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Study: Urbanization To Boost Water Demand

A projection of land-use change in California through 2062 finds that urbanization will be the primary cause of greater water demand in the state. Tamara Wilson of the U.S. Geological Survey, lead author of the study, explains her findings.

Written by Matt Weiser Published on Read time Approx. 7 minutes
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A luxury apartment complex is under construction in downtown Mountain View, Calif. A new study from the U.S. Geologic Survey finds that urban growth will drive up water demand in coming decades.Tara Lohan

Think the current California drought has been tough? It will get worse in the future, because the state is always growing. But quantifying that effect has largely been guesswork. Until now.

A new study in Environmental Research Letters by scientists from the U.S. Geological Survey (USGS) and the Nature Conservancy estimates how water demand will change in 2062 by projecting the present rate of land-use change. This includes urban growth and farmland conversion – both conversion of farms into urban areas and conversion of annual crops to permanent crops, like orchards and vineyards.

The results? California is likely to see a 4 percent overall increase in water demand in 2062 compared to 2012. That doesn’t sound like much. But that is nearly 1.5 times the capacity of Folsom Reservoir near Sacramento, and enough water to serve 3 million homes for a year.

And remember that 2012 was before the recent 25 percent cut in water use was ordered because of the drought. Compared with today, the projected growth in water demand is much greater than what Californians are using now.

Note, also, that the study addresses a single scenario based on a relatively normal water year that merely estimates agricultural water consumption and does not include the effects of climate change. Because of these factors, the authors concede that future water demand will probably be greater than their estimate.

The authors also conclude that California’s statewide 25 percent conservation rules should be maintained as a permanent requirement if the state wants to hold water consumption at 2012 levels. But that’s not what state officials have done. Last week, the State Water Resources Control Board ended the 25 percent requirement, and will now allow local water agencies to set their own conservation rules.

To understand the study better, Water Deeply interviewed Tamara Wilson, lead author and a research geographer with the USGS in Menlo Park.

Water Deeply: Your study notes it is the first of its kind. What do you mean by that?

Tamara Wilson: It’s the first to look at pure land use and projections of water use together. So it’s not a hydrological model. It’s not a future projection of climate. Nobody has approached this issue from this kind of an angle before. Land-use modeling is an interesting thing. It’s only really been taking off in the last decade or so. Land use is a pervasive factor in all of these examinations of what the future might look like, and it’s one that’s gaining traction as people see its value and importance.

In California, in drought years, groundwater pumping can go from a third to almost a half of all water use. That has a myriad of trickle-down effects. Natural systems, I think, are what are often overlooked in what humans do to the landscape. You know the water is in your pipe and you assume it hasn’t had an impact upstream, and that’s just simply not the case.

Part of this study shows that rangelands are going to be impacted greatly by all this new development. The more you eat up all this landscape, the less surface area you have for water to percolate back into these aquifers.

Water Deeply: How did this study come about?

Tamara Wilson: Our group at USGS is already deeply entrenched in analyzing historic land-use change in California and the Central Valley. Our group uses remote sensing technology – satellite imagery – and we’ve looked at the entire Landsat record from 1973 to 2000. We’ve carved out chunks of the state and looked at what kind of land-use changes have occurred.

We were approached by the Nature Conservancy. They wanted to see if we could work together to look at land change and the implications of change on carbon emissions. Some research shows planting of vineyards and orchards act as a carbon sink to take carbon out of the atmosphere. What we’re finding in that study is, it’s debatable that they actually take carbon out, because they usually rip those crops out after 20 years. My study is a spin-off of that.

Water Deeply: Your results found a 4 percent total increase in water demand statewide by 2062. Is this a mix of agriculture and urban?

Tamara Wilson: It’s a 4 percent increase in what we’re calling demand. It’s a net 4 percent increase. We used applied water for agriculture and a dataset from USGS for municipal and industrial. Under different scenarios, it could be different and likely would be. The actual overall crop water use declined. It increased dramatically for perennial (permanent) crops, but it dropped dramatically for annual cropland.

