Fundamentals of Hydrology: The Water Balance and Basin Response
Core concepts
Every hydrologic problem rests on the continuity equation: P = ET + Q + ΔS + ΔG, where precipitation is partitioned into evapotranspiration, streamflow, storage change, and deep recharge. Candidates must be fluent in the components and their measurement: precipitation (gages, radar, areal averaging methods such as Thiessen polygons and isohyetal), potential versus actual evapotranspiration (Penman-Monteith, Blaney-Criddle, pan coefficients), infiltration (Horton, Green-Ampt, Philip), and the soil-moisture accounting that links them. The hydrologic cycle is a budgeting exercise, not a list of processes.
Basin characteristics and travel time
Understand the difference between time of concentration, lag time, and travel time. The rational method (Q = CiA) applies only to small catchments, and its runoff coefficient is dimensionless while units must be checked. The unit hydrograph assumes linearity and superposition; SCS/NRCS curve numbers convert rainfall to runoff using antecedent moisture conditions (AMC I, II, III).
Traps to watch
Questions often swap potential and actual ET, or apply a curve number without adjusting for antecedent moisture. Another common trap is unit inconsistency: intensity in mm/h with area in hectares. Watch for the distinction between a gage catch and true areal precipitation in mountainous terrain, where undercatch is significant.
Worked example
A 50 ha catchment has C = 0.35 and receives 40 mm of rain in 2 hours. Peak intensity i = 20 mm/h. Q = C·i·A = 0.35 × 20 mm/h × 500,000 m² = 0.35 × 0.020 m/h × 500,000 m² = 3,500 m³/h ≈ 0.97 m³/s. Rule of thumb: 1 mm/h over 1 ha equals about 2.78 L/s.