Year of Award

2023

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Systems Ecology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Kelsey Jencso

Commitee Members

John Kimball, Douglas Brinkerhoff, Marco Maneta, Justin Huntington

Keywords

drought, evapotranspiration, irrigation, streamflow, sustainability

Publisher

University of Montana

Abstract

Irrigation represents our greatest intervention in the hydrological cycle, accounting for over 80% of extracted freshwater in the Western U.S. Despite its economic and ecological importance, irrigation’s spatial and temporal occurrence, magnitude, impacts on streamflow, and response during water shortages has not been characterized in our region. The major objective of this dissertation was to systematically assess irrigation over the Western US to answer the following questions: 1) Where and when does irrigation occur within the study region? 2) How has the intensity, area, and distribution of irrigation changed over the course of the past 35 years? 3) What impact is irrigation having on surface water (i.e., rivers) in the region, how do impacts differ across basins, and what is driving them? 4) Finally, how do irrigators respond to drought and what can be expected of the irrigated system during water scarce times? To answer these questions, we developed a 35-year dataset covering the western U.S. consisting of high resolution satellite-derived irrigation and evapotranspiration data alongside streamflow, field-scale agricultural boundaries, and detailed climate information. We used a combination of data-driven methods to infer the behavior of irrigation in time and space, Bayesian regression modeling to define relationships between climate, irrigation and streamflow, and detailed geospatial and economic data analysis to explore drivers of the behavior of irrigated systems. We show that both climate change and irrigation are impacting streamflows, and that contrary to government statistics, irrigation is expanding in intensity, area, and water use. We show evidence for the large-scale operation of the ‘paradox of irrigation efficiency’, where despite increasing on-farm irrigation efficiency enabled by advances in irrigation infrastructure, basin-scale crop water use increases. We show how streamflow is changing and where the changes are driven by changes in climate and irrigation. Finally, we show that crop prices appear to drive crop planting decisions (and thus irrigated water use) to a greater degree than seasonal climate conditions and that intensely irrigated regions are unresponsive to drought. This dissertation contributes to our understanding of the systems-level impact of irrigation and provides opportunities for basin-specific management actions to mitigate irrigation and climate impacts on streamflow.

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© Copyright 2023 David Granger Ketchum