Irrigation Water Requirements of Crops

1.      What are the quality of irrigation water? (Nov 2011 5m)

-          Explain the quality of irrigation water required.

-          What are the quality of irrigation water? And list the impurities of irrigation.

Ans. The quality of suitable irrigation water is very much influenced by the constituents of the soil which is to be irrigated. The water containing impurities can be injurious to the growth of the plant is not satisfactory for irrigation, therefore, the quality of irrigation water depends primarily on silt content and salt constituents of the soil.

The impurities which make the water unfit for irrigation are classified as follows;

·         Sediment concentration in water.

·         Soluble salts in water.

·         Proportion of Sodium ions.

·         Concentration of potentially toxic elements in water.

·         Bicarbonate Concentration.

·         Bacterial contamination.

2.      Explain the effects of impurities of irrigation.

Ans. Following are the effects of impurities of irrigation.

a)      Sediment concentration in water:

·         Fine Sediment increases the soil fertility.

·         Sediment from eroded areas decrease the soil fertility and permeability.

·         Sedimentation in water causes silt formation in irrigation canals, therefore, increasing the maintenance cost.

b)      Soluble salts in water:

·         Salts of calcium, sodium, potassium and magnesium, when present in excessive quantity causes damage to the growth of the plant.

·         These can cause reduction in osmotic activities of plants.

·         Prevent adequate aeration.

c)      Proportion of Sodium ions:

·         Soil containing sodium ions more than 5%, the soil becomes less permeable.

·         Soil grains breaks down, thus soil becomes clayey

d)     Concentration of potentially toxic elements in water:

·         The elements like boron, selenium, etc., are toxic to plants.

·         Traces of boron are very essential for the growth of the plant, but it should not increase above 0.3ppm it may prove to be toxic to certain plants.

e)      Bicarbonate Concentration:

·         High concentration of bicarbonate ions may result in precipitation of calcium and magnesium.

·         Increasing the amount of sodium ions may be hazardous for growth of plant.

f)       Bacterial contamination:

·         It causes serious damage when crops irrigated with highly contaminated water are directly consumed without being cooked.

·         Cash crops like cotton, nursery stock, etc. which are processed after harvesting, can use the waste water without any trouble.

3.      Define the following

a)      Crop period

b)      Base period

c)      Duty

d)     Delta

e)      Gross Commanded Area (GCA)

f)       Cultivable command Area (CCA)

g)      Intensity of irrigation

h)      Outlet factor

i)        Kor-period

Ans. Following are the definitions of the terms given in the question.

a)      Crop period: It is defined as the total time that elapses between the sowing of the crop and its harvesting. Thus crop period represents the total time during which the crop remains in the field.

b)      Base period: It is defined as the total time between the first watering done for the preparation of land for sowing of a crop and the last watering done before its harvesting.

Note: Generally crop and base period are represented by B, which expressed in terms of days.

c)      Duty: Duty of the water is the relation between the area of the land irrigated and the quantity of water required to be supplied for growing a crop. The Duty of water is represented by D. It is generally expressed in hectares per cumec for the case of direct flow irrigation from canals, it is called as flow duty water. This may be also expressed in terms of the total area of land in hectares per million cubic metre of water stored in tank or reservoir, this case is called as quantity duty of water.

[Alternative definition - It is usually defined as the area of land in hectares which can be irrigated for growing any crop if one cumec (one cubic metre per second) of water is supplied continuously to the land for entire base period of the crop]

Following are the importance of duty:

·         It helps us in designing an efficient irrigation canal system.

·         An area to be irrigated can be easily calculated.

·         It helps in calculating the discharge for designing the channel.

·         It helps in checking the efficiency of working of the canal system.

d)     Delta: It is defined as the total depth of water over the irrigated land required by a crop grown on it during the base period of the crop. It is usually denoted by ‘Δ’. The delta may also be defined as total water required by a crop for its growth in hectare-metre per hectare.

e)      Gross Commanded Area (GCA): The total area, including the roads, villages, etc., which can be irrigated economically from irrigation projects such as a dam, a head work, etc., is called Gross Commanded Area (GCA).

f)       Cultivable command Area (CCA): The Gross Commanded Area (GCA) minus the uncultivable area (such as roads, villages, ponds, etc.) is called Cultivable command Area (CCA).

i.e., CCA = GCA – Uncultivable area.

g)      Intensity of irrigation: The percentage of cultivable command area (CCA) to be irrigated annually is called Intensity of irrigation.

