Backup_Appendix E_ Preliminary Needs Analysis — original pdf

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DRAFT - Water Forward 2024 Appendix E APPENDIX E: PRELIMINARY NEEDS ANALYSIS Estimating Water Supply Needs Selecting water management strategies and their targeted yields first requires an estimate of the future water supply needs. For the purposes of this analysis, water supply needs are defined as water supply shortages in each decade of the Water Forward 2024 (WF24) 100-year planning period when no additional conservation, reuse, or supply are implemented. The needs in each decade are estimated using modeling and vary based on scenarios, which reflect plausible future conditions. Only existing water management strategies are included in the estimate of future water supply needs. The modeling effort used to estimate needs is known as the Preliminary Needs Analysis. Model The Texas Commission on Environmental Quality (TCEQ) Water Availability Model (WAM) is the modeling tool used to estimate future water supply needs. The WAM is a collection of model input files covering all state-granted water rights across all river basins in Texas and a generalized computer modeling platform called the Water Rights Analysis Package (WRAP). The WAM is used by state agencies, water right holders, and stakeholders to simulate water availability for a specified water management scenario through a given hydrologic sequence. Typically the WAM is used to simulate water availability during a repeat of historical hydrology which is comprised of monthly naturalized stream flows and net evaporation, generally covering the past 60 to 80 years for the state’s river basins. The following section describes the modified WAM inputs that are used to assess water availability and needs under a range of future conditions. Scenarios The Colorado River Basin WAM is modified for the Preliminary Needs Analysis. First, the WAM is modified to reflect basin-wide demands, including Austin’s demands, in future decades of the 100-year planning period. Secondly, future water supply strategies for non-Austin entities are added in the decade within which the strategy is expected to be operational. Lastly, the WAM’s hydrological inputs are also modified to include projections of future drought and climate conditions in addition to the standard historical hydrology. The pairings of future demands, regional supplies, and hydrological conditions are known as Scenarios. The scenario approach to modeling is intended to explore the range of uncertainty in each of the scenario components across the planning period. E - 1 DRAFT - Water Forward 2024 Appendix E Demands Future demand estimates by decade for Austin and customers of the Lower Colorado River Authority (LCRA) are used in the scenario modeling. Three levels of demand are considered for Austin in each decade and are described in Appendix C. The range of Austin demands cover low, medium, and high projections. The Austin historic baseline demand projections do not include any additional conservation, reuse, or supply strategies. Those strategies are considered later in the WF24 process. Demand estimates for LCRA customers are adopted from the most recent report by the Lower Colorado Regional Water Planning Group, also known as the 2021 Region K Plan. The regional planning process covers a 50-year planning horizon. Demands beyond the Region K horizon are extrapolated from the trend over the first 50 years. LCRA customer demands were expressed in the scenarios as a single set of demands by decade and not categorized as low, medium, or high. Regional Supplies Regional supplies are major non-Austin water management strategies contained in the 2021 Region K Plan. These strategies are almost entirely those identified as LCRA strategies. They include strategies such as new off-channel reservoir construction, importation of return flows, and enhanced conservation measures. Regional supplies are characterized in the scenarios using three pathways: low, medium, and high. The low pathway assumes that the Region K strategies are implemented at a slower pace than expressed in Region K’s 50-year planning horizon. The low pathway is characterized as a full build-out of major supply strategies not occurring until 2120. The medium pathway is characterized as completing the full build-out within Region K’s planning horizon and then no further supply strategies being implemented through 2120. Finally, the high pathway includes an accelerated full build-out within Region K’s planning horizon followed by additional and similar supply strategies being added through 2120. Figure E-1 shows the sum of the yields of the regional water supply strategies for each pathway, comparing the low, medium, and high development rates. E - 2 DRAFT - Water Forward 2024 Appendix E Figure E-1. Regional Supply Pathways based on Region K Hydrology The WAM hydrologic inputs used in the modeling scenarios cover four possible future conditions: historical, and three climate warming conditions. The hydrologic inputs are described in detail in Appendix D. Historical conditions are based on the stream flow, precipitation, and evaporation records for the Colorado River Basin since 1940, also known as the period of record (POR). The historical POR contains two major drought periods of the 1950s and 2010s, with the latter being noted as the worst recorded drought to