User’s Question:

The site in question is a 5-hectare site situated within a 60-hectare drainage catchment. 

The pre-existing condition was forest, and the post-development condition is cleared (deforested) land. For the purposes of comparing before and after conditions, the engineer selected “grass – building lot” as the deforested surface condition.

We were quite surprised to see a very minimal difference in discharge, losses and infiltration between the forested and deforested conditions.  Then we noticed that the analysis had been based on a 60-hectare catchment area. What is the significance of selecting 60 versus 5 hectares.

Jim’s Response:

“If the site has 5 ha then why is the modeller using 60 ha?  This is a very good way of masking the changes happening on a small site by averaging the lack of change across a much larger area.”

“In a very temperate coastal area we do not anticipate large snow pack development during winters. It is the large snowpacks that accumulate in clear-cut forests that alter the hydrology of the interior portions of the province.”

“On the coast there will be some increases in stream discharge resulting from clear-cuts, but not nearly as great as in areas that accumulate a great deal of snow.”

“If your site is up and away from the climate gauge used in the Water Balance Model, then there will be some differences. Even the 1 degree per 1,000 foot elevation loss as you go upslope will make a big difference in snow accumulation.”