On DWR’s water supply models and drought risks

The Delta Independent Science Board has just released their draft Water Supply Reliability Review. We strongly agree with their recommendation on water supply system models:

The next generation of state-sponsored water supply system models for reliability estimation should be built, updated, and evaluated by a broad consortium of state and federal agencies and external experts in order to better incorporate regional management needs and apply the best feasible science.

Below is one experience we had with recommendations for improving DWR’s modeling of drought risks.

In 2012, the California Department of Water Resources produced a draft climate adaptation strategy which focused on flooding, and asked for comments. California Water Research submitted comments entitled, “Incorporating Drought Risk From Climate Change Into California Water Planning.”

We were never contacted about our recommendations, but it has since become clear that they could have been useful in DWR’s water planning.

Our report documented the increase in severe droughts around the world and in the United States, and commented:

There are now enough studies of impacts of climate change on frequency of droughts in California for the Department of Water Resources to include potential increases in frequency of dry and critically dry years in planning and modeling. An assessment of possible increase in drought risk is essential for reliability planning for the State Water Project and the urban water agencies which contract with the State Water Project.

We also commented on DWR’s method of mapping hydrologic model outputs onto the 82 year historic sequence of water years, stating

The problem … is that it loses information from the global climate models that would change the frequency and severity of droughts, as well underestimating changes in annual and seasonal runoff. This can seriously underestimate impacts of climate change on both river flows and water supply.

And noted:

Global Climate Models contain information about persistence in increases sea surface temperatures, which can affect the ENSO and other circulation patterns, and influence the structure and persistence of droughts. Mapping output from global climate models onto the historic record of water years will inevitably lose much of this information.

This is just one experience, but based on it, more engagement with external experts would be beneficial for the Department of Water Resources in building, developing, and evaluating the next generation of water supply models.