The choice of irrigation method should be based on the surface topography, soil characteristics, crop to be grown, irrigation source, depth of water application, labour requirement, level of technology and layout and management. The criteria to be adopted for selecting irrigation method are given in Table 13.4.
The irrigation method generally adopted by the farmers in India is discussed below:
1. Surface Method:
This method is very simple and cost effective if followed according to soil characteristics. Surface method is adopted in more than 99% of irrigated area in India. According to the distribution of irrigation water over the field, three different methods namely, border strips, check basins and furrow methods are followed by the farmers in India.
The farmers on medium textured soil for irrigating narrow-spaced crops mostly adopt border strip method. Recommended dimensions for border strip methods for different soils have been worked out. However, this method of irrigation is not suitable in light textured soils having greater slope. Check basin method is more suitable for sloppy soil and flow of water is regulated as per slope and soil type. The size of check basin for different soil types and locations has been worked out.
Furrow method of irrigation is generally followed in widely spaced crops, and is more suited in the soils that develop crusts after irrigation and has low infiltration rate. The general recommendation about the length of furrow in relation to soil characteristics has been suggested. This method of irrigation involves high labour cost but irrigation efficiency is more than that in border strip and check basin method of irrigation as saving of water is reported in the range of 26-66.6%.
2. Micro-Irrigation Method:
Micro-irrigation is a technique in which water and plant nutrients can be directly placed at the rhizosphere so that plants can be cultivated at low soil moisture tension. As the water is supplied only at the root zone, the conveyance, distribution and evaporation losses are minimized. Micro-irrigation is a relatively new method, which was developed all over the world towards the later part of the last century.
This system has gained wide popularity in areas of acute water scarcity. There is many forms of micro-irrigation namely- sprinkler, mister, line sourcing and drip irrigation, practiced in world. However, only sprinkler and drip irrigation are most widely adopted by the farmers.
These two methods of irrigation are discussed briefly:
Under this method, crop is irrigated by overhead sprinklers, which can be either stationary or mobile. Sprinkler method ensures high degree of water control and enables judicious utilization of even small water flow on undulating and shallow soils.
It saves about 10-16% land than that of surface irrigation and can give overall irrigation efficiency as high as 80-82% as compared to 30-50% in surface irrigation. Sprinkler irrigation can be adopted for almost all crops and most soils. Multi-location trials conducted in Gujarat on various field crops showed 4-57% increase in crop yield besides substantial saving (11-69%) of irrigation water.
Sivanappan (1995) also reported significant increase in yield of various field crops (1-51%) and water saving (29-56%) when sprinkler irrigation was adopted compared to surface irrigation. At Junagadh (Gujarat), sprinkler system was found more cost effective than drip, but both these systems were superior to surface irrigation.
It has been observed that vegetables, groundnut, forage crops, tea and coffee gave higher yields, good quality produce and greater water economy when irrigated by sprinkler. In Gujarat, sprinkler improved yield of gram (57%), cowpea (3%), sugarcane (12%), lucerne (3%) and wheat (3%) besides saving in irrigation water in the range of 19-69% compared with surface irrigation.
This method suffers from an inherent drawback. It works under pressure and the wind direction plays a major role. Excess irrigation in the direction of wind seems to be occurring as against the portion, which is opposite to the wind direction. Also moving parts of sprinkler irrigation system pose serious maintenance problems. This system is also not recommended with water having salts (more than 6 dS m-1 electrical conductivity) particularly during summer months.
In drip irrigation, watering of plants is done at a rate equivalent to (and not more than) its consumptive use so that the plants would not experience any moisture stress throughout their life cycle. In this method, water is supplied only at the root zone of the crop with the help of emitters and a network of pipes. It acts as mechanical meter and ensures supply of water and plant nutrients at right time, in required quantity and at appropriate place. Thus, only part of soil in the vicinity of plant root is wetted and kept close to field efficiency.
This system can be adopted to a wide range of soils, topography, water quality and crops. This method can give irrigation efficiency as high as 90% besides substantial water saving to the extent of 40-80%.
Drip irrigation, however, has some limitations as indicated below:
i. High initial investment cost
ii. Poor uniformity of water application
iii. Durability of lateral and emitters
iv. Clogging of emitters and laterals
v. Sophisticated technology
To overcome some of these limitations, drip system has been improved. The improved drip systems have high adoptable values in the field.
Some of the improved drip systems are discussed below:
In this system, tubes and drip emitters are placed beneath the soil surface (30-50 cm) to irrigate crops and water moves upwards towards the relatively dry soil layers where the plants roots absorb water and dissolved nutrients.
This irrigation method has the following advantages:
a) Water saving up to 50% compared with sprinkler and 30% with surface drip irrigation
b) Reduced leakage and seepage
c) Causes less soil compaction
d) Fewer weeds
e) Minimal environmental damage, prevention of contaminated water reaching the water table and prevention of aerosol formation
f) Use of high concentration of saline water is possible
g) Improvement in yield
h) Long life span (lateral not exposed to environment)
ii. Minute Drip Irrigation:
There has been conceptual and technological development in Israel regarding minute drip irrigation. In this system, drippers pulsate at pulses of 2 sec. /70 seconds with discharge rate per hour of 100 cc as compared to 2 litre with regular drip irrigation. Thus with minute irrigation system, only one twentieth of the flow rate is required.
The advantages expected from this method are:
a. Slow humidification of root zone which will prevent leakage of water below the root zone especially in sandy soils.
b. Retains air mainly in heavy soils with low penetration ability, thus, optimal conditions for root growth are achieved.
c. Enables irrigation round the clock when plant needs available water and creates a situation of static “field capacity”.
It is assumed that the results of minute drip irrigation will lead to convince farmers to adopt this new technology that creates optimal conditions in soil for root growth and absorption of water and nutrients by roots with low energy investment leading to high crop yield and improved quality. The results will also explore the possibility for efficient soil less cultivation in greenhouses particularly in the medium where the use of common drip system causes quick damage of water, which affects the creation of a static and balance air water relationship in the medium.