The ecology of an irrigation system: wetland creation in an agricultural landscape
Date
2012
Authors
Sueltenfuss, Jeremy, author
Knight, Rick, advisor
Waskom, Reagan, committee member
Cooper, David, committee member
Wallace, George, committee member
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Abstract
Irrigation has increased the agricultural productivity of the arid American West, but has also greatly altered the natural landscape. Irrigation canals transport water to 17 million ha of currently irrigated land. Because water is a limited resource in the west, and irrigated agriculture uses approximately 90% of all the water diverted from rivers, much attention has been paid to the efficiency of irrigation systems. Irrigation canals have been shown to leak up to 50% of the water they transport, affecting both groundwater recharge and return flows to rivers, though little work has been done documenting the ecological effects of irrigation canal seepage on wetland ecosystems. This study sought to identify the hydrologic processes linking canals and reservoirs to wetlands, identify the types of wetlands supported by irrigation canal seepage, and document the area of wetlands supported by irrigation within the service area of an irrigation company. All wetlands within the North Poudre Irrigation Company service area in Larimer County were mapped and their hydrologic source determined from visual clues. Groundwater monitoring wells were installed in wetlands adjacent to canals and reservoirs to identify the hydrologic influence of canal seepage on wetland hydrologic regime. To further demonstrate the hydrologic source of wetlands, stable oxygen isotopes were analyzed within wetlands and possible adjacent water sources. Vegetation characteristics and species percent composition was related to environmental variables to highlight the types of wetlands supported by an irrigation infrastructure. A total of 176 wetlands covering 652 ha were mapped, 92% of which were visually connected to the irrigation infrastructure. Wetland water tables fluctuated with adjacent canal flow, with increases in the water table when canals started transporting water, and decreases in water table depth during times when canals did not carry water. Isotopic data indicate that canal leakage is the hydrologic source for adjacent wetlands within the study area. The isotopic signature of canal water matched that of wetlands closer to canals, with evaporatively enriched isotopic signatures in wetlands further from canals. Wetland vegetation composition was related to both salinity and groundwater depth, with salt flats dominated by Atriplex spp. forming in areas with high salinity, marsh communities dominated by Typha latifolia and Schoenoplectus acutus forming in areas with low salinity and deeper standing water, and meadow communities dominated by Carex nebrascensis and Schoenoplectus pungens forming in areas with low salinity and water tables closer to the ground surface. Though land conversion and water diversions have led to dramatic reductions in historic wetland area in some places, it is clear from this study that current agricultural landscapes create wetlands that rely on excess irrigation water for their hydrologic maintenance. Any future changes in irrigation practices or water distribution may have negative consequences on wetland ecosystems.
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Subject
wetland
agriculture
irrigation
canal seepage