Crop water requirement

By

Dr. Dipankar Roy

Delta

Some quantity of water is required for any crop to come to its maturity. The total quantity of water required for any crop during its base period(B) for its full fledged nourishment when expressed in depth of water(i.e. in ’cm’ or in ’inches’) is called its Delta.

The total quantity of water(i.e. volume of water) is divided by the total irrigated area to obtain Delta of crop of the irrigated area.

Duty

Duty of a water simply expresses the number of hectare of land that can be irrigated for the full growth of the given crop by supplying 1 cumec water continuously during the entire base period of that crop. It is generally represented by ’D’. Its unit is hectare/cumec.

For example, if water flowing at the rate of 1 cumec, runs continuously for B days of the crop matures 100 hectares then Duty of that crop is 100 hectare/cumec to the base of B days.

Duty varies from point to point. It increases as one moves to downstream from the head of main canal to the head of branches. It is due to the transmission losses in the channels.

Crop Seasons

The Indian cropping season is classified into two main seasons
based on the monsoon.

• Kharif and
• Rabi

The kharif cropping season is from July–September duringthe south-west monsoon and the Rabi cropping season is from October-March (winter). The crops grown between March and June are summer crops.

Important definitions

Kor watering: The first watering is done when the crop has grown to about three centimeters. This watering is known as Kor watering.

Kor Period: The period during which kor watering is done is known as kor Period.

Kor depth: The depth of water applied for the kor watering is called Kor depth.

Cash Crop: A crop produced for its commercial value rather than for use by the grower. e.g. Coffee.

Crop Rotation: The growing of different crops in succession on a piece of land to avoid exhausting the soil and to control weeds, pests, and diseases.

Irrigation Efficiencies

These irrigation efficiencies are brought about by the desire not to waste irrigation water, no matter how cheap or abundant it is.

The objective of irrigation efficiency concept is to determine whether improvements can be made in both the irrigation system and the management of the operation programmes, which will lead to an efficient irrigation water use.

Irrigation Efficiencies

Efficiency of water-conveyance

It is the ratio of the water delivered into the fields from the outlet point of the channel, to the water entering into the channel at its starting point. It may be represented by . It takes the conveyance or transit losses into consideration.

Efficiency of water-application

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 . It may also be called on-farm efficiency, as it takes into consideration the water lost in the farm.

Irrigation Efficiencies

Efficiency of water-storage

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 .

Efficiency of water use

It is the ratio of the water beneficially used, including leaching water, to the quantity of water delivered. It may be represented by .

Irrigation Efficiencies

Uniformity coefficient or Water distribution efficiency

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

where = Water distribution efficiency, = Mean depth of water stored during irrigation, = 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, uniformly throughout the field, the deviation from the mean depth is zero and water distribution efficiency is 1.0.

Example Problem

Example Problem

Net Irrigation and Gross Irrigation

Net Irrigation is the moisture that must be supplied by irrigation to satisfy evapotranspiration plus that needed for leaching and not supplied by off-season storage, and the effects of precipitation and groundwater storage.

Where: is the net irrigation, is evapotranspiration, is leaching requirement, is off-season soil moisture carry-over, is rainfall excess

Gross Irrigation is the Net Irrigation Requirement Divided by Irrigation Efficiency

Important Terms

Depth of Irrigation is the depth of the readily available moisture. This is the net depth of water normally needed to be applied to the crops during each irrigation

Irrigation interval is the time between successive irrigations. Irrigation interval is equal to: Readily Available Moisture or Net Irrigation divided by Evapotranspiration, ET The shortest irrigation interval is normally use in design. The irrigation interval varies with ET. It is equivalent to Readily Available Water divided by the Peak ET

Irrigation Period is the number of days allowed to complete one irrigation cycle in a given area.

Important Terms

Irrigation Interval and Period: In irrigation scheduling, the irrigation period should be less that the irrigation interval. This is because if the period is not smaller, before the latter parts of the area are to be irrigated, the earlier irrigated areas will need fresh irrigation.At peak evapotranspiration (used in design), irrigation interval should be equal to irrigation period. i.e. Generally IP < II.

