Stanford, California - This week California water officials performed one of their regular measurements of the state's snowpack and confirmed it is a startling 5 percent of normal - the lowest April 1 total on record. That accumulation of snow in the state's mountains - specifically the Sierra Nevada - serves as natural "frozen storage" for surface water supplies. Its runoff feeds streams, rivers and reservoirs throughout the year and provides about 30 percent of the water Californians depend on for drinking, growing food and other uses in a typical year.

The California snowpack is historically at its peak by this time of the year, but has been hindered by below-normal precipitation and record-warm temperatures. Those warmer conditions have caused more precipitation to fall as rain rather than snow, and have sped the melting of snow that has fallen.

This unprecedented low snowpack means that there will be very little water this summer to meet the needs of people, agriculture and the environment as California contends with a fourth year of drought. California Gov. Jerry Brown responded to Wednesday's snowpack results by announcing the state's first executive order mandating reduced water use statewide.

Two Stanford experts have been widely quoted on issues related to the ongoing California drought this Spring: Noah Diffenbaugh, an associate professor of Earth system science and a senior fellow at the Stanford Woods Institute for the Environment, and Leon Szeptycki, executive director of Stanford's Water in the West program and a Woods professor of the practice specializing in water law.

Stanford News Service recently interviewed Diffenbaugh and Szeptycki about why California's snowpack is in decline, and what it means for water management in the state.

What does this historic low mean for the drought and water supplies in California?

Diffenbaugh: This is the kind of extreme event that falls outside our historical experience. This is not only a low April 1 snowpack, but it is much lower than the previous record low. California's water infrastructure and management system relies on natural water storage from snowpack, which accumulates during the cool season and melts gradually into reservoirs and streams during the warm season. April 1 is typically the fulcrum between the snow accumulation season and the snowmelt season. Because we have such little snow right now, what is in the reservoirs is essentially what we will have between now and the next rainy season, which typically doesn't start until the autumn. Even in a normal summer without pre-existing drought conditions, it would be stressful to have essentially no snow supply. But we are already in severe drought conditions with record high temperatures so far this year, meaning this drought is very likely to worsen during the summer.

Szeptycki: In terms of impact on people and the environment, California's federally operated reservoir system will likely not provide any water to agricultural users for the second year in a row – an unprecedented situation. Urban water providers are expected to get 25 percent of their allocated water. Similarly, the state-operated reservoir system is currently expecting to provide 20 percent of water deliveries to the majority of its users. Snowmelt runoff plays an important role for fish and other aquatic species because it is the primary source of summertime flow for many of our rivers and streams. The lack of snowpack will mean acutely low flows for these streams, and many of them will run dry. The impacts on aquatic life and ecosystems is potentially staggering.

What do the statewide mandatory water reductions encompass?

Szeptycki: Gov. Brown just issued an executive order calling for a mandatory 25 percent cut in water use for all cities and towns. This is the first time in the state's history that this has been done. The order lays out specific actions such as replacing 50 million square feet of lawn throughout the state with drought-tolerant landscaping in partnership with local governments, giving rebates to homeowners to replace old appliances with more water- and energy-efficient models, requiring campuses, golf courses and other large landscapes to take big cuts in water use, and prohibiting new homes and developments from using potable water to irrigate unless drip systems are installed. These steps together represent a significant step forward for water conservation in the state, and we need to move toward making actions like these business as usual.

For agriculture, Brown is mandating more water use reporting to the state and better enforcement of illegal diversions and unreasonable uses of water, as well as strengthening standards for agricultural water management plans to prepare for a continuing drought.

What, if any, role is climate change playing in the drought?

Diffenbaugh: From our work and the work of others, we now have very clear evidence that global warming is increasing the risk of the conditions that are creating this drought. In particular, global warming is causing statewide warming in California, and that is increasing the odds of extremely high temperatures like what we have seen this year.

Low precipitation is not the only factor in creating drought – temperature also plays a critical role by impacting soil moisture, snowpack and demand for water. Looking at the historical record, we see that all droughts have had low precipitation, but we also see many years that have low precipitation but not drought. In fact, over the course of California's history, when low precipitation has occurred, it's been about twice as likely to result in drought if it occurred with warm conditions than if it occurred with cool conditions. And as California has warmed, we've seen big changes, including doubling of the frequency of drought years and doubling of the percentage of low precipitation years that produce drought. In addition, we have found very high statistical confidence that the increasing risk of these conditions is due to human-caused global warming.

Do recent findings reported by you and your lab (PNAS, March 2015) have implications for statewide water management?

Diffenbaugh: Our water management system, infrastructure and expectations for what is normal have all been built on an old climate. And it very clear from our research that California is in a new climate.

For short-term decisions, the hot conditions that we are experiencing are very important, because they are exacerbating the drought, and making it more difficult for the drought to break. Given the extremely low snowpack and the very high temperatures that we are seeing now, it's very hard to come up with a plausible scenario in which California won't be in a more severe drought situation at the end of the summer.

In the long term, decisions about how to manage the risks of extreme events such as this drought are going to be much more effective if they acknowledge that California is in a different climate now than it used to be. Developing a water management and infrastructure system that avoids disasters now and in the future will require an acknowledgement that California's climate has changed.

The weather forecast calls for snow this weekend – will that fix the problem?

Szeptycki: Unfortunately, it will take much more than one snowstorm to improve our current situation. Our water supply deficit built up over the past several years of drought, so it will take the state much more precipitation than we receive in an average year to make up the existing deficit. NASA estimates that it will take 11 trillion gallons of water to recover from California's continuing drought. Most of the water lost has been from groundwater pumping. So not only do we need a lot more precipitation over the long term to make up for the deficit, we need to put a good portion of that water back into the ground to store and help get us through future droughts. Otherwise we're truly in trouble.

How is Stanford responding to the drought?

Szeptycki: I'll convey what Tom Zigterman, the director of water resources and civil infrastructure at Stanford, said at a recent Water in the West panel discussion on the drought. The dry water fountains around campus may be the most visible signs of Stanford's response to the drought, but this token step is the tip of the iceberg in terms of what Stanford has done to get to this point in time. Stanford has so far gotten by in this drought in pretty good shape because it has taken measures to diversify its water supply portfolio and reduce its water use over time. Since the 1990s, water fixtures around campus have been retrofitted with water-saving devices such as dual-flush toilets. Stormwater has been recharged into the ground in dual-purpose basins and playing fields. Investment in a brand new energy plant will reduce overall campus water usage by 15 percent.

Stanford's potable water supply is from San Francisco's Hetch Hetchy system, but landscaping water is from local reservoirs and groundwater. It's important to think carefully about campus lawns and landscaping to incorporate drought-tolerant species to cut down on this water for landscaping – in fact, the governor's order contains a mandate for campuses to cut back on their landscape water use. In the long term, the campus is growing and potable demand is expected to increase over time, so more creative solutions will need to come into play to accommodate this growth while keeping under the limits of the SFPUC [San Francisco Public Utilities Commission] water allocation.

How do we cope with these drier, warmer conditions long-term?

Szeptycki: Reduced or non-existent surface water deliveries are likely to result in continued over-pumping of the state's groundwater aquifers. This is bad news for the future because we count on groundwater in dry times, and our groundwater won't last at this rate. We need to put in place systems that will help us manage water during wet periods in a way that helps us prepare for the next drought. This means managing surface and groundwater together, including, for example, setting aside some surface water during wet years to recharge aquifers we have depleted during the current drought.