Water Quality and Watershed Management
What this domain tests
Watershed hydrology, nonpoint source pollution, riparian management, and the BMPs that protect water quality during forest operations.
Core rules
- Forest cover generally reduces peak flows and erosion compared with converted land, but roads, compacted skid trails, and ditches concentrate water and can increase peak flows and sediment delivery.
- Sediment is the dominant forest pollutant. Turbidity is an optical measure; total suspended solids is a gravimetric measure. They correlate but are not interchangeable.
- Riparian management zones shade streams, filter sediment, stabilize banks, and supply large wood. Width and harvest restrictions come from state rules or a certification standard.
- Clean Water Act framework: section 303(d) lists impaired waters and TMDLs set load limits; section 404 regulates dredge and fill; section 401 gives states certification authority.
- Watershed delineation, curve number runoff, and time of concentration are standard tools; the curve number depends on soil group, land use, and antecedent moisture.
Worked example
Curve number method: S = 1000/CN - 10. For CN 75, S = 13.33 - 10 = 3.33 inches. With 4 inches of rain, Q = (4 - 0.2 times 3.33) squared divided by (4 + 0.8 times 3.33) = 11.1 divided by 6.67 = 1.66 inches of runoff. A road that raises effective CN to 85 would produce substantially more runoff from the same storm.
Traps
Assuming a buffer fixes all sediment; using a curve number without checking antecedent moisture; confusing turbidity with TSS; ignoring ditch connectivity to streams.