Date of Award:

8-2022

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Watershed Sciences

Committee Chair(s)

David G. Tarboton

Committee

David G. Tarboton

Committee

Julia Burton

Committee

Thomas C. Edwards

Committee

Sarah Null

Committee

John C. Schmidt

Abstract

Forested watersheds produce more than half of the water supply in the United States. Forests affect how precipitation is partitioned into available water versus evapotranspiration. This dissertation investigated how water yield and snowpack responded to forest disturbance following recent disturbances in western U.S. forests during the period 2000-2019.

Chapter 2 systematically reviewed 78 recent studies that examined how water yield or snowpack changed after forest disturbances. Water yield and snowpack often increased after disturbance, but decreased in some circumstances. Decreased water yield was most likely to occur following disturbances that did not remove the entire forest canopy. It was also more likely to occur in more arid watersheds at lower latitudes, such as in the southwestern U.S., and on south-facing aspects.

Chapter 3 examined 159 watersheds across the western U.S. to determine how often and where water yield increased or decreased following forest disturbance. Overall, more severe forest disturbances, particularly in relatively wet watersheds such as in the Northern Rocky Mountains or Pacific Northwest, were more likely to produce larger water yield. However, forest disturbances in very arid watersheds, such as those in the southwestern U.S., were more likely to result in less water yield.

Chapter 4 developed a new method for more precisely mapping forest canopies and understory forest vegetation. This method used data collected by the U.S. Forest Service’s Forest Inventory and Analysis Program. The maps of separate forest canopy and understory vegetation layers are expected to allow hydrologists to make more accurate predictions regarding the effects of future vegetation changes on water supply.

Previous studies that monitored water yield before and after clearcut timber harvests concluded that forest disturbances would lead to increased water yield. In contrast, the work presented here found that disturbances that do not remove the entire canopy (e.g., due to insects, drought, disease, thinning, low-severity wildfire) may lead to different water yield and snowpack responses than disturbances that remove the entire canopy (e.g., clearcut harvesting, severe wildfire). This work has therefore helped us better understand how future water supply, for people and for ecosystems, will be affected by future forest changes.

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