Chapter: Civil - Water Resources and Irrigation Engineering - Irrigation Engineering

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Water Requirements of Crops

Every crop requires a certain quantity of water after a certain fixed interval, throughout its period of growth. If natural rain is sufficient and timely so as to satisfy both these requirements, no irrigation water is required for raising that crop.


WATER REQUIREMENTS OF CROPS

 

General

 

Every crop requires a certain quantity of water after a certain fixed interval, throughout its period of growth. If natural rain is sufficient and timely so as to satisfy both these requirements, no irrigation water is required for raising that crop. In England, for example, the natural rainfall satisfies both these requirements for practically all crops, and therefore, irrigation is not significantly needed in England. But in a tropical country like India, the natural rainfall is either insufficient, or the water does not fall frequency of the rainfall varies throughout a tropical country, certain crop may require irrigation in certain part of the country .The area where irrigation is a must for agriculture is called the arid region, while the area in which inferior crops can be grown without irrigation is called a semiarid region.

 

 

The term ‘Water requirements of ae waycrop’inwhich amea crop requires water, from the time it is sown to the time it is harvested. It is very clear from the above discussion that the water requirement, will vary with the crop as well as with the place. In

 

other words, different crops will have different water requirements and the same crop may have different water requirements at different places of the same country; depending upon the climate, type of soil, method of cultivation, and useful rainfall, etc.

 

Crop Period or Base Period

 

 

The time period that elapses from the instant of its sowing to the instant of its harvesting is called the cropperiod. The time between the first watering of a crop at the time of its sowing to its last watering before harvesting is called the base period or the base of the crop. Crop period is slightly more than the base period, but for all practical purposes, they are taken as one and the same thing, and generally expressed in days. Hence, in future, the terms like growth period, crop period, base period etc., will be used as synonyms, each representing crop period, and will be represented crop period, and will be represented by B(in days).

 

Duty and Delta of a Crop

 

 

Delta: Each crop requires a certain amount of water after a certain fixed interval of time, throughout its period of growth. The depth of water required every time , generally varies from 5 to 10 cm depending upon the type of the crop . If this depth of water is required five times during the base period, then the total water required by the crop for its full growth, will be 5 multiplied by each time depth. The final figure will represent the total quantity of water required by the crop for its fullfledged nourishment.

The total quantity of water required by the crop for its full growth may be expressed in hectaremetre (Acreft) or in million cubic metres (million cubicft) or simply as depth to which water would stand on the irrigated area if the total quantity supplied were to stand above the surface without percolation or evaporation. This total depth of water (in cm) required by a crop to come to maturity is called its delta (').


Example: If rice requires about 10cm depth of water at an average interval of about 10 days, and the crop period for rice is 120 days, find out the delta for rice.

 

Solution:

 

Water is required at an interval of 10 days for a period of 120 days. It evidently means that 12 no. of waterings are required, and each time, 10 cm depth of water is required. Therefore, total depth of water required.

 

= 12 x10 cm = 120 cm.

 

Hence ' for rice = 120 cm. Ans.

 

 

Example: If wheat requires about 7.5 cm of water after every 28 days, and the base period for wheat is 140 days, find out the value of delta for wheat.

 

Solution:

 

Assuming the base period to be representing the crop period, as per usual practice, we can easily infer that the water is required at an average interval of 28 days up to a total period of 140

140 o

days. This means that 5 no. of waterings are required. 28

 

The depth of water required each time = 7.5 cm.

 

 Total depth of water required. In 140 days = 5 x7.5 = 37.5 cm

 

Hence, 'for wheat = 37.5 cm. Ans.

 

Delta for certain crops

 

The average values of deltas for certain crops are shown in table. These values represent the total water requirement of the crops. The actual requirement of irrigation water may be less, depending upon the useful rainfall. Moreover, these values represent the values on field, i.e.

 

‘delta             on  field’  which   includeslosses.  the  evaporation

 

Table: Average Approximate Values of ' for Certain Important Crops in India

 

S.No         Crop Delta on field

 

1 Sugarcane        120    cm  (4

2 Rice        120    cm  (4

3 Tobacco 75  cm  (3

4 Garden fruits   60      cm  (2

5 Cotton   50      cm  (2

6 Vegetables       45 cm (18?)

7 Wheat    40      cm  (1

8 Barley    30      cm  (1

9 Maize     25      cm  (1

10    Fodder        22.5  cm

11    Peas  15  cm  (6

 

 

Duty of Water:

 

 

The ‘duty’ of water is the relationship betw it matures. This volume of water is generally expressed by: a unit discharge flowing for a time

 

equal to the base period of the crop, called Base of a duty.

