A climate stress test of Los Angeles’ water quality plans

Abstract

Climate change can significantly affect water quality, in addition contributing non-stationarity and deep uncertainty that complicates water quality management. But most of the total maximum daily load (TMDL) implementation plans crafted to meet water quality standards in the USA are developed assuming stationary climate and at best a small number of land use futures, although neither assumption seems reliably justified. To address this challenge, this study employs robust decision making (RDM) methods, commonly used to help develop water supply plans, to stress test the proposed Enhanced Watershed Management Plan (EWMP), a TMDL implementation plan, for the Tujunga Wash, the largest subwatershed of the Los Angeles River, over a wide range of climate and land use futures. We find that climate change could significantly reduce the ability of the Tujunga EWMP to meet water quality goals; however, meeting the city's goals for increasing permeable land cover offsets the risk of non-compliance in the face of climate change uncertainties. This study also introduces innovations in RDM analyses, including: treatment of the deeply uncertain incidence of extreme precipitation events, an explicit link between RDM scenario discovery methods and the specification of signposts for adaptive policy pathways, and the use of (imprecise) probabilistic climate projections to inform the choice among robust adaptive policy pathways. The paper also contributes to a larger debate over how to address climate and other uncertainties in regulatory processes involving water quality.

How to evaluate a monitoring system for adaptive policies: criteria for signposts selection and their model-based evaluation

“…In Herman and Giuliani (2018), signposts and triggers are identified jointly using optimisation. Lempert and Groves (2010) and Tariq et al (2017) use signposts based on expert judgement. Ceres et al (2017) investigate the level of confidence provided by the 100-year peak storm surge, used as signpost, in detecting climatic change.…”

Building the Resilience of WSS Utilities to Climate Change and Other Threats

“…If resources were available, analysts could invest in exploring plausible future climates in depth (see appendix A for a deep dive into specific methods). Tariq et al (2017) provides an example of when and how one could consider more extensive analysis of climate information.…”

“…The authors use "(imprecise) probabilistic climate projections to inform the choice among robust adaptive policy pathways" (Tariq et al 2017). The study uses alternative sets of climate information to provide a bounding set of probability distributions for the future 90th percentile 24-hour rainfall, which is used to quantify the trade-offs between inaction and two alternative adaptive plans (admitting all caveats of the analysis).…”

Authors response #1

“…At the present time the Tampa Bay water supply system includes 90 MGD groundwater pumping permitted for the CWF, a 25 MGD desalination plant and permitted water withdrawals from the Hillsborough and Alafia rivers that vary daily to maintain ecologically protective instream flows. Scenario discovery analysis (Tariq et al, 2017) was used to explore the ability of Tampa Bay Water to meet 2045 water demand while maintaining or improving existing levels of compliance with surface and groundwater regulations. Figure 6 presents the results of the scenario discovery analyses that evaluates which climate and water use scenarios achieve these objectives in future 1 (2030-2060) using the Hargreaves ET 0 method.…”