© 1991 Heron Publishing—Victoria, Canada
Modeling the water balance of a small Pinus radiata catchment
D. Whitehead (1) and F. M. Kelliher (2)
1. Forest Research Institute, Private Bag 3020, Rotorua, New Zealand / 2. Forest Research Institute, P.O. Box 31-011, Christchurch, New Zealand /
Summary
An hourly biophysical model was used to calculate the water balance over a period of one year for an 8.7-ha catchment with
a closed-canopy, 13-year-old Pinus radiata D. Don forest in the central North Island, New Zealand. Components of the model are transpiration from the dry tree canopy,
evaporation from the partially wet tree canopy and stems, evaporation from the understory and soil, and drainage from a single-layer
root zone. The model requires input of hourly weather data (net radiation, air and wet bulb temperatures, windspeed, and rainfall),
tree stand characteristics (average height, tree number, leaf area index), physical characteristics of the site (root zone
depth, relationship between root zone matric potential and volumetric water content, the relationship between the rate of
drainage from the root zone and volumetric water content, and the area of open-stream channels). A submodel of the response
of stomatal conductance to air saturation deficit and root zone matric potential is also required.
Tree transpiration (704 mm year–1 or 50% of annual rainfall) was a dominant component of the catchment water balance. Estimated evaporation from the wet tree
canopy was 203 mm year–1 (15%). Evaporation from the understory was much less, amounting to 94 mm year–1 (7%) and an increase in water storage for the 3.5 m root zone depth was estimated to be 53 mm year–1 (4%). Estimated daily rates of drainage generally agreed well with measurements of streamflow, although estimated annual
drainage (349 mm year–1, 24%) exceeded measured streamflow (234 mm year–1).
The significance of the results is discussed in relation to closure of the hydrologic balance.
