Forest Water Dynamics (ForWaDy) Model for Simulating Forest Stand Hydrology
Purpose: Provide stand level moisture inputs to the FORECAST model that reflect alternative climate projections and silviculture systems.
Alterations in forest hydrology, whether induced by management or climate change, can have a significant impact on the partitioning of limited water resources among trees and understory species and ultimately on tree water stress and stand development.
ForWaDy is a 2-dimensional model designed to simulate the hydrologic dynamics of a forest stand under a given set of climatic and vegetation conditions. It was developed for integration with the FORECAST ecosystem model to simulate effects of forest water dynamics specifically on stand growth and development. When integrated, ForWaDy can be used for exploring the effects of water competition and climate scenarios (e.g. shifting rainfall patterns) as well as provide flexibility in predicting the effect of various silvilcutural systems on forest floor moisture contents.
ForWaDy uses an energy budget approach for calculating water loss in forest systems through evaporation and plant transpiration (evapotranspiration). Such an approach eliminates the need for certain experimentally determined data and allows the model to partition evapotranspiration into its primary components to simulate water competition. The daily energy available for evapotranspiraton to occur is divided among canopy trees, understory plants, and the forest floor based on the amount of sunlight intercepted by each of these forest layer.
Subsequently, competition for available soil moisture is simulated in ForWaDy. Canopy water stress is determined through the calculation of a cumulative water stress index. The water stress index represents a dynamic measure of tree water stress over a given time period and may be used as a parameter to limit tree growth rates based on light and nutrient availability in models such as FORECAST. Snowfall and snowpack dynamics is also simulated in ForWaDy.
ForWaDay has been validated on a reclamation site within the Oil Sands area, a Douglas-fir plantation at Shawnigan Lake, BC and a high elevation Montane Alternative Silviculture Systems (MASS) site on Vancouver Island.