between evapotranspiration and moisture, but demonstrate no significant improvement in connecting sensible heat flux and temperature. I also find that in these systems vegetation cover appears to have stronger controls on soil moisture and temperature than does soil texture. Further, I explore the influence of footprint vegetation composition on the measured fluxes, which reveals that during the monsoon season evaporative fraction tends to increase with footprint bare soil coverage for the New Mexico site and that the ratio of daily transpiration to evapotranspiration increases with grass coverage at the Arizona site. The thesis results are useful for understanding the land-atmosphere interactions of these ecosystems and for guiding future EC studies in heterogeneous landscapes.
ContributorsAnderson, Cody Alan (Author) / Vivoni, Enrique R (Thesis advisor) / Wang, Zhihua (Committee member) / Mays, Larry W. (Committee member) / Arizona State University (Publisher)
Created2013
Description
The history of outdoor water use in the Phoenix, Arizona metropolitan area has given rise to a general landscape aesthetic and pattern of residential irrigation that seem in discord with the natural desert environment. While xeric landscaping that incorporates native desert ecology has potential for reducing urban irrigation demand, there are societal and environmental factors that make mesic landscaping, including shade trees and grass lawns, a common choice for residential yards. In either case, there is potential for water savings through irrigation schedules based on fluxes affecting soil moisture in the active plant rooting zone. In this thesis, a point-scale model of soil moisture dynamics was applied to two urban sites in the Phoenix area: one with xeric landscaping, and one with mesic. The model was calibrated to observed soil moisture data from irrigated and non-irrigated sensors, with local daily precipitation and potential evapotranspiration records as model forcing. Simulations were then conducted to investigate effects of irrigation scheduling, plant stress parameters, and precipitation variability on soil moisture dynamics, water balance partitioning, and plant water stress. Results indicated a substantial difference in soil water storage capacity at the two sites, which affected sensitivity to irrigation scenarios. Seasonal variation was critical in avoiding unproductive water losses at the xeric site, and allowed for small water savings at the mesic site by maintaining mild levels of plant stress. The model was also used to determine minimum annual irrigation required to achieve specified levels of plant stress at each site using long-term meteorological records. While the xeric site showed greater potential for water savings, a bimodal schedule consisting of low winter and summer irrigation was identified as a means to conserve water at both sites, with moderate levels of plant water stress. For lower stress levels, potential water savings were found by fixing irrigation depth and seasonally varying the irrigation interval, consistent with municipal recommendations in the Phoenix metropolitan area. These results provide a deeper understanding of the ecohydrologic differences between the two types of landscape treatments, and can assist water and landscape managers in identifying opportunities for water savings in desert urban areas.
ContributorsVolo, Thomas J (Author) / Vivoni, Enrique R (Thesis advisor) / Ruddell, Benjamin L (Committee member) / Wang, Zhihua (Committee member) / Arizona State University (Publisher)
Created2013
Description
There is a considerable need for improved understanding of the outcome and amounts of water used to manage urban landscapes in arid and semiarid cities. Outdoor irrigation in urban parks consists of a large fraction of water demands in Phoenix, Arizona. Hence, ecohydrological processes need to be considered to improve outdoor irrigation management. With the goal of reducing outdoor water use and advancing the general knowledge of water fluxes in urban parks, this study explores water conservation opportunities in an arid city through observations and modeling.Most urban parks in Phoenix consist of a mosaic of turfgrass and trees which receive scheduled maintenance, fertilization and watering through sprinkler or flood irrigation. In this study, the effects that different watering practices, turfgrass management and soil conditions have on soil moisture observations in urban parks are evaluated. Soil moisture stations were deployed at three parks with stations at control plots with no compost application and compost treated sites with either a once or twice per year application instead of traditional fertilizer. An eddy covariance system was installed at a park to help quantify water losses and water, energy and carbon fluxes between the turfgrass and atmosphere. Additional meteorological observations are provided through a network of weather stations. The assessment covers over one year of observations, including the period of turfgrass growth in the warm season, and a period of dormancy during the cool season. The observations were used to setup and test a plot-scale soil water balance model to simulate changes in daily soil moisture in response to irrigation, precipitation and evapotranspiration demand for each park.
