Tuesday, January 22, 2013

Nevada's groundwater, dust, and Colorado River flows

In late December, the BLM granted Southern Nevada Water Authority the right-of-way for its 260+-mile water pipeline which will transport pumped groundwater from four remote valleys in east-central Nevada to Las Vegas. This ruling, following the Nevada State Engineer's decision last March to grant 84,000 AF/year of ground water rights to SNWA, is reportedly the final state and federal approval needed to begin construction of the project.

Among the zillion environmental impacts evaluated in the Final EIS for the SNWA pipeline and groundwater pumping project is the potential for the project to increase airborne dust due to long-term groundwater pumping.

It seems that projecting dust impacts is not at all simple or straightforward. But the analysis shows that the proposed action is expected to cause the greatest increase in airborne particulate matter out of all alternatives. From the Executive Summary, Section 4.8, page 61,
"Groundwater drawdown would likely result in windblown dust emissions due to drying of hydric soils and loss or reduction of basin shrubland vegetation. The estimated particulate matter for a size of 10-micrometer emissions by alternative are shown on Figure ES-29. The particulate matter emissions for a size of 2.5 micrometers would show the same pattern of drawdown effects by alternative, although the magnitude would be less than the 10-micrometer size. The level and extent of these predicted dust emissions are highly uncertain due to the assumptions involving dust increases from changes in vegetation."

And again, emphasizing uncertainty in calculations, but nonetheless connecting groundwater pumping to a possible increase in airborne dust, from Chapter 3 in the EIS:
 "Importantly, the expected changes in vegetation and soil surface conditions are critical for the estimation of future fugitive dust impacts due to groundwater drawdown. It is possible that there would be no net increase in soil erosion in groundwater drawdown areas undisturbed by construction activities. However, there is inherent uncertainty and spatial variability in plant communities’ response to drawdown, as discussed in Section 3.5, Vegetation Resources. Therefore, to provide an upper bound on the uncertainty associated with the effects of soil erosion and plant cover on air quality, a 10 percent decrease of the current plant cover is assumed to result from drawdown. This assumption provides a conservatively high estimate of potential air quality impacts from groundwater drawdown."
Based on these assumptions about dust generation, the EIS assesses the potential impact on visibility in certain areas around the west - including the Four Corners area with its high density of national parks. But as far as I can tell based on heavy skimming (but admittedly not a complete reading) of the document, there is no analysis of the potential impacts of airborne dust on annual runoff in the Colorado River, or any other, basin.

A recent opinion piece in the Salt Lake Tribune linked Utah's land management practices to increasing amounts of airborne dust and decreased Colorado River flows. In the same way that disturbing the land loosens the soil, groundwater pumping both dries out soil and kills off some vegetation that depends on shallow groundwater. In both cases, dry, loose, exposed soil is a source of windblown dust. 

dust on snow photo galleryRecent studies have documented that windblown dust generated by erosion of disturbed desert surfaces causes snowpack in the Rocky Mountains to melt earlier and run off at higher intensities, resulting in lower in downstream flow. The reason is that when snow melts faster, the growing season for plants is longer, resulting in more evapotranspiration by plants and less water available as runoff. One such study is here. 

 Photo: dust layers in snowpack profile,  Center for Snow and Avalanche Studies, Colorado Dust on Snow Program

It has been estimated that dust events in the Rocky Mountains of the Upper Colorado River Basin could cause up to a 5 percent reduction in annual runoff to the Colorado River. More information from the National Academy of Sciences is here, and here.  As an example, 5 percent reduction in runoff amounts to 700,000 AF of 14 MAF Colorado River of annual runoff. While this may be an overestimate both of the Colorado River's annual flow and the reduction caused by dust, even a 1 percent reduction in a record low flow would still amount to more than 100,000 AF/year - more than the amount of groundwater pumping approved for Nevada. As a comparison, Nevada has an annual allocation of 300,000 AF of Colorado River water.  So, its not clear to me why this potential impact was not addressed in the EIS. 

The bottom line for Arizona. Well, nothing is ever simple. On the one hand, development of a new water source for Nevada should be welcomed by Arizona - any new supplies in the CR Basin will help alleviate some pressure on demand. On the other hand, although large-scale water augmentation projects always involve trade-offs, there should be a clear quantification of all the impacts of a project - and ideally not a net reduction in available water supplies. 











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