Implications of projected climate change for groundwater recharge in the western United States – paper review

A group of 17 researchers, led by Thomas Meixner of the University of Arizona, attempted to assess the possible response of groundwater recharge to global warming in eight basins in the western U.S. In this context, “recharge” means the replenishment of water in aquifers from some surface source.

Meixner recharge fig 1

The paper: Meixner et al, 2016, Implications of projected climate change for groundwater recharge in the western United States, Journal of Hydrology, Volume 534, March 2016, Pages 124–138.

You can download the full PDF.

The authors assume future climate conditions will be as projected in the most recent National Climate Data Assessment (2014) which itself is very speculative and lacks any supporting physical evidence (see links to my articles below).

The authors acknowledge that their study comes with a great deal of uncertainty. That did not stop Tony Davis from asserting in the Arizona Daily Star that “groundwater recharge in the San Pedro will decline faster than in any of seven other Western groundwater basins…” Tony apparently did not read the part about the Central Valley of California nor the part about the San Pedro in the paper itself.

Here is an excerpt from the paper’s abstract:

“Eight representative aquifers located across the region were evaluated. For each aquifer published recharge budget components were converted into four standard recharge mechanisms: diffuse, focused, irrigation, and mountain-systems recharge. Future changes in individual recharge mechanisms and total recharge were then estimated for each aquifer. Model-based studies of projected climate-change effects on recharge were available and utilized for half of the aquifers. For the remainder, forecasted changes in temperature and precipitation were logically propagated through each recharge mechanism producing qualitative estimates of direction of changes in recharge only (not magnitude).”

“Several key patterns emerge from the analysis. First, the available estimates indicate average declines of 10–20% in total recharge across the southern aquifers, but with a wide range of uncertainty that includes no change. Second, the northern set of aquifers will likely incur little change to slight increases in total recharge. Third, mountain system recharge is expected to decline across much of the region due to decreased snowpack, with that impact lessening with higher elevation and latitude.”

“Factors contributing the greatest uncertainty in the estimates include: (1) limited studies quantitatively coupling climate projections to recharge estimation methods using detailed, process-based numerical models; (2) a generally poor understanding of hydrologic flow paths and processes in mountain systems; (3) difficulty predicting the response of focused recharge to potential changes in the frequency and intensity of extreme precipitation events; and (4) unconstrained feedbacks between climate, irrigation practices, and recharge in highly developed aquifer systems.”

In other words, this paper is long on speculation and short on physical data.

Meixner mapHere is what they predict for each of the eight basins due to global warming:

High Plains Aquifer (aka Ogallala aquifer):

The paper projects moderate increases in recharge in the north and shifting to moderate decreases in the south or an overall net decrease in recharge depending on which data set is used.

San Pedro River:

The authors relied on two other studies which project a 30% and a 27% decrease in recharge over the next 100 years. However, the authors also note that depending which climate model was used, “Future recharge varied from a 100% decline in recharge to a 30% increase in recharge across the GCMs used.” That large range of uncertainty detracts from the value of the study.

Death Valley:

The paper predicts less recharge due to decreased snowpack in the surrounding mountains. That project is hedged: “Sources of uncertainty include potential increases in summer precipitation and winter precipitation intensity, which could lead to increased focused recharge. This source of recharge is currently so small that even large relative changes would result in negligible changes to total recharge.”

Wasatch Front Aquifers:

The paper predicts a general decrease in recharge (unless there is more snowpack).

Central Valley Aquifer:

Here, recharge is dominated by irrigation. Modeling projects decreases on anywhere between 5% and 60%.

Columbia Plateau:

There might be a modest increase in recharge if it rains more.

Spokane Valley-Rathdrum Prairie aquifer:

The authors don’t know because of the great uncertainty of the data.

Williston Basin aquifer system:

Some “considerations suggest that diffuse recharge to the Williston Basin may decline. However, model-based projections in multiple studies of future recharge to the northern HPA [high plains aquifer], located directly south of the Williston Basin, indicate that diffuse recharge will increase. Given these inconsistent outlooks, uncertainty in future diffuse recharge is high.” In other words, the authors don’t know about that one either.

The goal of this study was to provide information that would help planners form water policy for the future. Considering the high level of uncertainty, do you think this study will help?

Study funding: This study receive support from the U. S. Geological Survey and the National Science Foundation through a concurrent award (EAR-1328505). Additional support to several authors was provided by the USGS National Research Program and the USGS Office of Groundwater.”

Your tax dollars at work.


