Browsing by Author "Butters, Gregory L., committee member"
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Item Open Access Exploring the contribution of crop water use to remotely sensed estimates of soil salinity in irrigated agriculture(Colorado State University. Libraries, 2023) Craig, Brian D., author; Chávez, José L., advisor; Gates, Timothy K., advisor; Butters, Gregory L., committee memberGlobally, 72% of the world's water withdrawals are used for agriculture. As the world's population continues to grow and increase its caloric intake, agricultural producers must provide more food and fiber with the same amount of water and soil, or less, due to expanding urbanization and climate change. In Colorado (CO, U.S.A.), agricultural producers in the South Platte and Arkansas River Basins, for instance, have been offered water transfer programs to temporarily or permanently transfer their water shares to municipalities and industry. Another challenge agricultural growers face is soil salinization, which needs to be monitored. In the Arkansas River basin, upflux from saline shallow groundwater tables consistently contributes to crop evapotranspiration (ET), leaving salts in the vadose zone. These salts accumulate over decades to the point where crop yields decline, threatening agricultural sustainability. Remote sensing is an economical tool to monitor salinity (e.g., soil electrical conductivity; EC, dS m-1) at large spatial scales. Existing remote sensing models that predict EC mostly utilize vegetation indices (VIs), which are arithmetic combinations of vegetation reflectances captured by discrete spectral bands. In this study, two additional explanatory variables were investigated: 1) the actual crop ET (ETa, mm d-1), and 2) the crop water stress index (CWSI). Calculations of ETa were performed using Landsat satellite multispectral imagery and a surface energy balance approach. This research was conducted over two growing seasons in commercial maize fields located within the Fairmont Drainage District near Swink, CO. Results indicate that models including ETa or CWSI with existing VIs improve the accuracy of soil EC mapping over models including VIs alone. The developed EC models are accurate within ±1 dS m-1 (Root Mean Squared Error), which is considered well within the precision required to make pragmatic field and ditch company-level management decisions.Item Open Access Membrane and diffusion behavior of a compacted sand-bentonite mixture for hydraulic and chemical containment applications(Colorado State University. Libraries, 2016) Meier, Amara Joy, author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Butters, Gregory L., committee memberDue to the ability of sodium bentonite (Na-bentonite) to exhibit high swell, low hydraulic conductivity, k (≤ 10-10 m/s), and semipermeable membrane behavior when exposed to water and dilute chemical solutions, Na-bentonite is commonly used as a component for engineered barriers (e.g., geosynthetic clay liners (GCLs)), or as an engineered barrier (e.g., compacted Na-bentonite buffers) used to contain solid wastes and liquid contaminants. Compacted sand-bentonite (SB) mixtures typically comprising 5 to 20 % Na-bentonite (by dry weight) are commonly used as alternatives to compacted clay liners (CCLs) for containment of solid wastes and liquid contaminants when a suitable natural clay source is not readily or economically available. In addition, membrane behavior, or the ability of a porous material to selectively restrict the passage of dissolved chemical species (solutes), has been shown to exist in many of these bentonite-based barriers, including GCLs, bentonite amended natural clays used for CCLs, and soil-bentonite backfills for in situ vertical cutoff walls. However, compacted SB mixtures suitable for use as engineered hydraulic and chemical containment barriers previously have not been evaluated for membrane behavior. As a result of these considerations, the purpose of this study was to evaluate simultaneously the membrane and diffusion behavior of a SB mixture that would be suitable for use as an engineered barrier for hydraulic and chemical containment applications. Accordingly, membrane tests were conducted on duplicate specimens of a compacted SB mixture comprising 15 % bentonite that was shown to exhibit sufficiently low k (≤ 2.7 x 10-11 m/s) to be suitable for use as a hydraulic and chemical containment barrier. In addition, the simultaneous diffusion of the principal salt species evaluated in the study, viz., Cl- and K+, was evaluated for one of the specimens. The results indicated that both specimens exhibited virtually the same magnitude of membrane behavior, with measured values of the membrane efficiency coefficients, ω, ranging from 0.395±0.053 to 0.063±0.012 when exposed to KCl solutions with source concentrations, Cot, ranging from 5 mM KCl to 80 mM KCl, respectively. In addition, the diffusion of both