 

 

If water flowing at a rate of one cubic meter per second, runs continuously for B days, and matures 200 hectares, then the duty of water for that particular crop will be defined as 200 hectares per cumec to the base of B days. Hence, duty is defined as the area irrigated per cumec of discharge running for base period B. The duty is generally represented by the letter D.

 

Relation between duty and delta:

 

 

Let there be a crop of base period B days. Let one cumec 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.

 

 

Volume of water applied to crop = V = (1 x60 x60 x24 xB) m3. = 86400 B (cubic metre)

 

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

 

 This quantity of water (V) matures D hectares of land or 104 D sq.m of area.

 

Total depth of water applied on this land

 

= Volume/ Area = 86,400 B/ 104 D . 8.64B/D metres

By definition, this total depth of water is called delta (').

 

'=8.64B/D (metres)

Where

 

 

'is in cm, B is in days; and

D is duty in hectares/cumec.



During the passage of water from these irrigation channels, water is lost due to evaporation and  percolation. These losses are called Transit losses or Transmission or Conveyance losses in channels. 

 



Figure: Layout of a canal system

 

Duty of water for a crop is the number of hectares of land which the water can irrigate. Therefore, if the water requirement of the crop is more, less number of hectares of land it will irrigate. Hence, if water consumed is more, duty will be less. It, therefore, becomes clear that the

 

duty of water at the head of the watercourse will be less than the because when water flows from the head of the watercourse and reaches the field, some water is

 

lost as transit losses. Applying the same reasoning, it can be established that duty of water at the head of a minor will be less than that at the head of the watercourse; duty at the head of a distributary will be less than that at the head of a minor, duty at the head of a branch canal will be less than that at the head of a minor, duty at the head of a main canal will be less that the duty at the head of a branch canal. Duty of water, therefore, varies from one place to another, and increases as we move downstream from the head of the main canal towards the head of the branches or watercourses. The duty at the head of watercourse (i.e. at the outlet point is generally the end point of Irrigation Department. The control of Irrigation Department finishes at the outlet point, and the water is carried into the fields through watercourses by the cultivators themselves.

 

Flow duty and Quantity duty:

 

 

In direct irrigation, duty is always expressed in hectares/cumec. It is then called as flowduty or duty.

 

 

In storage irrigation, duty may, sometimes be expressed in hectares/millions cubic metre of water available in the reservoir. It eventually means that every million cubic metre of water available in the reservoir will mature so many hectares of a particular crop. Hence, the irrigation capacity of the reservoir is directly known. When duty is reduced in this manner, it is called Quantity duty or Storage duty.

 

 

(i)    Climatic and season: As stated earlier, duty includes the water lost in evaporation and percolation. These losses will vary with the season. Hence, duty varies from season to season, and also from time to time in the same season. The figures for duties which we generally expresses are their average values considered over the entire crop period.

 

 

(ii)   Useful rainfall: If some of the rain, falling directly over the irrigated land, is useful for the growth of the crop, then so much less irrigation water will be required to mature the crop. More the useful rainfall, less will be the requirement of irrigation water, and hence more will be the duty of irrigation water.

 

 

(iii)  Type of soil: If the permeability of the soil under the irrigated crop is high, the water lost due to percolation will be more and hence, the duty will be less. Therefore, for sandy soils, where the permeability is more, the duty of water is less.

 

(iv) Efficiency of cultivation method: If the cultivation method (including tillage and irrigation) is faulty and less efficient, resulting in the wastage of water, the duty of water will naturally be less. If the irrigation water is used economically, then the duty of water will improve, as the same quantity of water would be able to irrigate more area. Cultivators should, therefore, be trained and educated properly to use irrigation water economically.

 

Importance of duty:

 

 

It helps us in designing an efficient canal irrigation system. Knowing the total available water at the head of a main canal, and the overall duty for all the crops required to be irrigated in different seasons of the year, the area which can be irrigated can be worked out. Inversely, if we know the corps area required to be irrigated and their duties, we can work out the discharge required for designing the channel.

