AIH Professional Hydrologist - Groundwater Examination common mistakes
AIH Professional Hydrologist - Groundwater Examination · Understand the traps and how to avoid them.
Computing specific yield as total porosity minus zero, i.e. treating all pore space as drainable, so storage-change estimates come out several times too large.
Candidates memorise 'porosity' as the single storage term and never separate specific retention from specific yield, so the distinction between total voids and gravity-drainable voids collapses.
What to do instead
Before any storage calculation, write out Sy = n - Sr explicitly and state the assumed Sy value with units; sanity-check that confined storativity (10^-5 to 10^-3) and unconfined Sy (0.01 to 0.30) fall in their expected orders of magnitude.
Using the full aquifer thickness in the Theis well function argument u = r^2 S / (4 T t) for a confined aquifer but substituting specific yield for storativity in an unconfined aquifer and still applying the confined solution without the delayed-yield caveat.
The two storage coefficients look interchangeable in the formula, and the unconfined early-time response (rapid release from the unsaturated zone) is not intuitive from the equation alone.
What to do instead
Label the storage term physically before substituting: S (confined, per unit thickness) versus Sy (unconfined, drainable). Note that Theis is valid for unconfined aquifers only at times after gravity drainage is complete, and state that assumption in the answer.
Drawing the water-table contour map with contours crossing or terminating in mid-aquifer, and inferring flow directions perpendicular to contours in the wrong sense (uphill rather than downhill).
Candidates focus on interpolation between point measurements and lose the physical constraint that a water table is a continuous single-valued surface.
What to do instead
Apply two checks to every finished map: contours never cross or dead-end except at a boundary, and groundwater flow is from high head to low head, perpendicular to contours. Verify each arrow by comparing the two adjacent contour values.
Reporting transmissivity from a pumping test without converting units consistently, mixing drawdown in feet with time in minutes and discharge in gallons per minute, producing T off by a factor of 60 or more.
Field data arrive in mixed imperial units and the standard analytical solutions are written for consistent time units, so the conversion step is invisible in the algebra.
What to do instead
Convert every input to a declared consistent set (e.g. m, m/day, m^3/day) in a written first line before any substitution, then convert the final T back to the requested unit with the factor shown explicitly.
Applying Darcy's law with the hydraulic gradient computed over the wrong distance, such as using aquifer thickness instead of the horizontal separation between observation points.
Thickness and length both appear as 'the distance' in diagrams, and gradient is taught as a simple ratio without emphasis on which two points define it.
What to do instead
Write the gradient as (h1 - h2)/(L) with L explicitly identified as the horizontal distance between the two head measurements, and cross out any use of saturated thickness in that term.
Computing a runoff volume with the rational method or a unit-hydrograph peak without checking that rainfall intensity, area, and time of concentration are in compatible units and that the storm duration equals or exceeds Tc.
The formulas are simple enough that candidates plug numbers straight in, and the duration condition is a conceptual requirement rather than an arithmetic one.
What to do instead
State the design storm duration and Tc side by side before calculating, confirm duration is at least Tc, and carry units through the multiplication explicitly (e.g. mm/h times km^2 gives m^3/s only after the 1/3.6 factor).
Confusing the return period of a rainfall or flood with the probability of at least one exceedance over a multi-year design life, reporting a 100-year event as 'will not occur for 100 years'.
The phrase '100-year flood' invites a deterministic reading, and the binomial/geometric relation between annual exceedance probability and life-span risk is not part of routine practice.
What to do instead
Convert explicitly: annual exceedance probability p = 1/T, and risk over n years = 1 - (1 - p)^n. Show the computation whenever a design life is mentioned, and state the result as a probability, not a schedule.
Estimating baseflow or groundwater recharge by reading the low-flow tail of a hydrograph without separating the recession curve from direct runoff, so stormflow is counted as baseflow.
The separation line is a judgement call on a graph, and candidates pick a convenient point rather than applying a stated method consistently.
What to do instead
Name the separation method (fixed-interval, straight-line, or a recession-curve filter) and apply it uniformly across the record; report the baseflow index as a ratio so the method and its effect are visible.
Mixing the water-balance sign convention mid-calculation, adding evapotranspiration as an inflow in one line and subtracting it in the next, so the residual storage term changes sign.
Balance equations are written in several textbook forms and candidates switch conventions between the setup and the arithmetic without noticing.
What to do instead
Write the balance once at the top with a declared convention (inflows positive, outflows negative), keep every term in that form, and check that the residual storage change has the physical sign implied by the season or stress period.
Reporting a hydrologic quantity to four or five significant figures from inputs measured to one or two, and omitting the units or datum on reported heads and elevations.
Calculator output is copied verbatim, and datum/unit conventions are treated as bookkeeping rather than part of the answer's meaning.
What to do instead
Carry significant figures through the calculation and round the final answer to the precision of the least precise input; state units and the vertical datum (e.g. metres above mean sea level) on every head, elevation, and gradient reported.