Striking a balance between water for food, energy, and the environment: a quantitative framework to guide sustainable water management for a changing future

Humans and hydrology are strongly interconnected. Growing populations and improving technologies have led to a rising demand for fresh water, increasing the difficulty of finding a sustainable balance between competing water needs in a changing world. A major challenge to achieving this balance is the complexity of the interactions between two changing systems. While current research has provided insights into either the impacts humans have on hydrology or, conversely, hydrological variability on human society, the dynamic nature of these interactions is still not well understood. This dissertation develops and applies a framework for considering and analyzing the human and hydrologic systems as a linked, co-evolving system, utilizing insights developed from past interactions to guide exploration of future ones. The aim of this work is to further our understanding of these dynamic interactions between the human and hydrologic systems using a novel combination of modeling tools to investigate possible future behavior for patterns that reveal emergent properties. This dissertation is presented around two main themes. The two studies of the first theme examine historical hydrologic data for patterns that provide insight into the underlying processes controlling catchment behavior. The first study comparatively analyzes many catchments to determine the dominant processes controlling the flow duration curve, while the second utilizes insights from the first to guide analysis of the historical co-evolution of two intensively-managed case study catchments to better understand human-hydrologic system interactions. Results from the first study suggested that the influence of catchment properties was strongest in the low flow tails of the flow duration curve, but also that in many places, including the case study region, the agricultural Midwestern US, humans were the dominant process. Building on this, the second study comparatively analyzed the two catchments as linked, co-evolving human-hydrologic systems and found their current hydrologic responses to recent climate reflect their unique histories and are highly heterogeneous due to differences in their geologic pasts and human modifications such as artificial drainage and reservoir operation. These differences manifest temporally, from annual to daily time scales, and spatially, both within and between the watersheds, suggesting a complex, nonlinear response to other, future external driving forces. The two studies of the second theme develop and then apply an integrated modeling framework to better address the dynamic interactions between the human and hydrologic systems. This framework employs a System of Systems optimization model to emulate human development decisions which are then incorporated into a watershed model to estimate the resulting hydrologic impacts. The two models are run interactively to simulate the co-evolution of coupled human-nature systems, such that reciprocal feedbacks between hydrologic processes and human decisions (i.e., human impacts on critical low flows and hydrologic impacts on human decisions on land and water use) can be assessed. The framework is applied to the two case study watersheds, in the context of proposed biofuels development. This operation is illustrated by projecting two possible future co-evolution trajectories, both of which use dedicated biofuel crops to reduce annual watershed nitrate export while meeting ethanol production targets. Imposition of a primary external driver (biofuel mandate) combined with different secondary drivers (water quality targets) results in highly nonlinear and multi-scale responses of both the human and hydrologic systems, including multiple tradeoffs, impacting the future co-evolution of the system in complex, heterogeneous ways. In one watershed, the strength of the hydrologic response is sensitive to the magnitude of the secondary driver; 45\% nitrate reduction target leads to noticeable impacts at the outlet, while a 30\% reduction leads to dominant impacts that are mainly local. The local responses are conditioned by previous human hydrologic modifications and their spatial relationship to the new biofuel development. This sensitivity is not as evident in the second watershed, where past human modifications to hydrology serve to both increase the importance of outlet flow and to partially mitigate some of the negative impacts of the external drivers. This emphasizes the importance of past co-evolutionary history in predicting future trajectories of change. The work presented here provides some insights into the dynamic interactions that emerge from external driving forces (both climate and policy) propagating through connected human and hydrologic systems, dependent on local geology and landscape properties and unique histories of anthropogenic modifications. This framework is a first step to a more integrated, dynamic approach to the study of coupled human and natural systems under change