Surface irrigation may be classified as flooding method, furrow method and contour farming method, with further divisions as illustrated in Fig. 4.1.
1. Flooding Method: In flooding irrigation, water is allowed to cover the surface of land in a continuous sheet, the water standing just long enough in the field for the soil to absorb the water applied to refill the root zone. A properly designed size of irrigation stream aims at proper balance against the intake rate of soil, the total depth of water to be stored in the root zone and the area to be covered so as to give reasonably uniform coverage of water over the entire field. The flooding may be (i) wild, and (ii) controlled.
(a) Wild Flooding: It is also called uncontrolled flooding. It is the primitive and most inefficient method of irrigation. In this method water is spread over the smooth or flat field without much control over the flow or prior preparation. The water is spread into the field from the ditch excavated either on the contour or up and down the slope. This method is applicable to inundation irrigation system or for pastures or forage crops where water is available in abundance at the highest elevation and is inexpensive or the crop values do not justify adoption of better method. The water distribution is quite uneven. The method is suitable for all medium to fine texture soils. It has low cost and does not interfere with tillage. The disadvantages of the method are (i) wasteful use of water, (ii) non-uniform distribution of water, (iii) excessive soil erosion on steeper slopes, (iv) require drainage arrangement to reduce ponding.
(b) Controlled Flooding: (i) Free flooding. Also called ordinary flooding. It is the commonly adopted method where irrigation water is in abundance and cheap. The land is divided into plots or kiaries of suitable size depending on porosity of soil. Water is spread over the field from watercourse. The irrigation operation begins at the higher area and proceeds towards the lower levels. The flow is stopped when the lower end of the field has received the desired depth of water. The field watercourse is properly spaced, the spacing depends on the topography, oil texture, depth of soil and size of stream. The spreading may vary from less than 15 m to more than 60 m. Porous soil requires close spacing than tight soil. The method is most suitable for soils of medium texture and with moderate slopes.
It is by far the most common type of irrigation. In surface irrigation method, since water is applied to the field in varied quantities and at different times, the flow remains unsteady. The method involves diverting a stream of water from the head of a field into furrows or borders, and allowing it to flow downward. Water infiltrates into the soil while traversing the furrow. By subsequent ponding and lateral movement, the soil is restored to its full water holding capacity to a depth that depends on the quantity of water introduced, the duration and rate of stream flow, the gradient and the soil structure and texture. Generally under open-ditch conveyance and surface irrigation methods, less than one-half of the water released reaches plants. Highly efficient irrigation can be achieved by an appropriate combination of size of the irrigation stream, uniform application of water, minimum soil erosion, minimum labour requirement, maximum land use, size and shape of field and use of machinery. The surface irrigation is essentially supplemented with efficient water disposal system.
Advantages: (i) Allows use of machinery for land preparation, cultivation and harvesting, and (ii) Helps to store the required amount of water in the capillary zone of the soil for supply to the root zone of plants.
Disadvantages: (i) Greater loss of water by surface runoff and deep percolation, larger requirement of water per unit area, (ii) Water is lost in infiltration and deep percolation, (iii) Low efficiency due to imperfect control over the water flow, (iv) Inferior quality crops with a low yield, (v) Wasteful use of water compared to better irrigation methods, and (vi) Costly and time consuming preparation of land.
In sub-surface or sub-irrigation water is applied beneath the ground by creating and maintaining an artificial water table at some depth, usually 30 to 75 cm, below the ground surface. Moisture moves upwards towards the land surface through capillary action to meet requirements of the crops in plant roots. Water is applied through underground distribution system consisting of a properly designed main field ditches, laterals, laid 15 to 30 m apart. Water may be obtained from wells, streams, lakes etc. Water is introduced into soil profile through open ditches, mole drains or tile drains. Open ditches are preferred because they are relatively inexpensive and suitable for all types of soils. Tiles and mole drains are suitable only for organic soils. Sub-surface irrigation requires little field preparation and labour. It entails minimum evaporation loss and surface waste. The irrigation water is essentially required to be of good quality to prevent excessive soil salinity. The flow rate in supply ditches is required to be low to prevent waterlogging of the field. The use of sub-irrigation is limited because it requires certain soil condition that is the soil is permeable in root zone, underlain by an impervious horizon or high water table.
The essential requirements for a successful sub-surface irrigation are (i) Availability of adequate supply of good quality water throughout growth period of the crop, (ii) Fields must be nearly level and smooth. Ground slope is moderate. Land is approximately parallel to water table, (iii) Availability of a layer of permeable soil such as sandy loam or loam immediately below the surface soil to permit free and rapid movement of water laterally and vertically, (iv) Availability of a relatively impervious layer at 2 to 3 m in the substratum to prevent deep percolation of water or a permanently high natural water table on which an artificial water table can be built, (v) A well planned distribupon system of main ditches, field laterals, etc., which raises the water table to a uniform depth below the ground surface over the entire area, (vi) Availability of adequate outlet for drainage of the area so irrigated particularly in humid areas, (vii) Subsoil water table is within 2 to 3 m below the ground surface, (viii) Topographical conditions are uniform, and (ix) Soil is capable of lifting moisture from the water table to the root zone. Also the soil permits lateral and downward movement of water. The efficiency of water use depends on soil characteristics, topography and operation and maintenance management. In good system, the efficiency is 70-75 per cent.
The essential requirements for adoption of any irrigation method are (1) Application is within desirable limits. Stream flow is adequate so that the quality of irrigation is such that the depth of wetting and hence the stand of crop are approximately uniform. The irrigator achieves high productivity so that during a day’s work he can irrigate a large hectarage, (2) Afford a uniform water distribution in root zone of a crop with as small as 6 cm applications for light irrigation, (3) Allow
heavy uniform application of 15 to 20 cm of water depth and under some conditions as much as
25cm per irrigation for salt leaching where such a problem exists, (4) Allow use of large concentrated water flows for reduction of conveyance losses, field channel network and labour cost,
(5) Suitable for use with economical conveyance structure, (6) Facilitate mechanized farming, (7) Occupy minimum land under bunds, etc, (8) Inexpensive and economically justifiable, (9) High efficiency of water application i.e., the ratio of water stored in the root zone to that delivered to the field should be maximum, (10) Minimum wastage of water either through surface runoff or through deep percolation below the root zone of a crop.
A suitable irrigation method shall ensure maximum yield at optimum water utilization, i.e., conservation of water resource. The choice of an irrigation method suitable to a specific situation is an important aspect of water management of crops.
The design, equipment and technique of replenishing the soil water deficit by applying irriigation water is referred to as irrigation method. Five basic methods of the application of irrigation water to the fields to suit different crops, soil type, topography, water availability and its quality, climatic conditions, individual holdings and costs are (i) flooding irrigation, (ii) furrow irrigation, (iii) sprinkler irrigation, (iv) drip irrigation, and (v) subirrigation, besides various subclasses.
The primary objective of any irrigation method is to supply water to soil so that moisture will be readily available at all times for crop growth but without indiscriminately adding to the water table, as well as avoiding influence of soil salinity. An efficient irrigation method is that which best suits local conditions such as (i) soil characteristics, (ii) kind of crop and its age, (iii) crop rotation, (iv) topographic condition, (v) available water flow, (vi) underground water table condition (high or low), and (vii) state of soil salinity.