Combining modeling and observations of climate-soil-vegetation processes, I provide guidance on irrigation schedules and management that could help minimize water losses while supporting turfgrass health in desert urban parks. The irrigation scenarios suggest that water savings of at least 18% can be achieved at the three sites. While the application of compost treatment to study plots did not show clear improvements in soil water retention when compared to the control plots, this study shows that water conservation can be promoted while maintaining low plant water stress.
ContributorsKindler, Mercedes (Author) / Vivoni, Enrique R (Thesis advisor) / Mascaro, Giuseppe (Committee member) / Garcia, Margaret (Committee member) / Arizona State University (Publisher)
Created2021
Description
The American Southwest is one of the most rapidly growing regions of the United States, as are similar arid regions globally. Across these landscapes where surface water is intermittent and variable, groundwater aquifers recharged by surface waters become a keystone resource for communities and are consumed at rates disproportional to recharge. In this study, I focus on the controls of runoff generation and connectivity at both hillslope and watershed scales along a piedmont slope. I also investigate the effects of plant phenology on hydrologic connectivity and runoff response at the hillslope scale during the summer monsoon season. To carry out this work, I combine existing hydrologic instrumentation, a new set of runoff plots with high-resolution monitoring, near-field remote sensing techniques, and historical datasets. Key analyses show that a rainfall intensity (I30) of 10 mm/hr yields runoff production at three scales (8, 12700, and 46700 m2). Rainfall, runoff, and soil moisture observations indicate a Hortonian (infiltration-excess) dominated system with little control imposed by antecedent wetness. Hydrologic connectivity analyses revealed that <15% of total rainfall events generate runoff at the hillslope scale. Of the hillslope events, only 20% of the runoff production leads to discharge at the outlet. Vegetation was observed to effect individual plot runoff response to rainfall. The results of this study show that 1) rainfall intensity is a large control on runoff production at all three scales (8, 12700, and 46700 m2), 2) proportions between bare and vegetated space effect runoff production at the hillslope scale, and 3) runoff connectivity decreases, and channel losses increase as you move downstream on an individual storm basis and across a 30-year historical record. These findings indicate that connectivity from the hillslope to outlet scale can be an evolving process over thehistorical record, reliant on both rainfall intensity, plant and bare soil mosaics, and available channel storage.
ContributorsKeller, Zachary Theodore (Author) / Vivoni, Enrique R (Thesis advisor) / Whipple, Kelin X (Committee member) / Semken, Steven (Committee member) / Arizona State University (Publisher)
Created2021
Description
Observational evidence is mounting on the reduction of winter precipitation and an earlier snowmelt in the southwestern United States. It is unclear, however, how these changes, along with forest thinning, will impact water supplies due to complexities in the precipitation-streamflow transformation. In this study, I use the Triangulated Irregular Network-based Real-time Integrated Basin Simulator (tRIBS) to provide insight into the independent and combined effects of climate change and forest cover reduction on the hydrologic response in the Beaver Creek (~1100 km2) of central Arizona. Prior to these experiments, confidence in the hydrologic model is established using snow observations at two stations, two nested streamflow gauges, and estimates of spatially-distributed snow water equivalent over a long-term period (water years 2003-2018). Model forcings were prepared using station observations and radar rainfall estimates in combination with downscaling and bias correction techniques that account for the orographic controls on air temperature and precipitation. Model confidence building showed that tRIBS is able to capture well the variation in snow cover and streamflow during wet and dry years in the 16 year simulation period. The results from this study show that the climate change experiments increased average annual streamflow by 1.5% at +1°C of warming. However, a 28% decrease in streamflow occurs by +6°C of warming as evapotranspiration (ET) increases by 10%. Forest thinning shifted the warming threshold where ET increases reduce streamflow yield until +4°C of warming as compared to no forest thinning when this threshold occurs at +2°C. An average increase in streamflow of 12% occurs after forest thinning across all climate scenarios. While the snow covered area is unaffected by thinning, the volume of snowmelt increases and is linked to the higher water yield. These findings indicate that water managers can expect decreases in streamflow due to climate change but may be able to offset these impacts up to a warming threshold by thinning forested areas within the Beaver Creek.
