Photo by ©Chris Boyer / Kestrelaerial.com
THE PARADOX OF IRRIGATION EFFICIENCY
While there are many obvious benefits of sprinkler irrigation, there is increasing recognition that conversion to sprinkler irrigation or the lining of canals may not always lead to reduced water use or consumption. In fact, a growing number of studies have shown that shifts to sprinkler irrigation can lead to increased water consumption, along with the gradual decline in alluvial groundwater levels, especially when these changes are considered at a larger watershed or basin scale.6-9
Mechanized sprinkler irrigation allows a more precise match between farm water application and crop water requirements, more consistent application of water throughout the irrigation season, and more uniform coverage, all of which can increase crop production. In turn, increased crop production (assuming the same crop type) leads to increased water consumption because of a direct correlation between crop yield (plant growth) and water consumption. Sprinklers may also enable producers to exercise their full water right on one or more fields more often due to time-saving mechanization and reduced labor costs, or irrigate acreage that was unproductive under flood irrigation, e.g., raised or sloped land. With the ability to apply water more precisely and more consistently, producers may also choose to fallow fields less often, or to grow more water-intensive crops. Moreover, in contrast to flood irrigation methods, sprinkler pumps can remove water from the source or canal at lower flows; consequently, a producer may be able to appropriate water more consistently and over a longer duration within the irrigation season. Finally, if more water is left in the ditch or stream early in the season due to conversion to sprinklers, downstream junior water users may be able to appropriate and consume more water than they had
previously, leading to increased consumption at a watershed scale.
The influences of changes to irrigation practice on total water consumption have not been well quantified in Montana, with the exception of a few cases. The landmark Supreme Court decision Montana v. Wyoming, which acknowledged that conversion to sprinkler irrigation in Wyoming led to increased water consumption and reduced return flows to the Tongue River and downstream irrigators in Montana (Case Study 1). A study in the Upper Missouri Headwaters Basin found increased riparian dryness along nearly half (42%) of the river reaches, and those sections with a drying trend had seen a greater land area converted to center-pivot irrigation than those with no drying trend.10 Other examples from the western US suggest that upgrades to irrigation infrastructure can lead to higher water consumption. For instance, conversion from flood to sprinkler irrigation in the Salt River watershed in Wyoming increased average hay production from 1.6 to 2.1 tons per acre11; in New Mexico, drip-irrigated fields had higher rates of production (8-16%) and water consumption than flood irrigated fields growing the same crop.12 In addition, some efforts to reduce water consumption have unexpectedly led to higher consumption. In Western Kansas, irrigators switched from center pivots to dropped nozzle irrigation to reduce water use but inadvertently increased water consumption due to changes in crop choice, fallow practices, and a general increase in water use per unit of land area.13 Although increased consumption with more efficient irrigation may not always occur, this topic clearly merits further investigation and quantification in Montana.
POTENTIAL OUTCOMES OF DIFFERENT IRRIGATION AND CONVEYANCE METHODS
| TYPE OF IMPACT | FLOOD IRRIGATION† OR UNLINED CANALS †† |
SPRINKLERS† OR LINED CANALS OR PIPES†† |
|---|---|---|
| Economic | • Labor and time intensive • Low power costs † • Increased potential need to add nitrate to soil due to leaching † • Annual canal maintenance required †† |
• Lower labor and time intensity † • Increased power costs † • High initial investment • Maintenance costs • Reduces application rates and cost for fertilizers and other agricultural chemicals † (due to precision application by sprinklers) • Increases ability for additional harvest † • Increases ability to irrigate sloped fields † • Increases ability of some producers to earn income from off-farm employment † |
| Water Supply | • Recharges aquifer • Supports groundwater contribution to streamflow • Requires more water diverted from streams/rivers (compared to pivot/pipe) • Reduces spring peak streamflow |
• Reduces aquifer recharge • Reduces groundwater contribution to streamflow • Leaves more water instream at the time and place of diversion • Potential increase in consumptive use at watershed scale † |
| Water Quality | • Leaches nitrates from the soil † • Results in fertilizer runoff into streams and leaching of fertilizer into GW † • Provides cooling effect on summer stream temperatures due to groundwater contribution to streamflow |
• Reduction in leaching of nitrates † • Reduced contribution of agricultural chemicals to surface and groundwater † • Reduced sedimentation of surface water † • Increase in summer stream temperatures due to reduced groundwater contribution to streamflow |
| Ecological (also see Box 4) |
• Maintains natural and/or incidental wetlands • Provides important migratory bird habitat† • Reduced spring peak flow results in: ° Reduced numbers of young cottonwood trees ° Impacts on fish habitat ° Impacts on channel maintenance • Higher diversion rates may negatively impact stream connectivity |
• Reduction in natural and/or incidental wetlands • Higher spring peak flows result in: ° Increased numbers of young cottonwood trees ° Improved fish habitat ° Improved channel maintenance • Lower diversion rates may improve stream connectivity |
Table of Contents | Key Messages | Water and Irrigated Agriculture | Irrigated Agriculture in Montana | The Paradox of Irrigation Efficiency | Hydrology of Irrigated Agriculture | Assessing Consequences of Changing Irrigation Methods | Water Policy and Irrigated Agriculture | Adapting to Change | Conclusion | List of Contributors | Glossary | Footnotes | References