Introduction

The Bell Rapids Mutual Irrigation District is located adjacent to the Hagerman Fossil Beds National Monument on the Bruneau Plateau. The irrigation company operates and maintains an irrigation system that pumps water from the Snake River to irrigate approximately 19,000 acres of land on the plateau. The water license priority for Bell Rapids Irrigation Company is dated December 16, 1963 and construction of the irrigation facilities began in 1969 with the first irrigation season in 1970 (Vector Engineering, 1994).

The irrigation system consists of river pumps, three penstocks, 10 miles of delivery canal system, a canal transfer pump station, regulating ponds, canal lateral line pumps, and over 110 miles of lateral and mainline distribution pipe. The main pumps lift water from the Snake River up 600 feet in elevation where it is emptied into the Fossil Gulch Canal and lateral lines along the remainder of the canal system pump water out of the canal for field application using sprinkler system technology (Vector Engineering, 1994). This system provides pressurized irrigation water to approximately 25 individual farms. The original irrigation project had two main pump stations named the Fossil Gulch and Bell Rapids pump stations. In 1987 a massive landslide eliminated the Bell Rapids pump station.

As a result of the landslide the Fossil Gulch station increased pumping rates and additional lateral lines were constructed to distribute water to areas formerly supplied by the Bell Rapids Canal. The Fossil Gulch Canal was realigned, straightened and lined with concrete for the first one-half mile after the 1987 irrigation season due to an increase in demand. The now buried first one-half mile canal segment was part of all the canal leakage studies prior to the realignment. The rest of the Fossil Gulch Canal system and storage ponds are unlined with synthetic materials but do have natural calcium carbonate (caliche) in segments that may act as a natural liner.

The landuse change of the plateau from sagebrush desert to farmland has also coincided with formation of multiple perched aquifer systems that discharge at various locations. The largest discharge volumes and most numerous discharge sites are located near Fossil Gulch Basin on the hillside about 250 feet below the plateau surface. Many studies have been conducted since 1984 indicating the source of recharge to the perched aquifers is the Bell Rapids Irrigation System.

Five separate seepage studies have been performed on the Fossil Gulch Irrigation Canal and pond. The first study was completed by the U.S.G.S. (Young, 1984) in cooperation with the B.L.M. in 1984. This study measured flow in the Fossil Gulch Canal at two locations. The amount of water leaking from the canal was calculated based on the difference between two flow rates. About 1,900 AC-FT of water leaks from the first mile of the Fossil Gulch Canal per irrigation season. A second study by Worstell (1985) was performed in response to a request from the Bell Rapids Irrigation District. Worstell measured canal leakage by a seepage meter technique that is commonly used for such studies. The first half-mile of canal length was analyzed using this technique and results indicate 193 AC-FT of water leak from the canal per irrigation season. Montgomery (1987) conducted a third canal leakage study in October 1987 by constructing a water impoundment in the first one-half mile of Fossil Gulch Canal. Montgomery (1987) calculated leakage at 299 AC-FT per season.

In 1992 the fourth study was implemented by the National Park Service (N.P.S.) Water Resources Division (Martin, 1992). Leakage tests were conducted on three sections of the Fossil Gulch Irrigation Canal and two irrigation holding ponds. The results from the N.P.S. study show 360 AC-FT of water per irrigation season is lost from the first one-half mile segment of unlined canal. The fifth study was performed in 1994 by Vector Engineering, Inc.. This study was designed to measure the canal flow directly every 500 feet for the entire length and use this flow rate data to indirectly calculate canal leakage similar to the U.S.G.S. study. The results of the Vector study were stated as inconclusive but a leakage rate of 5,000 AC-FT per season for the entire canal system was reported.

Summary of Canal Leakage Studies

Table 4 lists estimates from the five different studies for the volume of water leaking from the Fossil Gulch Canal for an average irrigation season. Changing geologic conditions in the canal base will influence extrapolation of canal leakage data. For instance, the Tuana Gravel Formation is below the canal at the inlet area near the edge of the bluffs. Conversely, near the end of the canal the Tuana Formation is observed higher in elevation on the hillsides above the canal where active gravel pits are located in section 31. Drill well logs from monitor wells N.P.S.- 4 and 5 lateral to the canal support this geologic condition. It is probable the canal crosses the contact between the Tuana Gravel and underlying finer grained Glenns Ferry Formation somewhere within the first mile segment of canal.

Table 4. Comparison of canal leakage rates. The measured data from short segments of the unlined canal was extrapolated for the entire length of the canal with the leakage rate assumed to be constant for an average length irrigation season of 180 days.

Young (1984), Worstell (1985) and N.P.S. (1994) canal leakage tests on the Fossil Gulch Canal were performed in proximity to this geologic contact. Young's (1984) flow measurement site #3 is located near the start of the canal and test site #4 is about one mile downstream (Figure 9). It is likely that somewhere between #3 and #4 flow measurement sites is the Tuana/Glenns Ferry contact. The study by Martin (1994) indicates a decrease of water leakage from the middle unlined test impoundment to the lower unlined test impoundment. This may be a function of the Tuana/Glenns Ferry Formation Contact. It is conceivable that the lower leakage test impoundment may have been performed in the finer grained Glenns Ferry Formation. Worstell (1985) notes gravel at the first two test sites which indicates Tuana Formation in this area. It appears most of the leakage studies have been performed within the Tuana Formation and therefore biasing the projected results for the remainder of the canal system. The Fossil Gulch Irrigation Pond leakage study performed by Martin (1994) indicated a water leakage of 140 acre-feet per season. A degree of uncertainty is associated with the reported pond leakage data due to problems with data collection.

Generally speaking there may be a bias in the reported canal leakage data due to changing geologic conditions along the canal. This would effect extrapolation of the data to other segments of canal. The Fossil Gulch Canal appears to intercept the Glenns Ferry Formation for most of its length base on the canal geology investigation.

Description/Purpose

In 1995 Vector Engineering conducted a water budget analysis for the total Bell Rapids irrigation system. The system was assessed as a single unit that includes approximately ten miles of canal and four ponds. Water loss due to infiltration was calculated at 10 percent of the total volume of water pumped into the irrigation system during 1991 (Vector Engineering, 1995).

Results

The results of the water balance study are illustrated in Table 5.

Inflows to the canal system were calculated at 48,600 acre-feet while irrigation outflows were calculated to be approximately 43,400 acre-feet. Evaporation was estimated to be 150 acre-feet and precipitation 30 acre-feet for the Bell Rapids Irrigation District in 1991. Canal leakage estimate for the entire Bell Rapids Irrigation District amounts to 5,080 acre-feet for the 1991 season.

Conclusions

Vector Engineering (1995) calculated that 10 percent of the total volume of water pumped into the Bell Rapids irrigation system is lost to infiltration. This loss amounts to about 5,000 acre-feet of water. It is important to note this amount is for the entire irrigation system consisting of ten miles of canals and four ponds and not just the first three miles of canal and Fossil Gulch Pond.

Table 5. Results of water balance study from the Vector Engineering illustrating estimates of water inflows and outflows for the total Bell Rapids Irrigation District (modified from Vector Engineering, 1995).

Table of Contents
Chapter 1 | 2 | 3 | 4 | 5
Appendix A | B | C | D

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