Types of irrigation systems & Water Requirement of Crops Study Notes

Types of Irrigation Systems

Major aim of irrigation systems is to help out in the growing of agricultural crops and vegetation by maintaining with the minimum amount of water required, maintenance of landscapes and re-vegetation of disturbed soils. Irrigation systems are also used for dust repression, removal of sewage, and in mining.

On the contrary, agriculture that relies only on direct rainfall is referred to as rain-fed or dry-land farming.

Techniques of Irrigation

In India, the irrigated area consists of about 36 percent of the net sown area. There are various techniques of irrigation practices in different parts of India. These methods of irrigation differ in how the water obtained from the source is distributed within the field. In general, the goal of irrigation is to supply the entire field homogeneously with water, so that each plant has the amount of water it needs, neither too much nor too little. Irrigation in India is done through wells, tanks, canals, perennial canal, and multi-purpose river valley projects.

A) Surface Irrigation

In this technique, water flows and spreads over the surface of the land. Varied quantities of water are allowed on the fields at different times. Therefore, it is very difficult to understand the hydraulics of surface irrigation. 

Surface irrigation technique is broadly classified as

1. Basin irrigation - Basin irrigation is common practice of surface irrigation. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin. 

2. Furrow irrigation - In furrow irrigation technique, trenches or “furrows” are dug between crop rows in a field. Farmers flow water down the furrows (often using only gravity) and it seeps vertically and horizontally to refill the soil reservoir. Flow to each furrow is individually controlled. Furrow irrigation is suitable for row crops, tree crops and because water does not directly contact the plants, crops that would be damaged by direct inundation by water such as tomatoes, vegetables, potatoes and beans. 

There are numerous advantages of Furrow technique of irrigation:

1. Large areas can be irrigated at a time.
2. It saves labor since once the furrow is filled, it is not necessary to give water a second time.
3. It is a reasonably cheaper method.
4. Plants get proper quantity of water by this system.

Major drawback of furrow system of irrigation is ensuring uniform dispersal of water over a given field. . Other problem with furrow irrigation is the increased potential for water loss due to runoff. 

• Uncontrolled flooding: There are many cases where croplands are irrigated without regard to efficiency or consistency. These are usually situations where the value of the crop is very small or the field is used for grazing or recreation purposes.
• Free flooding - In free flooding method, water is applied to the land from field ditches without any check or guidance to the flow. The land is divided into plots or kairines of suitable size depending on porosity of soil. Water is spread over the field from watercourse. 

This technique is beneficial for newly established farms where making furrows is very expensive. This method is economical and can be effectively used where water supply is in plenty. This method is suitable for the fields with an irregular surface in which other techniques are difficult to apply.

The major drawback of this method is that there is no perfect control over the flow of water to attain high efficiency. Sometimes the flow of water over the soil is too rapid to fulfil soil moisture deficiency. On the other hand, sometimes water is retained on the field for a very long time and consequently, the water is lost in infiltration or deep percolation.

3. Border Strip Method - In this technique of irrigation, a field is divided into number of strips. The width of strip varies from 10 to 15 meters and length varies from 90 m to 400 m. Strips are separated by low embankments or levees. The water is diverted from the field channel into the strips. The water flows gradually towards lower end, wetting the soil as it advances. 

Classification Based on Availability of Water

1. Gravity Irrigation:

Gravity or flow irrigation is the type of irrigation in which water is available at a higher level as to enable supply to the land by gravity flow. In flow irrigation water is supplied to the fields though the canals off taking from head works.

Perennial irrigation may be either direct or indirect, as follows:

1.1.1 Direct irrigation:

In direct irrigation system, water is directly diverted from the river into the canal by the construction of diversion weir or barrage across the river without attempting to store water. 

1.1.2 Indirect irrigation:

It is also termed as storage irrigation. Here water is stored in reserved during monsoon period by the construction of a dam across the river for supply into the off taking canals. 

1.2 Non –Perennial Irrigation:

 Also called restricted irrigation. Canal supply is generally made available in non-monsoon period from the storage in small dams as in Kandi areas which inadequate to feed all the year round, and/or canal water is not required during monsoon due adequate rainfall in the command area.

1.3 Inundation Irrigation:

Inundation irrigation is done by a canal taking off from a river in flood without any diversion work. It depends on the periodical rise in water level of the river and the supply is drawn through open cuts in the river bank or creeks which are called heads. 

