(a) Field capacity: The field capacity of soil describes the maximum amount of water that a soil will retain after free drainage. This situation usually exists one to three days after the soil has been thoroughly wetted by irrigation or rain. The field capacity is the upper limit of available moisture range in soil moisture and plant relations. The soil moisture tension at field capacity varies from soil to soil, but it generally ranges from 1/10 to 1/3 atmospheres. At field capacity, the micro pores are filled with water and the large soil pores are filled with air.
Permanent wilting point: The permanent wilting point is the soil water content at which plants can no longer obtain enough moisture to meet transpiration requirements and remain wilted unless water is added to the soil. At the permanent wilting point, the water film is held tightly around the soil particles so much so that the roots in contact with the soil cannot remove the water at a sufficiently rapid rate to prevent wilting of the plant leaves. It is considered equivalent to the water held by the soil against an applied pressure of 15 bars.
(b) Irrigation scheduling is defined as the frequency with which water is to be applied based on needs of the crop and nature of the soil. Irrigation scheduling is a decision making process involving when to irrigate and how much of water to apply. Irrigation scheduling may also be defined as scientific management techniques of allocating irrigation water based on the individual crop water requirement under different soil and climatic condition, with an aim to achieve maximum crop production per unit of water applied over a unit area in unit time. The objectives of irrigation scheduling are as follows:
High water – use efficiency
High crop productivity
Low irrigation cost
Minimal damage to soil ecosystem
Minimal weed menace
High fertilizer-use efficiency
Critical stage approach
The approaches to irrigation scheduling include direct and indirect approach. Critical stage approach is an indirect approach to irrigation scheduling. Irrigation may be scheduled according to the physiological stages. The growth ** of an annual crop can be divided into four growth stages.
Initial stage : Sowing to 10% ground cover
Crop development stage : 10 to 70% ground cover
Mid-season stage : Flowering to grain setting stage
Late season stage : Ripening to harvesting stage
Certain physiological stages are critical at which there should not be moisture stress. These stages are called as critical stages of crop growth. Water supply should be ensured at these stages to get high yield. The critical crop growth stages of few crops are as follows:
| *.No. | Crop | Critical stages |
| 1 | Wheat | Crown root initiation and flowering |
| 2 | Paddy | Early tillering and flowering |
| 3 | Maize | Tasselling and silking |
| 4 | Potato | Sprouting and tuberization |
(c) Drip irrigation is one of the latest methods of irrigation. Drip irrigation involves application of water near the plant roots at short intervals through a network of piping system. It allows an irrigator to limit watering closely to the consumptive use of the plants. The crop yields are higher by adopting this method on account of increased water use efficiency. The water conveyance and application losses are minimized in this irrigation method. It is suitable for water scarcity and salt affected soils. Drip irrigation is highly suitable to wide spaced and high value crops like coconut, grapes, lime, banana, vegetables, mango, pomegranate, etc.
Components of drip irrigation system:
A drip irrigation system consists of a pump or overhead tank, main line, submains, laterals and emitters. The mains, sub-mains and laterals are usually made of ***** PVC (poly vinyl chloride) tubing.
The main line delivers water to the sub-mains and the sub-mains to the laterals.
The emitters which are attached to the laterals distribute water for irrigation.
Other components include pressure regulator, filters, valves, water meter, fertilizer application components, etc.,
Pump :
The pump creates the pressure necessary to force water through the components of the system. Centrifugal pump operated by engines or electric motors are commonly used. The laterals may be designed to operate under pressures as low as 0.15 to 0.2 kg/cm2 and as large as 1 to 1.75 kg/cm2 .
Chemical tank
A tank may be provided at the head of the drip irrigation systems for applying fertilizers, herbicides and other chemicals in solution directly to the field along with irrigation water.
Filter
It is an essential part of drip irrigation system. It prevents the blockage of pipes and drippers/emitters. The filter system consists of valves and a pressure gauge for regulation and control.
Emitters
Drippers are provided at regular intervals on the laterals. The drippers allow water to trickle out at very low rates. The amount of water dripping out of each emitter in a unit time will depend on the pressure and size of the opening. The water coming out of the emitters is almost at atmospheric pressure.
