Surface Water Resources and Hydrology practice questions

Answer: D) Baseflow

• AIncorrect. That's a loss to the air.
• BIncorrect. That's surface runoff.
• CIncorrect. That's rain caught on vegetation.
• DCorrect. Direct runoff comes from storms.

Answer: A) An increase of 7 units

• ACorrect. 10 + 50 − 8 − 45.
• BIncorrect. Inputs exceed outputs.
• CIncorrect. Subtract the losses.
• DIncorrect. That isn't the balance.

Answer: B) A higher reaeration rate

• AIncorrect. Turbulence increases it.
• BCorrect. Turbulence speeds oxygen transfer.
• CIncorrect. It increases.
• DIncorrect. It affects reaeration.

Answer: B) More runoff

• AIncorrect. Wet soils absorb less.
• BCorrect. Less infiltration capacity remains.
• CIncorrect. Antecedent moisture matters.
• DIncorrect. Infiltration is reduced, not zero.

Answer: D) Area-weighting between equal-rainfall contours

• AIncorrect. That overstates rainfall.
• BIncorrect. That's the arithmetic method.
• CIncorrect. That's the Thiessen method.
• DCorrect. It can reflect terrain effects.

Answer: B) It weights each gauge reading by the area closer to that gauge than to any other

The Thiessen method area-weights each gauge's rainfall by the polygon of watershed area nearest that gauge, unlike the arithmetic-mean, isohyetal, and normal-ratio methods.

• AIncorrect. This is the arithmetic-mean method, which weights every gauge equally no matter how much area it represents.
• BCorrect. Perpendicular bisectors between neighboring gauges form polygons. Each gauge's depth is weighted by its polygon area inside the watershed, divided by the total area.
• CIncorrect. This describes the isohyetal method, which uses contour lines of equal precipitation rather than polygons around gauges.
• DIncorrect. This is the normal-ratio method. It estimates missing data at a single station and does not compute an areal average.

Answer: A) The post-development peak discharge for the design storm doesn't exceed the pre-development peak discharge

A detention basin temporarily stores runoff and releases it through a controlled outlet so the post-development peak discharge doesn't exceed the pre-development peak. It attenuates peaks rather than reducing total volume.

• ACorrect. Detention stores runoff temporarily and releases it through a restricted outlet. This flattens the outflow hydrograph so the post-development peak stays at or below the pre-development (allowable) peak, which protects downstream channels from flooding and erosion.
• BIncorrect. A dry detention basin mainly changes the timing and peak of the outflow. Most of the stored water is eventually released, so the total runoff volume is not substantially reduced.
• CIncorrect. This describes a retention basin, not a detention basin, which releases stored water through an outlet.
• DIncorrect. Peak attenuation depends on temporary storage. The basin fills during the peak inflow and drains over a longer period afterward.

Answer: B) It decays exponentially with time from an initial rate toward a constant final rate

Horton's equation, f = fc + (f0 − fc)e^(−kt), has infiltration capacity decaying exponentially from an initial rate f0 toward a constant final rate fc as the soil wets up.

• AIncorrect. Horton's model shows capacity falling, not rising. Surface sealing and filling of the soil pores reduce infiltration as the storm goes on.
• BCorrect. Horton's equation is f = fc + (f0 − fc)e^(−kt). Capacity starts at f0 and decays exponentially toward the constant final rate fc.
• CIncorrect. A constant rate is only the limit the curve approaches over time. Early in the storm, infiltration capacity is well above that final rate.
• DIncorrect. The decay is exponential, not linear. It levels off at a positive constant rate fc and does not drop to zero.

Answer: A) Second-order stream

• ACorrect. Order rises only when equal orders meet.
• BIncorrect. Two first-order streams make a second-order stream.
• CIncorrect. That needs two second-order streams.
• DIncorrect. That isn't a stream order.

Answer: C) +15 million m³

• AIncorrect. Evaporation is a loss.
• BIncorrect. Subtract evaporation.
• CCorrect. ΔS = I − O − E.
• DIncorrect. Recheck: 100 − 80 − 5.