See also:

National Climate Assessment lacks facts, an analysis

National Climate Assessment = science fiction and politics

San Pedro River Geology – Implications for water law

Trends in groundwater levels around Tucson

A story in the Arizona Daily Star notes that depletion of our groundwater supply is diminishing in some areas due to use of CAP water (water imported via canal from the Colorado River).  Much of the CAP water is being used to recharge the groundwater aquifer.  The maps below show the state of the aquifer levels for the periods 1970-1979 and 2000-2008.  On the maps, red indicates a falling water table, blue indicates a rising water table, and yellow indicates a stable water table.



The data come from the U.S. Geological Survey.  See an overview page here, and an interactive map page here.

The maps show that the recharge project in Avra Valley and retirement of central city wells have made quite a difference.

Note that USGS provides this disclaimer: “All information on this website should be considered provisional and subject to revision. No judgment on the presence or availability of ground water should be made on the sole basis of information on this website. Neither the USGS nor ADWR will be held responsible for any loss or damages due to the use of this information.”  Comforting, isn’t it.

According to the Star story: “the city this year will put into the ground 140,000 acre feet of CAP water and take out 80,000. This has raised the water table 9 feet a year at the city facility in the central Avra Valley facility for the past decade and 140 feet in the three years that a second city recharge facility and well field has existed in the southern Avra valley.”

Back in 2009 I posted an assessment of Water Supply and Demand in the Tucson area based on information from  the Water Resources Research Center at the University of Arizona (WRRC). I summarize from that post here:

The Demand:

In 1999, total usage in the Tucson Active Management Area was 323,000 AF according to WRRC.  Municipal usage was 154,000 AF which included 17,000 AF used by golf courses (35% was effluent from the sewer plants), and 20,000 AF used by “turf” facilities such as parks, schools, cemeteries (33% was effluent).  Agriculture used 132,000 AF (of which 20,000 AF came from imported CAP water).  Mines used 39,207 AF, sand and gravel operations used 5,167 AF and “other” industrial use totaled 4,026 AF.  Sewer treatment plants produced 70,000 AF per year and are projected to produce 115,000 AF by 2025.  Currently 84% of effluent discharge is released into the Santa Cruz river channel where it infiltrates into a shallow aquifer.  (Alert readers might notice that these official figures from 1999 add up to more than 323,000 AF, so some categories must have been counted twice.)

By 2003, total usage increased to about 350,525 AF.  This is projected to rise to 396,000 AF by 2025 assuming increased municipal and industrial demand, and decreased agricultural use.  Natural recharge provides only about 60,000 AF per year.  In 2003, municipal usage totaled 185,199 AF.  Municipal use includes all domestic and small business consumption.  Industries used 47,430 AF; agriculture used 102,959 AF; Indians used 14,196; all others used 3,705 according to WRRC.

This total usage is about 169 gallons per day per capita, with residential use pegged at 110 gallons per day per capita, a figure that has remained constant for many years.  In contrast, the Phoenix area uses 238 gallons per day per capita, but gets 73% of its water from “renewable” resources such as rivers, CAP, and effluent.

The Supply:

In 2003, groundwater supplied 256,233 AF, CAP supplied 64,554 AF, use of effluent supplied 11,360 AF.  The rest was due to incidental and natural recharge.

Tucson gets most of its water by mining groundwater stored in aquifers down to 1200 feet deep in the Tucson and Avra Valley basins.  This is mainly fossil water deposited during the wet Pleistocene glacial periods.  However, there is even more water in deeper aquifers, but as depth increases, water quality decreases, and water becomes briny with salts and toxic metals.

The 1999 WRRC report states that, “In 1940, when Tucson began to increase its groundwater pumping, these aquifers held approximately 70 million AF of groundwater at depths less than 1,200 feet below the surface.” This resource is equivalent to all the water in Lake Mead and Lake Powell combined.  Since 1940, 10% of this groundwater has been withdrawn.  Simple arithmetic implies at that rate, the remaining groundwater supply shallower than 1200 feet could last about 150- to 200 years.  This time will be extended by increasing use of CAP water and effluent.

See my post linked above for more information.

Earth Fissures in Arizona

The Arizona Geological Survey has been busy mapping the many earth fissures in Arizona. Most earth fissures occur in the corridor between Tucson and Phoenix, and from Phoenix west along interstate 10 (see map below).

Earth fissures develop from soil compaction associated with extensive pumping of groundwater. The fissures range in width from a few inches to tens of feet. Length of the fissures ranges from hundreds of feet to miles.

These fissures pose danger to cattle, wildlife, and unwary humans. The presence or possibility of a fissure-prone area also has implications for municipal planners, developers, highways, and railroads. And, because they tend to develop perpendicular to surface drainage, they can capture runoff and develop into large gullies.

The gateway to the Survey’s many maps and reports is the Earth Fissure Viewer, an interactive map that allows you to zoom in on areas and get detailed maps. Maps, reports, and photos are also available at the Survey’s Earth Fissure Center.