Agricultural landscapes across the Western United States are rapidly urbanizing, causing substantial land use and land cover changes as well as shifts in our water resources. Canal systems originally designed to deliver water to agricultural fields are now distributing water to homeowners and cities to irrigate lawns. Urbanization increases the amount of impervious surfaces – surfaces that don’t allow water to pass through them, such as concrete – covering the landscape. Changing the land cover and reducing the overall green space impacts the amount of water needed to irrigate a plot of land, but how this changes irrigation demand is still being explored. Urbanization also impacts the volume of water draining from the irrigated acres and returning to a river. Water enters drains through direct field runoff, through discharge from the groundwater into the drains, and through direct flow from the end of a canal into the drain. So, decreasing crop field coverage and, in turn, direct field runoff, and decreasing the amount of water that can enter the ground through increased impervious surfaces, will likely diminish drain flows into a river.

 Simultaneously, climate change is causing hotter and drier summers, increased flow in our rivers earlier in the year from earlier snowmelt, and more frequent drought. Characterizing how both urbanization and climate variables impact drains returning irrigation water to a river can help water managers, who rely on using the returned water for downstream irrigators, to better manage the system.

In this project, Bridget Bittmann will work with the Idaho Water District 63 watermasters in developing a model to determine how climate and urbanization are affecting the amount of water draining to the Boise River in the Treasure Valley of Idaho. The watermasters rely on accurate return flow estimates to redistribute the water to downstream water users and will use the information in this model to better understand how the drains have changed over the past 34 years. The model will help watermasters understand whether the drain flows will continue to be a reliable source of available irrigation water in the coming years. The information learned from this project can help the watermasters identify downstream water users who are particularly vulnerable to changes in return flows and will help the watermasters communicate with these water users.