Integrated ecohydrological modeling at the catchment scale

Abstract

Water resources managers increasingly face the challenge of balancing water allocation in environments of water scarcity, high food production demand, and rising pollution levels. Scientists face the challenge of providing managers with accurate predictions on the outcome of management alternatives that help to guide the decision process. The need to take a holistic approach and evaluate interrelated factors that impact the availability of water resources, such as climate, landscape processes, and surface water interaction with groundwater at the catchment scale is increasingly recognized. It is through this approach that human interaction with the environment can be properly assessed. The field of ecohydrology is an interdisciplinary science that seeks to understand the links between the physicochemical stressors with biological receptors to support policy for the sustainability of natural resources. This PhD study focuses in developing integrated ecohydrological models at the catchment scale that quantify the changes in receptors caused changes in environmental stressors as a results of management alternatives. The modeling approach involves coupling spatially distributed and physically based hydrological models to process-based ecological models. The output is a measure of ecological status as a result of a changing environment. The models include the dynamic interactions between the main components of the hydrologic cycle, with a focus on surface water and groundwater interactions, which are key drivers in aquatic ecology. Another key driver in aquatic ecology is stream temperature, which traditionally has been simulated at the local stream scale with point source thermal loads. This study has extended the previous work on stream temperature model development to include diffuse loads at the catchment scale. The research methodology was applied in two case studies, both located in agricultural catchments. One case, located in Idaho, US, deals with the issues of water scarcity, intensive agricultural practices, high stream temperatures, and fish habitat degradation, which are widespread problems in the Western US. To evaluate management alternatives for an intensively cultivated valley an integrated surface water-groundwater and stream temperature model was developed. The model was coupled to an ecological model that predicts fish growth as a function of temperature and other factors. Among the main findings of this study is that groundwater flow has a strong influence on stream temperature levels and dynamics in areas with high surface water and groundwater exchange. Moreover, the strong relationship between stream temperature and the volume and source of streamflow (snowmelt, groundwater, urban and agricultural runoff) demonstrate the value of temperature data in an integrated flow model calibration. Land use and water use changes impact both the surface water and groundwater resources and can thus substantially change stream temperature dynamics. Local scale factors such as stream vegetation and geomorphology also play an important role in determining stream temperature. Thus, a combination of restoration strategies must be evaluated to find the optimal thermal conditions. Fish optimal growth and sustainability is dependent on a specific range of temperatures, but is also affected by seasonal variability, which should be taken into account when evaluating restoration alternatives. The other case, located in an agricultural catchment west of Copenhagen, Denmark, deals with the issue of pesticide toxic effects in a stream ecology. Pesticides are among the most prevalent contaminants of surface waters worldwide. The composition and abundance of aquatic biota, such as benthic macroinvertebrates, are commonly used as indicators of water quality in the streams. The effects of toxicity have different effects on different organisms, which leads to a change in the abundance of certain species and the community structure. To evaluate these effects a general equilibrium ecosystem model was linked to a hydrologic and solute transport model of the catchment. The ecohydrological model transports pesticide loads from agricultural fields to the streams where the concentrations of the pesticides are simulated and linked to the ecosystem model. The ecosystem model includes several macroinvertebrate taxonomic groups and fish and accounts for the mechanisms that affect the stream community structure: energy availability and flow, foodweb interactions, and the toxic responses. Under polluted habitat conditions, physiological changes in an organism can affect not only its metabolic functions and the ability to optimize its energy use, but also its defense mechanisms to avoid predation. This can cause changes in the food availability and the biomass flow, which can lead to changes in the community structure. The model results show than these effects can vary in magnitude and direction under different habitat conditions. However, there is a need to better understand the physical mechanisms that impact an organism in response to stress so that these can be properly parameterized.