Desired Irrigation Design Capacity: This is the flow rate determined by the water requirement, irrigation time, irrigation period and the irrigation application efficiency.
It is the flow rate of flow of the water supply source e.g. pumps from a reservoir, or a borehole required to irrigate a given area.

Soil Moisture Relationship

Field Capacity

is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This usually takes place 2–3 days after rain or irrigation in pervious soils of uniform structure and texture.

Permanent wilting point

is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.

Readily Available Moisture

Readily available water (RAW) is the water that a plant can easily extract from the soil. RAW is the soil moisture held between field capacity and a nominated refill point for unrestricted growth. In this range of soil moisture, plants are neither waterlogged or water-stressed.

Soil Moisture Deficiency

is the water required to bring the soil to Field Capacity

Equivalent moisture

is the water retained after 30min of centrifuge at 1000g

Irrigation Scheduling

This means Predicting when to Irrigate and how much to Irrigate.

For efficient water use on the farm, the farmer needs to be able to predict when his crops need irrigation. This can be done by:

• Observing the plants;
• Keeping a Water Balance Sheet
• By Measuring the Soil Moisture Content or
• Computer Software

Irrigation Scheduling

Estimating Depth and Frequency of Irrigation based on soil moisture regime concept:

Irrigation Scheduling

Estimating Depth and Frequency of Irrigation based on soil moisture regime concept:

Irrigation water quality

Irrigation water must not have direct or indirect undesirable effects on the health of human beings, animals, and plants.

The irrigation water must not damage the soil and not endanger the quality of surface and ground waters with which it comes into contact.

The presence of toxic substances in irrigation water may threaten the vegetation besides degrading the suitability of soil for future cultivation.

Surface water, ground water, and suitably treated waste waters are generally used for irrigation purposes.

Irrigation water quality

The various types of impurities, which make the water unfit for irrigation, are classified as:

• Sediment Concentration in water
• Total concentration of soluble salts in water
• Proportion of Sodium ions to other cations
• Concentration of potentially toxic elements in water
• Bi-carbonate concentration as related to the
• concentration of Calcium plus Magnesium in water
• Bacterial Contamination

Irrigation water quality

Total concentration of soluble salts in water The salt content is generally measured by determining the electrical conductivity of water.

Irrigation water quality

Proportion of Sodium ions to other cations The percentage of sodium ions is generally less than 5% of total exchangable cations. If this percentage increases to about 10% or more, the aggregation of soil grains breaks down. The soil becomes less permeable and poorer tilth.

Irrigation water quality

Irrigation water quality

Waterlogging

Waterlogging

If water table rise to such a level through capillary action to the surface & root zone, that it cannot conveniently permit an anticipated activity this situation is called WATER LOGGING

Cause of Waterlogging

• Seepage from canal system
• Inadequate Drainage of irrigated areas
• Poor irrigation management by cultivators
• Natural obstruction to the flow of ground water
• Obstruction of natural drainage
• Poor quality of irrigation water
• Inadequate capacity of natural drainage
• Flat topography
• Geological barrier
• Inadequate surface drainage

Effects of Waterlogging

• Lack of aeration
• Creation of salinization
• Growth of natural weeds and their competition with crops
• Delayed cultivation operation
• Inhibiting activities of soil bacteria
• Reduction in soil temperature
• Low yield
• Adverse effects on plant community health
• Loss of cash crops
• Diseased crops

Measures to Control Water Logging

Preventive measures

Curative measures

Preventive Measures to Control Water Logging

Reduce flow to the subsoil by controlling:

• Percolation from rainfall
• Percolation from canals
• Percolation from irrigation fields
• Sub soil flow in ground water reservoirs

Curative Measures to Control Water Logging

• Percolation from rainfall
• Percolation from canals
  • Lining of canals and watercourses
  • Lowering of water levels in canals
  • Underground water conveyance system
• Percolation from irrigation fields
• Sub soil flow in ground water reservoirs

Curative Measures to Control Water Logging

• Provision of intercepting seepage drains
• Arresting percolation from irrigation in the fields
  • Lining of canals and watercourses
  • Controlling intensity of irrigation
• Open well irrigation

