Test: Irrigation Engineering- 2

In this method Irrigation water is applied to the land in the form of spray. The sprinkler irrigation is used for all the crops except rice and jute and for almost all the soils except very heavy soils with very low infiltration rates.

In this method, borders and field channels are not required and hence more land is available for cropping.

The rate at which water can be extracted from the tank is known as capacity of the tank. Capacity of irrigation tank depends on the type of crop and its duty.

Due to the presence of injurious salts like Na₂CO₃, Na₂SO₄ and NaCl, Soil will become saline which lead to efflorescence and if it continuous for a longer time then the soil will become alkaline. These soils are reclaimed by leaching, crop rotation, providing sub surface drainage area, addition of gypsum etc.

On the other hand, acidic soils can be reclaimed by basic salts such as limestone.

Lacey’s silt factor is given by:

Check dams are small barriers built across the direction of water flow on shallow rivers and streams for the purpose of water harvesting. The small dams retain excess water flow during monsoon rains in a small catchment area behind the structure.

The check dam

- could be designed to create small reservoirs

- reducing the velocity of flow in the channel

- can counteract erosion

- avoid silting

- increases groundwater recharge

For elementary profile of the dam,

Base width of the dam is given by:

When uplift is ignored, C =0

Minimum thickness of floor required is,

Silt excluders are constructed on the bed of the river, upstream of the head regulator. While silt ejectors are constructed on the bed of the canal and a little distance downstream from the head regulator.

As, Silt excluders remove silt from the water before it enters the canal and silt ejectors extract the silt from the canal water after the silted water has travelled a certain distance in the off-take canal.

Afforestation means planting trees. As we know, the roots of the trees help to hold the soil layer firmly, it is evident that it helps not to make the soil loose and prevents erosion.

Thus by planting more and more trees, the top layer of the soil becomes less prone to erosion by wind, water etc. Hence it prevents erosion and helps in soil conservation.

Note: Contour bunds and check bunds are basically water harvesting techniques.

The duty at the head water course is called as “outlet Discharge factor”.

The ratio between the area of the crop irrigated and the quantity of water required during its entire period of growth is known as duty. Duty is measured in hectares per cumec.

Hence, 

Outlet factor = 

i) The canal which is aligned along the *water-shed line* is known as ridge canal. Canal can also be aligned along contours of the country, right angle to contours of the country but the most economical is ridge canal. The advantage of this type of canal is that it can irrigate the areas on both the sides.

ii) As per financial perspective of the country, canal should not be located near-by our valuable properties because of problems in flood like situations.

iii) It should be so aligned such that minimum number of cross drainage work is required.

According to Bligh’s creep theory, the percolation water creeps along the base profile of the apron with the sub-soil. The path traced by the percolating water is known as creep length and the head loss per unit creep length is known as hydraulic gradient or percolating coefficient.

Groynes or spurs are constructed transverse to the river flow extending from the bank into the river. This form of river training works performs one or more functions such as:

i) Training the river along the desired course to reduce the concentration of flow at the point of attack,

ii) Creating a slack flow for silting up the area in the vicinity and protecting the bank by keeping the flow away from it.

Canal fall is solid masonry structure which is constructed on the canal if the natural ground slope is steeper than the designed channel bed slope.

If the difference in slope is smaller, single fall can be constructed. If it is of higher then falls are constructed at regular intervals.

Design discharge

Heavy scour of the river bed at curved heads and shanks of guide banks can cause undermining of stone pitching thereby resulting in failure of guide banks. Such a failure can be prevented by providing launching apron beyond the toe of guide banks. The width of launching apron is generally taken as 1.5D.

Irrigation Efficiencies:

i) Water Conveyance Efficiency, (η_c)

where, W_f = Water delivered to the field 

W_r = Water delivered from the reservoir

ii) Water Application Efficiency (η_a)

where, W_s = Water stored in the root zone 

W_f = Water delivered to the field

iii) Water use Efficiency (η_u) 

where, W_u = Water use consumptively. 

W_f = Water delivered to the filed

iv) Water storage Efficiency (η_u) 

where, W_s’ = Actual water stored in the root zone

W_f = Water needed to store to bring the water content up to field capacity.

v) Water distribution Efficiency, (η_d) 

where, y = Average numerical deviation in the depth of water stored from the average depth of irrigation stored

d = Average depth during irrigation

vi) Consumptive use Efficiency (η_cu) 

where, W_cu or C_u = Water used by plant consumptively. 

W_d = Net amount of water depleted from root zone.