- Multipurpose management of hydrosystems face a number of uncertainties related to hydrologic variability and nonstationarity. Anticipated air temperature increases in the Pacific Northwest region are projected to alter the timing and quantity of streamflow associated with precipitation shifting from snow to rain, including shorter winter runoff periods, earlier spring runoff, and longer and drier summers. Changes in land use, such as urbanization, can reduce infiltration and groundwater recharge, thus lowering base-flow levels. Furthermore, these future changes in water supply are likely to vary across catchments with sensitivity to climate and land use changes. In this dissertation, I investigate hydrosystem sensitivities to climate and land use change considering modeling uncertainty across two different hydrologic settings in the Santiam River Basin in Oregon (Chapter 2), the reliability of reservoirs to accommodate these changes given current operating procedures (Chapter 3), and the performance of alternative reservoir operations scenarios in mitigating projected future hydrologic changes (Chapter 4).
This research is based on modeled future streamflows forced by temperature and precipitation projections from eight global climate models and two greenhouse gas emissions scenarios. To represent the uncertainty associated with future streamflow, I apply global climate model projections to a groundwater-surface water model (GSFLOW), coupled with a formal Bayesian uncertainty analysis. The land use changes were simulated in GSFLOW by adjusting model parameters based on the proportion of change in land use area within hydrologic response units. I apply streamflow projections as inputs to a reservoir operation model (HEC-ResSim) to analyze reservoir system reliability under future climate. I then use historical records to identify what outcomes are unacceptable to stakeholders, a condition labeled as vulnerable, and establish thresholds of reservoir reliability. I then use projections of future hydrology to identify the likelihood of the system being pushed to that vulnerable state under current and alternate reservoir operations.
Modeling and analysis is conducted in the North and South Santiam River basin, Oregon. The North Santiam sub-basin is sourced by the High Cascades, with high elevations but low in relief, deep groundwater and spring-dominated drainage system that sustain base flow during the dry summer months. In contrast, the South Santiam sub-basin is entirely sourced by the Western Cascades geology, with steep drainage network and relatively impermeable rock that generates rapid runoff responses, high peak flows, high flow variability and little groundwater storage.
In the context of water scarcity, Chapter 2 presents an analysis of the influence of climate and land use change on the future availability of water resources across sub-basins with different hydrogeological and land use characteristics. In this analysis, I investigate how sub-basin characteristics, including elevation, intensity of water demands, and apparent intensity of groundwater interactions, contribute to hydrologic sensitivity, to climate and land use change response, and to water scarcity. Across the entire SRB, water demand exerts the strongest influence on basin sensitivity to water scarcity, regardless of hydrogeology, with the highest demand located in the lower reaches dominated by agricultural and urban land uses. Results highlight how seasonal runoff responses to climate and land use change vary across sub-basins with differences in hydrogeology, land use, and elevation.
In Chapter 3, I investigate the impact and importance of climate-related uncertainties and hydrologic variability on reliability and sensitivity of current reservoir operations for meeting water resources objectives. I assess whether and how projected future changes in the timing and quantity of water resources affect the reliability of reservoir systems to meet flood management, spring and summer environmental flows, and hydropower generation objectives. I evaluate which sub-basin and reservoir operations are more sensitive to hydrologic variability, and the sensitivity of different elements of reservoir operations to climate variability. Despite projected increases in winter flows and decreases in summer flows, results suggested little evidence of a response in reservoir operation performance to a warming climate, with the exception of summer flow targets in the SSB. Independent of climate impacts, historical prioritization of reservoir operations appeared to impact reliability, suggesting areas where operation performance may be improved. Results also highlighted how hydrologic uncertainty is likely to complicate planning for climate change in basins with substantial groundwater interactions.
In Chapter 4, I apply a bottom-up approach to identify reservoir system sensitivities and vulnerabilities to changes in operations considering hydrologic variability and uncertainty. I compare historical reservoir reliability to projected future reliability to evaluate how well climate information can capture historical conditions that determine when the system is in a vulnerable state, and evaluate the effectiveness of implementing variable rule curves to current reservoir operations. Results highlight the poor fit between coupled GCM and hydrologic models and historical summer streamflows in this basin. Results illustrate how increases in air temperature appear to reduce the reliability of meeting summer flow targets, but have negligible impacts on reservoir refilling and flooding. Variable rule curves appear to mitigate the impact of atmospheric warming on summer target reliability to some extent, without compromising flood risk. Across the two hydrogeologic settings, results indicate that the mixed groundwater-surface water basin has higher sensitivity to changes in climate and reservoir operations than the basin with streamflows derived primarily from surface water.
The studies presented herein provide useful information about the causes (e.g. climate change, land use change, water demands) and degree of future changes in the performance of hydrosystem, as well as the potential benefits of changes in reservoir operations. The results highlight several important conclusions. First, in addition to hydrogeology and elevation, considering water demands is a key mechanism needed to translate the analysis of atmospheric warming on low flows into the impacts on people. Second, the impact of climate variability on reservoir reliability was only evident for summer low flow targets. However, implementing variable rule curves to current reservoir operations appears to be an effective strategy to reduce the impact of atmospheric warming on summer target reliability, without increasing the risk of flooding. Finally, higher sensitivity to changes in climate and reservoir operations were projected for the mixed groundwater-surface water basin than the basin sourced primarily from surface water. Though, higher uncertainty is related with the basin with substantial groundwater interactions complicating the planning for climate change because water resources may be less predictable at these locations. While the results from this study are specific to the Santiam River Basin in Oregon, USA, the analyses and general findings regarding governing mechanisms for vulnerability to climate and land use change will be relevant to hydrosystems with similar hydrogeologic characteristics around the world.