Intensity of irrigation (%) = (Area to be irrigated/CCA) x 100

h)      Outlet factor: Duty based on the actual supply on the field through the outlet excluding all the losses in the canal system is called as outlet factor or outlet discharge factor. It is actually the duty at the head of the water course. Generally, it is the end point of the irrigation department.

i)        Kor-period: The limited period during which any crop requires more quantity of water for its growth is called Kor-period. The first watering given to a crop when it is few centimetres high is referred as Kor-watering. It must be applied within a limited period.

4.      What are the factors affecting the Duty of water?

Ans. The following are the factors affecting the Duty of water:

1)      Type of crops.

2)      Climate condition of the area.

3)      System of irrigation.

4)      Method of irrigation.

5)      Method of cultivation.

6)      Quality of irrigation water.

7)      Type of soil and sub soil of the irrigated field.

8)      Time of irrigation and frequency of cultivation.

9)      Type of soil and subsoil of the area through which canal passes.

10)  Canal conditions.

11)  Skill of the cultivator.

12)  Method of assessment of irrigation water.

13)  Topography of land.

14)  Base period of crops:

Note: For description of factors affecting duty, refer ‘Water Resource Engineering’ by P.M. Dhotrad, pg 48-49

5.      What are the method of improving the Duty of water?

Ans. The following are the method of improving the Duty of water,

1)      The land should be properly ploughed to a required depth and levelled before sowing the crop.

2)      The land should be cultivated frequently, which reduces the loss of moisture from the soil.

3)      Suitable methods of irrigation should be used.

4)      The canal should be lined. This will reduces the transmission losses.

5)      The canal should be close to the land to be irrigated.

6)      Volumetric assessment of irrigation water should be made.

7)      Good quality of water should be used for irrigation. The canal should not be allowed to get polluted.

8)      The alignment of the canal either in sandy soil or in fissured rock should be avoided.

9)      The cultivators must be trained to use irrigation water effectively and efficiently.

10)  Instead of large canals, smaller sized parallel canals with lower full supply level should be made.

11)  The canals should be properly maintained to reduce loss of water.

12)  Proper distribution of the land among the cultivators for effective use of water.

13)  Increasing the water holding capacity of the soil through suitable measures.

14)  The canal administration staff should be well trained to maintained the same.

15)  The cultivators must be encouraged to adopt modern methods of cultivation.

6.      Explain the crop seasons – Rabi and Kharif.

Ans. There are two main crop season in India, namely Rabi season and Kharif season.

·         Rabi season: In India, the winter season is called as Rabi season, normally starting from 1^st October and ends on 31^st March. The crops are sown in October and harvested in February, these crops are called as ‘winter crops’. Following are some of the crops grown in Rabi season;

a)      Wheat

b)      Barley

c)      Gram

d)     Linseed

e)      Mustard

f)       Potatoes, etc.

·         Kharif season: In India, the summer season is called as Kharif season, normally starting from 1^st April and ends on 30^st September. The crops are sown in April, May or June are harvested in September or early October, these crops are called as ‘summer crops’. Following are some of the crops grown in Kharif season;

a)      Rice

b)      Bajra

c)      Maize

d)     Cotton

e)      Tobacco

f)       Groundnut, etc.

Note –

§  The type of bifurcation of seasons are mostly applicable to North India, but in South India, there is no clear cut winter, spring, summer and autumn seasons as they are in North India.

§  These dates are not rigid dead line. The time may vary up to three months on either side.

§  Sugarcane, which is a cash crop, extends over both seasons.

7.      

Problems:

Relation between duty and delta:

Let there be a crop of base period B days.

Let one cumec (m³) of water be applied to this crop on the field for B days.

Now the volume of water applied to this crop during B days,

V = (60*60*24*B) m³

V = 86,400 m³

By definition of duty (D), one m³ supplied for B days matures D hectares of land.