 

Irrigation Efficiencies

 

 

Efficiency is the ratio of the water output to the water input, and is usually expressed as percentage. Input minus output is nothing but losses, and hence, if losses are more, output is es and, therefore, efficiency is less. Hence, efficiency is inversely proportional to the losses. Water is lost in irrigation during various processes and, therefore, there are different kinds of irrigation efficiencies, as given below:

 

 

(i)  Efficiency of waterconveyance (ηc): It is a ratio of the water delivered into the fields from the outlet point of the channel, to the water pumped into the channel at the starting point. It may be represented by ηc. It takes the conveyance or transit losses into account.

 

(ii)  Efficiency of waterapplication (ηa): It is the ratio of the quantity of water stored into the root zone of the crops to the quantity of water actually delivered into the field. it may be represented by ηa. It may also be termed as farm efficiency, as it takes into account the water is lost in the farm.

 

(iii)   Efficiency of waterstorage (ηs): It is the ratio of the water stored in the root zone during irrigation to the water needed in the root zone prior to irrigation (i.e., field capacity 'existing moisture content). It may be represented by ηs.

 

Efficiency of water use (ηu): It is the ratio of the water beneficially used, including leaching water, to the quantity of water delivered. It may be represented by ηu.

Example: Once cumec of water is pumped into a farm distribution system. 0.8 cumec is delivered to a turn out, 0.9 kilometres from the well. Compute the conveyance efficiency.

 

Solution: By definition

ηc = Output/ Input  x 100 = 0.8/1.0 . 100 = 80%

 

Example: 10 cumecs of water is delivered to a 32 hectare field, for 4 hours. Soil probing after the indicated that 0.3 metre of water has been stored in the root zone. Compute the water application efficiency.

 

Solution:

 

Volume of water supplied by 10 cumecs of water applied for 4 hours (10 =(4 60x 60)m3 = 1,44,000 m3

 

= 14.4  x104 m3 =   14.4m  x 104m2 =   14. 4ha.m.

Depth of water applied =        volume/area =      1,44, 000/32,0, 000  =  144         /320  

= .45

 

 Input = 14.4 ha.m

 

Output = 32 hectares land is storing water upto 0.3 m depth,

 Output = 32 x0.3 ha.m = 9.6 ha.m

 

Water application efficiency (ηa) = Output/ Input   x 100

= 96/14.4 = 67%

 

 (v) Uniformity coefficient or Water distribution efficiency:

 

 

The effectiveness of irrigation may also be measured by its water distribution efficiency (ηd), which is defined below:

ηd = |1d(D)

 

 

Where ηd = Water distribution efficiency

D = Mean depth of water stored during irrigation.

d = Average of the absolute values of deviations from the mean.

 

 

The water distribution efficiency represents the extent to which the water has penetrated to a uniform depth, throughout the field. When the water has penetrated uniformly throughout the field, the deviation from the mean depth is zero and water distribution efficiency is 1.0.

 

 

Examples: The depths of penetrations along the length of a boarder strip at points 30 metres aprt from probed. Their observed values are 2.0, 1.9, 1.8, 1.6 and 1.5 metres. Compute the water distribution efficiency.

 

Solution:

 

The observed depths at five stations are 2.0, 1.9, 1.8, 1.6 and 1.5 metres, respectively.

Mean depth = D = 2.0+1.9+1.8+1.6+1.5 / 5 = 8.8/5 = 1.76metres

 

Values of deviations from the means are (2.0 '1.76), (1.9 '1.76), (1.8 '1.76). (1.6 '1.76), (1.5

'1.76) i.e.,

 

0.24, 0.14, 0.04,  0.16 and '0.26.

 

The absolute values of these deviations from the mean are 0.24, 0.14, 0.04, 0.16 and 0.26.

 

The average of these absolute values of deviations from the

Mean = d=  (0.24 + 0.14 + 0.04 + 0.16 + .26. )/5

= 0.84/5 = 0.168 metre

 

The water distribution efficiency

Hence, the water distribution efficiency = 0.905. Ans.

 

 

Example: A stream of 130 litres per second was diverted from a canal and 100 litres per second were delivered to the field. An area of 1.6 hectares was irrigated in 8 hours. The effective depth of root zone was 1.7 m. The runoff loss in the field was 420 cu. M. The depth of water penetration varied linearly from 1.7 m at the head end of the field to1.1 m at the tail end. Available moisture holding capacity of the soil is 20 cm per metre depth of soil. It is required to determine the water conveyance efficiency, water application efficiency, water storage efficiency, and water distribution efficiency. Irrigation was started at a moisture extraction level of 50% of the available moisture.

