January 29, 2010

Water, water everywhere. But does it runoff or does it sink?

Article reviewed:  Effects of forest biomass use on watershed processes in the western United States

By W.J. Elliot. Published in the Western Journal of Applied Forestry Vol. 25 12-17 available for download here.

The plot line: This article is a review of the potential influence of forest harvest operations, including biomass harvests, on hydrologic processes in forests. The author explains some of the processes by which forests can alter both water yield and water quality. While there are often increases in runoff and sedimentation (i.e. decreases in water quality) following harvests, the magnitude of effects can be minimized by maintaining roads and with operational practices that reduce compaction and soil exposure. The net result of biomass removal harvests can be an improvement in water quality if operations are effective at reducing wildfire severity because wildfires are usually the greatest source of sedimentation in dry western forests. An example of how managers can estimate the net effect of a harvest operation on erosion is given.

Relevant quote: When evaluating the risks of erosion associated with biomass use, managers should also consider risk associated with wildfire, as that risk will decline as forest fuels are removed.”

Relevance to landowners and stakeholders:

Removal of trees from a forest can increase short-term yield in two ways:

1.     Less interception. Snow and rain falls directly on tree canopies, and some of that water is evaporated directly back into the atmosphere without hitting the ground. This can be especially relevant where it snows, as snow sticking to trees evaporates via sublimation (remember that from high school chemistry?). Interception losses can be high- from 10 to 35% depending on how much precipitation falls as snow

2.     Less transpiration. Trees take up water from the soil and use it for photosynthesis (remember that from high school biology?). Trees aren’t perfectly efficient in how they use water, however, and a lot of it is evaporated into the atmosphere. Kind of like when we breathe, water vapor comes out.

Water utility companies, farmers, or city dwellers with water meters might conclude that we should cut down all the trees and thereby increase water yield to reservoirs. There can be a tradeoff, however, between water quantity and water quality. Cutting down all the trees would indeed increase water yield, but it would also decrease water quality if erosion increases. When impacts are minimized, harvest activities that effectively reduce wildfire severity can improve both water quality and water yield because one severe wildfire can be a huge source of erosion that dwarfs all other sources associated with harvest activities. The key, of course, is that the operation is actually effective at reducing wildfire severity. Methods for reducing wildfire severity were discussed in this post.

Relevance to managers:

Wildfires are the #1 source of sediment. Roads are #2. This article reviews some of the practices for minimizing soil movement associated with roads, including:

  • removing unwanted road segments, especially segments with culverts and stream crossings
  • outsloping roads with rolling dips
  •  closing roads to vehicle use in the winter
  • monitoring culverts and drains for blockages during storms
  •   using gravel where roads cross streams

The article gives an interesting method for estimating how much erosion a given operation might cause. The key input to the equation seems to be estimating how much a severe wildfire would contribute to erosion, versus a low-severity fire. I’m not sure how feasible it is to accurately predict this estimate, but the equation makes sense anyway. This tool, as well as others for designing roads, are available here.

Critique (I always have one, no matter how good the article is) for the pedants:

The title and introduction suggests that this article will be specifically about effects of biomass removal, but there was little detail given about what distinguishes biomass harvests from any other type of harvest. I would have liked to have seen more evidence or examples to support the suggestion that “harvesting trees for biomass has the potential for greater onsite impacts than common with current forest harvesting practices [no references given].” In other words, what is it about biomass harvests, per se, that makes them have greater potential impacts ompared to timber harvests?

Perhaps I am thinking too narrowly about the types of biomass harvests I have seen in California. Most that I have observed involve removal of smaller trees in conjunction with thinning projects that keep much of the overstory intact. In such a case, I suppose it is possible that a biomass harvest could have a greater impact than a timber harvest if:

  •   there is a greater frequency of entry to harvest small trees on a short rotation
  •   there are larger landings and roads to accommodate the different equipment used (chip vans, chipper, etc.); this is indeed often the case
  •  road networks are expanded into areas to harvest biomass that would not have been entered to harvest timber

-Rob York

Send comments and article or topic suggestions to ryork [@] berkeley.edu

copyright 2010 Robert York