After more than two years of public debate about the best ways to meet the population’s water needs in the coming decades, Plan for water The final technical memorandum was published by the Nevada Irrigation District (NID).

The memorandum describes in detail strategic alternatives and how future supply and demand scenarios can be integrated into the district’s water management practices and capital improvement projects.

The Plan for Water (PFW) process began in November 2021 and included a review of the NID’s available water supplies and the long-term impacts on diverse water needs. The public was invited to participate in a series of workshops covering 11 phases, including: the NID delivery system, water rights, watershed, risk, strategic planning, hydrology and hydrography, and local water supply and demand.

This memorandum documents the modeling results and associated evaluation presented in March 2024. The public is invited to read the report and will have an opportunity to provide comments before the NID Directors when the Board discusses the memorandum at a meeting scheduled for September 25. Please note that there is no formal public comment and response period for this document. Comments were accepted and responded to throughout the model development process.

Details of the memorandum:

The memorandum focuses on the three numerical models developed to collectively represent how the NID’s water supply system works: a hydrology model to represent the performance of watersheds, a demand model to estimate how much water is needed to meet customer and government needs, and an operations model to simulate the functions of the NID’s water storage and transportation system.

Together, the models were used to evaluate a range of alternative operating strategies and their ability to meet the future needs of NID customers.

Model development

PFW modelling (Phases 7, 8 and 9) consisted of the development of three numerical models that together represent how NID’s water supply system operates.

1. Hydrological model: A physically based hydrological model representing current and future flow conditions in NID watersheds. Flow scenarios were developed using precipitation and temperature projections as inputs. These projections were generated from three representative global climate models (extreme dry, intermediate and humid climate conditions).
2. Demand model: A demand model was configured and applied to estimate forecasted demand for a 50-year planning horizon. The demand model analyzed forecasted customer demand under three different demand scenarios (high demand, base demand, and low demand). The model used local data to account for the relative impacts of estimated future changes in land use, climate, irrigation practices, soil properties, and other factors affecting demand.
3. Development of operating models: A reservoir operating model was developed to simulate the operation of NID’s current storage, transportation and delivery system. The operating model used inflows from the hydrology model, operating rules and regulations to assess how well customer requirements were met. The operating model simulated three main scenarios:

• Dry future climate with high demands
• Medium future climate with baseline requirements
• Humid future climate with low demands

Using the models described above, the district is projected to experience an average annual unmet demand in each of the three scenarios.

Strategic alternatives

Seven strategic alternatives were examined to assess their potential to reduce the forecast unmet demand. These alternatives included:

• An extended irrigation season, expected to include two additional weeks in October
• A storage expansion of the Rollins Reservoir by 10,000 acre-feet (AF), representing an 18 percent increase in existing storage capacity.
• A storage expansion of the Rollins Reservoir by 50,000 AF, which corresponds to an increase of 91 percent of the existing storage capacity
• The addition of Centennial Reservoir, a new reservoir on the Bear River below Rollins Reservoir with a storage capacity of 96,660 AF. A reduction in overflow storage targets in NID reservoirs at the end of the irrigation season by a total of 50,000 AF, which is 38% of the current overflow storage target.
• Additional water purchases from PG&E based on the existing Coordinated Operating Agreement (COA) between NID and PG&E from 2018
• A combination of revised storage targets (50,000 AF) and additional water purchases from PG&E based on the existing 2018 COA

These seven alternatives were individually simulated by the operating model. The results of each strategic alternative simulation were compared to the baseline climate change scenario and are summarized below:

In all three climate scenarios, the alternative of increasing storage by 50,000 AF in Rollins Reservoir resulted in the largest relative increase in average annual supplies, with comparable reductions in average annual unmet demand.

The Centennial Reservoir scenario and the Revised Transmission Targets + PG&E Water Purchase scenario also resulted in relatively large increases in average annual deliveries and a reduction in projected unmet demand.

Despite the greater increase in storage capacity of Centennial Reservoir (96,000 AF) compared to Rollins Reservoir (50,000 AF), the lower location of Centennial Reservoir limits its potential benefit to the NID water supply system.

The report also finds that increasing storage capacity at Rollins has greater potential benefit because it can be used to supply water to a much larger percentage of NID customers. Revised storage targets resulted in the least beneficial increase in average annual water delivery.

Each of the strategic alternatives considered in this analysis resulted in a net increase in water deliveries to NID customers. All alternatives, except the extended irrigation season, reduced the projected unmet demand.

Next Steps: Recommendations on storage alternatives will be presented to the Board on September 25, 2024. Please note that any alternative selected by the Board for further analysis will be further investigated and evaluated as part of the Raw Water Master Planning process.

Read the full memorandum:​