Attachment C TECHNICAL MEMORANDUM TO: Rob Morrow, WSC CC: Joshua Haggmark, Dakota Corey and Catherine Taylor, Santa Barbara PREPARED BY: Warren Greco and Emily Honn, Woodard & Curran REVIEWED BY: Brian Van Lienden, Woodard & Curran DATE: October 30, 2020 RE: Santa Barbara Surface Water Supply Analysis 1. INTRODUCTION The purpose of this technical memorandum is to focus on the City of Santa Barbara’s (City) surface water supplies from Gibraltar Reservoir and Lake Cachuma and to identify their performance against future constraints, risks, and uncertainties, including climate change. Hydrologic analyses were performed using the RiverWare Model of the Santa Ynez River (RiverWare Model) to determine impacts to water supply to the City under a series of scenarios. The City’s surface water supplies are summarized on an average annual basis and for a critical drought period. This analysis includes three operational scenarios for Lake Cachuma and Gibraltar Reservoir, described below: Scenario 1 – Existing Conditions: This scenario simulates Lake Cachuma and Gibraltar Reservoir under the existing operational scenarios and storage capacities. Scenario 2 – Pass Through Operations: This scenario simulates Gibraltar Reservoir under future operations based on the implementation of Pass Through Operations, and the existing storage capacities. Scenario 3 – Gibraltar Sedimentation: This scenario simulates Gibraltar Reservoir under future operations based on the implementation of Pass Through Operations and future storage capacity based on continued sedimentation of Gibraltar Reservoir. This analysis also includes the following two additional scenarios of future hydrology under climate change. Both climate change scenarios are applied to Scenario 2 and Scenario 3. The two climate change scenarios, both considered equally likely to occur for the purposes of this analysis, are described below: 2050 Central Tendency: This scenario simulates modified projections of streamflow, precipitation, and evaporation based on projected climate change impacts for the year 2050 that approximate the median projections from an ensemble of climate models and emissions scenarios. 2070 Hot and Dry: This scenario simulates modified projections of streamflow, precipitation, and evaporation based on climate change impacts for the year 2070 based on a single climate model selected to represent drier and hotter conditions as compared to the median climate projections. 2. MODEL DESCRIPTION Hydrologic modeling of the Santa Ynez River was conducted using the RiverWare Model. The model simulates the operations of Jameson, Gibraltar, and Cachuma reservoirs, and includes operational logic for fishery releases, City of Santa Barbara 1 Woodard & Curran, Inc. Santa Barbara Surface Water Supply Analysis October 2020 Attachment C contractual deliveries to the Cachuma Member Units (City of Santa Barbara, Goleta Water District, Montecito Water District, and Carpinteria Valley Water District), and water rights releases to downstream users in accordance with State Water Resources Control Board (SWRCB) Water Rights Order 89-18. The model also simulates coordinated operations of the City’s water supplies in Gibraltar Reservoir and Lake Cachuma under the 1989 Upper Santa Ynez River Operations Agreement (USYROA), also known as the Pass Through Agreement. The City’s RiverWare Model runs on a daily time step, with a simulation period from 1942 through 2017 (76 years) on a water year (WY) basis (October to September). The RiverWare Model does not currently include carryover water from the previous water year held in Lake Cachuma by the Cachuma Member Units or State Water Project water delivered by the Central Coast Water Authority to Lake Cachuma for delivery to the Member Units. 3. OPERATIONAL SCENARIOS 3.1 Gibraltar Reservoir Pass Through Operations The USYROA is a settlement agreement developed in response to the continual sedimentation of Gibraltar Reservoir. The City agreed not to move forward with plans to expand Gibraltar Reservoir in exchange for an operations agreement that uses Lake Cachuma facilities to store water to replace some of the Gibraltar yield that has been lost due to sedimentation, while addressing the effects of these operations on the Cachuma Project and other downstream interests. The USYROA describes two modes of operation: 1) a “Mitigation Mode” whereby the City relinquishes a defined portion of its Cachuma Project entitlement in response to increased diversions at Gibraltar Reservoir through Mission Tunnel, and 2) a “Pass Through Operations” mode whereby the City will receive a portion of its Gibraltar entitlements through the Cachuma Project. Under “Mitigation Mode” of Pass Through Operations, the USYROA provides for operations under which the City could continue to approximate historical levels of Gibraltar diversions through Mission Tunnel. When diversions through Mission Tunnel increase beyond 4,550 acre-feet per year (AFY), the City is required to relinquish the Cachuma Project contractual entitlement to mitigate the water impacts to the Cachuma Project. As the sedimentation of Gibraltar