Implications of climate change for the water supply of the Chisos Mountains developed area: Big Bend National Park technical assistance request 4945

Lawrence, David; Runyon, Amber

Big Bend National Park (BIBE) needs to replace or repair a 1950s drinking water system in the Chisos Mountain Basin (estimated cost, $8-10M). In addition to the issue of aging infrastructure, the current water source for the system (Oak Spring) has experienced intermittent reductions in flow, prompting the park to also consider developing an alternative water source, rather than investing in Oak Spring improvements. Prior studies indicate discharge from Oak Spring reflects contemporary climate conditions. To help evaluate the long-term reliability of Oak Spring as a water source, we examined this potential climate connection and also assessed projected future climate conditions at BIBE to provide supporting information for this water development decision. Our analysis of ten years of data suggests flows from Oak Spring are sensitive to contemporary changes in precipitation. Oak Spring flows correlate with monthly precipitation falling in the local area, with a two month lag time, where precipitation explained 58% of the variation in spring flow. Although there is significant unaccounted for variation in this relationship, and thus error, we used the correlation between Oak Spring flow records and local precipitation to develop a statistical model relating precipitation to flow. This model was then paired with precipitation projections from two climate change scenarios – a “Warm Wet” and “Hot Dry” scenario derived from 20 global climate models – to explore how discharge of Oak Spring may change through a time period centered on 2065 (2050-2080). We recognize the limitations of discharge projections, given unaccounted for variation in the statistical model, but considered projections (with appropriate caveats) useful to the management question at hand. The premise of this approach is that, given contemporary climate change, historical conditions may not be a reliable indicator of future Oak Spring discharge. Global climate models are quantitative representations of our understanding of how the earth responds to increasing greenhouse gas concentrations in the atmosphere and provide a tool to evaluate how key parameters such as temperature and precipitation may change in the future. The scenarios we developed are intended to bracket the range of potential futures for BIBE, and in turn Oak Spring as a water source. The Warm Wet scenario represents the largest projected increase in annual precipitation and the smallest increase in temperature of any of our global climate models; a “best case scenario”. The Warm Wet scenario also assumes humans use a range of strategies and technologies to reduce carbon emissions and ultimately stabilize emissions through time. The Hot Dry scenario represents the largest projected decrease in annual precipitation and the largest increase in temperature from our model set; a “worst case” scenario. This scenario assumes business as usual, with emissions of greenhouse gases increasing through time. Under the (best case) Warm Wet scenario, precipitation increases and so does interannual precipitation variability. This increased variability results in flows at Oak Spring that fall below 20 gallons per minute (gpm) in the 2060s at about the same number of months per decade as that observed during the 1950-2000 historical period. This threshold is directly relevant to the park’s decision, as flows below 20 gpm are generally inadequate to support BIBE operations in the Chisos developed area, and may invoke a drought conservation plan. Warm Wet projections through time (2020-2100) indicate there will be decades where flows fall below this threshold more often than the past, and others less often. Under the Hot Dry scenario, the number of months per decade in which Oak Spring flows fall below 20 gpm in the 2060s is more than double the historical average. If this worst case scenario is realized it may substantially challenge the reliability of Oak Spring as a water supply. Our findings are....

Implications of climate change for the water supply of the Chisos Mountains developed area: Big Bend National Park technical assistance request 4945

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