2.1 Crops & crop seasons

Water requirement of a crop means the total quantity and the way in which a crop requires water, from the time it is sown to the time it is harvested.

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

If natural rain is sufficient and timely to satisfy water requirement of crop, no irrigation water is required for raising that crop. But since magnitude as well as frequency of the rainfall varies throughout tropical country certain crop may require irrigation.

Functions of irrigation water

· It acts as solvent for nutrients.

· Irrigation water supplies moisture which is essential for the life of bacteria beneficial to plant growth.

· Irrigation water supplies moisture which is essential for chemical action within the plant leading to its growth.

· Some salts require presence of water for its reaction.

· Water cools the soil and atmosphere thus making favorable environment for plant growth.

· It reduces hazard of soil piping

· It softens the tillage pans.

Preparation of land for irrigation

· Cleaning of land - Removal of trees and bushes from roots.

· The land should be made level. High patches should be scraped and depressions should be filled. Unless this is done irrigation water will fill depression and duty will be less.

· Land should be given regular slope in direction of falling gradient.

· Land should be divided into suitable plots as per method of irrigation.

· Permanent supply ditches and drainages (to carry waste water) should be excavated.

· Proper drainage system should be adopted to prevent water logging.

Availability of soil water

Hygroscopic water – When an oven dried sample is kept in atmosphere, it absorbs some amount of water from atmosphere. This is known as hygroscopic water, it not capable of movement by gravity or capillary forces.

Capillary water – This is excess of hygroscopic water which exists in the pore spaces of soil by molecular attraction.

Gravitational water – is that part in excess of hygroscopic and capillary water which will move out of the soil if favorable drainage is provided.

Principal crops & crop seasons in Maharashtra

Crop period or Base period

Base period- It is defined as the interval of time in days between first watering of crop at sowing to last watering before harvesting.

Crop period- It is time interval in days between sowing of crop to its harvesting.

Although base period of crop will be slightly less than crop period, for all practical purposes they can be taken as one and same. Expressed as B number of days.

Delta -∆

Every crop requires certain quantity of water at fixed interval. This water is given to crop in 4 to 5 watering (each varying from 5 to 12 cm) within growth period.

This depth to which water would stand on the irrigated area if the total quantity supplied was to stand above the surface without percolation or evaporation.

∆ (delta) is total depth of water (in cm) required by a crop to come to maturity.

Duty of water (D)

Duty of water is the relationship between the volume of water and the area of crop it matures.

Duty is defined as the area irrigated per cumec of discharge running for base period B.

Relationship between duty and delta

Let one cubic meter per second (Cumec) water is applied to crop for base period B

Then, total volume of water applied V

One cumec supplied for B days matures D hectare of land ( 1 Ha = 10⁴ sqm)

Therefore total depth of water supplied =

Factors affecting duty of water

Method and system of irrigation- In inundation system there is wasteful use of water hence lower duty than perennial system. Flow irrigation system has lower duty due to conveyance losses while lift irrigation has higher duty as command area is nearer.

Mode of applying water – Surface irrigation has lower duty than subsurface irrigation. Conventional methods of water application have lower duty than modern methods.

Method of cultivation – Properly ploughed soil will have high water retention increasing its duty.

Type of crop – duty varies from crop to crop. Duty will be less for crops requiring more water and vice versa. e.g for rice it is less while for wheat it is more.

Climatic conditions – temperature, wind, humidity and rainfall affects the duty. In high temperatures evaporation losses will be more and duty will be less. Rainfall during crop period will reduce crop water requirement and duty will be more.

Method of assessment – In volumetric assessment farmer will have tendency to use water economically increasing duty of irrigation water. While in area based assessment farmer will have tendency to use more water lowering duty.

Canal conditions – In unlined canal seepage and percolation losses will be more lowering duty while in lined canal, losses will be less and duty will be more.

Character of soil in irrigation field – if soil in the field is coarse grained, percolation will be high lowering the duty

Quality of water – if salt content is more in water more quantity of water will be required to consider leaching, in turn reducing duty.

