The Use of a Numerical Ground Water Flow Model to Assess the Effects of Leakage from Irrigation Canals and Laterals on Domestic Water Supply Wells in the Helena Valley, Montana
The Helena Valley Irrigation District (HVID) consists of more than 15,000 irrigated acres served by more than 50 miles of irrigation canals and laterals. Leakage from the HVID canal and laterals and infiltrating excess irrigation water applied on fields currently accounts has been estimated to be about 25 percent of the ground water recharge to Helena Valley aquifer system. About 20,000 people in the valley rely on ground water pumped from more than 6,000 wells as their only source for domestic supply. Any hydrologic changes in the valley that diminish ground-water recharge could cause water levels to drop and wells to go dry. Lining the irrigation canal and laterals would diminish the amount of recharge to the valley-fill aquifer system, but how this would affect wells in the valley is unknown.
The focus of this research project is to use a numerical ground-water flow model to assess how lining the HVID canal and laterals would affect the ground-water flow system and water wells.
Need and Benefit
The demographics of Montana have changed rapidly in the past 20 years. Areas in the valleys of western Montana that were once rural and used for agriculture have experienced urbanization and increased population growth. Because city services do not extend to these areas, residences depend on wells for their domestic water supply.
The Helena Valley is no exception to the rapid growth that the valleys of western Montana has experienced during the past two decades; more than 20,000 people currently living in the valley depend on ground water from 6,000 wells for their drinking water source. Changes in ground-water recharge---such as lining the canal and laterals---to the valley-fill aquifer could potentially adversely impact the static water level in wells. Leakage from the canal, laterals, and infiltration of excess water has been estimated to be about 25 percent of the recharge to the valley-fill aquifer system; the effects of diminishing the recharge by lining the canal and laterals have never been quantified.
Lining the main canal and laterals or even putting the water in a pipe is an attractive alternative. Lining the canal and ditches would conserve water; replacing the main canal with a pipe would eliminate siphons and other hydraulic structures that are attractive nuisances as these once rural areas house more families and children. The pipe alternative would greatly reduce the need for weed and grass control along the 22-mile canal.
Lining the main canal and laterals, or any proposed action that would decrease recharge to the valley would almost certainly be met with opposition from some of the valley residents as they may perceive a threat to the viability of their drinking-water supply. Conversely, some planned development of the ground-water resources has been scrutinized because it is hypothesized that these new wells would depend largely on leakage from the canal and lateral, and that future leakage from the irrigation system could not be guaranteed. Understanding how the canals and laterals interact with the ground-water flow system through the use of a ground-water flow model would be the first step toward assessing the impacts of lining the canal and laterals.
Because of numerous ground-water studies in the Helena Valley, an abundance of hydrologic data exist that can be used to calibrate a steady-state and transient ground-water flow model. These data include up to about 25 years of water levels in monitoring wells; long-term stream-flow records; leakage studies on the canal, laterals, and streams; and hydraulic properties of the valley-fill aquifer. The Montana Bureau of Mines and Geology also maintains a database containing well-completion reports for the more than 6,000 wells in the valley.
The benefit of a ground-water flow model includes a quantitative understanding of how the irrigation canal and lateral leakage affects the ground-water flow system. Using this understanding, decisions can be made in regards to how lining the canal and laterals would impact the 6,000 wells serving the 20,000 citizens. Indirectly, a potential benefit may be water saving if it can be shown that the impacts of lining the canal and lateral are minimal. Other districts may benefit if it can be demonstrated that a numerical ground-water flow model is a useful tool for investigating ground water/surface water interactions such as leakage from irrigation features.
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