Background

Changing hydrology due to climate shifts in western Washington forestlands will require landscape level interventions to preserve the timing, flow, quantity, and quality of water that ecosystems and Tulalip treaty trust resources depend upon. In addition, current and historic forest practices, road building, and fire suppression have changed the pattern, condition, and functions of forest ecosystems in the Snohomish River watershed.

New and innovative ways of capturing water within and across the landscape are needed to best maintain the habitats and ecological functions salmon and downstream users depend on. Different forest treatments such as changing stand density or stocking levels, extending harvest rotations, or designing the shape, size and location of harvest units will be modeled in key areas within the watershed to estimate the long term effects on instream hydrology. Increased water quantities during periods of low flow in the summer and autumn will have beneficial impacts for salmon and aid with adaptation to future climate conditions.

This project was focused on two areas:

1. understanding the landscape as a physical topography of depressions and patch openings able to retain water and affect water flows (i.e., can forest management help lengthen snow retention and increase ground water storage in streams and wetlands)
2. understanding how soils and vegetation affect water holding capacity and instream flows

Our Work

To better understand the impacts of forest managment on water storage and instream flows in the Snohomish River watershed, we developed two basin-wide forest condition datasets:

1. a "business as usual scenario" which follows current ownership patterns, management practices, and harvest targets. This is the baseline scenario, used to model the hydrology in the watershed if current management practices continue into the future.
2. an "extreme scenario" in which the forest conditions in the watershed have been altered to increase snow storage at high elevations and water retention in streams and wetlands. This scenario assumes that the theoretically optimal forest treatments can be applied anywhere and anytime they are needed regardless of land ownership, regulations, or economics. This provides the outer limit of what could happen with forest managment, and should provide the largest changes in modeled hydrology.

Harvest unit boundaries were developed for each year in the project timeline for both scenarios, so that the hydrologic modellers could know where and when different treatments needed to occur.

These forest condition datasets and harvest unit boundaries were provided to the larger project team to use as the starting points in hydrologic modeling efforts using both DHSVM, which was used to estimate the contributions of the storage potential/snow melt, and VELMA, which was used to estimate the effects of forest stand structure and age on instream flows.

Products

Forest Management Alternatives for the Snohomish Basin Final Report. This report describes the development of the scenarios and harvest unit boundaries.