In this article we will discuss about:- 1. Surface Irrigation Methods 2. Calculating Depth of Irrigation 3. Mathematical Models 4. Implements.
Surface Irrigation Methods:
These can be broadly classified as follows:
1. Wild flooding or uncontrolled flooding.
2. Controlled flooding, further classified as:
(i) Border method,
(ii) Check basin method or flat-bed method, and
(iii) Basin method.
3. Furrow irrigation, further classified as:
(i) Deep furrow method, and
(ii) Corrugation method.
Wild or uncontrolled flooding consists in applying water to the field without any bunds to guide the flow. Land is also generally not graded or levelled. This method is practised when irrigation water is abundant and also where land grading has not been undertaken.
Pasture lands are sometimes irrigated by this method. Uncontrolled flooding will involve minimum cost in terms of labour and land preparation. The efficiency of water application, however, will be low as some parts of the land irrigated will get more water and some less.
In controlled flooding the land to be irrigated is levelled or graded and further subdivided by means of channels and ridges. Water is guided to each of the plots or subdivisions to complete irrigation.
Depending upon the manner in which the land is divided, irrigation methods under controlled flooding are classified into several methods as follows:
1. Border Method:
Border method of irrigation is a method of surface flooding wherein the water is applied to the field divided into strips separated by parallel ridges (Fig. 15.1a). Each strip is levelled transversely, but has the natural slope in the longitudinal direction so that the water turned at the upper end of each strip moves down the slope in the form of a thin sheet.
Border strips may be designed either to allow runoff at the tail end or to retain the entire amount of water introduced into the strip. Border method of irrigation is suited to a wide variety of crops and soils. Close growing crops like wheat, barley, groundnut, barseem etc., are conveniently irrigated by this method. The method can be designed to suit different soil types.
Three different kinds of border irrigation are practised.
These are:
i. Graded Borders:
The graded borders have some slope in the direction of irrigation. The slopes generally range from 0.1 per cent to 0.5 per cent, but higher slopes are used under unavoidable conditions. The slope selected should not cause soil erosion problems.
Each strip is irrigated by turning in the stream of water at the upper end. The stream size, dimensions of the border, time of irrigation are so adjusted as to get uniform application of water throughout the strip.
ii. Level Borders:
These borders have no slope in the direction of irrigation and are closed at the ends. Water is ponded until it infiltrates into the soil. Level borders are same as the checks or the flat beds.
iii. Contour Borders:
While irrigating steep lands (lands greater than 2 to 3 per cent slope), if borders are made longitudinally, it is difficult to get uniform application of water. Such borders also cause severe soil erosion problems.
In such cases the land is converted into a series of borders in the transverse direction. The borders are given the required grade in the transverse direction. Such borders are referred to as contour borders. Because of land topography sometimes it may not be possible to get uniform width in case of contour borders.
2. Check Basin Method:
The check basin method of irrigation consists in dividing the area into square or rectangular plots and irrigating each plot (Fig. 15.1b). The plots are generally level or have a very mild slope. The terms flat-bed method, check borders and level borders are also used to describe this method of irrigation.
The check basin method is used to irrigate a wide variety of crops. This method can be adopted either when the soils are having high infiltration rates or low infiltration rates. In case of high infiltration rates the soils are to be quickly covered to avoid deep percolation losses and in case of soils of low infiltration rates the water is to be allowed to stand for the required time to enable the water to penetrate to the desired depth.
3. Basin Method:
The basin method of irrigation is a modification of the check basin method used for irrigation of orchards. Basins are constructed for each tree or a group of trees (Fig. 15.1c). Water is conveyed to each basin, either by flowing through one basin into another or through a channel separately constructed.
4. Furrow Irrigation:
Furrow irrigation consists in making the land into ridges and furrows and irrigating the area through the furrows (Fig. 15.1d). This method of irrigation is used for nearly all the row crops. Crops like potato, maize, sugarcane, etc., which are grown on ridges are suited for this method of irrigation.
When furrow irrigation is practised under saline and alkaline conditions, the lateral movement of soil moisture coupled with evaporation causes salt accumulation in the ridges or beds between furrows. If the salt concentrations reach harmful proportions, planting is done in the relatively salt-free bottoms of the furrows following pre-irrigation.
Other ways consist of pre-irrigation by the furrow method, removal of the top parts of the ridges where salt accumulates, and seeding the crop in the middle of the reduced ridges.
The furrow irrigation systems can be broadly classified into five systems. Their principal characteristics and adaptability are given in Table 15.1.
Corrugation Method:
Corrugations or rills are shallow furrows running down the slope from the irrigation channel. Water moves down through several corrugations at a time. Unlike in deep furrows, water in corrugations over-top them during flow. Corrugation irrigation is well adopted for medium and heavy textured soils, particularly if the soil tends to bake and crust after irrigations.
Corrugations are made using suitable implements for the purpose. The corrugator is a simple wooden frame with short lengths of circular bamboos fitted on it. It is drawn over a well prepared field forming the corrugations on the surface.
Corrugations should be spaced such that the lateral movement of water will provide an adequate irrigation between the corrugations by the time sufficient water has been added to refill the soil profile. Corrugation method is useful in irrigating small grain crops especially wheat.
Calculating Depth of Irrigation:
The information obtained from the soil moisture characteristics can be used in calculating the depth of water to be applied in irrigation. Whenever a crop is irrigated, the governing principle is to get the rootzone of the crop to field capacity.
Depending on the root zone of the crop, the depth of the soil to be irrigated is known. Let this depth be denoted by D. Considering a soil column of unit area and depth D, moisture content of the soil can be represented as depth ‘d’ obtained by multiplying the percentage volume Pv by the depth of soil D; or
If the moisture content at field capacity is Pf and the actual moisture content in the soil Pa, (both expressed as percentages by weight) then the depth of irrigation d is given by –
Since, Pf, Pa and As, are dimensionless, d will have the same dimensions as D. When the soil profile is layered and information is available for each layer, D is calculated separately for each layer and summed up.
Mathematical Models in Surface Irrigation:
Mathematical models based on computer methods are being increasingly used for the design of surface irrigation systems. These models predict accurately the advance and recession phases so that the designs based on these models are more precise.
Most of these approaches are based on the fundamental principles of conservation of mass, momentum, and energy. Detailed discussion of these approaches is given in Walker and Skogerboe (1987).
For the spatially varied and unsteady flow that occurs during surface irrigations, application of the conservation of mass principles yields the following equation (called the continuity equation).
Surface irrigation models based on Eqs. 15.39 and 15.41 are called zero-inertia models. These have been applied to the design of borders, furrows and level basins. BASCAD developed by Boonstra and Jurriens (1988) for the design of level basin systems is an example.
Implements for Surface Irrigation Systems:
Efficient application of irrigation water is made possible by suitable land grading and the use of appropriate farm irrigation structures and appliances. There are several implements which are useful in laying out surface irrigation systems.
Some of such implements are as follows:
1. A-Frame Ridger:
This implement consists of two wooden planks fixed in a tapering form with the help of cross bars. It can be both tractor drawn or animal drawn. This implement is useful for making ridges in border method or check method of irrigation. The land should be well prepared and the soil should be in workable condition for the use of this implement.
2. Bund Former:
This is also an implement used for similar purposes as the A-Frame ridger. This has two metallic plates arranged in a tapered manner to help the formation of bunds.
3. Furrow Opener:
There are several tractor drawn and animals drawn implements for formation of furrows. One such animal drawn implement consists of M.S. pipe filled with concrete to add to its weight. A disk is fixed in the front to open the furrow. For efficient use of this implement, the soil should be in good working condition.