Reservoir has continued, the ability for the City to approximate the historical levels of Gibraltar diversion has become impractical, and the City has historically diverted an amount that does not result in a mitigation requirement from the City’s Cachuma Project entitlement. Under Pass Through Operations1, the City receives a portion of its Gibraltar entitlements through a Pass Through Account (PTA) created in Cachuma Reservoir. The PTA accrues credits when the actual amount of water spilled at Gibraltar Dam exceeds the amount of water that would have spilled under historical operations, referred to as the 1988 Base Operations (based on a capacity of 8,567 acre-feet and at surface elevation 1,400 feet). The USYROA includes accounting for conveyance loss, and evaporation and spills in Lake Cachuma, with the PTA being the first water to spill from Lake Cachuma. The USYROA also includes relinquishments of Cachuma Project contractual entitlement to ensure that the City does not receive water in excess of what would have been diverted under the 1988 Base Operations. The Cachuma Member Units approved the USYROA, and the City has been working toward securing a Warren Act contract with United States Bureau of Reclamation (USBR) for the creation of a new storage account in Lake Cachuma to implement Pass Through Operations in Lake Cachuma under the USYROA. In 2013, Stetson Engineers prepared Hydrologic Analysis of the Pass Through Operations at Gibraltar Reservoir (2013 Pass Through Report) based on 1 The USYROA includes two methods of Pass Through Operations. This analysis is based on “Method A.” Implementing “Method B” is not currently considered a viable option for the City. City of Santa Barbara 2 Woodard & Curran, Inc. Santa Barbara Surface Water Supply Analysis October 2020 Attachment C hydrologic modeling completed using the RiverWare Model of the Santa Ynez River. This analysis was used to support a draft Environmental Assessment (EA) in 2016. As of 2020, no Warren Act contract has been put in place. Scenario 1 is based on the current operations of Gibraltar Reservoir under Mitigation Mode, while Scenario 2 and Scenario 3 are based on the future operations under Pass Through Operations. The operational scenarios for Gibraltar Reservoir are summarized in Table 3-1. One constant is a planned maximum diversion of 4,550 acre-feet per year from Gibraltar Reservoir through Mission Tunnel. This figure is based on the amount of water that the City is entitled to divert without relinquishing a portion of its Cachuma Project entitlement under Mitigation Mode from the USYROA. Table 3-1. Gibraltar Reservoir Operational Scenarios Gibraltar Reservoir Maximum Mission Scenario Operating Mode Tunnel Diversions (AFY) Scenario 1 Mitigation Mode 4,550 Scenario 2 Pass Through Operations 4,550 Scenario 3 Pass Through Operations 4,550 3.2 Reservoir Capacity The capacity of Lake Cachuma in all scenarios is based on the most recent bathymetric survey from 2013, with a storage capacity of 184,121 acre-feet at surface elevation 750 feet. The capacity of Gibraltar Reservoir for each scenario is shown in Table 3-2. The capacity of Gibraltar Reservoir for Scenario 1 and Scenario 2 is the recent capacity of 4,583 acre-feet of storage at surface elevation 1,400 feet based on the 2019 bathymetric survey. For Scenario 3, the storage capacity in Gibraltar Reservoir is based on a Substantial Sedimentation scenario developed as part of the 2013 Pass Through Report. This scenario has an assumed capacity of 2,000 acre-feet at surface elevation 750 feet based on a hypothetical scenario of continued sedimentation. This scenario is consistent with historical sedimentation trends, which are dependent on several factors, including future wildfires. Under all scenarios, there is an assumed minimum pool in Gibraltar of 500 acre-feet. Table 3-2. Gibraltar Reservoir Capacity Scenarios Gibraltar Reservoir Gibraltar Reservoir Scenario Storage Capacity (AF) Minimum Pool (AF) Scenario 1 4,583 500 Scenario 2 4,583 500 Scenario 3 2,000 500 3.3 Cachuma Project Operational Yield All scenarios assume that the 1996 Master Contract between the Santa Barbara County Water Agency and Reclamation remain in place, limiting the full operational yield of the Cachuma Project to 25,714 acre-feet per year. The City’s share of the annual yield is 32.19 percent or 8,277 acre-feet per year in normal years. 3.4 State Water Resources Control Board Water Rights Order All scenarios include the future implementation of the SWRCB Water Rights Order (WR 2019-0148), issued in September 2019, for the Cachuma Project for water rights permits held by the USBR. The SWRCB Water Rights Order provides for additional fish flows in Wet and Above Normal water years, based on the Santa Ynez River hydrological classification, shown in Table 3-3. Until the cumulative inflow threshold of 33,707 acre-feet of inflow is reached within a water year the operating criteria for fish water releases is the same as under the 2000 National Marine Fisheries Service Biological Opinion (2000 Biological Opinion). City of Santa Barbara 3 Woodard & Curran, Inc.