Quantifying the hydrological effects of converting from flood irrigation to sprinkler irrigation in a semi-arid stream aquifer system

Abstract

Flood irrigation is commonly practiced in many semi-arid alluvial river valleys, particularly in the Upper Colorado River Basin. Due to high inefficiencies, particularly regarding field runoff and deep percolation, this practice can be seen as wasteful at the field scale, leading to calls for a switch to sprinkler irrigation. However, the hydrologic and ecosystem impacts of such a switch is not well understood. This thesis seeks to understand the hydrologic implications of a regional conversion from flood irrigation to sprinkler irrigation in a semi-arid alluvial aquiver system. The region of interest in this study is the area in and around Meeker, Colorado (180 km2), which lies along the White River and its alluvial aquifer system, within the Upper Colorado River Basin. Hydrologic fluxes subject to change include ditch diversions, ditch seepage, irrigation runoff, irrigation recharge, and groundwater-river exchange. To accomplish this objective, we use a MODFLOW groundwater model, modified to include an irrigation package that accounts for all major hydrologic flows in the study region, for the 2019-2024 period. The model is calibrated and tested against measured values of groundwater head, ditch seepage, river flow, and groundwater return flows, with the latter performed for three sections of the White River. From model results, we conclude that during the period of study, approximately 90% of water diverted from the White River for irrigation is replenished by groundwater discharge. Irrigation efficiency is very low (40%), but the "lost" water recharges the aquifer, increasing groundwater storage and groundwater gradients, thereby inducing discharge to the White River and its tributaries. From the scenario of converting all fields to sprinkler irrigation, we conclude the following: 1) Converting from flood to sprinkler irrigation results in a re-timing of river flow, as river flow increases in the summer months due to lower ditch diversions but decreases in the ii fall and winter months due to a decrease in groundwater return flows. On average, groundwater return flows to the White River decrease by 70%. 2) Converting from flood to sprinkler irrigation results in a drastic decrease in groundwater levels, with average declines of 5 ft in riverine wetlands, and 5% of total wetland area experiencing a decrease of over 5 ft. These changes occur due to an overall decrease in hydrologic fluxes and water storage in the soil and aquifer. Less water is diverted from the White River to ditches, which leads to a decrease in ditch seepage recharge, applied irrigation, deep percolation, and groundwater recharge, leading to a decline in groundwater levels, groundwater storage, and groundwater gradients, resulting in less groundwater discharging to the White River and its tributaries, particularly during late-season months. This re-timing of river flow, i.e., less river water in the winter months, can have profound impacts on downstream hydropower and aquatic ecosystems. Furthermore, the decline in water table likely will result in the eventual drying of wetlands, impacting associated habitat and ecosystem services. This study highlights the importance of understanding the unique hydrologic circumstances of individual regions when making major decisions about changing an area's long-standing agricultural practices.