Hydrology: The Hydrologic Cycle & Runoff Estimation

The Hydrologic Cycle

Water continuously moves through:

1. Precipitation — rain, snow, sleet
2. Infiltration — water entering soil
3. Evapotranspiration — water returning to atmosphere
4. Runoff — water flowing over land surface
5. Groundwater flow — subsurface movement

Rational Method

For small urban watersheds (<200 acres):

Q = CiA

• Q = Peak runoff (cfs)
• C = Runoff coefficient (0–1)
• i = Rainfall intensity (in/hr)
• A = Drainage area (acres)

Runoff Coefficients

| Surface Type | C Value | |---|---| | Lawns, good condition | 0.25–0.35 | | Pavement | 0.70–0.95 | | Rooftops | 0.75–0.95 | | Parks / Green areas | 0.10–0.25 |