date for the Highland Lakes watershed. Retaining and repeating the historical POR inputs in some of the WF24 modeling scenarios assumes that future climate remains similar across the 100-year planning horizon. The hydrologic inputs associated with the three climate warming conditions are derived from global climate model (GMCs) outputs and include a lower, medium, and higher level of warming across the basin. The warming and impacts to hydrology progress in each decade and in each level of hydrologic inputs. The hydrological component of the modeling scenarios facilitates testing of water availability under drought conditions. WF24 does not predict or assign a likelihood to drought occurrence or severity in the future. Instead, the hydrological component explores a wide range of possible droughts to address uncertainty about future climate change impacts. This is accomplished by developing a large dataset of potential droughts for each decade. The historical POR is adjusted to reflect stream flows derived from the GCM outputs to provide an estimate of how major droughts in the historical POR might have occurred under future warming conditions. In addition to the historical and climate change-adjusted PORs, stochastic drought sequences are included in the modeling scenarios. The historical-based and climate-adjusted stochastic droughts are E - 3 DRAFT - Water Forward 2024 Appendix E created by re-sequencing the POR datasets to generate new droughts that are worse than those of the underlying POR. Lastly, the hydrology derived from the GCM outputs is included in the analysis as-is and without mapping onto the historical POR for each decade or stochastically re-sequencing. Figure E-2 shows an overview of the scenario modeling methodology. For each decade, 74 hydrologic inputs, 3 levels of Austin demands, and 3 levels of regional supply strategies are used in the WAM modeling. The combinations of scenarios across 7 decades result in 4,662 scenarios for estimating Austin’s water supply needs over the entire planning horizon. Figure E-2. Scenario Modeling Methodology E - 4 DRAFT - Water Forward 2024 Appendix E Results Within the scenarios, Austin’s water demand grows over time along with the demands of other users in the basin. The growth in demand and the associated use of stored water from Lakes Buchanan and Travis creates increasingly lower combined storage in the lakes, particularly during drought periods when run-of-river supplies are limited. In the model, Austin water supply needs occur during times of severe drought when there is insufficient Colorado River flow and low or zero storage in Lakes Buchanan and Travis. Water supply needs can also occur outside of droughts as the decades progress and Austin’s demands exceed its existing water supply contract with LCRA for 325,000 AFY. Figure E-3 shows the simulated minimum combined storage content of Lakes Buchanan and Travis in each modeling scenario across the decades. The dots on the figure are colored according to the hydrologic inputs used in the scenario. The modeling scenarios using historical POR hydrology are darker red and noted in the legend as HIST.POR. The results indicate that if future climate remains similar to the recent past and if regional supplies are developed according to the Region K Plan, then combined storage in the lakes will decrease over time during extreme drought but remain above empty. Likewise, droughts based on stochastically re- sequenced historical hydrology, noted as orange dots and HIST.STOC in the legend, produces lower simulated storage over time but again remaining above empty. However, the climate adjusted POR (green dots and ADJ.POR legend), the climate adjusted stochastic droughts (blue does and ADJ.STOC legend), the unmodified GCM-derived droughts (purple dots and GCM.FUT) show simulated lake storage falling over time to lower levels and achieving zero lake storage in some scenarios. E - 5 DRAFT - Water Forward 2024 Appendix E Figure E-3. Modeled Lake Storage Content During Drought Figure E-4 shows the scenario results for the size of Austin needs, or unmet demand, during simulated droughts across the decades. Needs are grouped into four categories in the figure: no needs, needs up to 100,000 acre-feet in the worst 12-month period of drought, needs up to 200,000 acre-feet, and needs up to 300,000 acre-feet. There are 666 scenarios results shown for each decade within Figures E-3 and E-4. The 2120 blue bar in Figure 4, for example, is sized to 39% which indicates that 260 of the modeling scenarios have no unmet demand. E - 6 DRAFT - Water Forward 2024 Appendix E Figure E-4. Percent of Modeling Scenarios with Unmet Austin Demand During Drought The results of this scenario modeling for the Preliminary Needs Analysis show increases over time in future water supply needs due to extreme drought with climate change impacts. Hotter and drier climate change impacts result in lower levels of run-of-river supplies as well as lower inflows that replenish storage in the upstream reservoirs. In parallel with increasing climate change impacts over time, other LCRA water supply customers have increasing demands which also exert pressure upon the same shared surface water resources of the basin. These results show that additional conservation, reuse, and supply strategies are required to meet the range of possible future water supply needs for Austin. E - 7