Managing Water Quality in the Face of Uncertainty: A Case Study of the Effects of Future Land Use and Climate Change on Stormwater Management in the Los Angeles Region
In accordance with the 2014 Los Angeles County MS4 Permit, several regional municipalities have combined efforts to develop Enhanced Watershed Management Programs (EWMPs) that include proposed management strategies and control measures. A key requirement of the EWMPs is that they demonstrate reasonable assurance of attaining water quality objectives in receiving waters. Each EWMP contains detailed recipes for compliance and associated milestone dates by which different water quality control measures must be put into practice in order to meet water quality compliance objectives. Although the EWMP models included a margin of safety for required management strategies, projected impacts of land use change and climate change were not explicitly represented.
The City of Los Angeles Bureau of Sanitation (LASAN) is the lead agency for the Upper Los Angeles River EWMP. LASAN and Paradigm Environmental supported a RAND study to explore the impacts of regional land use change and climate change on the proposed management strategies. Tujunga Wash, in the Upper Los Angeles EWMP, was selected for this study because it houses several climate-sensitive facilities operated by different agencies and is representative of regional orographic variability (rainfall differences due to elevation changes). Some goals of this study were to: (1) test how robust proposed strategies will be in the face of future land use and climate change uncertainty, (2) identify possible implications of different combinations of land use and climate change uncertainty, and (3) highlight practical adaptive management approaches that can boost resilience in the face of projected future uncertainties.
This research builds upon extensive regional watershed and BMP modeling efforts. The research team performed spatiotemporal analyses to derive a 5 × 47 matrix of projected future land use and climate change scenarios, which was then projected upon the baseline EWMP model. The baseline condition explicitly represented the network of cost-optimized BMP treatment trains for the final EWMP compliance milestone. Each of the five future land use projections represented varying degrees of stormwater management for newly developed areas. The ensemble of forty-seven climate change scenarios represent regionally-downscaled General Circulation Models (GCM) projections, ranging from “business-as-usual” to “optimistic” carbon-controlled climate futures. LID implementation had synergistic effects on Tujunga EWMP performance among the various climate futures. Results also varied among the ensemble of climate futures in terms of the percent of futures attaining or exceeding the baseline-EWMP Zn critical condition. This presentation describes the methodology and findings from this study, and highlights key insights gained for increasing the resilience of water quality management in Los Angeles in the face of land use and climate change uncertainty.