Design of Surface Irrigation Systems:
In every irrigation system a particular size of steam is available to irrigate the land with a particular topography and soil characteristics. These should be matched in such a way that the required amount of water is applied all over the field as uniformly as possible and at the same time no soil losses occur during irrigation.
Data Required for Design:
The data required for design of surface irrigation systems may be broadly grouped as follows:
1. Soil characteristics – Soil properties required are the infiltration rates as a function of time and expected variability between irrigations,, field capacity, wilting point and bulk density. Other properties like salt content, effects of surface flooding such as crusting and cracking are useful.
2. Crop data – Types of crops proposed to be raised in the area, their agronomical requirements like ridging etc., rooting habits, allowable soil water deficits at various stages of growth and relative sensitivity to inundation are the information needed.
3. Information about water – Stream size available at the field to be irrigated, quality of the irrigation water, expected amount and distribution of rainfall are the needed data.
4. Information about topography – Slope of the area to be irrigated, size and shape of the field are the factors that influence the design of the surface irrigation system.
Design Principles:
The surface irrigation system should be able to apply an equal depth of water all over the field without causing any erosion. After the water is allowed to enter the plot it will advance towards the end of the filed.
Recession of the water starts from the beginning of the plot after the water is shutoff. Most of the time it becomes necessary to stop the inflow of water when the water front reaches a particular length of the plot, so that the irrigation of the remaining length is completed with the water already introduced.
The reduction of the stream size partially or fully is known as cut-back. The difference between the advance and recession periods at any point is known as the opportunity time. To minimise the percolation losses, the opportunity time should be uniform throughout the plot and also equal to the time required to put the required depth of water into the soil.
Design of Border Irrigation:
The design of the border irrigation consists in deciding:
(1) Width of the border,
(2) Length of the border.
(3) Stream size,
(4) Slope, and
(5) Duration of irrigation.
All these factors are interrelated. The width of the border should be such that there should be no cross slope within the width and the stream size available should flow with non-erosive velocity.
The length should be such that a uniform intake opportunity is obtained all along the border. When the field sizes are small, the field size may govern the length. The stream size allowed into each border should be non-erosive and at the same time should give the desired advance rate.
The slope of the border conforms to the land slope and as such cannot be changed’ like other parameters. The land slope is given at the time when land grading operations are carried out. The general recommended land slopes for different soil types and the maximum length of borders are given in Table 15.2.
The duration of irrigation depends upon the depth of water to be applied. As the depth of water to be applied could change with the different irrigations the design of the border should consider all the possible depths to be applied.
Design of Check Basin Method:
The design of check basin system of irrigation consists in determining the size of the plot suitable for a particular stream size and also finding out the duration of irrigation in order to replenish the rootzone of the crop. As water is introduced into the check, it starts advancing and ultimately reaches the tailend. The bund at the lower end prevents any runoff.
The amount of water theoretically required to irrigate a particular size of the check can be obtained using the formula –
The quantity of water calculated by the above formula will irrigate the entire check if the opportunity time is same all over the check. In a check as some time is required for the water to spread, the opportunity time, and consequently the depth absorbed will not be same over the entire area. It can be said that at the tailend of the check basin, the opportunity time available is less to an extent of the time required for the water to spread (or advance) all over the check.
As such, if the area is to be fully irrigated, an additional opportunity time equal to the time of spread should be allowed at the end. It is assumed that after the water has spread all over the check basin initially, the rest of the water introduced into the check is uniformly available over the entire area i.e., the advance and recession curves are as shown in Fig. 15.9.
Percolation Losses and Application Efficiency:
Consider the case when the advance of water front in the check basin is described by a power function of form
Using Kostiakov’s infiltration function, the percolation losses and hence the application efficiency can be calculated as per the approach suggested below.
The total volume of water V absored during the advance and recession over a length L, for a unit width of the check basin is-
To achieve complete irrigation, water should be allowed to flow till the end of the basin and allowed to stand to infiltrate the desired depth, certain amount of percolation losses in the upper reaches are unavoidable. If application efficiency Ea is considered as the ratio of the amount of water stored in the rootzone and the total water (including the percolation losses) this is given by –
It can be seen that the use of equation q . t = a . d will not adequately irrigate the check basin as the essential percolation is not included.
An approximate but convenient expression can be obtained from Eq. 15.31. Neglecting the term Tn tL θ which is relatively small (as the exponent n is negative), Eq. 15.31 reduces to
Gupta et al. (1983) approached the problem of calculating percolation losses in check basins, using the concept of average ponding time.
