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Santiam River Basin, Oregon - Effects of landuse and climate change on water resources resiliency: implication for reservoir management

Santiam River Basin, Oregon

Effects of landuse and climate change on water resources resiliency:  implication for reservoir management

Using the hydrologic modeling output from the uncertainty analysis, this study compares hydrologic variability across the projected land use, climate, and policy changes in the Santiam Basin, the reliability, resiliency and vulnerability of reservoirs to accommodate these changes given current operating procedures, and evaluation of alternative operating scenarios in improving the ability of reservoirs to meet multiple operating objective under future change scenarios.

Objectives for reliability and resiliency analysis:

  • Chapter 1: Compare hydrologic variability across the projected land use, climate, and policy changes in the basin;
  • Chapter 2: Evaluate the reliability and resiliency of reservoirs to accommodate these changes given current operating procedures;
  • Chapter 3: Evaluate alternative operating scenarios in improving the ability of reservoirs to meet multiple operating objective under future change scenarios.

Modeling framework

This modeling-based research utilizes the results of two existing models. Results obtained from 1) GSFLOW, a coupled Groundwater and Surface-Water Flow model used to simulate groundwater/surface water flow across land surfaces and 2) PNW-ERC land use analysis of alternative future scenarios based on climate, land use and policy changes will be used to calculate future supply and demand in the basin. I will combine the results of these two existing modeling efforts with CE-QUAL-W2 and ResSim to evaluate dam operations under current and future hydrologic variability.

 

This project will focus on the Santiam Basin; both North and South Santiam present geomorphic conditions to make useful comparisons between a snowmelt, groundwater hydrological system (N. Santiam) and a rainfall driven, mixed surface-groundwater system (S. Santiam).  Detroit Reservoir and Big Cliff located in the North Santiam Subbasin, and Green Peter and Foster Reservoirs, located in the South Santiam Subbasin, will be studied to evaluate the success of dam operations in meeting multiple operating objectives.

Chapter 1 - An analysis of the resiliency of water supply and demand in the Santiam River under the influence of changing land use, climate, and policies.

To understand how the hydrology of the Santiam  River System may change in response to forcing functions such as land use, climate and policy changes, an analysis of water supply and demand will be calculated for 2020 and 2050. I will apply the simulated hydrology from GSFLOW to evaluate various characteristics of hydrograph.  A water budget will be constructed for 11 subbasins within the Santiam  (Figure 1) and effects of the three scenarios to explore a) how hydrogeology of the subbasins influences their vulnerability to changes in land use and climate and b) the relative effects of climate and land use change and conservation policies on vulnerability.

Figure 1.  Santiam SubBasins analysis points and PODs

 

Analysis of Supply and Demand

Vulnerability  =   Supply   =   Q + ST
                          Demand      CU + IS

  • Mean Stream Flow (Q): 50% & 80% Exceedance.
  • Existing storage (ST): mean stream flow change from one month to the next.
  • Consumptive uses (CU): irrigation, municipal, industrial, commercial, domestic, agricultural and others.
  • Instream uses (IS): water rights and scenic waterway flows.

Chapter 2 - Reservoir resiliency:  assessing the success of current rule curves in meeting water demand considering the influence of climate, land use, environmental demands, and policy changes on downstream hydrology.

For this section, simulated (GSFLOW) inflow hydrographs will be used as inputs for an analysis system that combines a 2D hydrodynamic and water quality simulation model (CE_QUAL-W2) with a reservoir operation model (HEC-ResSim) to evaluate when current reservoir operations and rule curves will be adequate for meeting the projected demands of water.  In addition, W2 will be used to model the temperature dynamics of the Santiam River downstream of the dams to evaluate ecological outcomes of current operations under future supply and demand scenarios.  Results from this analysis will be used to measure the performance of reservoirs (Table 1) to assess the effects of stream flows on water resources objectives (reliability) and their ability to support and accommodate (resiliency and vulnerability) changes in land use, climate and policy given current operating procedures.  

Chapter 3 - Hydrogeological and ecological resiliency:  analysis of alternative reservoir operations

Given future land use, policy, and climate change scenarios described in Chapter 1, I will apply ResSim to evaluate the effects of modifying current operating rules to evaluate whether alternate operational strategies can be more effective at meeting operational targets, thus improving system resiliency and reliability.  CE-QUAL-W2 and ResSim models will be applied with a) varying inflows representing the climate, land use, and policy scenarios from Ch. 1, and b) varying operations scenarios for each inflow.
 
The proposed changes to operating rule curves will take into account a) tradeoffs that will occur when modifications to current rule curves are made; b) modifications described by the National Marine Fisheries Service (NMFS) and the US Fish and Wildlife Service (USFWS) in the Biological Opinion (BiOp) for the Willamette Project.  Using the same model structure as outlined in Ch. 2, those policies will be implemented within W2 to evaluate the effects of the operations strategies on resiliency, reliability and vulnerability in meeting targets for downstream habitat suitability, water supply, hydropower generation, and flooding.
 
Tradeoffs to be considered:

  • Earlier spring runoff will result in higher flood risk as reservoirs are filled earlier. However, if you decide to refill later in the season to be able to maintain flood storage there will be the risk of not meeting refill requirements for late summer supply. At the same time, this late refill will affect the ability of the reservoir to meet minimum spring flow requirements for fish.
  • In dry years, adequate storage to meet summer supply and temperature targets will compete with storage requirement for recreation purposes.
  • Maintaining an adequate spill for minimum flow requirements will affect the ability of the reservoir to refill for summer supply.

Alternate scenarios and key considerations for the Willamette BiOp:

  • Rule curves created for wet, dry and normal years
  • Tradeoffs between flood risk and water scarcity
  • Adjustments during flood control season and late summer months
  • Habitat needs
  • Environmental flow targets (minimum flow levels)
  • Temperature
  • Spill passages
  • Increase hydropower generation
  • Flood control period

This material is based upon work supported by the National Science Foundation under Grant No. 0846360.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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