FRST 557. Forest Operations Module. Lecture 5a. Hydrology Objectives

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1 FRST 557 Forest Operations Module Lecture 5a Hydrology Objectives Lesson Overview: Anyone that has observed forest landscapes will have recognized that water is a significant factor in shaping and maintaining that landscape. The forest plays a significant role in the behaviour of water. Forest management needs to consider the interrelationship of forest and water in planning and developing of forest operations to optimize these two resources and the many resources that are dependent on them. Lesson Preparation: Skim through the following publication. You are not expected to become intimately familiar with the details of this process for the purpose of this course. You will, however, be expected to understand the purpose of the process, the inputs, and the importance of the outcome units as related to forest operations. BC Ministry of Forests. April Watershed Assessment Procedure Guidebook. Forest Practices Code of British Columbia. Lesson Objective: Upon completion of this lesson you will be able to: 1. Recognize some of the effects that forests have on water. 2. Describe risks associated with forest development. 3. Identify factors important to a watershed evaluation. 4. Use watershed assessment as an operational planning tool.

2 This area was clearcut logged over eighty years ago and is now supporting a second growth stand. Some recent harvesting has occurred. What is the cumulative hydrologic impact of all forest operations activity to date? Watershed Processes This lesson will not attempt review details of a course in Forest Hydrology. The fundamental premise here is that trees contribute to water related processes. Included in this contribution are: precipitation interception, retention, and runoff evaporation and transpiration stabilization of soils Many of these processes can be correlated to the closure of the forest canopy which in turn can be correlated to stand density and individual tree size. If forest cover is removed through harvesting, road construction, or any other natural or human activity, then water processes are altered. If this alteration is excessive, the cumulative effects can be detrimental to water quantity and water quality. These changes impact related or dependent resources or resource uses. Cumulative adverse effects are reduced as harvested areas regenerate and grow. Restoration activities on roads or other physical modifications can also reduce detrimental impacts. 2

3 Watershed Assessment A watershed assessment measures the cumulative effects of forest practices and natural processes on the water in a geographic unit. When completed, the watershed assessment will indicate potential risks of: 1. changes to peak runoff and stream flow 2. accelerated surface erosion 3. landslide activity 4. stream bank erosion, channel morphology changes, and channel changes By using a watershed assessment, forest managers can establish limits to development and harvest activities, can mitigate adverse effects, and can prescribe restorative activities to reduce cumulative effects. Measurement Definition: For the purpose of this lesson on watershed evaluation, a clearcut is a polygon (or block ) where all trees in the polygon have been harvested and where the distance to the polygon perimeter exceeds a distance which will produce edge effect. The referenced publication uses a term equivalent clearcut area or ECA as the unit of measurement relating to modifications due to forest management. It includes areas harvested, cleared (e.g. landings or roads), or burned. The unit is modified to consider silvicultural systems, regeneration, and location within the watershed. Refer to Appendix 2 in the Watershed Assessment Procedure Guidebook for details. Note that natural clearings (e.g. grasslands or marshes) are included in the watershed area but are not considered to be clearcut, A polygon harvested to a non-clearcut silvicultural prescription is assigned a reduced ECA to reflect a partially retained canopy. Definition: Edge Effect is defined in this case as an area along a block boundary that is within a horizontal measure equivalent to two tree heights from the boundary. To indicate cumulative effect on the watershed, ECA is expressed as a percentage of the actual area of the watershed. As a regenerated stand is established and grows in height, the value of ECA is reduced as indicated in the following illustration. 3

4 % ECA FRST 557 Lecture 5a Hydrology Objectives Equivalent Clearcut Area Fully Stocked Stands With Crown Closure of % to 7% Average Height of Main Canopy (m) Example 1 If an area of 1 hectares is clearcut harvested, it will be assigned an ECA of 1 ha. That value will remain until regeneration reaches a height of 3 meters, at which time the stand will be 25% recovered or 75% impacted. The ECA is then recalculated at 75% of 1 ha or 7 ½ hectares. At 9 meters height, the ECA of the block will be 1 hectare. Example 2 A 2 hectare watershed has 8 ha of original forest. Harvesting has produced a mosaic of height classes as indicated in the table below. When ECA is calculated, the 12 cumulative harvested area has an ECA of 58 hectares. 4

5 Equivalent Clearcut Area Block Group Area (ha) Average Height (m) ECA % ECA (ha) A 1 B C 2 5 D E F 8 >4 Total Watershed 2 58 Total ECA is 58 / 2 = 29 % of the total watershed area. Detailed Measurement Hydrological impact of a given modification will vary based on relative location in a watershed. Actual measurements for ECA calculations include: 1. Total watershed (or sub-watershed) area. 2. Total area harvested. 3. Equivalent clearcut area by elevation band. 4. Total road (kilometers) and density (km/ha). 5. Length of road in high erosion areas. 6. Total number of landslides. 6. Length of roads on unstable terrain. 7. Number of stream crossings. 8. Percent of stream banks logged separated by stream classification. 9. Length of disturbed stream channel. Refer to the referenced publication for details. In Summary The purpose of a watershed assessment is to provide the forest manager with the information needed that will allow for forest activities while maintaining the hydrological integrity of both individual and complex watersheds. 5