Curative Measures to Control Water Logging

• Economical use of water
• Arresting subsoil flow of ground water reservoir
  • Pumping from subsoil
  • Seepage drains
  • Natural drainage rivers
  • Seepage cum pumping
  • Local reservoirs

Curative Measures to Control Water Logging

• Cropping pattern
• Puncturing underground barriers
• Adequate waterway under drainage works
  • Tube well pumping
  • Shallow tube wells
  • Deep discharging tube wells
• Land grading
• Tree plantation

Land Reclamation

Land reclamation is the Process of restoring to cultivation lands which have been rendered uncultivable or have suffered reduction in yield.

Salt content

Main salts found in Punjab soils are sodium sulphate, sodium chloride and sodium carbonate.
Salt content above 0.25% renders soil infertile. Up to 0.18% is desirable.

Salinity

The problem of soil salinity has existed for centuries. History records that the collapse of ancient Mesopotamia was partly due to crop failure caused by saline soils
If the concentration of harmful salts in the root zone of a plant increases to such on extent that plant growth is Affected, this situation is called Salinity. Electrical conductivity less than 4 mmhos/cm

Causes of Salinity

The factors contributing towards the problem of salinity are almost same as that of water logging.

Every agricultural soil has certain mineral salt is also called alkali salts in it like NaCl, Na2 CO3 , Na2 SO4 etc. When these soluble alkali salts are excess in soil and further, ground water table is very near to ground, these salts get mixed with ground water and with upward movement of water not only accumulated in first 3 to 4ft of soil layer below ground surface but also form a tin 2” to 3” crust on surface.

Alkalinity

Accumulated salt reaches the surface- Rain washes it below the surface - Evaporation brings it to surface again. Upward-downward movement replaces soil calcium with
sodium- soil becomes impermeable to water and air.

Land reclamation

pH value

The pH value or hydrogen-ion-concentration is a measure of the intensity of acidity or alkalinity (basicity) of a soil. Its value ranges from 0 to 14, of which the figure 7 is neutral in the sense of chemical reaction. Below 7 the soil is acidic above 7 alkaline.

Soil sickness

Over-exploitation leads to soil sickness with depletion of soil organic matter and the reaction of the soil changes to acidity. In such situations addition of bulky organic matter, either in the form of farm-yard manure, rural or city compost or green manure is needed. Organic matter helps build up the soil microflora.

Effects of Salinity

• Reduction in crop yield and crop failure in extreme cases
• Limited choice of crop cultivation
• Nutritional imbalances causing diseases in livestocks
• Very low intake rate of highly deteriorated alkali soils with the result that most of the rain falling on such soils runs off and causes floods and damages crops in adjoining areas.

Land reclamation process

Reduction in salt content

It is achieved by leaching and rice cultivation. Sufficient depth of water in rice cultivated land tends to maintain the salt at a safe depth below the surface from crop growth point of view. It is the first step of reclamation.

Reduction in soil alkalinity

Reduction in soil alkalinity is obtained by the roots of the rice crops. Carbon dioxide generated by rice roots on the soil replaces the sodium which brings about the improvements in soil such as (i) pH value of soil is lowered, (ii) rate of percolation is increased, and (iii) more of the exchangeable sodium of the soil is brought into solution.

Land reclamation process

Restoration of nitrogen

Nitrogen, essential for plant food, is reduced during rice cultivation due to absence of air and excess water content of the soil. A leguminous crop is grown during rabi season to restore this nitrogen. Guava and San possess maximum percentage of nitrogen.

Land Reclamation Methods

Methods depend on soil characteristics, salt distribution and their nature in soil, pH value and total soluble salt content in the soil, quality of irrigation water and present state of salts

• Leaching and cropping
• Crop rotation
• Addition of ammendments like Gypsum, Acids of acid
• forming fertilizers
• Ponding
• Dug out ponds

Land Reclamation Methods

• Deep ploughing
• Sub-surface drainage
• Addition of waste products
• Earthworms
• Electro-dialisis
• Green manuring
• Natural method
• Surface drainage
• Through agronomic practices

The End