Cl- and K+ was found to be restricted relative to the case in which the specimen would not exhibit membrane behavior (i.e., ω = 0). Despite the imposition of chemical conditions in the tests that were more complex than those imposed previously, the measured values of ω and the effective diffusion coefficients, D*, for Cl- were in good agreement with those reported in the literature for other bentonite-based engineered barriers when exposed to similar or the same types of salts and salt concentrations. Thus, this study provides the first results to illustrate that a compacted SB mixture that is suitable for use as a hydraulic and chemical containment barrier behaves as a semipermeable membrane that can restrict aqueous-phase diffusion of chemical species to an extent that the chemical containment function of the barrier is improved.Item Open Access Membrane behavior and diffusion in unsaturated sodium bentonite(Colorado State University. Libraries, 2015) Sample-Lord, Kristin M., author; Shackelford, Charles D., advisor; Bareither, Christopher A., committee member; Butters, Gregory L., committee member; Lu, Ning, committee member; Sale, Thomas C., committee memberSodium-bentonite (Na-bentonite) is a highly active clay commonly used as a barrier or a component of a barrier for chemical containment applications (e.g., landfills, waste impoundments, vertical cutoff walls) due to the ability of Na-bentonite to limit solute (contaminant) transport resulting from high swell and low hydraulic conductivity. However, Na-bentonite also may exhibit semipermeable membrane behavior or solute restriction, which can result in enhanced performance of the barrier by reducing liquid and contaminant flux. Experimental studies to date have focused on the correlation between membrane behavior and diffusion of solutes almost exclusively under fully saturated conditions (i.e., degree of water saturation, S, of 1.0). However, clay barriers can exist at various degrees of water saturation (S < 1.0), and, based on our current, conceptual understanding of the mechanisms causing membrane behavior in saturated clays, the influence of membrane behavior on solute transport is likely to be even more significant in clays under unsaturated conditions. Based on these considerations, an innovative testing apparatus was developed to allow for the simultaneous measurement of membrane behavior and diffusion in unsaturated Na-bentonite. The test specimens were prepared using a dialysis method that allowed for control of the cation species on the exchange complex of the bentonite, removal of excess soluble salts, and estimation of diffusion properties. Membrane efficiencies (ω) and effective diffusion coefficients (D*) of bentonite specimens with S ranging from 0.79 to 1.0 were measured by performing multistage tests using solutions of potassium chloride (KCl). The source concentrations (Cot) of the KCl solutions were 20 mM, 30 mM, and 50 mM, which resulted in average concentrations in the specimen at steady-state diffusion (Cave) of approximately 10 mM, 15 mM, and 25 mM. For all values of S, a decrease in S correlated with an increase in ω and a decrease in D*. For example, for Cot of 50 mM, ω increased from 0.31 to 0.41 and D* for chloride decreased from 4.1 x 10-10 m2/s to 3.1 x 10-10 m2/s as S decreased from 1.0 to 0.84. The results of this study advance our fundamental understanding of solute transport mechanisms in Na-bentonite and contribute to the base of knowledge that must be established prior to incorporating membrane behavior effects in the design of barriers for chemical containment facilities.Item Open Access Occurrence and transport of salinity and selenium in a tile-drained irrigated agricultural system(Colorado State University. Libraries, 2017) Daly, Miles Brian, author; Bailey, Ryan T., advisor; Gates, Timothy K., advisor; Butters, Gregory L., committee memberTo view the abstract, please see the full text of the document.Item Open Access Snow sublimation and seasonal snowpack variability(Colorado State University. Libraries, 2016) Sexstone, Graham A., author; Fassnacht, Steven R., advisor; Clow, David W., committee member; Hiemstra, Christopher A., committee member; Butters, Gregory L., committee memberIn the western United States, seasonal melt from snow in mountainous regions serves as an essential water resource for ecological and anthropological needs, and improving our abilities to quantify the amount of water stored in the seasonal snowpack and provide short-term forecasts of snowmelt inputs into river systems is a critical science endeavor. Two important uncertainties in characterizing the seasonal evolution of snow in mountainous environments are related to the inherent spatial variability of snow in complex terrain and the magnitude and variability of snow sublimation fluxes between snow and the atmosphere; these uncertainties motivate this collection of research which includes three studies