The model described here called ForWaDy (Forest Water Dynamics) was constructed around the foundation of FORHYM, a general water balance model designed by Arp and Yin (1992) for the simulation of water fluxes through temperate forest ecosystems. The major difference between the two models lies in the nature of the central potential evapotranspiration (PET) algorithms. While FORHYM uses a PET equation based on relationships with air temperature, ForWaDy drives PET using an empirically based energy budget approach. The energy budget approach employed in ForWaDy allows the model to partition evapotranspiration into its primary components, thereby facilitating the capacity to simulate water competition. The daily energy available for evapotranspiraton is divided among canopy trees, understory plants and the forest floor based on the proportional interception of incoming solar radiation by each layer adjusted for reflection depending on surface albedos. Subsequently, a passive competition for available soil moisture is simulated through the use of an algorithm that combines species-specific root occupancy information with energy-limited transpiration and evaporation demands.

Canopy water stress is determined as a function of energy-limited canopy transpiration demand and soil-limited actual canopy transpiration through the calculation of a cumulative water stress index. This 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 and FORCEE. The simulation of snowfall and snowpack dynamics in ForWaDy is based on the RHYSSys Snow Model (RSM) (Coughlan and Running 1997).

All equations describing water flow within the model are solved using a simulation dt of 0.25 days. The use of a dt less than one day enables the model to divide the flux of water from a particular soil reservoir among competing outflows.

Figure 1. Schematic of the forest hydrology model indicating the various flow pathways and storage compartments in the model. Compartments with bold borders indicate areas of the model which will facilitate a future integration with FORECAST and FORCEE.