Irrigation Engineering — Part 1 MCQs with Explanations

Irrigation water is classified based on its sodium concentration relative to the concentrations of calcium and magnesium ions. The classification is determined by the sodium absorption ratio (SAR), which is calculated using the following formula: SAR = (Na ++ / √(Ca ++ + Mg ++ )) * 100 Given the concentrations provided: Na ++ = 20 meq/L Ca ++ = 3 meq/L Mg ++ = 1 meq/L Plugging these values into the SAR formula: SAR = (20 / √(3 + 1)) * 100 SAR ≈ (20 / √4) * 100 SAR ≈ (20 / 2) * 100 SAR ≈ 10 * 100 SAR ≈ 1000 According to the classification: SAR < 10: Low sodium water 10 ≤ SAR < 18: Medium sodium water 18 ≤ SAR < 26: High sodium water SAR ≥ 26: Very high sodium water Since the calculated SAR is approximately 100, the irrigation water falls into the category of very high sodium water .

In irrigation engineering, the difference in level between the top of a bank and the supply level in a canal is termed as the free board . This term refers to the vertical distance between the water level in the canal and the top of the embankment or bank. The free board provides a safety margin to prevent overflow or breach of the canal embankment during high water flow or flooding events.

The correct answer is Permanent wilting point . In irrigation engineering , the term Permanent wilting point refers to the state of the soil when plants are unable to extract sufficient water for their needs. At this point, the soil moisture content is so low that plants wilt permanently, even if moisture is added to the soil. So, the sentence should read: "The state of the soil when plants fail to extract sufficient water for their requirements, is Permanent wilting point ."

Infiltration Rate =2/6=1/3cm/hr Infiltration=1/3*12=4cm Run-off= 10-4=6 cm

The main function of a divide wall is to separate the undersluices from the weir proper and provide a smooth flow of water by guiding it efficiently. It helps in: - Preventing crosscurrents and turbulence that may interfere with the proper functioning of the undersluices. - Controlling sedimentation by ensuring that silt-laden water flows towards the undersluices for easy disposal. - Protecting the structural integrity of the weir by minimizing the impact of high-velocity flows.

Let's understand what Infiltration is in simple terms, especially when we talk about water and soil, like in Irrigation Engineering . Imagine it raining, or you're watering your garden. What happens to the water? Infiltration is essentially how water gets *into* the soil from the ground surface. Let's look at the options: Option B: absorption of water by soil surface This is definitely a part of infiltration! The very first step for water to enter the soil is for the soil's surface to 'soak up' or 'absorb' the water. Think of a sponge soaking up water. This is the starting point of the whole process. Option A: movement of water through the soil Once the water is absorbed by the surface (as explained in Option B), it doesn't just stay right at the top. It starts to move *downward* into the soil's deeper layers, flowing *through* the tiny spaces (pores) between the soil particles. This downward movement into and through the soil is also a crucial part of what we call infiltration. So, infiltration isn't just about the water sitting *on* the surface; it's about it getting *in* and then moving *through* the soil. Therefore, the most complete and correct answer is Option C: both (A) and (B) . Infiltration involves both the initial absorption of water at the soil surface and the subsequent movement of that water downwards through the soil itself. It's a very important concept in Irrigation Engineering because it tells us how much water actually enters the soil to be used by plants, and how much might run off.

A fall is a structure constructed across a canal to lower the water level and dissipate surplus energy caused by a difference in bed levels. A low weir fall is specifically designed to maintain the depth of flow in the canal. It allows water to flow smoothly over a broad-crested weir while keeping the upstream depth nearly constant. This type of fall helps in energy dissipation and prevents scouring and erosion downstream by stabilizing the flow conditions. Trapezoidal notch fall and rectangular notch fall are primarily used for measuring discharge and controlling flow rather than maintaining depth. Hence, among the given options, the fall that maintains its depth is the low weir fall . Therefore, the correct answer as per exam and technical standards is Option B: A low weir fall .

The correct answer is Option A: 0.2 x 10 6 The question involves understanding S-curve hydrographs and their relationship to unit hydrographs . An S-curve hydrograph is created by summing an infinite series of unit hydrographs , each lagged by a duration equal to the unit hydrograph duration. In this case, we have a 6-hour unit hydrograph. The equilibrium discharge of the S-curve represents the constant rate of runoff when the rainfall continues indefinitely at a rate of 1 cm (or 1 unit) over the basin for the duration of the unit hydrograph. The equilibrium discharge (Q eq ) is calculated as: Q eq = (Area of catchment x Rainfall depth) / Duration of Unit Hydrograph Where, Area = 120 km 2 = 120 x 10 6 m 2 Rainfall depth = 1 cm = 0.01 m Duration = 6 hours Q eq = (120 x 10 6 m 2 x 0.01 m) / 6 hours Q eq = (1.2 x 10 6 m 3 ) / 6 hours Q eq = 0.2 x 10 6 m 3 /hour Therefore, the equilibrium discharge of the S-curve is 0.2 x 10 6 m 3 /hour .

An aqueduct is a type of cross drainage work where the canal passes over the drainage. In this case, the R.L. (Reduced Level) of the canal bed is higher at 212.0 m compared to the high flood level of the drainage at 210.0 m. This difference in levels makes an aqueduct the appropriate structure, allowing the canal to safely cross above the drainage.

