AIH Professional Hydrologist - Surface Water Examination common mistakes
AIH Professional Hydrologist - Surface Water Examination · Understand the traps and how to avoid them.
Computing a runoff volume or peak discharge without first confirming that rainfall depth and catchment area are in consistent units, then reporting a result in the wrong order of magnitude.
The exam mixes SI and imperial data, and candidates carry over a conversion factor from a worked example without re-deriving it for the new units.
What to do instead
Convert every input to one coherent unit system before any arithmetic, and sanity-check the output against a rough mental estimate (e.g. 1 mm over 1 km² = 1000 m³).
Treating the time of concentration as the storm duration that produces the maximum peak, rather than as the duration at which the entire catchment first contributes.
The rational-method shortcut 'peak occurs at duration equal to tc' is memorised without understanding that the critical duration depends on the rainfall intensity-duration relationship and can exceed tc.
What to do instead
State explicitly that tc is the minimum duration for full areal contribution, then test durations at and beyond tc against the IDF curve to find the true critical duration.
Using arithmetic-mean rainfall over a catchment when the gauge network is sparse or orographically biased, without applying a Thiessen or isohyetal weighting.
Arithmetic averaging is the simplest method taught first, and candidates default to it under time pressure without checking gauge spacing relative to catchment size.
What to do instead
Assess gauge density against the catchment area; if spacing is irregular or relief is significant, use Thiessen polygons or isohyetal interpolation and show the weighting.
Reading a stage-discharge (rating) curve or hydrograph and reporting peak discharge from the wrong limb, ignoring hysteresis between rising and falling stages.
The looped rating curve for unsteady flow is subtle, and candidates assume a single-valued curve for all conditions.
What to do instead
Note whether flow is steady; for rapidly rising or falling stages, apply the appropriate looped rating or use the discharge measured at the time of interest rather than the steady-state value.
Confusing the return period of a rainfall event with the return period of the resulting flood, and assuming they are numerically equal.
Frequency language is shared between rainfall and flood analysis, so the two are conflated even though catchment response transforms the distribution.
What to do instead
Treat rainfall and flood frequency as separate analyses; derive flood frequency from the transformed/computed flows, not by copying the rainfall return period.
Applying the water-balance equation with a sign convention that double-counts or omits a storage term, producing an apparent imbalance that is then dismissed as measurement error.
Different textbooks write the balance as inputs minus outputs equals change in storage or its reverse, and candidates mix conventions mid-problem.
What to do instead
Write the balance explicitly with a declared sign convention (inflows positive, outflows negative, storage change = sum) before substituting numbers.
Estimating evapotranspiration with a temperature-only method (e.g. Thornthwaite) for a windy, arid site where the aerodynamic term dominates.
The simplest formula is the one most practised, and candidates do not check whether its underlying assumptions match the climate.
What to do instead
Match method to climate: use combination or Penman-type approaches where advection and wind matter, and state the assumption when a temperature-index method is used.
Computing baseflow separation or a unit hydrograph without verifying that the direct-runoff ordinates sum to the correct effective rainfall volume.
Candidates focus on the shape of the hydrograph and skip the volume check that validates the separation.
What to do instead
Always close the loop: integrate direct runoff, divide by catchment area, and confirm it equals the effective rainfall depth within rounding.
Applying Darcy's law with a hydraulic conductivity taken from a different material or with head gradients computed over the wrong distance.
K values span many orders of magnitude and are often quoted without units or material context; gradient distance is confused with elevation difference.
What to do instead
Confirm K matches the aquifer material and carries consistent units, and compute gradient as head difference divided by flow-path length, not by vertical drop alone.
Ignoring the difference between confined and unconfined aquifer responses when using storativity, and applying a confined-aquifer S value to an unconfined system.
Theis-type solutions are practised on confined cases, and candidates reuse the same storativity magnitude without checking the aquifer type.
What to do instead
Identify the aquifer type first; for unconfined systems use specific yield (often two orders of magnitude larger than confined storativity) and apply the appropriate solution.