Hydrology: The Water Cycle & Precipitation

The Hydrologic Cycle

Water continuously moves between atmosphere, land, and oceans:

Precipitation → Interception/Infiltration/Runoff → Evapotranspiration → Atmosphere

Key Components

| Component | Description | |-----------|------------| | Precipitation | Rain, snow, sleet, hail | | Evaporation | Water to vapor from surfaces | | Transpiration | Water loss through plants | | Infiltration | Water entering the soil | | Runoff | Surface flow to streams | | Groundwater | Sub-surface storage and flow |

Precipitation Analysis

Return Period (Recurrence Interval)

T = 1/P (years)

• P = annual probability of exceedance
• 100-yr storm: P = 1% chance any given year

Intensity-Duration-Frequency (IDF) Curves

• Shows rainfall intensity vs. storm duration for various return periods
• Used in design of drainage systems
• Higher intensity → shorter duration (for same return period)

Rainfall Frequency Analysis

• Log-Pearson Type III distribution (USGS standard for floods)
• Gumbel distribution for extremes

Rational Method (Small Watersheds)

Q = C × i × A

• Q = peak runoff (cfs)
• C = runoff coefficient (0–1)
• i = rainfall intensity (in/hr) for time of concentration
• A = drainage area (acres)

Runoff Coefficients (C values)

| Surface | C | |---------|---| | Rooftop | 0.90–0.95 | | Pavement | 0.70–0.90 | | Lawn (flat) | 0.25–0.35 | | Forest | 0.10–0.20 |

Time of Concentration (t_c)

Time for runoff to travel from the furthest point of the watershed to the outlet.

Kirpich formula: t_c = 0.0078 × L^0.77 × S^(-0.385)

Summary

The hydrologic cycle drives watershed behavior. Precipitation analysis, runoff coefficients, and the rational method are foundational for storm drainage design.