ContributorsCederstrom, Charles Joshua (Author) / Vivoni, Enrique R (Thesis advisor) / Mascaro, Giuseppe (Committee member) / Svoma, Bohumil (Committee member) / Arizona State University (Publisher)
Created2021
Description
Accelerated climate and land use land cover (LULC) changes are anticipated to significantly impact water resources in the Colorado River Basin (CRB), a major freshwater source in the southwestern U.S. The need for actionable information from hydrologic research is growing rapidly, given considerable uncertainties. For instance, it is unclear if the predicted high degree of interannual precipitation variability across the basin could overwhelm the impacts of future warming and how this might vary in space. Climate change will also intensify forest disturbances (wildfire, mortality, thinning), which can significantly impact water resources. These impacts are not constrained, given findings of mixed post-disturbance hydrologic responses. Process-based models like the Variable Infiltration Capacity (VIC) platform can quantitatively predict hydrologic behaviors of these complex systems. However, barriers limit their effectiveness to inform decision making: (1) simulations generate enormous data volumes, (2) outputs are inaccessible to managers, and (3) modeling is not transparent. I designed a stakeholder engagement and VIC modeling process to overcome these challenges, and developed a web-based tool, VIC-Explorer, to “open the black box” of my efforts. Meteorological data was from downscaled historical (1950-2005) and future projections (2006-2099) of eight climate models that best represent climatology under low- and high- emissions. I used two modeling methods: (1) a “top-down” approach to assess an “envelope of hydrologic possibility” under the 16 climate futures; and (2) a “bottom-up” evaluation of hydrology in two climates from the ensemble representing “Hot/Dry” and “Warm/Wet” futures. For the latter assessment, I modified land cover using projections of a LULC model and applied more drastic forest disturbances. I consulted water managers to expand the legitimacy of the research. Results showed Far-Future (2066-2095) basin-wide mean annual streamflow decline (relative to 1976-2005; ensemble median trends of -5% to -25%), attributed to warming that diminished spring snowfall and melt and year-round increased soil evaporation from the Upper Basin, and overall precipitation declines in the Lower Basin. Forest disturbances partially offset warming effects (basin-wide mean annual streamflow up to 12% larger than without disturbance). Results are available via VIC-Explorer, which includes documentation and guided analyses to ensure findings are interpreted appropriately for decision-making.
