IRRIGATION - TERMS & DEFINITIONS

Introduction: 

Irrigation may be defined as the process of artificially supplying water to the soil for raising crops.

Importance of irrigation

1. Area of less rainfall

2. Non-Uniform rainfall

3. Commercial crops with additional water

4. Controlled Water Supply

Types or Systems of Irrigation:

Lift Irrigation: It is that system of irrigation in which irrigation water is available at a level lower than that of the land to be irrigated and hence water is lifted by pumps or other mechanism (Hydraulic ram and siphon action) and then conveyed to agriculture fields by gravity flow. 

Inundation Irrigation: It is that system of irrigation in which large quantity of water flowing in a river is allowed to flood or inundate the fields to be cultivated. 

Perennial Irrigation: It is that system of irrigation in which irrigation water is supplied as per the crop requirements at regular intervals throughout the crop period.

Direct Irrigation:It is a type of flow irrigation in which water from rivers and streams are conveyed directly to agricultural fields through a network of canals, without making any attempt to store water this is practiced in areas where the rivers and streams are perennial.

Storage Irrigation:Dams are constructed across rivers which are non- perennial. 

Bandhara Irrigation: It is a special irrigation scheme adopted across small perennial rivers. This system lies somewhere between inundation type and permanent type of irrigation. A Bandhara is a low masonry weir (obstruction) of height 1.2m to 4.5m constructed across the stream to divert water into a small canal. The canal usually takes off from one side and the flow into the canal is controlled by a head regulator. 
 

Methods of Irrigation: 

 Surface irrigation method: In this method the irrigation water is applied by spreading water as a sheet or as a small stream on the land to be irrigated. 

1. Wild flooding: 

- In this method water is applied by spreading water over the land to be irrigated without any preparation. 

- There is no restriction for the movement of water. 

- It follows the natural slope of the land. 

- The water may be applied to the land directly from a natural stream during season of high flow as in inundation irrigation. 

- This method is suitable for flat and smooth land but involves wastage of water and hence it can be practiced where water is abundant and inexpensive.

2. Controlled flooding: 

- In this method water is applied by spreading it over the land to be irrigated with proper control over the flow of water and as well as the quantity of water to be applied. 

- In such methods prior land preparation is essential.

Various controlled flooding methods are as follows

i) Free flooding: This method is also known as irrigation by plots. Here the field is divided into a number of small sized plots which are practically level. Water is admitted at the higher end of the plots and the water supply is cut off as soon as the water reaches the lower end of the plots.

ii) Border strip method: In this method the land to be irrigated is divided into a series of long narrow strips separated from each other by levees (Earthen bunds) or borders. 

iii) Checks or Levees: In this method a comparatively large stream of water discharged into a relatively level plot surrounded by check or levees or low rise bunds. 

iv) Basin flooding: This method of irrigation is adopted for irrigating banana (enclosures of fruit trees).

v) Zigzag flooding: This is a special method of flooding where the water takes a circuitous route before reaching the dead end of each plot. Each plot is subdivided with help of low bunds. This method is adopted in loose soils to prevent erosion at the higher ends

vi) Furrow method: In this method water is applied to the land to be irrigated by a series of long narrow field channels called furrows. A furrow is a narrow ditch 75 to 125 mm deep excavated between rows of plants to carry irrigation water.

vii) Contour farming:Contour farming is practiced in hilly regions where, the land to be irrigated has a steep slope. Here the land is divided into a series of strips usually known as terraces or benches which are aligned to follow the contour of the sloping area. This method also helps in controlling soil erosion.

b. Sub Surface Irrigation: This method consists of supplying water directly to the route zone through ditches at a slow rate which are 0.5 m to 1 m deep and 25 to 50 cm wide.

c. Sprinkler Irrigation: This method consists of applying water in the form of a fine spray as similar to rain fall. Stationary over head perforated pipes or fixed nozzle pipes installations were earlier used.

Drip or trickle irrigation: This is the latest irrigation method, which is becoming popular in water scarcity areas and water with salt problems. In this method, small diameter plastic or PVC pipes with drip nozzles commonly called emitters or drippers are adopted to deliver water to the land surface near the base of the plant. Water can be applied at a rate varying between 2 to 10 liters per hour to keep the soil moisture within the desired range for plant growth. 

