|Abstract or Summary
- A model was developed for analysis of the soil water balance of
individual reforestation sites in western Oregon. The numerical
procedure for this model was programmed in compiled BASIC language and
calculations for an entire season are made in about 180 seconds on an
IBM AT microcomputer.
The processes that define the water balance are represented in
five model components: 1) An estimate of potential evapotranspiration
using the energy-based Jensen-Haise method, 2) Partitioning of
evapotranspiration from estimates of percentage cover by vegetation
type and soil surface cover, 3) Actual evaporation at the soil
surface, 4) Actual transpiration from calculations of soil water
supply limit and the Nimah-Hanks uptake method, and 5) Soil water
flow, including redistribution and drainage.
The input data requirement consists of 20 parameters for
characterizing site variation of climate, soil, and ground cover.
Surface cover conditions (slash, mulch, and litter) and plant
competition (grasses, forbs, and shrubs) are featured in the model
because these ground cover variables are a major reforestation concern.
The minimum input data requirement has been simplified in order to
accomodate users with limited data for practical applications.
Output of the model includes estimates of solar radiation, soil
water storage, total water loss, and water allocation. To test the
validity of model output, calculations of water loss were compared to
measured values from a detailed field study of the environmental
effects of site management practices. The results confirm that the
model performs reasonably well and that theory and assumptions of the
model are appropriate.
Results of a sensitivity analysis are given in order to evaluate
the importance of input parameters and to consider the effects of data
error. The analysis demonstrates limitations and possible uses of the
model. The predictive ability of the model was found to be most
limited by the availability of precipitation data; this does not limit
the model as a comparative tool. One important use of the model is the
identification of harsh sites and potential water-limiting conditions.
Model estimates will also facilitate the consideration of forest
management options regarding type of harvest, modification of the
seedling environment, species selection, and choice of stock