conducted in the north-central Colorado Rocky Mountains. The first study uses fine resolution airborne lidar snow depth datasets to evaluate the spatial variability of snow within areas comparable to coarse scale model grids (i.e. subgrid variability at 500 m resolution). Snow depth coefficient of variation, which was used as a metric for evaluating subgrid snow variability, exhibited substantial variability in mountainous terrain and was well correlated with mean snow depth, land cover type, as well as canopy and topography characteristics. Results highlight that simple statistical models for predicting subgrid snow depth coefficient of variation in alpine and subalpine areas can provide useful parameterizations of subgrid snow distributions. Given that snow sublimation fluxes are expected to exert important influences on snow distributions, the second and third studies focus on measuring and modeling the variability and importance of snow sublimation. To evaluate the relative merits and measurement uncertainty of methods for quantifying snow sublimation in mountainous environments, a comparison was made between the eddy covariance, Bowen ratio-energy balance, bulk aerodynamic flux, and aerodynamic profile methods within two forested openings. Biases between methods are evaluated over a range of environmental conditions, which highlight limitations and uncertainties of each method as well as the challenges related to measuring surface sublimation in snow-covered regions. Results provide guidance for future investigations seeking to quantify snow sublimation through station measurements and suggest that the eddy covariance and/or bulk aerodynamic flux methods are superior for estimating surface sublimation in snow-covered forested openings. To evaluate the spatial variability and importance of snow sublimation, a process-based snow model is applied across a 3600 km2 domain over five water years. In-situ eddy covariance observations of snow sublimation compare well with modeled snow sublimation at sites dominated by surface and canopy sublimation, but highlight challenges with model evaluation at sites where blowing sublimation is prominent. Modeled snow sublimation shows considerable spatial variability at the hillslope scale that is evident across elevation gradients and between land cover types. Snow sublimation from forested areas (canopy plus surface sublimation) accounted for the majority of modeled sublimation losses across the study domain and highlights the importance of sublimation from snow stored in the forest canopy in this region. Model simulations suggest that snow sublimation is a significant component of the winter water balance, accounting for losses equivalent to 43 percent of total snowfall, and strongly influences snow distributions in this region. Results from this study have important implications for future water management and decision making.Item Open Access Use of global datasets for downscaling soil moisture with the EMT+VS model(Colorado State University. Libraries, 2017) Grieco, Nicholas R., author; Niemann, Jeffrey D., advisor; Green, Timothy R., committee member; Butters, Gregory L., committee memberSatellite remote sensing and land-surface models provide coarse-resolution (9-40 km) soil moisture estimates, but various applications require fine-resolution (10-30 m) soil moisture patterns. The Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model downscales soil moisture using fine-resolution topography, vegetation, and soil data. It has been shown to reproduce temporally unstable soil moisture patterns (i.e. patterns where the spatial structure varies in time). It can also reproduce hillslope dependent patterns (wetter locations occur on hillslopes oriented away from the sun) and valley dependent patterns (wetter locations occur in valley bottoms). However, the EMT+VS model requires several parameters to characterize the local climate, soil, and vegetation characteristics. In previous applications, the parameters were calibrated using point soil moisture data, but many regions of interest may not have such data. The purpose of this study is to evaluate EMT+VS model performance when the parameters are estimated from global datasets without site-specific calibration. Reliable and accessible global datasets were identified and methods were developed to estimate the parameters from the datasets. The global model (without site-specific calibration) was applied to six study sites, and its results were compared to local soil moisture observations and the results from the locally calibrated model. The use of global datasets decreased downscaling performance and the spatial variability of soil moisture was underestimated. Overall, only 5 of the 16 parameters can be estimated from global datasets. However, the global model still provides more reliable soil moisture estimates than the coarse-resolution input for most sampling dates at all six study sites.