AIH Fundamentals of Hydrology Examination common mistakes
AIH Fundamentals of Hydrology Examination · Understand the traps and how to avoid them.
Computing runoff depth as if the entire rainfall depth becomes runoff, instead of separating abstractions (interception, depression storage, infiltration) from effective rainfall before applying it to the catchment area.
Candidates conflate rainfall depth with runoff depth because both are expressed in millimetres, and the unit match hides the fact that only effective rainfall generates direct runoff.
What to do instead
Write the water-balance line explicitly (P = Ia + F + Q) before any calculation, and convert volumes to depths by dividing by catchment area as a deliberate, labelled step.
Averaging rainfall or potential evapotranspiration station values with a simple arithmetic mean regardless of station spacing or influence area, when the terrain or gauge network is uneven.
Arithmetic mean is the first method taught and is easy to apply, so candidates default to it without checking whether Thiessen polygons or isohyetal weighting is warranted.
What to do instead
State the criterion for method choice (uniform network and flat terrain permit arithmetic mean; sparse or orographically varied networks require area weighting) and show the weighting factors.
Reading a stage-discharge (rating) curve by interpolating linearly between tabulated points instead of recognising the curve is nonlinear and may have looped hysteresis during unsteady flow.
Candidates treat rating tables like linear lookup data and ignore that discharge scales roughly with stage to a power, and that rising and falling limbs differ.
What to do instead
Use the fitted rating equation or logarithmic interpolation, and flag that a single-valued curve assumes steady, uniform flow conditions.
Using total porosity as the value for specific yield when estimating drainable groundwater storage, so the calculated usable volume is far too large.
Porosity and specific yield are both dimensionless fractions, and the distinction between total void space and gravity-drainable water is easily collapsed.
What to do instead
Compute storage change as specific yield times area times water-table change, and explicitly note that specific yield is always less than porosity because of retained water.
Applying Darcy's law with the hydraulic gradient calculated over the screen or well interval rather than between the actual water-level measurement points in the aquifer.
The gradient term invites using whatever two elevations are printed on the problem, and the well casing head difference is visually convenient.
What to do instead
Identify the two piezometric head measurements that bound the flow path, compute gradient as head difference over that separation distance, and confirm units are dimensionless.
Treating a unit hydrograph as valid for any storm depth, including storms far larger than the one used to derive it, without checking the linearity assumption.
The unit hydrograph is presented as a general tool, so its underlying assumptions of linearity and time invariance are forgotten under exam time pressure.
What to do instead
Before applying a unit hydrograph, state the assumptions (linear response, uniform rainfall intensity and distribution, constant baseflow separation) and scale only within the linear range.
Mixing units within a single water-balance or flow calculation, such as inserting rainfall in millimetres per day with area in square kilometres and reporting a volume without conversion.
Hydrology problems routinely combine mm, m, km, hectares, seconds and days, and candidates track the numerical values while losing track of the dimensional bookkeeping.
What to do instead
Carry units through every line of the calculation and perform one explicit conversion step to a consistent base set (metres and seconds) before substituting numbers.
Confusing the time of concentration with the lag time, or using the time of concentration as the time to peak of the unit hydrograph.
Both are travel-time concepts measured in hours and both appear in rational-method and unit-hydrograph problems, so the definitions blur.
What to do instead
Define each in writing before use: time of concentration is rainfall-excess to hydrograph inflection or end of rise, lag is rainfall centroid to runoff centroid, and time to peak differs from both.
Assuming a confined aquifer has a water table and computing storage from specific yield, rather than using storativity with aquifer thickness and compressibility effects.
The word aquifer triggers the unconfined mental model, and the potentiometric surface of a confined aquifer is often drawn like a water table.
What to do instead
First classify the aquifer from the problem statement (presence of a confining layer and whether the water level rises above the top of the aquifer), then select storativity for confined and specific yield for unconfined.
Interpreting a groundwater contour map so that flow is drawn parallel to the contours or from low head to high head.
Contours look like flow paths on a topographic map, and the perpendicular relationship is stated once and easily reversed under pressure.
What to do instead
Apply the rule explicitly: groundwater flows perpendicular to contours, from higher to lower head, and flow lines converge toward discharge zones and diverge at recharge areas.