User’s Questions:

When I read the engineer’s report that summarized the results of the scenario comparison, I was surprised to find that there really was not a significant difference in the VOLUME summary between the pre-logged and post-logged conditions.  So I decided to try playing with some of the Water Balance Model input variables.  What I found left me confused. 

  1. When I varied the soil type, infiltration was the highest and discharge was the lowest with Clay and silty clay. 
  2. The next thing I did was vary the soil depth.  The deeper the soil, the higher the infiltration.  But with all other parameters being the same, and 300 mm soil depth, when I then removed the forest and changed it to grass (“building lot”), the infiltration was higher, discharge was only slightly higher, and the total losses  were lower, than when the site was forested.

Because of the trees’ evapotranspiration capabilities, we were expecting that discharge and losses would be significantly increased following deforestation.  I am wondering if I am doing something wrong, or interpreting something wrong (i.e. the terms “discharge”, “losses”). Any insight you may have would be extremely helpful!

Jim’s Response:

“The volume summary in the Water Balance Model report does not describe surface runoff alone. Rather, it describes water entering the stream from the project site, or contributing shallow groundwater to the system feeding the stream.”

“This description ignores completely the complex and variable soil moisture reservoir that is always operating just below the surface. The typical engineering approach is extremely simplistic which is why it is really only effective for sizing culverts or storm sewers.”

About Varying of Soil Type:

“While sand can see the greatest volumes being absorbed into the soil, that water cannot be retained as the sand’s water holding capacity is the smallest of any soil. In fact what happens is the water passes through the sand and discharges to the stream in a short period of time.”

“Only a soil with some fines of silt and clay can effectively retain water within its reservoir for use by a plant. This is why sandy soils need far more water to maintain a vegetative cover or to produce a crop.”

About Varying of Soil Depth:

“The soil depth is a factor that can make a difference, but only if it is not over-estimated. The hydrologically active zone may be 300 mm deep, but most forested areas do not have this depth of soil. For example, I am working on a couple of watersheds that behave as though they have only about 100 mm of soil. This may be a result of either bedrock, or an underlying marine clay.”

“It is important to provide a reasonable estimate of depth, and to not overstate it. It is also not reasonable to expect that increasing the depth to values of 300 mm or more will show a change in the hydrology of a project area.  I see many people input soil depths which are overly large into the model.”

“The best way to judge the best value to use is to go to the site and look at an excavation or road cut in a ditch. The colour variation near the surface as compared to the underlying soil horizons will be quite obvious, and will correspond to the appropriate depth for the model describing that project.”

About Forest versus Grassland Surface Condition:

“The thought that a forest will absorb a great deal more water than a healthy grassland is not correct. Both will have quite similar values. The forest, however, will intercept slightly more rainfall because of its greater surface area.”

“Where forests make a significant impact on interception is in colder climates where they intercept snow and allow it to sublimate  from the branches without ever touching the ground or being absorbed and transpired.”

“Trees in an urban setting behave in a different manner due to the enhanced tree canopy which is generally thicker and larger than for trees in close proximity in a forest.”

“If you wish to visualize the difference between a forest and grasslands imagine yourself as an ant on the ground looking upward. You might also go into your yard and spray your long grass for a few minutes. Then examine the ground below the grass to see if it is wet. Next, see how far the water has penetrated into the soil. This will replicate a very typical rainfall day with very little in the way of actual penetration of the water into the soil. Even an hour of sprinkling your lawn, equivalent of 25 mm of rainfall, will only have moisture wetting the upper 75 mm of the soil.”

“Hydrology is much more complex that the engineers have led us to believe; and observing the natural processes provides us with a great deal of insight.”