Hydrology
Hydrology covers precipitation, infiltration, evaporation, runoff, hydrographs, flood estimation and groundwater. The exam favours the rational formula, unit hydrograph and return-period concepts.
Core rules
- Mean catchment rainfall can be found by arithmetic mean, Thiessen polygons or isohyetal method; Thiessen is preferred when raingauges are unevenly spaced.
- Infiltration index φ = (P − R)/t for a storm; it represents average abstraction.
- Rational method: Q = CIA/3.6, with C as runoff coefficient, I in mm/h and A in km², giving Q in m³/s.
- A unit hydrograph is the direct runoff hydrograph from 1 mm of effective rainfall over a unit duration; it assumes linearity, uniform intensity and time invariance.
- Return period T = 1/p; Gumbel and log-Pearson methods are used for flood frequency.
Worked example
A 25 ha urban catchment has C = 0.4 and experiences 60 mm/h rainfall. Convert area: 25 ha = 0.25 km². Q = CIA/3.6 = 0.4 × 60 × 0.25/3.6 = 6.0/3.6 = 1.67 m³/s. Using 25 directly in the formula would give 166.7 m³/s, a 100-fold error.
Traps
Feeding hectares into a formula requiring km²; mixing rainfall intensity and depth; using a 1-hour unit hydrograph for a 3-hour storm; ignoring base-flow separation; treating φ index as a soil property.