 

Solution:

 

(i)                Water conveyance efficiency (C)

=( Water delivered to the fields/ Water supplied into the canal at the head) x 100

          = 100/130 x 100 =77%

 

(ii)Water application efficiency (ηa)

 Water stored in the root zone during irrigation / Water delivered to the field  x 100

Water supplied to field during 8 hours @ 100 litres per second

 

= 100x8  x60 x 60 litres = 2880 cu. m.

 

Runoff loss in the field = 420 cu. M.

 

the water stored in the root zone

 

= 2880 '420 = 2460 cu. m.

 

Water application efficiency (ηa)

=   2460 /100 = 85.4% Ans. 2880

orage efficiency

 

 = (Water stored in the root zone during irrigation /

Water needed in the root zone prior to irrigation)  x 100

 

Moisture holding capacity of soil

 

= 20 cm per m depth x1.7 m depth of root zone = 34 cm

 

Moisture already available in the root zone at the time of start of irrigation

 

50/100  x  34 =17cm.

Additional water required in the root zone

 

= 34 '17 = 17 cm.

= 2720 cu. m.

But actual water stored in root zone = 2460 cu. m.

 

 Water storage efficiency (ηs)

 

=2460 /2720  x 100 90% (say)

 (iv) Water distribution efficiency

 

Where D = mean depth of water stored in the root zone

D = ( 1.7+1.1 )/2  = 1.4m

d is computed as below:

 

Deviation from the mean at upper end (absolute value) =  |1.7 -1.4| = 0.3

 

Deviation from the mean at lower end = | 1.1 -1.4 | =0.3

 

d = Average of the absolute values of deviations from mean =

 

0.4 +0.3/2 = 0.35

Using equations, we have,

ηd = 75 or 75%    Ans.

 

Consumptive Use or Evapotranspiration (Cu)

 

 

Consumptive use for a particular crop may be defined as the total amount of water used by the plant in transpiration (building of plant tissues, etc.) and evaporation from adjacent soils or from plant leaves, in any specified time. The values of consumptive use (Cu) may be different for different crops, and may be different for the same crop at different times and places.

 

 

In fact, the consumptive use for a given crop at a given place may vary throughout the day, throughout the month and throughout the crop period. Values of daily consumptive use or monthly consumptive use, are generally determined for a given crop and at a given place. Values of monthly consumptive use over the entire crop period are then used to determine the irrigation requirement of the crop.

 

Effective Rainfall (Re)

 

 

Precipitation falling during the growing period of a crop that is available to meet the evapotranspiration needs of the crop, is called effective rainfall. It does not include precipitation lost through deep percolation below the root zone or the water lost as surface run off. Average ratios, applicable to effective rainfall, are shown in table.

 

Table: Average Ratios Applicable to Effective Rainfall





Average annual rainfall in cm. :

 

                   Per cent chance of occurrence 

                                               

          50      60      70      80      90

10:     0.84   0.72   0.61   0.50   0.38

20:     0.90   0.81   0.71   0.62   0.51

30:     0.93   0.85   0.78   0.69   0.58

40:     0.95   0.88   0.81   0.73   0.63

50:     0.96   0.90   0.83   0.75   0.67

60:     0.97   0.91   0.84   0.78   0.70

70:     0.97   0.92   0.86   0.80   0.72

80:     0.98   0.93   0.87   0.81   0.74

90:     0.98   0.93   0.88   0.82   0.75

100:   0.98   0.94   0.89   0.83   0.76

120:   0.98   0.94   0.90   0.85   0.78

140:   0.99   0.95   0.91   0.86   0.80

160:   0.99   0.95   0.91   0.87   0.82

180:   0.99   0.95   0.92   0.88   0.84

200:   0.99   0.95   0.92   0.89   0.85

 

Consumptive Irrigation Requirement (CIR)

 

It is the amount of Irrigation water required in order to meet the evapotranspiration needs of the crop during its full growth. It is, therefore, nothing but the consumptive use itself, but exclusive of effective precipitation, stored soil moisture, or ground water. When the last two are ignored, then we can write

 

CIR = Cu  Re

 

Net Irrigation Requirement (NIR)

 

It is the amount of irrigation water required in order to meet the evapotranspiration need of the crop as well as other needs such as leaching. Therefore, N.I.R. = Cu 'Re + Water lost as percolation in satisfying other needs such as leaching.

