Hydrology

From rainfall to runoff: storm characteristics, rational and NRCS runoff, hydrographs, intensity- duration-frequency and exceedance probability, time of concentration, rain and stream gauging, losses such as evaporation and infiltration, and stormwater management. It carries 8–12 of the 80 questions on the Water Resources & Environmental exam across eight subtopics.

What the problems look like

Heavy on reading exhibits: IDF and depth-duration-frequency curves, cumulative rain records, gauge networks, rating curves and hydrographs. Recurring shapes — Q = CiA with an area-weighted C; NRCS curve-number runoff depth and a TR-55-style peak; unit hydrograph derivation, convolution and changing duration; time of concentration by sheet, shallow-concentrated and channel flow or the NRCS lag; return-period risk over a design life; Thiessen, normal-ratio and inverse-distance rainfall averaging; and detention volume and orifice outlets.

By exam day you should be able to —

• compute a composite C and a rational peak, with i taken at the right duration and return period;
• compute S, Ia and runoff depth by the curve-number method;
• separate baseflow, derive a unit hydrograph, and lag it to a new duration;
• build t_c from its segments, or from lag;
• convert return period to annual exceedance probability and design-life risk;
• average rainfall over a watershed and fill a missing gauge;
• read a stream rating from stage, and estimate evaporation and consumptive-use losses;
• size detention storage and its outlet, and apply infiltration, swale and wetland criteria.

Where people lose points

• Composite coefficients are area-weighted, and only for area inside the drainage divide — not a simple average, and not the pre-development C for a developed site.
• Intensity vs. depth: a gauge depth over a short window is not an hourly intensity — divide by the duration in hours before reading an IDF curve, and enter the design return period, not its reciprocal.
• Curve-number arithmetic — S = 1,000/CN − 10 (subtract after dividing); Ia = 0.2S, a property of the watershed, not of the storm.
• Unit hydrographs: remove baseflow before scaling; the "unit" is a depth of excess, not the duration; a summed, lagged hydrograph is divided by the number of lags.
• Time of concentration: take the path with the longest travel time, not necessarily the longest length; parallel subareas don't add their times; keep each exponent on its own term.
• Return period is an average, not a schedule — p = 1/T each year, and "at least one" and "exactly one" exceedance are different questions.
• Detention is the maximum of inflow minus outflow across durations, not the longest storm; orifice head runs to the centroid.

Have these at your fingertips

The rational and curve-number relations, the unit-hydrograph method, t_c relations, and the probability relations for risk — plus where they live in the Handbook.

Going deeper

• Hydraulics–Open Channel — where the runoff goes next
• Groundwater and Wells

Part of the Water Resources & Environmental exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.