While talking about climate change and water supplies in the future, an important component for consideration in water management is inflow projections; that is, the effect of a warmer climate on how much water might enter reservoirs each year.

Consultant Western Hydraulics discussed different scenarios during its presentation during the Oct. 10 Plan for Water (PFW) workshop.

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Through public collaboration, the PFW will develop a range of potential scenarios for NID’s Board of Directors to consider when determining the best ways to meet the community’s demand for water for the next 50 years.

NID faces the new challenge of planning for climate change impacts. The District is already seeing the impacts through increased temperatures and changes in precipitation patterns.

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Less water available for customers

One particularly important impact of climate change may be the decrease in surface water availability for irrigated agriculture and treated water customers.

Over the next 50 years, available water (inflow) on average will be reduced in both the snowmelt season (April–July) and the full year.

Simulations indicate that reduced inflows are a result of warmer temperatures, reduced snow, and drier soils, but not a result of changes in precipitation quantities. The reduction of inflows can result in increased frequency and duration of drought conditions.

Cumulative means the total inflow to NID reservoirs over time.

This graph shows the 50-year projection of cumulative inflows, which is the total amount of water entering a reservoir.

First, the black dashed line takes the average annual inflow value of 1,444 thousands of acre-feet (TAF) from the historical dataset of 1976-2021. This is used to create a cumulative trend line using this average value.

The chart projects the total inflows during wet and dry years.

• The wet (blue) has the highest cumulative total inflow after the 50-year projection
• The dry has the lowest (red)
• The median is in the middle (green).

One takeaway is that wet years are projected to have less total inflow than what we’ve experienced as a historical “average.”  

Inflow changes at Bowman Lake to occur earlier in the year

This slide shows average inflows over the 50-year projection at each day of the water year at Bowman Lake.

Notice the inflow patterns are changing with climate change. Inflows are occurring earlier in the year as compared to historical patterns.

The historical (black) line shows inflow peaks in May and June.

Inflow peaks during a dry  year (gold line) are in January and February. Inflow peaks during a wet year (blue line) are during February and March. The median (green line) is spread, with peaks in February and also during May.

Unmet Demands Scenarios

The presentation also addressed potential unmet demands of future hydrologic conditions and future demands. Of nine scenarios, all exhibit more average annual unmet demand than the baseline of historical hydrology with current demand held constant into the future.

Dry (Scenario #1) 

For the dry future hydrology with high demand, the worst 3-year consecutive drought over the period 2022-2071 was determined.

Single year unmet demands ranged from about 70,000 acre-feet (AF) to about 85,000 AF. Total unmet demand over the worst 3-year period is more than 225,000 AF.

Median (Scenario #5)

For the middle of the road future hydrology coupled with baseline demand, the worst 3-year drought was identified. Unmet demands totaled about 120,000 AF over the three-year period.

Wet (Scenario #9)

For the wet future hydrology coupled with low demand, the worst 3-year drought was identified. Unmet demands totaled about 85,000 AF over the three-year period.

These scenarios:

• Select modeling scenarios to move forward to model Strategic Alternatives
• Select Strategic Alternatives studies
• Perform Strategic Alternatives studies
• Final documentation

Going forward, the Board of Directors will look at potential strategies. Alternatives that may be studied include:

• Canal improvements to reduce losses
• Optimized operations
• Increase reservoir carryover storage
• Additional purchase contracts
• Reoperate existing facilities
  • Storage augmentation
  • Sediment removal
• Increase storage
• Demand side management