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Environment / Sustainability

Potential changes in groundwater acquisition by native plants in response to climate change

____, 2010

Graduate student Julie Kray recently conducted research in the arid Colorado San Luis Valley, examining the variations in groundwater use by native plants in an effort to understand how plants' water use may change in response to climate variability.

Plants that use groundwater impact humans

An arid region of southern Colorado where many native shrubs use groundwater to survive.

Across the arid intermountain regions of western North America, precipitation is limited, yet much of the natural landscape supports plant communities. Some plants in these near-desert environments are able to thrive in spite of drier surface soil conditions by developing roots deep enough to tap into a more stable water source: groundwater.

Phreatophytes, or plants that can use groundwater, cover vast areas of our western landscape. Native plant communities dominated by phreatophytes are ecologically valuable for soil stabilization, wildlife habitat, and forage for domestic livestock. However, phreatophyte communities can substantially influence total water outflow on a basin scale through groundwater evapotranspiration, or ET.

Reliable water supply essential

Groundwater resources in the West are essential to human populations, sustaining regional agriculture and municipalities by providing a reliable water supply in arid regions with unpredictable climates.

Accurate estimates of groundwater use by native phreatophyte communities are therefore critical to managing groundwater in arid intermountain basins. Additionally, we need to understand how phreatophyte water use may change in response to climate variability.

Climate variability, water availability

Changes in the timing and amount of precipitation are likely throughout western North America, and warming temperatures are expected to increase ET by native plant communities and agricultural crops. It is unknown whether different species of phreatophytes will vary in their sensitivity to altered precipitation patterns, and how these differences will affect groundwater use at the plant community scale. Changes in plant community composition and water acquisition patterns may in turn influence water availability for agriculture and other human uses.

Graduate student examines native Colo. plants

Julie Kray, an ecology graduate student, has examined the following questions:

• How do water acquisition patterns (groundwater versus rain-recharged soil water) vary among native phreatophyte species under the current climate? Are some species more dependent on groundwater than others?

• How will phreatophyte water acquisition patterns respond to a change in growing season precipitation? Will increased monsoon rainfall lead to increased plant use of soil water and reduced use of groundwater?

• Conversely, if growing season precipitation decreases, will plants become more reliant on groundwater?

Research in San Luis Valley

Native phreatophyte communities occupy over 1.2 million acres in the San Luis Valley, Colo. These include the shrubs (1) Sarcobatus vermiculatus and (2) Ericameria nauseosa, and the grasses (3) Sporobolus airoides, and (4) Distichlis spicata.

The study took place in the San Luis Valley, or SLV, a high-elevation intermountain basin located in southern Colorado. The SLV is the most arid region in Colorado, receiving only 180-250 mm of precipitation annually; yet, a shallow unconfined aquifer recharged by snowmelt runoff from the surrounding mountains supports:

  • over 600,000 acres of irrigated agriculture
  • substantial water transfers out of the valley
  • more than 1.2 million acres of native rangeland plant communities

The dominant native plant species in the SLV are phreatophytes, and evapotranspiration by phreatophyte communities accounts for nearly one-third of the total annual groundwater consumption.

Between 50-70 percent of the total annual precipitation in the SLV occurs from mid-July through September, through rain events generated by the North American monsoon system. Some SLV phreatophytes may be adapted to use predictable pulses of late summer monsoon precipitation to reduce or supplement their groundwater consumption.

However, current precipitation patterns are likely to vary with climate change. Existing climate model projections for the SLV are inconclusive, with some suggesting an increase and others projecting a decrease in monsoon rainfall. The goal of our study was to understand the interactions between precipitation and plant water use patterns for both wetter and drier futures.

Long-term study site near Crestone, Colo.

The rainfall manipulation experiment compared control plots receiving ambient rainfall with one of two treatments: (a) decreased rainfall using rainout shelters and (b) increased rainfall through addition of rain captured by shelter roofs.

Kray and her team conducted a rainfall manipulation experiment at our long-term study site near Crestone, Colo. The experiment compared plants in control plots receiving natural rainfall with plants receiving one of two treatments: (1) decreased rainfall using rain out shelters (“rain out”), and (2) increased rainfall by applying rain captured from shelter roofs (“rain add”).

Study results indicated that distinct differences occur in the water acquisition patterns of four common native phreatophytes in the SLV. These phreatophytes will vary in their sensitivity to changes in soil water availability, which may affect basin-scale groundwater use by native plant communities over time.

More research needed

Future research should focus on quantifying the total annual groundwater use by each phreatophyte species and understanding how variations in soil water availability affect plant production.

Results from this work and future research on phreatophytes in the SLV will be incorporated into the Rio Grande Decision Support System groundwater model that is used to manage the SLV aquifer.


Excerpt from an article by Julie Kray, M.S. candidate, Forest, Rangeland, and Watershed Stewardship. Originally published in Colorado Water, the newsletter of the Water Center of CSU, May/June 2010, Volume 27, Issue 3.