ContributorsWhitney, Kristen Marie (Author) / Vivoni, Enrique R (Thesis advisor) / Mascaro, Giuseppe (Committee member) / Whipple, Kelin X (Committee member) / White, Dave D (Committee member) / Xu, Tianfang (Committee member) / Arizona State University (Publisher)
Created2022
Description
In the southwestern United States, water is a precious resource that influences landscapes and their respective ecosystems. Ephemeral lakes, known as playas, are drainage points for closed or endorheic basins and serve as important locations for plant productivity, biogeochemical processes, and groundwater recharge. In this study, I explore the hydrologic dynamics of eighteen (18) instrumented playas in the Jornada Basin of the Chihuahuan Desert with respect to the drivers of playa inundation and how their behaviors vary in space and time. To this end, I combine water level observations in playas with gauge-corrected radar precipitation estimates to determine hydrologic dynamics over the more than 6-year period of June 2016 to October 2022. Results indicate that all playa inundation events are associated with precipitation and that 76% of events occur during the warm season from April to September that is characterized by the North American monsoon. Mean annual runoff ratios in the playa catchments range from 0.01% to 9.28%. I observe precipitation depth and 60-minute intensity thresholds for playa inundation ranging from 16.1 to 71.3 mm and 8.8 to 40.5 mm/hr, respectively. Although playa inundation is typically caused by high rainfall amounts and intensities, other factors such as antecedent wetness conditions and the spatial variability of rainfall within the playa catchment also play a role. The magnitudes, durations, and occurrence of inundation events vary among playas, but their responses to precipitation generally agree with groupings determined based on their geological origin. Logistic and linear regressions across all playas reveal the relative importance of catchment variables, such as area, sand fraction, slope, and the percentage of bare ground. It is shown that larger catchment areas are strongly associated with a lower likelihood of inundation and higher precipitation thresholds for inundation. An analysis of precipitation data from 1916 to 2015 leads to the estimation of historical playa inundation and suggests that an increase has occurred in the frequency of large rainfall events that may be associated with increasing frequency of playa inundation. This study highlights the complex nature of playa inundation in the Jornada Basin, which can change over time in an evolving climate and landscape.
ContributorsKimsal, Charles Robert (Author) / Vivoni, Enrique R (Thesis advisor) / Whipple, Kelin X (Committee member) / Li, Jiwei (Committee member) / Arizona State University (Publisher)
Created2023
Description
In arid and semiarid areas of the southwestern United States and northwestern México, water availability is the main control on the interactions between the land surface and the atmosphere. Seasonal and interannual variations in water availability regulate the response of water and carbon dioxide fluxes in natural and urban landscapes. However, despite sharing a similar dependance to water availability, landscape characteristics, such as land cover heterogeneity, landscape position, access to groundwater, microclimatic conditions, and vegetation functional traits, among others, can play a fundamental role in modulating the interactions between landscapes and the atmosphere. In this dissertation, I study how different landscape characteristics influence the response of water and carbon dioxide fluxes in arid and semiarid urban and natural settings. The study uses the eddy covariance technique, which calculates the vertical turbulent fluxes within the boundary layer, to quantify water, energy, and carbon dioxide fluxes within local patches. Specifically, the study focused on three main scopes: (1) how vegetation, anthropogenic activity, and water availability influence carbon fluxes in four urban landscapes in Phoenix, Arizona, (2) how access to groundwater and soil-microclimate conditions modulate the flux response of three natural ecosystems in northwestern México during the North American monsoon, and (3) how the seasonal hydrologic partitioning in a watershed with complex terrain regulates the carbon dioxide fluxes of a Chihuahuan Desert shrubland. Results showed a differential response of landscapes according to their land cover composition, access to groundwater or functional traits. In Chapter 2, in urban landscapes with irrigation, vegetation activity can counteract carbon dioxide emissions during the day, but anthropogenic sources from the built environment dominate the carbon dioxide fluxes overall. In Chapter 3, across an elevation-groundwater access gradient, low elevation ecosystems showed intensive water use strategies linked to a dependance to shallow or intermittent access to soil moisture, while a high elevation ecosystem showed extensive water use strategies which depend on a reliable access to groundwater. Finally, in Chapter 4, the mixed shrubland in complex terrain showed an evenly distributed bimodal vegetation productivity which is supported by an abundant water availability during wet seasons and by carry-over moisture in deeper layers of the soil during the dry season. The results from this dissertation highlight how different forms of water availability are responsible for the activity of vegetation which modulates land surface fluxes in arid and semiarid settings. Furthermore, the outcomes of this dissertation help to understand how landscape properties regulate the flux response to water availability in urban and natural areas.
ContributorsPerez Ruiz, Eli Rafael (Author) / Vivoni, Enrique R (Thesis advisor) / Sala, Osvaldo E (Committee member) / Throop, Heather L (Committee member) / Whipple, Kelin X (Committee member) / Yepez, Enrico A (Committee member) / Arizona State University (Publisher)
Created2021