Understanding Unit Hydrographs: Imagine a unit hydrograph as the standard response of a watershed (an area of land where all water drains to a common outlet) to a unit depth of rainfall . It is a tool to predict how a river will react to rainfall. How is the unit hydrograph derived? To derive a unit hydrograph, we isolate the effect of rainfall from the direct runoff hydrograph (DRH) , which shows the flow in a river directly caused by rainfall. What do we divide by? We scale down the DRH to represent the response to a unit depth of rainfall . The steps are: 1. Divide the ordinates of the DRH (values at different times) by the period of the storm . 2. This ensures that the hydrograph ordinates correspond to a unit depth of rainfall excess for the specified duration. Why not the other options? - Direct Runoff Volume: While this shows the total runoff, dividing the DRH by volume does not yield a unit hydrograph. - Total Rainfall: Not all rainfall contributes to runoff; some is lost to infiltration and evaporation. - None of the above: This is incorrect because the correct method is dividing by the storm period. Correct Answer: Option B: Period of Storm

All the above is the correct answer to “Finally formed berms in canals are provided for”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Drains is the correct answer to “Canals constructed for draining off water from water logged areas, are known”. This question is standard in competitive examination syllabi for Irrigation Engineering.

high head scheme is the correct answer to “A hydroelectric scheme operating under a head of 80 m will be classified as”. This question is standard in competitive examination syllabi for Irrigation Engineering.

x = r2d2 is the correct answer to “If d 1 is the depth of cutting, d 2 is the height of the bank from bed level r 2 : 1 and r 1 : 1 are the slopes in filling and cutting respectively, the horizontal distance n between the bed and bank, is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Over 60 cumecs is the correct answer to “A minimum of 90 cm free board is provided if the discharge in the canal is between”. This question is standard in competitive examination syllabi for Irrigation Engineering.

25 cm is the correct answer to “If the irrigation efficiency is 80%, conveyance losses are 20% and the actual depth of watering is 16 cm, the depth of water required at the canal outlet, is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Syphon aqueduct is the correct answer to “The structure constructed to allow drainage water to flow under pressure through an inverted syphon below a canal, is called”. This question is standard in competitive examination syllabi for Irrigation Engineering.

45° is the correct answer to “The downstream expansion head of a guide bank is extended to subtend an angle at the centre, equal to”. This question is standard in competitive examination syllabi for Irrigation Engineering.

(i), (ii) and (iii) is the correct answer to “The uplift pressure on a dam can be controlled by i) constructing cutoff under upstream face ii) constructing drainage channels bet-ween the dam and its foundation iii) by pressure grouting in foundation The corret answer is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

$$\frac{1}{{1 - {{0.9}^ \circ }10}}$$ is the correct answer to “If the risk of a flood occurring in the next 10 years is accepted to 10%, then the return period for design should be”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Apex point of one curve and apex point of the reserve curve is the correct answer to “The width of a meander belt is the transverse distance between”. This question is standard in competitive examination syllabi for Irrigation Engineering.

greater than 1 is the correct answer to “The ratio of average values of shear stresses produced on the bed and the banks of a channel due to flowing water is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

637 hectares/m3/sec is the correct answer to “The kor depth for rice is 190 mm and kor period is 14 days. The outlet factor for this will be”. This question is standard in competitive examination syllabi for Irrigation Engineering.

To raise water level is the correct answer to “The main function of a diversion head works of a canal from a river, is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

consumptive irrigation requirement is the correct answer to “The amount of irrigation water required to meet the evapotranspiration needs of the crop during its full growth is called”. This question is standard in competitive examination syllabi for Irrigation Engineering.

1.50 hw is the correct answer to “For wave action in dams, the maximum height of freeboard is generally taken to be equal to (where hw is height of wave.)”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Percentage of culturable commanded area to be irrigated annually is the correct answer to “The intensity of irrigation means”. This question is standard in competitive examination syllabi for Irrigation Engineering.

capacity/inflow ratio is the correct answer to “Trap efficiency of a reservoir is a function of”. This question is standard in competitive examination syllabi for Irrigation Engineering.

drain trenches is the correct answer to “Seepage through embankments in an earthen dam is controlled by”. This question is standard in competitive examination syllabi for Irrigation Engineering.

All the above is the correct answer to “A river is said to be of”. This question is standard in competitive examination syllabi for Irrigation Engineering.

water is under pressure between two impervious strata is the correct answer to “An artesian aquifer is the one where”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Its breadth and depth ratio must only be known is the correct answer to “For a unique design of a channel by Kennedy's theory”. This question is standard in competitive examination syllabi for Irrigation Engineering.

D = 0.47 $${\left( {\frac{{\text{Q}}}{{\text{f}}}} \right)^{\frac{1}{3}}}$$ is the correct answer to “The scour depth D of a river during flood, may be calculated from the Lacey's equation”. This question is standard in competitive examination syllabi for Irrigation Engineering.

cubic metre/sec is the correct answer to “Unit of runoff in M.K.S. system is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

convex water surface is the correct answer to “Under the same conditions, which of the following shapes of water surface will give the highest rate of evaporation”. This question is standard in competitive examination syllabi for Irrigation Engineering.

the extra turbulence generated by the excess of river sediment during floods is the correct answer to “The main cause of meandering is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

V0 = 0.55 mD0.64 is the correct answer to “According to Kennedy, the critical velocity (V 0 ) in meters in a channel is the mean velocity which keeps the channel free from silting or scouring. Its value is given by (where m is critical velocity ratio and D is the depth of the channel).”. This question is standard in competitive examination syllabi for Irrigation Engineering.

rain deficiency of 30% is the correct answer to “A 70% index of wetness means”. This question is standard in competitive examination syllabi for Irrigation Engineering.

Capillary water is the correct answer to “Useful soil moisture for plant growth, is”. This question is standard in competitive examination syllabi for Irrigation Engineering.

At right angle and parallel to weir crest is the correct answer to “The flow of water after spilling over the weir crest in chute spillway and side channel spillway respectively are”. This question is standard in competitive examination syllabi for Irrigation Engineering.