Exploring frameworks for simulating historical and future hydrology in the Upper Deschutes River basin
What approach to quantifying/simulating historical streamflow will best reproduce historical unregulated conditions and will be best suited for considering climate non-stationarity in the Deschutes River basin, where groundwater and surface water interactions are a significant driver of streamflow and, where common approaches have resulted in unrealistic streamflow volume and/or timing or have required overly intensive modeling efforts?
In answering these Research Questions, we aim to identify a suitable approach for estimating unregulated streamflow in the Deschutes River basin for both historical and projected future conditions that does not require a computationally intensive coupled groundwater-surface water model. We also seek better understanding of uncertainty in those estimates. A suitable approach would consist of one in which resulting estimated streamflow is similar in volume and timing to reconstructed unregulated streamflow datasets that may already exist. The Deschutes River flow can be characterized as dominated by groundwater discharge (Gannett et al. 2017). We hypothesize that an approach similar to that taken in the St Mary and Milk River Basin Study Update (ongoing) could be effective, which includes a precipitation-runoff model with routing module that represents both a storage term that simulates the quick component of the runoff, as well as a lower storage term which describes the slow component of the runoff and the groundwater extraction and losses. Previous application of this approach in the St Mary and Milk River Basin Study Update applies a variable drainage area concept that brings further definition to the groundwater contribution to streamflow. However, exploration of model configuration specific to the Deschutes River basin would be needed to further identify applicability.
Need and Benefit
Numerous studies evaluating hydrology of the Deschutes River basin have been completed to date that have resulted in less than useful hydrologic datasets and thereby necessitate exploration of an alternative approach. Despite development of a coupled groundwater-surface water model called GSFLOW, Gannett et al. (2017) found that differences in simulated historical streamflow at Deschutes River near Madras were about 4.6 percent, while differences at Tumalo Creek below Tumalo Feed Canal were 47.2 percent and 167.7 percent at Crescent Creek at Crescent Lake, over a period 1985 – 2008. Further, the Upper Deschutes River Basin Study (Reclamation 2018), which relied on over 160 historical and future streamflow scenarios from the River Management Joint Operating Committee Phase II (RMJOC-II) effort, concluded climate change effects on water supply and instream flows can be more effectively evaluated when refinements to basinspecific hydrologic modeling become available. In the case of the RMJOC-II effort, despite broad federal and university partnership in developing a hydrologic dataset, the physically-based hydrologic models and traditional techniques to remove systematic model biases did not yield useful results in the Deschutes River basin.
Since traditional and computationally intensive methods have failed to develop reliable historical and projected future unregulated flows, water managers in the basin do not have the tools they need to identify vulnerabilities to climate change, nor do they have the baseline information to evaluate strategies to adapt to changing conditions. Without reliable information, water managers may have to continue pursuing qualitative assessment of climate impacts, as was done in the Upper Deschutes River Basin Study. Alternatively, without better information, water managers may need to develop overly conservative adaptation strategies. Both of these outcomes result in lower stakeholder confidence in future proposed structural or operational changes. Once available, this information will be immediately incorporated in ongoing studies and will be used to evaluate potential vulnerabilities in the system for better water management. It is imperative that a new method for developing future climate adjusted unregulated flows be developed prior to newer investigations such as the "Crooked River Water Quality and Supply Reliability Pumping Plant Appraisal Study". The study involves moving the existing North Unit Irrigation District Pumping Plant (NUID) pumping plant, which is currently on the lower Crooked River, to below the confluence with the Upper Deschutes River. This study is commencing during fiscal year 2022 and climate adjusted hydrology data will be needed for that effort.
New reliable hydroclimate information for the Deschutes River basin, that would ultimately result from this scoping work, could have a meaningful impact on the understanding of historical and future streamflow, particularly as we anticipate a continued shift toward decreased snowpack, increased rainfall, and increased streamflow variability (floods and droughts) in the Pacific Northwest. The information generated from this study will be immediately used to analyze already existing Deschutes climate change information, such as that developed through the RMJOC-II effort. Improved understanding of historical and future conditions may also improve Reclamation's ability to meet its mission to provide reliable water supplies for irrigation, municipal, and ecological needs in the Deschutes River basin.
One immediate benefit, assuming a suitable approach will be identified through this scoping project and a following Science and Technology (S&T) conducting project will further develop the approach and apply it in the Deschutes River basin, could be to inform future Bipartisan Infrastructure Law (BIL) studies that will come over the next five years.
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