Fundamentals of Hydrology: The Water Balance, Catchment Processes, and Flow Measurement
The water balance as the organizing principle
Nearly every fundamentals question reduces to the catchment water balance: P = ET + Q + ΔS, with P precipitation, ET evapotranspiration, Q runoff/streamflow, and ΔS change in storage (soil, groundwater, snow, channel). Candidates must be able to close the balance over a chosen period and recognize that ΔS ≈ 0 only for long multi-year averages, never for a single storm.
Precipitation, losses, and runoff generation
Know the difference between rainfall depth (a volume per unit area, e.g. mm) and intensity (mm/h). Distinguish infiltration-excess (Hortonian) overland flow from saturation-excess flow, and know that the runoff coefficient Q/P rises with antecedent wetness, storm intensity, and imperviousness. Traps: confusing gross rainfall with effective (net) rainfall after abstraction; forgetting that unit conversion (1 mm over 1 km² = 1,000 m³) is tested routinely.
Flow measurement and hydrograph reading
Discharge Q = velocity × cross-sectional area. Stage is converted to discharge through a rating curve, valid only within its calibrated range. On a hydrograph, separate baseflow from quickflow using a straight-line or fixed-base method, and read time of rise, peak, and recession. Traps include reading peak discharge off a different point than the stated time, and assuming the rating curve is linear.
Worked rule of thumb
A 50 mm storm falls on a 10 km² catchment with runoff coefficient 0.4. Effective rainfall = 0.4 × 50 = 20 mm. Runoff volume = 20 mm × 10 km² = 20 × 10⁻³ m × 10×10⁶ m² = 200,000 m³. Sanity check: 1 mm over 1 km² equals 1,000 m³, so 20 mm over 10 km² = 20 × 10 × 1,000 = 200,000 m³.