Gallatin Valley Resources Evaluation

The Montana Department of Natural Resources and Conservation (DNRC) is proposing sweeping state-wide legislation which would limit the flexibility of applicants to obtain new beneficial use permits for ground-water appropriations. This legislation pertains to public water supply wells, agricultural wells and exempt wells. DNRC believes “cumulative impacts” are occurring because of ground-water use.

DNRC further hypothesizes that this is adversely impacting an unnamed group of senior appropriators. DNRC has proposed exempt well legislation (HB 104) and augmentation legislation (HB138) without conducting thorough evaluations of stream flow data or ground-water levels at a watershed or sub-watershed scale. Such analysis is necessary before proposing sweeping legislation that would affect all sectors of Montana.

It is important to determine “what the data are showing” before it can be conclusively ascertained if “cumulative impacts” are occurring. NE&W is currently evaluating the available data at a watershed scale in various areas of Montana to test the validity of DNRC’s “cumulative impacts” hypotheses. This report discusses an evaluation performed for the Gallatin Valley which is located at the northern end of the Gallatin watershed. This valley has one of the highest exempt well densities and number counts of any area in Montana. Hence, if DNRC’s hypothesis is true, then the data should provide statistically definitive evidence that ground-water levels are generally declining and that stream flows are being reduced. Otherwise, the DNRC “cumulative impacts” hypothesis should be rejected. Climatic, stream flow, ground-water data, and other information were used to test DNRC’s hypothesis.

Based upon the above evaluation, the following were key findings:

1) Stream flow of the Gallatin River and at Logan for a given water year is highly dependent upon each given year’s mountain snow pack in the Gallatin Watershed. Snow pack in the last seven years has been far below average. This has led to a period of lower than average stream flows in the Gallatin River and other streams entering the valley.

2) By far the most significant human-induced influence on stream flow in the valley is surface-water diversions for irrigation.

3) Careful scrutiny of the data from the 1930s to 2000s demonstrated there was no trend for change in stream-flow behavior over this lengthy period of time.

4) There is no evidence of “cumulative impacts” on stream flow from wells.

5) Ground-water use from wells is inconsequential when compared to stream flows. For instance, total domestic (household) consumption of groundwater from exempt wells is negligible and equates to about 0.01 % of Gallatin River flow entering the valley annually. A worst case estimate for consumption from lawn and garden irrigation in the Gallatin County associated with exempt wells equates to about 0.2 % of the water entering the valley annually. For another perspective, the total amount of consumptive use from all exempt wells combined in Gallatin County equates to about 3 to 9 percent of the total ground-water consumption lost to cottonwoods and willows in the Gallatin Valley. A worst case estimate of consumption from other irrigation wells equates to less than 1.7 % of the water entering the valley annually.

6) The actual net effect of wells is much less than the computed percentages provided above because most of these wells are simply used to irrigate land that had been irrigated previously with surface water.

7) There is no evidence that consumptive use has increased in the valley with the growth of city/rural subdivisions and their accompanying use of ground water. In order for consumptive use to increase, there must be an increase in irrigated acreage compared to historically irrigated acreage. Most subdivisions have been placed in areas that had been irrigated historically.

8) The amount of irrigated acreage in Gallatin Valley may have actually decreased with time, especially in areas where rural subdivisions exist. A strong argument can be made that overall consumptive use of water in the valley is declining as a result of subdivision growth.

9) Ground-water levels in Gallatin Valley have not changed significantly since the 1950s.

10) In order to reliably assess the overall implications of ground-water use on stream flows, it is necessary to define the land use both before and after wells are used for irrigation purposes. In a majority of the instances where wells are being used for irrigation purposes that same land had been irrigated before by surface water. Based upon a review of infrared imagery in the valley, it is apparent that there is less irrigation in areas where subdivisions are present. In order accurately quantify the relative significance of wells on the overall water budget in the valley, it is necessary to add and subtract to determine the net changes that may or may not occur.