Modeling Climate Change Impacts

We have utilized the most recent downscaled climate change products for the Tahoe basin to: (1) provide a set of climatic conditions for the Tahoe basin that capture a range of extreme events likely to occur in the next one hundred years; (2) produce a time series of hydrological conditions for the basin, including a set of statistical functions that can be used to improve the planning of Environmental Improvement Plan (EIP) projects; (3) use the hydrologic data to produce revised estimates of the constituent loads of nutrients and sediment that will be delivered to the lake; and (4) describe the impacts of the changed climate and hydrology on lake water quality and quantity, primarily due to changes in the lake stratification and external nutrient loads.

The basic building blocks used in this project include the outputs of four downscaled General Circulation Models (GCMs) and the PRISM hydrologic model, provided by the Southwest Climate Center; the Tahoe Watershed hydrologic model LSPC that had been previously calibrated for the Tahoe basin as part of the TMDL development; and the one-dimensional Lake Clarity Model that had also been developed as part of the TMDL. That latter model was only used to compare changes in stratification and lake water level and not for evaluation of future clarity changes.

The four GCMs used were CanESM2, CNRM-CM5, HadGEM2-ES, and MIROC5. in a suite of 20 candidates, these models have been found to be the best in terms of their ability to model historic precipitation for the period 1980-2005 at a global scale. For each model two Representative Concentration Pathways (RCPs) were used to denote universally established conventions for future carbon concentrations.  At the time of writing, the RCP 4.5 pathway seemed somewhat optimistic and may be considered a fanciful best case. The RCP 8.5 pathway represented a higher level of greenhouse gases in the atmosphere. The use of these two pathways in this study should not be interpreted as being a statement of what we believe future conditions will be.