The expressions derived for percolation losses can be used in design of the check basin system. For a given stream size and plot size, calculate the percentage percolation losses. If these losses are high, either the size of the plot is to be reduced or the size of the stream to be increased.
Smaller size plots will involve more ridges and channels. Larger stream sizes will cause the soil to erode. Thus a compromise between percolation losses, area going under ridges and channels and soil erosion has to be accepted in the design of the check system.
Knowing the percolation losses, size of the plot and depth of water to be applied, the total amount of water to be applied can be calculated. Then, knowing the stream size, the total time of irrigation can be calculated.
Example 1:
This example is given to illustrate the procedure.
Data : Size of the check basin = 10 m x 40 m.
Cumulative infiltration function given by y = 9.95 t0.43, (y in mm and t in min).
Depth of irrigation (calculated using soil moisture deficit) = 75 mm.
Advance function defined by:
I = 5.6t10.63 (for a stream size of 1.71 l/s/m) or, alternatively, time to cover the entire area of the check = 22.66 min.
Calculations:
Application efficiency as calculated by Eq. 15.32 = 95 per cent. The values obtained by Eq. 15.31 are slightly higher than the values obtained by Eq. 15.32, and therefore can be used safely for general field practice.
Design of Furrow Irrigation:
The design of the furrow irrigation system consists in deciding:
(i) Spacing of furrows,
(ii) Shape and size of the furrows,
(iii) Stream size,
(iv) Furrow slope, and
(v) furrow length.
The spacing of the furrow depends upon the row spacing required for the particular crop to be grown. Sugarcane, potato and maize are some of the common crops irrigated by furrows. It is also desirable that the spacing is such that the lateral movement of the soil moisture wets the ridges by the time irrigation is complete. As such sandy soils which tend to have vertical wetting patterns should have closer furrow spacing than clayey soils.
Furrow shapes depend upon soil stability, slope of the land and type of the implements used for making the furrows. For steeper slopes (slopes beyond 0.5 per cent) broad based furrows are recommended as steeper slopes cause larger flow velocities and less depth of flow.
Conversely, for shallower slopes steep sided furrows are useful as the furrow capacities will be less. The shape of the furrow and position of planting has special significance in irrigating with saline water. The height of the ridge and the land slope determine the water carrying capacity of the furrow. Too high ridges are advantageous in areas where excess rain water is to be disposed but require large stream sizes to irrigate the top parts of the ridges.
The stream size of the furrow selected should be non-erosive and should be within the capacity of the furrow. The stream size should be adjusted such that the entire furrow length should be irrigated as uniformly as possible.
For this purpose, after sometime the size of the stream is cut back. Knowing the intake rate at various times, the optimum cut-back stream for a particular furrow length can be worked.
In order to minimize the land grading operations, longitudinal and cross-slopes used should be adapted to natural topography. The slope of the furrow should be such that it should not cause erosion problems and at the same time help in efficient irrigation.
Some general values of furrow slopes are given in Table 15.4. When the land slope exceeds the safe limits for furrows, the furrows can be laid across the slope on the contour with desired slopes. Such furrows are known as contour furrows.
The furrow length is influenced by the slope, rate of advance and the depth of application. The stream size, slope and furrow length should be so adjusted that the deep percolation losses are a minimum.
Table 15.5 can be used as a rough guide for selecting the furrow length.
The time of advance of the water should be such that minimum percolation losses are caused at the head of the furrow. Criddle et al. (1956) suggests a value of T/4 (for the stream to reach the end of the furrow) where T is the time required to infiltrate the required depth of water.
In such a system it is assumed that the water flows out at the end of the furrow and the water is used subsequently. In South East Asian Countries almost all the furrow irrigation systems are designed not to allow runoff at the tailend.
Design of Sprinkler Systems:
The design of a sprinkler system for a given situation needs consideration of the soil, crop, climate and topography besides the equipment availability.
The different steps involved are as follows:
1. Inventory of Resources:
This consists in obtaining information about the available land, water and equipment. A topographical map of the area to be irrigated should be prepared. The soil type in the area under consideration should be known.
Information about the source of water, quality and its availability during the entire year should be collected. The water available should be of sufficient quantity and its quality should be satisfactory for irrigation.
Amount of sediment present in the water is an important consideration in sprinkler system design. The type of sprinkler equipment available and its specifications are necessary in the proper selection of the equipment. The power source to be utilized at the site needs to be decided depending on the availability of electric power.
2. Quantity of Water to be Applied:
The quantity of water to be applied and the period of irrigation depend upon the crops, climate and soil. The methods of scheduling of irrigations are also applicable in case of sprinkler systems. The proposed crops, their water requirements along with the water holding capacities of the soils of the areas should be known.
