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