Methods of improving duty

• · Suitable method of applying water to crops to be used.
• · Land should be properly ploughed and leveled before sowing the crop.
• · The canals should be lined. This reduces seepage and percolation losses.
• · Canals should be so aligned that areas to be cultivated are concentrated along it.
• · Water should be of good quality.
• · Rotation of crop must be practiced.
• · Volumetric method of assessment should be used.
• · Farmers must be trained to use optimum quantity of water.

Certain important definition

Kharif Rabi ratio or Crop ratio – The area to be irrigated for Rabi crop is generally more than that for Kharif crop. This ratio of proposed areas to be irrigated in Kharif season to that Rabi season is called Kharif Rabi ratio or crop ratio.

Paleo irrigation – Sometimes in the initial stages before the crop is sown, the land is very dry. This particularly happens at the time of Rabi crops. In such a case application of irrigation water to land before sowing of a crop is required to moisten the soil. This is known as paleo irrigation.

Kor watering – After sowing the crop when it grows few cm above the ground, it needs large quantity water for its healthy growth and is known as Koe watering and depth of this watering is known as Kor depth. The period during which this watering is applied is known as Kor period.

Cash crops – Crops other than food grains, which cannot be consumed directly by the cultivators and are to be sent to the market for further processing before encashment are called as cash crops e.g. sugarcane, tobacco, tea, jute etc.

Crop rotation- If the same crop is grown in agricultural land every year, then fertility of land decrease because the soil becomes deficient in certain nutrients required for particular crop and yield from land gets reduced. In order to maintain fertility of the soil, it is advisable to grow another crop that does not require same nutrients as earlier crop or to keep the land fallow without any cultivation. Such a systematic arrangement of growing different crops in rotation in same land is termed as crop rotation.

Gross command area (GCA) – It is total area that is enclosed in between two drainage boundaries which can be brought under the irrigation canal system without considering the limitation of the quantity of water available. It includes cultivable as well as non cultivable area. Such as ponds, residential area, roads etc.

Culturable Command area (CCA) - Cultivable land in the gross command area is called as culturable command area (CCA). This is equal to GCA minus the uncultivable land such as forest, roads, ponds, res area etc.

Intensity of irrigation – Entire CCA is not proposed to be irrigated at one time as intensive irrigation (irrigating all the fields of the same area at the same time) causes over irrigation & water logging, and due to shortage of irrigation water. Hence only a certain percentage of culturable land is brought under irrigation which is 30 to 40% of CCA every season. This percentage of CCA proposed to be irrigated seasonally is called intensity of irrigation (II).

Intensity of irrigation is defined as the ratio of actually irrigated area during crop season to the net culturable irrigable area.

Assessment of irrigation water

The water supplied for irrigation to the farmers is at government expenses. Some nominal charges must therefore be levied on farmers who use this water. Fixation of such charges is known as assessment of irrigation water. The returns on the capital investment is based upon the estimate of such water charges.

Following are the reasons of assessment

· To recover cost of construction incurred in irrigation projects such as dams, reservoirs, canals etc.

· To recover maintenance cost of various works and staff.

· To collect revenue to build more such structures.

· To check cultivators against uneconomical and careless use of water.

Methods of assessment

Assessment on area basis or crop rate basis – in this method area under cultivation is recorded and charges are levied on the area under cultivation. This method has certain disadvantages like cultivators near head of canal use more water and deprive the area near tail and cultivators have tendency to make wasteful use of water.

Volumetric assessment – The method involves levying charges on actual volume of water supplied. Although this is ideal method of assessment is not suitable in India due to necessity of installation of numerous water meters on irrigation outlets, which is difficult to maintain besides heavy initial cost.

Assessment on seasonal basis – In this method rate of assessment is based upon type of crop grown in particular tract in that crop season. Not vogue in India.

Permanent assessment or betterment levy basis – Some farmers may have their own sources of water supply under ordinary condition. Farmers in such area are levied at a fixed rate every year for the provisions called as betterment levy.

Composite rate basis - Composite rate is a combination of water charges and land revenue.