This quantity of water (V) matures D hectares (10⁴) of land.

Therefore,

Total depth of water applied on this land =  unit in meters.

i.e Δ = (8.64*B)/D m

     Δ = (864*B)/D cm

Where, Δ is in meters or cm, B is in days and D is duty in hectares/cumec.

8.      A crop requires 90 cm of water for base period of 120 days. Find the duty of water. (May 2015 5m)

Sol. Given:

                  B = 120 days

                  Δ = 90 cm

      We know that,

Δ =  864B / D

90 = 864*120/D

            Therefore,

                        D = 1152 hectares/cumec Ans.

9.      The duty of canal water for a tobacco crop was assumed to be 2200 hectares/cumec. First watering was done on 30^th of Nov. whereas last watering was done on 29^th March. Calculate delta for the crop. (Nov 2011 6m)

Sol. Given:

                  Duty of the crop ‘D’ = 2,200 hectares/cumec

                  Base period of the crop ‘B’ = 120 days, i.e. Nov 30^th to March 29^th

        We know that,

Δ =  864B / D

Δ =  864*120 / 2200

Δ = 47.12 cm Ans.

10.  An irrigation canal has GCA of 80,000 hectare the CCA is 85% of GCA and the intensity of irrigation for kharif and rabi crops or seasons are 30% and 60% respectively. If the duties of water at the canal are 800 hectares/cumec and 1700 hectare/cumec for kharif and rabi crops respectively. Determine the head discharge of the canal. (May 2011 9m)

Sol. Given:

Gross commanded area (GCA) = 80,000 hectares

Cultivable commanded area (CCA) = 85% of GCA

i.e. CCA = 0.85 * 80,000 = 68,000 hectares

              We know that,

                        Area to be irrigated in kharif crop at 30% intensity = 0.3 * 68,000

                                                                                                          = 20,400 hectares

            Therefore,

                        The canal discharge = 20,400/800   = 25.5 cumec Ans.

                       

                        Area to be irrigated for rabi crop at 60% intensity = 0.6 * 68,000

        = 40,800 hectare

            Therefore,

                        The canal discharge = 40,800/1700 = 24 cumec Ans.

11.  Find out the capacity of reservoir if its cultivable area is 60,000 hectares, from the following data:

Type of crop	Sugarcane	Wheat	Rice
B in days	330	120	120
D in hect/cumec	2,200	1,800	800
Intensity of irrigation %	15	20	10

Assume canal losses as 6% and Reservoir losses as 10%.

Sol. Step 1: Delta Δ

                  Sugarcane

Δ =  8.64B / D

Δ =  8.64*330/ 2200 = 1.296 m

Wheat

Δ =  8.64B / D

Δ = 8.64*120 / 1800  = 0.576 m

Rice

Δ =  8.64B / D

Δ =  8.64*120 / 800 = 1.296 m

             

              Step 2: Area

                        Note: Area under sugarcane =  = 9,000 hect.

                                                            Or

                        Area under sugarcane = 0.15 * 60,000 = 9,000 hect.

                        Area under wheat = 0.20 * 60,000 = 12,000 hect.

                        Area under rice = 0.10 * 60,000 = 6,000 hect.

     

              Step 3: Quantity of water

                        Sugarcane: 9,000 * 1.296 = 11,664 hect. / meter

                        Wheat: 12,000 * 0.576 = 6,912 hect. / meter                         

Rice: 6,000 * 1.296 = 7,576 hect. / meter

Total quantity of water required = 11,664 + 6,912 + 7,576

    = 26152 hect. / meter

              Step 4: Losses

                        Canal losses = 6%

                        Reservoir losses = 10%

                        Therefore,

                        Total losses = 16%

                                            = 0.16 * 26152

                                            = 4184.32 hect.-meter.

              Step 5: Capacity of the reservoir = 26152 + 4184.32 = 31336.32 hect.-meter

  Say 31340 hect.-meter Ans.

Reference:

·         Irrigation and Hydraulics Structures, by S.K. Garg. Khanna Publisher, New Delhi, India.

·         Irrigation and Water Power Engineering, by B.C. Punmia, Pande, B.B. Lal. Lakshmi Publications,       New Delhi, India.