Groundwater water resources must be accurately characterized in order to be managed sustainably. Due to the cost to install monitoring wells and challenges in collecting and managing in-situ data, groundwater data is sparse in space and time especially in developing countries. In this study we analyzed long-term groundwater storage changes with limited times-series data where each well had only one groundwater measurement in time. We developed methods to synthetically create times-series groundwater table elevation (WTE) by clustering wells with uniform grid and k-means-constrained clustering and creating pseudo wells. Pseudo wells with the WTE values from the cluster-member wells were temporally and spatially interpolated to analyze groundwater changes. We used the methods for the Beryl-Enterprise aquifer in Utah where other researchers quantified the groundwater storage depletion rate in the past, and the methods yielded a similar storage depletion rate. The method was then applied to the southern region in Niger and the result showed a ground water storage change that partially matched with the trend calculated by the GRACE data. With a limited data set that regressions or machine learning did not work, our method captured the groundwater storage trend correctly and can be used for the area where in-situ data is highly limited in time and space.

High latitude mountain environments are experiencing disproportionately adverse effects in a currently changing climate. The Gulf of Alaska (GoA) region is an exemplar of this. Dramatic shifts are occurring in the region’s freshwater reservoirs as glaciers retreat more with each passing year. Research in the region places much focus on observing and predicting climate driven shifts in glacier mass balance, surface discharge, and associated nutrient fluxes to the ocean. On the other hand, coastal groundwater discharge (CGD) is given very little attention. Global and near-global estimates of CGD indicate variable results spanning an order of magnitude. Focusing on regionally specific processes may provide more reliable estimates of CGD and allow isolation of CGD hotspots. This is of particular importance in the GoA region where complex topography, geology, and climate are coupled with recharge derived from rain, snow, and ice. I estimate CGD to the GoA with a water balance that integrates high temporal and spatial resolution recharge inputs and distinguishes between high conductivity surficial deposit and bedrock catchments. I find that CGD contributes nearly 3% of the total freshwater flux to the GoA, equivalent to a mean annual flux of 20.8 km3, and that CGD has been increasing by 0.5%/year over the past 4 decades. Although freshwater discharge to the GOA is well-constrained, the importance of fresh CGD to the GoA has, thus far, been overlooked.

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There are many different controlling factors on the partitioning of rainfall into runoff. However, the influence of each of these controls varies across different temporal scales. Consequently, numerous water balance models have been developed in the literature for application across various time scales. These models are usually developed for a particular time scale so that the controls with the greatest influence on rainfall partitioning are captured. For example, the SCS curve number method was developed to simulate direct runoff at the event scale; the "abcd" model was developed as a monthly water balance model; and the Budyko model was developed for long-term water balance. More recently, the proportionality hypothesis, which traces its origins from the SCS curve number method, has been identified as the commonality between these three hydrologic models, suggesting that this hypothesis may be the unifying principle of hydrologic models across various time scales. The objective of this thesis is to develop a conceptual hydrologic model structure for continuous simulations for multiple time scales. The developed model is applicable to daily, monthly, and annual time scales. Direct runoff is computed by a proportionality relationship in the SCS curve number method. In the "abcd" model, evapotranspiration and storage at the end of each time period are computed by a proportionality relationship, however evapotranspiration is computed based on an exponential relationship of storage and potential evapotranspiration while base flow is computed based on a linear reservoir model. In the Budyko model, runoff and evapotranspiration are computed by a proportionality relationship. The primary difference with the proposed model in this thesis in comparison with the other three water balance models is the application of the proportionality hypothesis to the partitioning of surface runoff and continuing abstraction as well as the partitioning of continuing evapotranspiration and subsurface flow. The proposed model structure is implemented in Matlab. The developed model includes six parameters, which are estimated for 71 case study catchments in the United States using a genetic algorithm. The model performances at the daily, monthly and annual time scales are evaluated during calibration and validation periods, and compared with the "abcd" model and a Budyko-type model developed for multiple time scales. Evaluation of the models shows that the proposed model performs better or comparable to the other models at all time scales.

A fish kill was observed in a historically freshwater region of the Everglades, coinciding with a spike in salinity. Sea level rise and altered hydrology allow freshwater wetland systems to be susceptible to saltwater intrusion. Most wetlands are characterized by highly organic soils with microbial communities starved of oxygen, seeking alternative electron acceptors. Seawater contains sulfate (SO42-), which is one of the alternative electron acceptors. Provided with sufficient quantities, sulfate-reducing bacteria (SRB) outcompete other anaerobes and produce toxic hydrogen sulfide. This study asked, what combination of soil organic matter (SOM) and SO42- are needed to produce sulfide concentrations potentially lethal to freshwater fish? Soil samples were collected from two freshwater wetlands with varying SOM content and incubated in microcosms at four SO42- concentrations (0, 0.75, 4.0, and 12.0 mM Na2SO4). Sulfide concentrations produced were compared to published data on lethal sulfide toxicity levels for aquatic life. This study demonstrated that high SOM soils (89.3 ± 0.2 % moisture) incubated with SO42- concentrations > 0.75 mM, rapidly produced toxic sulfide concentrations (> 5 ppm S2-) within 24 hrs.. While many freshwater species may tolerate a salinity of 1 ppt (0.75 mM SO42-), this SO42-concentration is sufficient to support SRB and lead to toxic sulfide production. ese results support the need to restore freshwater hydrology in wetland systems, such as the Everglades, thereby protecting against the rapid ecological effects of saltwater intrusion.

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