 

Factors Affecting Consumptive Use

 

Consumptive use or evapotranspiration depends upon all those factors on which evaporation and transpiration depend; such as temperature, sunlight, humidity, wind movement, etc, as detailed in article.

 

Example: The following table gives the values of consumptive uses and effective rainfalls for the periods shown against them, for a Jowar crop sown at Bellary in Karnataka State. The period of growth is from 16th October to 2nd Feb., i.e. (110 days). Determine the net irrigation requirement of this crop, assuming that water is not required for any other purpose except that of fulfilling the evapotranspiration needs of the crop.

 

Table:

Dates Cu in mm.  Re  in mm.

         

October           1631       37.0   30.8

November   130    84.2   20.4

December    131    154.9 6.7

January           131         188.1 2.4

February     12          13.3        1.0



Solution: The given table is extended as shown in Table 2.5 (b). Values in col. (4) are obtained by subtracting values of col. (3) from values of col. (2).

 

Table 2.5 (b)

Dates Cu     Re     NIR = Cu  Re

 

October       1631  37.0   30.8   6.2

November   130    84.2   20.4   63.8

December    131    154.9 6.7     148.2

January       131    188.1 2.4     185.7

February     12      13.3   1.0     12.3

                                      = 416.2 mm

                                      = 41.62 cm

 

Hence, the net irrigation requirement = 41.62 cm. Ans.

 

Estimation of Consumptive Use:

 

Although various methods have been developed in order to estimate evapotranspiration (consumptive use) value of a crop in an area, but the most simple and commonly used methods are:

 

(1)   Blaney 'Criddle Equation, and

 

(2) Hargreaves class A pan evaporation method. They are described below:

 

1. BlaneyCriddle Formula. It sates that the monthly consumptive use is given by

 

 

C u = k.p / 40  [1.8t + 32]

 

where, Cu  = Monthly consumptive use in cm.

 

k = Crop factor, determined by experiments for each crop, under the environmental conditions of the particular area.

 

t = Mean monthly temperature in oC.

 

p = Monthly pet cent of annual day light hours that occur during the period.

 

If       p/40  [1.8t +32]is represented by f, we get         

                                     

          Cu  ?k.f       ...(2.6)        

 

Consumptive irrigation requirement

= Cu  - Re   =  32.72 - 8. 38 = 24.34 cm.

 

Field irrigation requirement = C.I.R./ ηa

 = 24.34/0.8 = 30.43 cm.

 

Example: The monthly consumptive use values for Paddy are tabulated in Table 2.7. Determine the total consumptive use. What is the average monthly consumptive use and peak monthly consumptive use?

 

Table




          Dates                    Rice (Loam Soil) 

                                      Cu  in cm   

                                               

June  130    Nursery                26.69

July   112                       8.76  

                                               

July   1331                     14.38

August        131                       22.73

September  130                       21.29

October       131                       25.50

November   124                       15.06

 

Solution: The summation of consumptive uses

 

= 29.69+8.76+14.38+22.73+21.29+25.50+15.06 = 137.41 cm

Hence, total consumptive use for paddy = 137.41 cm.

Average daily consumptive use =

      137.4/Period of growth in days =

     = 137.41/31+31+31+30+31+24

= 137.41/177 = 0.77 cm. = 0.77x30=23.1  mm.           

 

Average monthly consumptive use = 0.77 - 30 = 23.1 mm.

Peak monthly consumptive use.

 

= 26.69 cm.            (Highest value given)

 

I.  Provision of farm roads.

II.  Land formation to suitable slopes.

III.  Introduction  of  the  ‘warabandi’  system   for   r

 

 

For future irrigation projects, the National Commission on agriculture suggested that the project report should be prepared in the following three parts(8):

 

Part I: All engineering works from source of supply to outlets, including  drains.

 

Part II: All engineering works in the command area comprising land leveling and shaping, construction of watercourses, lined or unlined field channels, field drains, and farm roads.

 

Part III: All other items pertaining to agriculture, animal husbandry, forestry, communication, and cooperation.

 

 

Upto 1985,102 projects covering an ultimate irrigation potential of 16.5 Mha has been included in command Area Development Authorities (CADA)(11).In financial terms, the allocation in the sixth and the seventh fiveyears plans was Rs.856 crores and Rs1,800 crores, respectively(5).



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