Duty and Delta of Crops, Precipitation, Spillways

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Duty of Crops (D)
The amount of water(i.e. 1cumec of discharge) available for the crop for full growth during the entire base period is called duty.
Suppose we are giving 5 cumecs of discharge for 500 hectares of land for a particular crop then
Duty = 500/5 =100 hectares/cumec
Here, 1 Cumec = 1 m3/s

1 hectare = 10000 m2

{ or think that we all have a duty to do homework (hectares) and for that effort (cumec) required to complete }.

Delta ()

It is the depth of water required by crop during the entire base period is called Delta.
Suppose, wheat requires 10 cm depth of water, if 5 waterings are provided during the entire base period then

∆(Delta for crop) = 10 × 5=50 cm.

Hence there is a relation between Duty, Delta and Base period

So, the depth of water required by the crop is

Questions Asked in Comments

The duty of different crops :

Rice: 900 ha/cumec

Wheat: 1800 ha/cum

Cotton :1400 ha/cum

Sugarcane: 800 ha/cum

Vegetables: 700 ha/cum

Various types of Precipitation:

    • Cyclonic Precipitation(may be frontal or non-frontal) :- This type of precipitation occurs when front of warm air mass lifts up due to difference in pressure by unequal heating of earth’s surface then warm air mass meets cool air mass hence condenses forms clouds and results precipitation.
    • Convective Precipitation :- This type of precipitation occurs when natural warm air mass rises and meets with colder surroundings( i.e. low pressure surrounded by high pressure,hence forced air mass up) hence leads to condensation and formation of clouds at last precipitation.
    • Orographic Precipitation:- Here evaporated air mass from large water body is lifted upward by high mountain barrier and then at certain height it condenses and precipitates.

Spillway is a waterway structure used to displace excess water from reservoir to the downstream area before reaching predetermined maximum reservoir capacity. Spillway can be located either entirely in body of the dam or at one end of dam or away from dam.

They act as a safety valves for dam structure by keeping the water level below maximum reservoir level, hence prevents over-topping of dam and damaging of structure(i.e. eroding and undermining the downstream toe of the dam)). If spillway capacity is over estimated then it results in an uneconomical design, hence it must have adequate capacity.

Types of Spillways

  • Straight drop or free overfall spillway
  • Ogee or overflow spillway
  • Chute or open channel or trough spillway
  • Side channel Spillway
  • Shaft or drop inlet or morning spillway
  • Tunnel spillway
  • Siphon Spillway

Factors affecting the required spillway capacity:

Inflow Flood Hydrograph: The Inflow Flood Hydrograph should be selected according to the degree of protection that ought to be provided to the dam. It will depend upon the type and height of the dam, its location with respect to inhabited and developed area, and consequence of its failure.
Available Storage Capacity: If the available storage capacity of the reservoir is quite large as compared to the inflow; a spill way of smaller capacity will normally be required.
Capacity of Outlets: If the dam outlets can be used to discharge a portion of the flood, its capacity can be correspondingly reduced.
Gates in Spill-way: If the it is gated; its discharge capacity can be modified. For a gate controlled spill way the water can be stored up to the top of the gates, whereas in the case of an un-gated spill-way, the water can be stored only up to the crest level.
Possible Damage: If there is possibility of extensive damage on the downstream, large spill-way capacity should be provided.


Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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