To commemorate World Water Day, happening this Sunday, Daniel Collins, an Australian (correction - Kiwi) hydrologist who blogs at Crikey Creek, suggested that all bloggers who focus on water dedicate a post to the issue of transboundary waters - water resources that are shared by more than one country (or state).
Transboundary waters are commonly discussed in the context of surface water sources - i.e. the Colorado River in the Southwest U.S. that is shared by 7 states and a portion of Mexico - but just as contentious can be conflict over shared groundwater resources, which is what I will focus on in this post because of both the importance of groundwater as a water supply in Southern Arizona (where I reside) and the importance of groundwater in terms of its interaction with the few remaining surface water sources in this area.
The two locations arguably of greatest importance in this regard, in Southern Arizona, are the upper Santa Cruz valley in the vicinity of Nogales, Arizona and the upper San Pedro valley, near Sierra Vista, Arizona. These are locations of historically rich riparian areas supported by perennial streams that depended on baseflow from adjacent aquifers to maintain streamflow during the driest parts of the year. These are also northward flowing rivers that travel from Mexico into the United States.
Baseflow to the Santa Cruz River has been significantly compromised by urban growth and development near the border with Mexico. Nogales, Arizona has a population of approximately 21,000 while Nogales, Sonora has an official population of about 200,000 but some estimates place the actual number closer to 300,000. Although residents on the American side of the border undoubtedly use much more water per capita than their poor neighbors in Mexico, the sheer numbers south of the border in addition to poorly maintained, leaky water systems result in significant groundwater use in Mexico. What Mexico does provide to the river/aquifer system, however, is effluent. This is why the Santa Cruz River, which historically was perennial over much of its course between Nogales and Tucson is currently perennial in two distinct stretches - north of Nogales and north of Tucson, where the flow is supported by effluent.
Among the management goals for the Santa Cruz Active Management Area (AMA), which encompasses the valley from Nogales north about 45 miles, is to maintain the riparian area and local aquifers. This goal is only possible with the contribution of the effluent from a border treatment plant that handles most of the sewage from Nogales, Sonora. North of the border, much of the historically irrigated acreage along the river has been retired to permit further population growth (which has been rapid in the past 20 years). Interestingly, pursuit of this goal requires recognition of the connection between surface and groundwater, something the common law in Arizona typically ignores. Hopefully the new residents of the area will appreciate the riparian area in their neighborhood and understand the potential impact future growth can have on that resource.
The story along the San Pedro River, about 40 miles east of the Santa Cruz, is similar in that the major factor impacting the aquifer (and hence the river) is rapid population growth. However, in this case the growth is almost entirely on the Arizona side of the border. This area is not within an AMA so there is little regulation of groundwater pumping under state law.
The San Pedro drainage located south of the border is still largely undeveloped apart from ranching and some mining so the focus of efforts to maintain the aquifer/river system has been in the vicinity of Sierra Vista, the largest city in the area and a rapidly growing community during the last several decades. There has been much concern about the fate of the San Pedro River since the creation of the San Pedro Riparian National Conservation Area in 1988. Fortunately the Federal Government has considerable leverage over the growing cities and towns in the area, not just because of the federal reserved water rights associated with the Conservation area but also because the main economic engine in the area is a military base, Fort Huachuca. The fort has been a leader in the region in promoting water conservation measures so that constraints on the local aquifer can be minimized. Local political and business leaders are terrified of losing the fort, resulting in an alliance of business, governmental, environmental, and military interests to ensure that river remains flowing (and the area continues growing).
So far they have conducted numerous studies of the river and aquifer, the connection between the two, and the sensitivity of the river to groundwater pumping in various areas. They have reduced per capita water use in the area, constructed projects to capture and recharge both stormwater and reclaimed water in an attempt to minimize the extent of the effects from local groundwater pumping (hopefully providing long-term protection to the river) and different organizations have purchased land adjacent to the river to retire irrigation rights, which provides the best short-term protection of the river, although there is still considerable pumping occurring adjacent to the river (again the issue of legally distinct surface and groundwater in Arizona rears its ugly head). But the biggest threat of all could ultimately be from climate change which might alter recharge patterns to the aquifer (less winter precip and more summer precip, meaning more flash floods that are less effective recharge sources than the widely dispersed, cold weather precip that historically occurred during winter months).
The saving grace for the San Pedro is likely to be the fact that there are lots of people watching to see what happens and lots of data is being collected, so hopefully someone will notice when changes need to be made in the region and they will have the necessary data to support those changes. But the population growth that has already occurred or has been approved for the near future could be too much for the aquifer to support long-term. That would mean augmenting water supplies in the region somehow. They have already been looking at that but funding will be a significant issue.