Tamil Nadu PSC — Agricultural Engineering (Code 164, degree standard) · India · Tamil Nadu Public Service Commission
Original practice questions written to the published syllabus · not past papersRain to runoff: measuring precipitation, subtracting losses, estimating runoff and routing floods.
In the exams: GATE AG Section 4 Hydrology (SWCE is the heaviest GATE section, ~16 marks a year 2019–26); ICAR AIEEA-PG Unit VII; ICAR AICE subjects 40 and 43; NCEES PE hydrology; state PSCs
How our questions for Tamil Nadu PSC spread across the units of Watershed Hydrology. Difficulty: 19 easy, 14 medium, 4 hard.
| Drought classification | 6 | |
| Interception and evaporation (pan) | 5 | |
| Missing rainfall data and double-mass analysis | 4 | |
| Runoff estimation: rational method | 4 | |
| The hydrologic cycle | 3 | |
| Frequency analysis, return period and IDF curves | 3 | |
| Streamflow measurement | 3 | |
| Precipitation measurement and rain-gauge networks | 2 | |
| Runoff estimation: SCS curve number | 2 | |
| Hydrographs and the unit hydrograph | 2 | |
| Flood routing: Muskingum and reservoir (Puls) methods | 2 | |
| Infiltration: Horton, Philip and the phi-index | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Which rain gauge produces a series of electrical pulses, each representing a fixed small depth of rain, and is therefore well suited to telemetry?
C. Tipping-bucket gauge
Each tip of the bucket closes a contact, so the count per unit time gives intensity. The natural-siphon gauge draws a mass curve on a chart, and non-recording and storage gauges give only totals.
Further reading: Subramanya, Engineering Hydrology
A double-mass curve shows a slope of 0.84 for the older part of a station's record and 1.05 for the recent part. The record is to be adjusted to current conditions. An old annual value of 72 cm becomes:
A. 90.0 cm
Pcx = Px × Mc/Ma = 72 × 1.05/0.84 = 90.0 cm, where Mc is the slope of the period adjusted to. 57.6 cm inverts the ratio; 75.6 cm multiplies by only one slope.
Further reading: Subramanya, Engineering Hydrology
Double-mass analysis of rainfall records is used to:
C. check and correct the consistency of a station's record
A break in slope of the double-mass curve shows a change in gauge site, exposure or practice. Missing data are filled by the normal ratio or averaging methods, not by double-mass analysis.
Further reading: Subramanya, Engineering Hydrology
The probability that a 50-year flood is equalled or exceeded in any one year is:
B. 0.02
p = 1/T = 1/50 = 0.02. 0.98 is the probability of non-exceedance; a 50-year event is not guaranteed once in 50 years. 0.64 is the chance of at least one exceedance in 50 years (1 − 0.98⁵⁰), and 0.36 is the chance of none (0.98⁵⁰).
Further reading: Subramanya, Engineering Hydrology
A 25 ha catchment has a runoff coefficient of 0.4 and a design intensity of 60 mm/h. The peak runoff by the rational method is:
B. 1.667 m³/s
With A in hectares, Q = CiA/360 = 0.4 × 60 × 25/360 = 1.667 m³/s. Using the km² constant 3.6 with hectares gives 166.7 m³/s (100 times too large), dividing by 36,000 gives 0.0167 m³/s, and omitting the constant gives CiA = 600.
Further reading: Subramanya, Engineering Hydrology
A grassed waterway drains 30 ha. The longest flow path is 700 m at a slope of 0.02, C = 0.45, and the local IDF relation is i = 1000 T^0.2/(D + 20)^0.8 (i in mm/h, D in min). For a 10-year return period, the design peak using Kirpich's tc is about:
D. 3.57 m³/s
tc = 0.01947 × 700^0.77 × 0.02^−0.385 = 13.6 min. i = 1000 × 10^0.2/(13.6 + 20)^0.8 = 95.2 mm/h. Q = 0.45 × 95.2 × 30/360 = 3.57 m³/s. Using a 60-minute duration instead of tc gives 1.78; entering tc in hours in the IDF formula gives 5.36 m³/s; the 3.6 constant with hectares gives 357 m³/s, 100 times too large.
Further reading: Subramanya, Engineering Hydrology
Hydrologic soil group A in the SCS curve number method refers to soils with:
D. high infiltration rates and low runoff potential, such as deep sands
Group A has the lowest runoff potential. Clays, high water tables and shallow soils over rock are group D.
Further reading: USDA NRCS National Engineering Handbook Part 630
A 60 ha catchment yields 35 mm of direct runoff in a storm. The runoff volume available for a farm pond is:
D. 21,000 m³
1 mm over 1 ha = 10 m³, so 35 × 10 × 60 = 21,000 m³. 2,100 and 210,000 m³ are factor-of-ten slips in the unit conversion, and 350 m³ forgets the area.
Further reading: Subramanya, Engineering Hydrology
A D-hour unit hydrograph is the direct runoff hydrograph resulting from:
B. one unit of effective rainfall spread uniformly over the catchment at a steady rate for D hours
The unit refers to effective (excess) rainfall, not total rainfall, and the duration D must be stated.
Further reading: Subramanya, Engineering Hydrology
For a district and week, current NDVI is 0.42, and the historical minimum and maximum NDVI for that week are 0.30 and 0.70. The Vegetation Condition Index is:
D. 30%
VCI = (NDVI − NDVImin)/(NDVImax − NDVImin) × 100 = (0.42 − 0.30)/(0.40) × 100 = 30%. 60% divides by the maximum only; 70% measures distance from the maximum instead. 42% simply reads the current NDVI as a percentage.
Further reading: WMO Handbook of Drought Indicators and Indices
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