Groundwater shortage predicted for southwest

Bad news for everybody.
Well, except Payson.

by Peter Aleshire – Courtesy of The Payson Roundup – Nov 28, 2017

The global warming trend will mean hard times and hard choices for communities dependent on groundwater in the Southwest, according to a regional study led by researchers from the University of Arizona.
Up to this point, that would have included Payson — which has been almost entirely dependent on groundwater for the past century.
But next year, Payson will start taking annual delivery of 3,000 acre-feet of water from the C.C. Cragin pipeline. Since Payson currently uses about 1,800 acre-feet annually from its wells, the town plans to inject water back into the water table through a network of existing wells. So not only will the town no longer draw out groundwater for the nine months a year the pipeline delivers water — the project will likely inject 1,000 to 1,500 acre-feet of C.C. Cragin water back into the water table annually.
However, the rest of the groundwater-dependent Southwest may face a much bleaker situation when it comes to finding water to sustain its growth in coming decades, according to the study of groundwater prospects throughout the West.
The study projected groundwater recharge both for the near future (2021-2050) and the far future (2070-2100).
The research predicted a significant decline in groundwater recharge from rainfall and snowmelt in the southern region of the western U.S. — including Arizona, New Mexico and Texas. However, the predicted shift of storm tracks to the north will likely increase groundwater recharge in the northern regions of the West.
“For those places already having problems, climate change is going to tighten the screws,” said lead author Thomas Meixner, UA professor and associate department head of hydrology and atmospheric sciences.
Generally, dry areas will get drier — and wet areas will get wetter, according to the climate projections.
However, geology, ecology and terrain will all have a big impact on the response from one water table to another.
For instance, the granite that lies beneath Payson creates a complicated water table — with pockets of crushed rock that hold water. During the past 50 years, Payson’s water table dropped by 100 to 200 feet as the area grew rapidly and more and more wells sucked water out of the underground water bank. Conservation measures and the lack of building since the recession have stabilized most well levels for the past decade.
On the other hand, much of the Valley relies on wells sunk into the deep, porous layers of sand and sediment deposited over the course of millions of years by the Salt, Gila and Verde rivers have created a deep groundwater basin in the Valley. Water readily soaks into the groundwater basin, but relentless pumping has dropped the top of the water table by hundreds of feet. Wells have drawn out so much water that the surface has subsided across wide areas of the Valley.
The researchers divided the West into five “hydro-climate” regions. The Southwest region included Utah, Colorado, Arizona and New Mexico.
The researchers then combined the results of 11 different global climate models to estimate future groundwater recharge rates. The buildup of heat-trapping pollutants in the atmosphere together with other changes have convinced most climate scientists the planet will undergo significant, continued warming in the course of this century.
The researchers used the actual measurements of rainfall and groundwater recharge in each region for the period of 1971-2000 to create a baseline for the future projections.
The researchers suggested recharge will increase in the Northern Rockies and the Plains region, but decrease in the West and the Southwest in the 2021-2050 time period.
“Groundwater represents a bank,” said Meixner. “We can store water from decade to decade and arguably millennium to millennium — but when we take a withdrawal from that bank, we have to hope there are deposits making up for our withdrawal. If not, we have less water in the future to face water challenges.
“The future is saying there’s going to be less recharge. That doesn’t mean you drain the aquifers dry. Whether we drain the aquifers dry is a management decision,” he concluded.
Conservation measures could extend the life of the underground reservoirs. A decade ago, Payson imposed the toughest conservation measures in the state — halting the fall in the water table and sharply reducing average use. The average Payson residents uses between one-half and one-third as much water as the average Phoenix resident.
Thanks to both conservation and the pipeline project, Payson may turn out to be one of the few communities in Arizona with an ample water supply —even if rainfall and groundwater recharge rates decline.
Payson invested $52 million in the C.C. Cragin Reservoir pipeline, which will nearly triple the community’s long-term water supply. The town’s current, projected build-out population is about 38,000, so the combination of Cragin water and well water provides an ample supply for build-out.
The town will have a chance to recharge the water table with Cragin water every year until it reaches build-out, thanks to the well-water injection system included in the project.
Moreover, the town will only use the wells during the three winter months when the snows on the Rim shut down the pipeline. That means most of the 1,800 acre-feet that fall on the watershed in an average year will also wind up in the water table.
However, ranchers, farmers, developers and residents have taken about 150,000 acre-feet out of the water table in the past century, which means it would take a century for the water table to recover to pre-settlement levels.
Still, thanks to the pipeline Payson’s water table will likely rise, while water tables throughout the region fall.

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