Exploring the Water We Can't See

Larry Hipps, professor of climate science at Utah State University, has worked on water use in agricultural and rangeland systems for more than 30 years, and is currently developing and testing ways to examine and measure how much water is “lost” by urban landscapes (lawns, parks etc.) by evaporation and transpiration. Most current mathematical models for calculating evapotranspiration (ET) are based on agricultural crops and soils. Urban landscapes very different systems, and require new approaches.

“It’s the frontier of water resource management,” Hipps said. “Meteorologists, climatologists and plant scientists have been working on the problem for some time, but with better satellites, imaging, sensors and scientific techniques we can make much better estimates in near real time.”

Hipps and his colleagues are doing the proof-of-concept work with satellite imaging and measurements on the ground as a foundation for more extensive research and ultimately an operational product that will help water managers and policy makers. The team, which brings together plant, water and climate scientists, was organized by Roger Kjelgren, USU professor of plant science, and is using satellite images of urban landscapes along the urbanized Wasatch Front. Hipps points out that “urban” in this case doesn’t just mean the middle of a city with its abundant concrete and asphalt. The study area includes roads, homes, sidewalks, lawns, shopping centers, gardens and parks, all the components of urban and suburban life. It’s a lot different measuring ET there than in agricultural fields with acres of a single type of crop and well-understood soil. The work also involves instruments on the ground to help verify the satellite data. Hipps pointed out that reliable “measurements” of ET require sophisticated instruments and complex calculations because there is, as yet, no single instrument that can measure it.

“As water supply has more and more trouble keeping up with demand, the first thing you need to know for sure is how much water we are getting then what are we doing with it,” Hipps said. “Uncertainty in future water supply makes it even more important to know those things.”

Hipps and colleague Simon Wang, USU assistant professor of climate science, point out that climate change is exposing more unknowns about our water supply, and that extreme weather events will make management even more challenging. Climatologists know that drought and excessive precipitation are cyclical in the arid West, but the extremes are becoming more pronounced and more frequent. Just out of curiosity, Hipps plotted precipitation values from 1896 through 2014 in California’s Sacramento Basin. He picked the location because California’s Central Valley is the nation’s salad bowl and fruit basket and the impacts of extreme drought there reach across the country to restaurants, grocery stores and family dinner tables. The graph that resulted from those data shows a fairly tidy cluster around the average that would be considered “normal” precipitation, or what he terms a “basin of attraction”. However, in recent decades the oscillations of dry and wet are growing larger, meaning more severe droughts and heavier wet episodes. Most of the extreme dry and wet periods have been since 1976.

 

This exercise further illustrated what Utah Climate Center scientists have found before, that our “normal” isn’t normal in the context of historical and paleoclimate evidence and that those extreme wet and dry cycles make average temperatures and average precipitation poor planning tools. [In fact, Hipps has found that northern Utah has three separate basins of attraction for precipitation: a large wet one and dry one, and a small “neutral (in between) and the state’s climate cycles through the extremes. That’s why their work to build better climate models is so critical for projecting and planning water supply, Wang said. For example, their models show an overall wetter future for California, but it’s a wetter-than-average decade punctuated by years of intense drought.

Wang and Hipps agree that recent severe droughts that have dried out much of California should be regarded as cautionary tales for Utahns, especially with regard to groundwater. Hipps and Wang noted that climate and hydrology above and below ground are all part of the same puzzle. The Utah Climate Center has used a combined observation and modeling method to project groundwater depletion in Utah.

“Groundwater is going to be the ultimate water resource,” Wang said. “In Utah, all but two counties tap more than seventy-five percent of groundwater, mostly for agriculture. The groundwater reservoirs should be replenished every year, but their levels have decreased and are projected to decrease with a rate proportional to the rate of climate warming. Not just warming here in Utah, but because overall the Pacific warms and changes the pattern of our precipitation.”

The result is that pumping groundwater without thought to how or if it will recharge is not sustainable, especially since demand for water will increase as the population grows. Hipps said unregulated pumping in California is understandable in the short term because farms need water and if it’s not coming from the sky, they will pump it from the ground.

“I think of groundwater like a retirement or savings account,” Hipps said. “When someone runs into financial difficulty, in the short term they can get by spending some of that account. But eventually that will run out and you’re not just in trouble right then, you’ve also spent away your future.”

Wang said that in California, groundwater levels and the climate wet and dry cycles have gone hand in hand, rising and falling in the same year (with a slight seasonal lag). But data from satellites and hundreds of wells revealed that over the past 15 years, there has been a significant lag in groundwater recharging after a drought meaning that groundwater levels continue to decline for another year or longer even after precipitation increases.

“Rapid population growth and current levels of water use in Utah will clash with the limits to water imposed by the climate, Hipps said. “This is a realm where supply does not respond to demand!”

Water managers, policy makers and water users ? in short, everyone ? faces a future where there will be changes in water supply and the ways it is used. Emerging knowledge of Utah’s climate will help in planning for short-term and long-term drought cycles, and in preparation for increasing uncertainty of expected water supply.