Natural Resources and Environmental Systems
Watershed thinking
Natural resource questions start at the field or watershed scale. Know the difference between soil erosion (detachment and transport on site) and sediment yield (what actually leaves the field), and between peak-flow estimation (Rational method, TR-55) and volume or continuous simulation (curve number).
Core tools
Rational method: Q = CiA, with i in in/h, A in acres, and Q in cfs (the constant is about 1.008). Use a storm duration equal to the time of concentration.
NRCS curve number: S = 1000/CN - 10 inches; Q = (P - 0.2S)^2 / (P + 0.8S) for P greater than 0.2S. A higher CN means wetter soils, poorer cover, or more impervious area.
RUSLE: A = R K LS C P, with A in tons/ac/yr. Only the C and P factors respond to management.
Water quality and waste
Understand animal waste management (nutrient management plans, lagoon sizing, land application rates set by crop nitrogen uptake), buffers and riparian zones, and groundwater protection. Regulatory hooks include NPDES CAFO permits, TMDLs, and wetland rules under Section 404.
Traps
- Unit mismatch in the Rational method: intensity must be in in/h for Q in cfs with acres.
- Applying a curve number without adjusting for antecedent moisture or land-use change.
- Confusing peak discharge with runoff volume.
- Using a storm intensity for a duration other than the time of concentration.
- Assuming erosion control changes R, K, or LS; it only changes C and P.
Worked example
CN = 80, P = 4.0 in. S = 1000/80 - 10 = 2.5 in. Initial abstraction = 0.2S = 0.5 in. Q = (4.0 - 0.5)^2 / (4.0 + 0.8 x 2.5) = 12.25 / 6.0 = 2.04 in of runoff.