So this increase is really dominated by development. We don’t include density or population. It’s strictly land use. Although the population projections do show, I think, 14 million more people by 2060. We didn’t attribute the model for how many people live in each pixel or cell in the landscape. We basically looked at historic rates of change and applied them into the future, assuming people are populating those cells. We looked at land use and we projected growth of those uses based on present trends.

Water Deeply: Should people be concerned about a 4 percent net increase? It doesn’t sound like a lot, but is it?

Tamara Wilson: Four percent works out to about 1.4 million acre-feet (1.7 billion cubic meters) – enough to serve 3 million households for a year. It’s something to be concerned about if you’re in a drought.

Keep in mind, this is a 50-year projection. So imagine even San Bernardino County 50 years ago, what it looked like. A lot changes over 50 years. It makes it a little bit easier if you put it in that context. It seems like not a lot, but if you look in the past for context, it has been a lot.

Water Deeply: How realistic is it to assume present growth rates will continue, which is the basis of your “business-as-usual” modeling scenario?

Tamara Wilson: The projections are really of land use, and what we’ve simply done is attribute a developed pixel, or land-use cell, with a water-use demand. They are one possible outcome out of a number of additional possibilities. It’s a reasonable possibility if things persist as they are today.

Water Deeply: What about farming? What farmers choose to plant often changes from year to year, based on market conditions.

Tamara Wilson: Exactly. Our study just basically projects into the future what’s been happening historically. There’s a lot of ways this could change. In scenario work, you can do multiple types of scenarios. You can impose a conservation scenario. But we did not do that. We kind of took today and projected that out into the future.

Water Deeply: Do you plan to try additional scenarios? For instance, in 2009 the state stopped funding the Williamson Act rangeland preservation program, and by 2013 nearly a half-million acres (roughly 200,000 hectares) were scheduled to drop out of the program.

Tamara Wilson: That’s the plan, yes. I’ve run this with removing the Williamson Act, and I haven’t been able to dive into the results of that, so I don’t want to say too much about that. But the outcome is definitely different. We’re developing an additional scenario for that.

Water Deeply: Your study concludes there is a need to continue 25 percent mandatory statewide water conservation to hold water use at 2012 levels through 2062. But that’s not what the state is doing. Should people be concerned about this?

Tamara Wilson: We are not out of this current drought. So even now we should be concerned with future growth. But growth will happen and humans are very innovative. This probably wouldn’t be a problem if more conservation measures at whatever level were taken. Even with existing technology, this would be easily managed for. Population trends could change. Projections are challenging. There are so many factors.

Water Deeply: What effect will climate change have?

Tamara Wilson: We did not model climate at all in this study. Climate change can impact farmers’ decisions, which is something we don’t account for in our model. There are a wide array of global climate models. All of them agree we’ll warm, which will increase your evapotranspiration. A long-term drought could affect different mandated water-use restrictions at whatever level of government. It’s extremely tricky.

Water Deeply: You state in the study that actual water consumption in the future is likely to be greater than you’ve estimated, mainly because most agricultural water use is not measured at all. How much greater?

Tamara Wilson: Agricultural water use is a really tricky thing. It’s not like municipal, where there’s a valve you turn and you measure how much comes out. It’s a really tricky value to estimate, and it’s often underestimated. It’s likely a lot higher than what we project. The values are often underestimated by 20–30 percent, especially since groundwater is not managed yet.

Water Deeply: Do we have the ability to manage the increase in water demand that you project?

Tamara Wilson: I do believe we can if the supplies – the amount of rain, the timing of that rain – sort of stay within the historic range of variability, meaning the last 100 years. There are a lot of factors in terms of when it falls. Does it stay as snow or does it melt immediately? But California developed its whole water system in an anomalously wet century. If we go past that, I’m not so certain. Megadrought is going to be a problem if that comes to pass.

Water Deeply: Is this reason to reconsider how we use land as we grow and develop in the future?

Tamara Wilson: If your goal is to keep open space, it should be considered.

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