Sewage Irrigation Sewage is a water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as waste water, it is more than 99% water.There can be significant health hazards related to using water loaded with pathogens in this way, especially if people eat raw vegetables that have been irrigated with the polluted water.

Classes of soil water: Water in the soil may be present in the following forms 

1. Hygroscopic water: 

2. Capillary water 

3. Gravitational water

Hygroscopic water: When an over dried soil sample is exposed to atmosphere, the soil absorbs some amount of water from the atmosphere. This water absorbed by the soil is called hygroscopic water and it is not capable of moving either under capillary action or gravitation.

Capillary water: It is that part of water in excess of hygroscopic water, which exists in the pore spaces (voids) of the soil due to molecular attraction.

Gravitational water is that part of water in excess of hygroscopic and capillary water which will drain out under favourable conditions.

Saturation Capacity (Maximum moisture holding capacity): It is defined as the amount of water required to fill all the pore spaces between soil particles by replacing all the air.

Field Capacity: It is defined as the maximum amount of moisture which can be held by a soil against gravity, thus immediately after the gravitational water has drained off from a saturated soil mass. 

Permanent wilting point or coefficient: It is that water content at which plants can no longer extract sufficient water from the soil for growth and become permanently wilted.

Available moisture: It is the different in water content of the soil between field capacity and permanent wilting point.

Readily available moisture: It is that portion of available moisture which is most easily extracted by plant roots. Only about 75% of available moisture is readily available. 

Duty of Water: 

- Duty represents the irrigating capacity of a unit of water. 

- It is usually defined as the area of land in hectares which can be irrigated to grow a crop of one cumec of water is continuously supplied for the entire period of the crop.  

Delta: 

It is the total depth of water required by a crop during the entire crop period and is denoted as ( ∆ )

Let 

D = Duty of water in hectares/cumec 

∆ = Total depth of water required during base period in  m 

B = Base period in days, 

∆= 8.64𝐵/𝐷

Crop seasons of India: 

Sowing of crops in irrigation of crops in India is usually done in 2 seasons, known as Crop seasons. They are 

1. Kharif season 

2. Rabi season

- Kharif season begins with the onset of south west monsoons sowing of crops in Kharif season is done during June-July and these crops are harvested in October-November. 

- In Rabi season the crops are sown during September-October and harvested during march-April. 

Sugarcane has a delta of about 90cms but has a period of 1 year or more. It is as such termed as a perennial crop.

IRRIGATION EFFICIENCIES

1. Water Conveyance Efficiency (ὴs ) 

This takes into account the conveyance or transit losses and is determined from the following expression : 

 (ὴs )= Wf / Wr 

Wf= water delivered to the farm or irrigation plot

Wr = water diverted to the canal

2. Water Application Efficiency (ὴa ) 

The water application efficiency is the ratio of the quantity of water stored into the root zone of the crops to the quantity of water delivered to the field. 

3, Water Use Efficiency (ὴu) 

It is the ratio of water beneficially used, including leaching water, to the quantity water delivered,

4. Water Storage Efficiency (ὴs )

ratio of water stored in the roots of the plant to the water required to make the soil to field capacity 

DETERMINATION OF IRRIGATION REQUIREMENTS OF CROPS

Consumptive Irrigation Requirement (CIR) 

Consumptive irrigation requirement is defined as the amount of irrigation water that is required to meet the evapo-transpiration needs of the crop during its full growth 

Net Irrigation Requirement (NCR)Net irrigation requirement is defined as the amount of irrigation water required at the plot to meet the evapo-transpiration needs of water as well as other needs such as leaching etc.

Field Irrigation Requirement (FIR)Field irrigation requirement is the amount of water required, to meet 'net irrigation requirements, plus the water lost in percolation in the field water courses, field channels and in field applications of water.

Gross Irrigation Requirement (GTR) Gross irrigation requirement is the sum of water required to satisfy the field irrigation requirement and the water lost as conveyance losses in distributaries upto the field. 

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