Scoping proposal focused on quantifying temperature change to a water body influenced by subsurface return flow
This scoping proposal is designed to evaluate the potential for developing a model that can help answer the following question:
* Can changes in river temperature be quantified, in degrees and thermal units, as a result of subsurface water returns?
The tool will quantify the potential temperature change in the hyporheic zone and its influence on the thermal regime of a stream to illustrate the effects of current and changing irrigation practices. The focus of this effort will be to investigate tools to simulate thermal surface and groundwater interaction. Models are used in understanding and predicting system responses to altered operations: total maximum daily loads (TMDLs), climate change, irrigation practices, and water delivery management. Therefore, this scoping proposal is intended to identify a suite of complementary modeling tools that can be enhanced to evaluate how changes in subsurface flow conditions, in response to changes in surface water practices can influence the thermal properties of a river.
Need and Benefit
Developing advanced surface water and groundwater management tools is paramount to Reclamation's ability to reliably deliver water and generate power as effectively as possible. Reclamation is responsible for understanding the impacts of its project operations and resulting water quality to the surrounding system in meeting standards. Quantitative models are powerful tools capable of simulating a system response under various water management or operation scenarios. Typically, ground water and surface water processes are viewed and managed separately. In many situations, surface water and groundwater processes comprise a single system that must be understood and managed together if the resources are to be used effectively.
Groundwater hydrologists use AEM models and FD models to answer hydrologic questions about changes in land use practices. Both models solve the general equation for groundwater flow and can be used to simulate the interactions between features such as streams and wells; however, AEM model solve the equation analytically and FD models solve the equation numerically. A numerical solution is based on the definition of a grid which can limit the resolution of the model solution.
On the other hand, an analytic solution is only limited by the distance of one element to another and the resolution of the input data. FD and AEM models can quantify the interaction between surface and groundwater; however, models that incorporate the specific behavior of a stream are necessary to fully evaluate water quality. Steady state and fully hydrodynamic models are available to describe surface water conditions and evaluate physical conditions under different hydrologic regimes. Some of the available models also incorporate water quality constituents that are processed accordingly as they are routed downstream. Reclamation is faced with satisfying temperature requirements below projects while at the same time meeting water delivery obligations. The ability to operate models that can simulate groundwater surface water interactions with models that can incorporate water quality constituents will allow Reclamation to analyze various water delivery management scenarios by analyzing the potential thermal regime change to the stream bed.
This proposal explores the opportunity to identify a complementary suite, with minimal enhancements, of models that can quantify the net temperature change of the subsurface flow to a stream that can be realized under a variety of surface water management scenarios or as a result of constructing a pathway for irrigation returns, via groundwater, to reach a river. This type of analysis would be valuable when demonstrating temperature compliance with a proposed mitigation measure. Additionally, this proposal will investigate the data needs and requirements for model population.
A specific case study in identifying and testing modeling tools is the Luther Wetland Project. This project was developed with a grant, awarded in 2001 from the Oregon Watershed Enhancement Board with work completed in 2003. The main objective of this project was to reduce the following constituents from irrigation drain returns via constructed wetland cells: sediment, ortho-phosphorus and nitrates. A secondary objective was to improve wildlife habitat. However, temperature reduction, via subsurface flow, to the nearby Malheur River has not been investigated. Instrumentation between the various cells of the wetland exist, including temperature measurement, although, no data are collected to identify what returning subsurface water temperatures are to the river. By investigating the thermal benefit of returning irrigation drains via wetlands or subsurface pathways, and in turn quantifying the benefit, Reclamation can demonstrate compliance and effects to river reaches while complying with stipulated water quality standards and effectively satisfying its water delivery obligations.
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