Fundamentals of Hydrology: The Water Balance and the Processes That Shape It
The organizing idea
Nearly every question in this domain traces back to the catchment water balance: P = ET + Q + ΔS (precipitation equals evapotranspiration plus streamflow plus change in storage, with ΔS positive when storage rises). Learn the terms, their typical magnitudes, and how each is measured, and most distractors become visible.
Concepts to master
- Precipitation: gage catch versus areal estimates (arithmetic mean, Thiessen polygons, isohyetal). Know that a single gage represents a point, not a basin.
- Evapotranspiration: potential versus actual ET; reference-crop ET and crop coefficients; the fact that actual ET is limited by water availability, potential ET is not.
- Infiltration and soil water: infiltration capacity, field capacity, wilting point, saturation excess versus infiltration excess overland flow.
- Runoff generation and the hydrograph: baseflow separation, time of concentration, lag, rising limb, recession. Unit hydrograph logic: same duration, linear response.
- Statistics: return period T = 1/p for annual exceedance probability p; the 100-year flood has a 1% chance each year, not once per century.
Traps
Mixing units (mm vs m³/s), confusing potential with actual ET, treating return period as a schedule, and assuming a gage reading is basin-average rainfall.
Worked rule of thumb
Annual balance check: P = 1200 mm, Q = 400 mm, ΔS ≈ 0 over a year, so ET ≈ 800 mm, a runoff coefficient of 400/1200 ≈ 0.33. If a question gives three terms and asks for the fourth, write the balance before reading the options.