2. Tank Irrigation: Tanks on local streams form a significant source of irrigation especially in the peninsula area in the States of Karnataka, Maharashtra and Tamil Nadu. Tank irrigation belongs to category of storage irrigation. Tanks are small sized reservoirs formed by small earthen embankments to store runoff for irrigation. 

3. Lift Irrigation: 

In lift irrigation water is lifted from a river or a canal to the bank to irrigate the land which are not commanded by gravity flow.

Lift Irrigation vs. Gravity Irrigation:

Lift irrigation

1.Costly means of irrigation

2.Less manorial silt in water

3.Working  dependent on the operation of machinery

4.Higher water rates.

5. Lift irrigation is a complex system and by and large costly.

Gravity flow irrigation

1.Cheapest means of irrigation

2. Silt in water has manorial value

3. Lifting equipment is not involved

4. Lowest water rates

5. Simple and economical system of irrigation.

4. Well Irrigation

Groundwater:

Groundwater is generally a more dependable source of irrigation than surface water and is free from seeds and plant organisms. The first cost of installation is, however, high. The best water bearing stratum or aquifer is coarse gravel free from sand but such formation are rare to find..

Subsurface Irrigation:

It is termed as subsurface irrigation because in this type of irrigation, water does not wet the soil surface. The underground water nourishes the plant roots by capillarity. 

5. Sprinkler Systems

In the sprinkler irrigation network, we have the mains and the subdomains, through which water under pressure is made to flow. Revolving sprinkler heads are then usually mounted on rising pipes attached to the laterals.

The advantage of sprinkler irrigation are enumerated below:

1. Seepage losses, which occur in earthen channels of surface irrigation methods, are completely eliminated.
2. Moreover, only the optimum quantity of water is used in this method.

3. Land levelling is not required, and thus avoiding removal top fertile soil, as happens in other surface irrigation methods.

4. No cultivation area is lost for making ditches, as happens in surface irrigation methods. It, thus, results in increasing about 16% of the cropped area.

Limitations of sprinkler irrigation are also enumerated below:

1. High winds may distort sprinkler pattern, causing non-uniform spreading of water on the crops.
2. In areas of high temperature and high wind velocity, considerable evaporation losses of water may take place.
3. They are not suited to crops requiring frequent and larger depths of irrigation, such as paddy.
4. The initial cost of the system is very high, and the system requires a high technical skill.
5. Only sand and silt free water can be used, as otherwise pump impellers lifting such waters will get damaged.

6. Drip irrigation Method

Drip irrigation, also called trickle irrigation, is the latest field irrigation technique and is meant for adoption at places where there exists acute scarcity of irrigation water and other salt problems. In this method, water is slowly and directly applied to the root zone of the plants, thereby minimizing the losses by evaporation and percolation.

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.

• 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 crop period. 

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. 

• Delta of a Crop (Δ)

Each crop requires a certain amount of water after a certain fixed interval of time, throughout its period of growth.

The total quantity of water required by the crop for its full growth may be expressed in hectare meter (ha.m) 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 (Δ).

• Delta for certain crops

The average values of deltas for certain crops are shown in the 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 the field, i.e. ‘delta on field’ which includes losses.

• Duty of Water (D)
  

The term duty means the "area of land" that can be irrigated with the unit volume of irrigation water. Quantitatively, duty is defined as the area of land expressed in hectares that can be irrigated with unit discharge, that is, 1 cumec flowing throughout the base period, expressed in days.

• Relation between Duty(D) and Delta(Δ)

Δ = 8.64B/D (meters)

Where

Δ is in meter, 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. 

Layout of 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.

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 the watercourse (i.e. at the outlet point is generally the endpoint of Irrigation Department.

Factors Affecting Duty of Water

1. Climatic and season
2. Useful rainfall
3. Type of soil
4. The efficiency of cultivation method 

Irrigation Efficiencies

Efficiency is the ratio of the water output to the water input and is usually expressed as the percentage. Input minus output is nothing but losses, and hence, if losses are more, the 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 water-conveyance (η_c)

(ii) Efficiency of water application (η_a)

(iii) Efficiency of water storage (η_s)

(iv) Efficiency of water use (η_u) 

(v) Uniformity coefficient or Water distribution efficiency

vi) Consumptive Use or Evapotranspiration (Cu)

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 runoff.

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.

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

Blaney-Criddle Formula: 

It sates that the monthly consumptive use is given by

C _u = K.(P/ 40  [1.8t + 32])

where, C_u  = 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