Series: Colorado Water Law Basics #7 - Not-Nontributary Water Must Mean it is Tributary Water, Right?

Not-nontributary is a classic double negative, so if water is not from a nontributary source, the only option is that it is tributary water. If you are an English major that would be the only logical conclusion; however, who has ever alleged that Colorado Water Law is logical? In fact, the term “not-nontributary,” as applied to some of the groundwater in the Denver Basin bedrock aquifers, is not tributary water. Actually, it’s not even close to being considered tributary water in terms of how it is administered by the CO State Engineer’s Office.

The term originated with the passage of Senate Bill 5 in 1985 and included the definition of both “nontributary” water, as well as “not-nontributary” water, which applied only to groundwater contained in the Denver Basin aquifers. The Denver Basin system generally extends from approximately Greeley on the north, to Limon on the east, Colorado Springs on the south, and the foothills on the west. A map of the Denver Basin is shown in Figure 1. Nontributary water is defined as groundwater when pumped continuously for a period of 100 years affects flow in any natural stream by less than 0.1 percent of the volume pumped. Conversely, not-nontributary water is defined as groundwater when pumped continuously for a period of 100 years affects flow in any natural stream by greater than 0.1 percent of the volume pumped. Keep in mind that tributary water, when pumped, can affect flow in a natural stream by 100 percent of the water pumped.

Figure 1: Map of Denver Basin aquifer system (USGS).

Figure 2: Map indicating “nontributary” and “not-nontributary” areas (CO DWR).

The difference between nontributary water and not-nontributary water seems like a relatively minor distinction but it is not. Nontributary groundwater can be developed simply by proving ownership of the groundwater underlying a property, while not-nontributary water cannot be utilized without a visit to Water Court to obtain an augmentation plan so the stream depletions can be shown to be replaced to the affected stream(s) so there is no injury to senior water rights. Not only does this make the development of not-nontributary water more expensive to develop but also delays the ability to utilize this water. In some cases, obtaining an acceptable augmentation water source to allow the plan to be adjudicated is difficult.

The origin of not-nontributary water was based on the numerical modeling of the Denver Basin, simulating not only the pumping of groundwater from each aquifer, but also simulating streams that transect the basin on the surface. The hydraulic connection between the surface streams flowing over the bedrock strata of the Denver Basin had to be assumed because the specific hydraulic interaction throughout the basin is an unknown. The method for estimating not-nontributary water was based on a calculation of the estimated groundwater available in each aquifer for each section of each township and range; one-one hundredth (1/100) of that volume was then pumped continuously for 100 years from each section individually. If the model results from this pumping from a specific section indicated that more than 0 .1 percent of the water pumped was being contributed by a stream, or streams, then the groundwater in that section was considered to be not-nontributary. If not, it was considered nontributary. This process was followed for each section that overlays the Denver Basin for each aquifer. The result was maps for the aquifers that delineated presumptively what areas in each aquifer are nontributary versus not-nontributary. An example of a “nontributary map” from the Denver Basin Rules is shown in Figure 2.

Once your groundwater has been “determined” to be not-nontributary, a source of augmentation water is necessary to allow an augmentation plan to be adjudicated in Water Court. One of the simpler means to provide an augmentation source is to reserve a like amount of deep nontributary water beneath your property to allow the shallower not-nontributary groundwater to be developed. This only works if there is minimal development on the property such that there is no expectation that the deeper nontributary water will ever be developed, as any nontributary water reserved for augmentation can never be developed for other purposes.

If all of the groundwater beneath a property needs to be developed at some point in the future then reserving some of the water reduces the total Denver Basin aquifer water available. In this case, an alternate augmentation source can be sought. To evaluate how much water has to be obtained for augmentation, the withdrawal of not-nontributary water is simulated using the State Engineer’s AUG3 model, which is a numerical model of each individual aquifer in the basin. The results from the AUG3 model provide the timing of stream depletions during the 100-year pumping period, as well as ongoing depletions after pumping ceases. This method will identify the maximum amount of annual depletion so it is known how much augmentation water has to be provided. Somewhat ironically, if the AUG3 model is run instead of reserving deep, nontributary water, much less water has to be provided for augmentation. However, there is the potential additional cost of acquiring alternative water supplies for augmentation. Therefore, the benefits of each alternative need to be weighed when deciding how to structure an augmentation plan.

LWS has assisted in many augmentation plans involving not-nontributary water, including both reserving deep, nontributary water as well as evaluating the timing and amount of augmentation water needed using the AUG3 model. If you need assistance with deciding about the structure of an augmentation plan or need technical assistance in putting together an augmentation plan to utilize not-nontributary water, give us a call (303-350-4090) or an email.

Bruce Lytle, P.E. bruce@lytlewater.com

Chris Fehn, P.E., P.G. chris@lytlewater.com

Anna Elgqvist, E.I. anna@lytlewater.com

Ben Bader ben@lytlewater.com