Soil scientist Barry Dutton clarifies compact water study
By Barry Dutton
From 1989 to 1992 I measured the actual amount of irrigation water applied to crop plants on the Flathead Indian Irrigation Project (water requirement at-the-plant). This was part of a program funded by the Bonneville Power Administration and the State of Montana. Recently, Flathead project irrigators asked me to summarize those historic irrigation measurements and compare them with the amounts proposed in the new Compact. My measurements were made in the field with rain gauges to determine the actual amount of irrigation applied and not by remote sensing, satellite data interpretation or other indirect modeling method.
My historic measurements show that local irrigators applied up to 18 inches of irrigation water (at the plant) to alfalfa crops, up to 20 inches to mixed hay crops and up to 24 inches to potatoes. This is the actual amount of water applied to crop plants in the field and does not include rainfall. My original draft of this report unintentionally included rainfall bringing the total for potatoes to 28 inches for potatoes but that mistake has been corrected in the final report. This change did not affect any conclusions.
After summarizing the historic data documenting irrigation applications up to 24 inches, I tried to compare these numbers with the amount offered in the current Compact proposal. However, the Compact does not actually say how much water individual irrigators will receive. It appears to leave this decision to future irrigation project managers with no commitment at this time to any historic amount for individual irrigators. Although there is mention about providing historic amounts to irrigators, no actual figures are provided for what irrigators will get.
There are hints of how much water irrigators will receive but this requires significant calculation, far beyond the abilities of most legislators and many irrigators. The proposed Compact assigns blocks of water to large geographic areas of the project (River Diversion Allowances). It does not commit to delivering specific amounts of water to individual irrigators. According to DNRC, the RDAs were based on delivering 12-16 inches of water to individual irrigators. This is significantly less water than some irrigators have historically applied. The RDAs do not seem to consider the efficiency of the delivery system which is approximately 50% efficient. The other 50% is lost to seepage and evaporation. It also does not consider the efficiency of the irrigation system. Flood systems are about 50% efficient and sprinklers 65-80% efficient. When these efficiencies are included it is obvious that the proposed RDAs cannot satisfy the irrigation amounts I measured in the past.
Paul Guenzler, Flathead Irrigation Commissioner recently criticized my report and indicated that Little Bitterroot Valley Irrigators would receive nearly 2 acre-feet of water. This is miss-leading since this is the amount that would be diverted from the stream and not the amount that would be available to farmers at their fields. The Flathead Irrigation Project has approximately a 50% loss rate in its delivery system so 2 acre-feet at the diversion becomes 1 acre-foot (12 inches) at the farm turnout. When you consider the efficiency of irrigation systems (50-80%) this becomes 6-10 inches delivered to the crop plants. This compares with the up to 24 inches I measured in the past.
The proposed Compact language is confusing about the amount of water that will be provided to individual irrigators. It does not seem to provide the amounts of water local irrigators used in the past. The amounts appear to be based on models and predictions instead of actual measurements. It leaves the final decision of how much each individual irrigator will receive up to future project managers. There is no clear process for irrigators to provide documentation and obtain their historic water amounts similar to that available to off-reservation water users.
Barry Dutton
Professional Soil Scientist
Land & Water Consulting