by Bea Ferrigno
State Engineer Dick Wolfe and several staff members visited the San Luis Valley for the second time this fall on December 11 and 12. On the morning of the 11th a meeting was held in Saguache to update the public on the progress of modeling efforts that will establish baselines and targets for subdistricts still to be formed. On the 12th Wolfe convened a meeting of the advisory (rules) committee to follow up on the work that was done at the previous meeting in October (see the November Eagle.) A further set of similar meetings is planned for later this month; at that time the models for Saguache and San Luis creeks should be complete so that estimated depletions can be calculated.
In Saguache, Deputy State Engineer Mike Sullivan and Lead Modeler James Heath gave an informative presentation on the fundamentals of the Rio Grande Decision Support System (RGDSS) or model, on which ground water regulations will be based. After a brief description of the formation of the San Luis Valley, which is far more complex than other river basins in Colorado, they discussed the water budget which takes into consideration many interactive variables—basically varied inputs and outflows—and forms the basis of the RGDSS model.
While pumping from the unconfined aquifer draws down the water table, there is enough interaction between the confined and unconfined aquifers that the confined, or lower aquifer, cannot be dewatered; instead, it is depressured, an effect demonstrated by reduced flows from artesian wells and springs. Another difference in effects of pumping from either aquifer occurs in the extent of cones of depression or the area of influence around wells: in the unconfined aquifer the area of lessened water extends only a mile around the well, whereas a well into the confined aquifer affects a circle whose radius is approximately 23 miles.
The effects of pumping are also magnified over time: with intermittent pumping, stream depletion rises and then recovers when pumping stops, but with continuous or long-term pumping, stream depletion continues to increase even after pumping ends. Extended pumping lowers the water table of the unconfined aquifer as well as hydraulic pressure in the confined aquifer. There are also different effects with different irrigation methods: in terms of crop production, flood irrigation is 50 to 60% efficient, while sprinklers are 80% efficient. Flood irrigation, however, provides more recharge to the aquifers and streams: all these factors go into the model.
According to the legislation that authorized the formation of subdistricts, the aims are to minimize reduction in pumping in order to maintain agriculture, restore the aquifers to sustainable levels, and remain in compliance with Rio Grande Compact provisions. While these might seem to be incompatible goals, the model will provide a scientific approach that considers all the many variables and moving parts. The definition of “sustainable” at present means an unconfined aquifer level equivalent to the average seen between 1978 and 2000. Twenty years are allowed for each subdistrict to replace its portion, based on historical water use, of the deficit. When subdistricts are in place, year-end direct measurements will monitor progress toward recovery of the aquifers. Once the stipulated level has been reached, it cannot be allowed to drop below the 1978-2000 average minimum but must be maintained between that level and the average maximum.
At the December 12th Alamosa rules committee meeting, which included a presentation similar to those given on the 11th, one participant questioned whether current plans for fallowing in subdistrict 1 would be adequate. Wolfe responded that he doesn’t want to dictate to the subdistricts as they will be addressing widely differing conditions. Instead, he chooses to allow for flexibility between subdistricts – the same plan won’t fit all of them. Another participant suggested there should be stronger guidelines for attaining sustainability and that there should be a tie-in with actual conditions, as there are for compliance with the Rio Grande Compact, so that the amount of fallowing would be based on the amount of water available. Again, Wolfe expressed a preference for guidance rather than rules.
Complete graphics, including detailed depictions of well pumping effects, can be seen under “General Concepts” at
(Note: the graphics for this article appeared in the January Crestone Eagle, however, the article did not. The article, with graphics, will run in the February Eagle.)
State Engineer Provides More Information on SLV Groundwater Conditions
by Bea Ferrigno