3. Capacity of the System:
The capacity of sprinkler system or the capacity of the pump to be used depends upon the area to be irrigated, the depth of water applied at each irrigation, the time allowed to apply this water and the application efficiency. The capacity is given by the formula –
4. Application Rate:
The rate of application of the sprinkler system is limited by the infiltration capacity of the soil. Soil types, crop cover and slope need to be taken into consideration in deciding the application rates. Application rates in excess of the infiltration capacity of the soils will cause surface runoff which will result in water loss, poor distribution of water and soil erosion.
5. Selection of Sprinklers:
After the rate of application is known, the sprinkler spacing along the laterals and the spacing of the laterals is decided using the formula –
The distribution pattern of sprinklers is affected by the spacing of the sprinklers, nozzle pressure, speed of rotation and wind velocity. As a single sprinkler does not provide uniform coverage sprinklers are used with overlapping patterns. The uniformity of distribution obtained with a sprinkler can be experimentally determined.
In finally deciding the discharge from an individual sprinkler, the pattern efficiency should be taken into consideration.
The discharge from an individual sprinkler is given by –
The coefficient of discharge for the sprinkler nozzles varies from 0.80 to 0.95. Normally, larger the nozzle, lower is the coefficient. This equation is useful in calculating the area of the nozzle of the sprinkler.
The type of the sprinkler to be selected is related with the operating pressure of the sprinkler, nozzle discharge and sprinkler spacing. Sprinklers are designed to operate under different pressures.
Table 15.10 shows the different operating pressures of sprinklers and their adaptability. Higher the operating pressures, finer will be the spray and larger coverages, but the operating costs will be higher. For field crops the intermediate pressure sprinkler is generally used.
6. Design of Laterals:
The sprinkler lateral is connected to the main and has the risers and sprinklers located on it. As the flow goes along the lateral, its volume decreases because of the discharge through the sprinklers. However, it is inconvenient to design the, lateral for a tapering section.
A uniform diameter of the lateral is adopted. The rate of flow entering the lateral is calculated and a trial diameter of the pipe is selected. Assuming that the flow is through the entire length without sprinklers, the frictional loss in the lateral is calculated using Scobey’s formula –
Standard tables based on Scobey’s formula are available and can be used for the purpose. The frictional loss is multiplied using a correction factor (F) as given in Table 15.11 corresponding to the number of sprinklers on the lateral. To the frictional loss thus calculated, the elevation is added if the lateral goes uphill or the drop is subtracted if the lateral goes downhill. It is recommended that the total pressure variation in the lateral should not be more than 20 per cent of the average pressure.
The F factor accounting for the decreasing flow rate along the sprinkler lateral is called the Christiansen friction factor. When the first sprinkler is located at the same distance as the sprinkler spacing, F can be calculated using the equation-
He will be positive or negative depending upon the lateral is uphill or downslope respectively. Hn is to be determined for the most critical lateral i.e., a lateral which gives maximum value of Hn.
Due to frictional loss, the operating pressure along the lateral decreases which influences the discharge of the sprinklers along the line. In order to narrow the range of operating pressure between the first and the last sprinklers, the lateral diameter is chosen so that the friction losses plus any elevation change between the lateral ends is not greater than 20% of the sprinkler operating pressure.
This will ensure that all sprinklers in the lateral operate within the permissible range of the designed discharge.
7. Design of the Main Line:
The main line diameter is selected taking into consideration the rate of flow and the frictional losses. The larger the pipe size the lesser will be frictional losses but the cost of a larger size diameter pipe is more than a pipe of smaller diameter.
The diameter of the main line is selected taking this aspect in consideration and with the objective of keeping the annual water-application cost as low as possible.
8. Selection of Pump and Power Unit:
For selecting the pump and the power unit for the sprinkler system, the total head against which the units are to operate is to be determined. The total head H, is given by –
Knowing the amount of water to be delivered and the total head the horse power of the prime mover is determined.
9. General Consideration for Layout:
The layout of the sprinkler system depends largely on the local conditions. Some general guidelines can only be given. The main lines should be laid along the slope and the laterals across the slope or nearly on the contour. In portable systems the laterals should be of the same size so that changing of the laterals is easy and convenient.
If tubewells are used as water supply points they may be located within the area to be irrigated. The layout should facilitate sequential movement of the laterals and lateral movement should be a minimum. It should also provide for application of different rates of water wherever necessary.
Design of Drip Irrigation Systems:
The design of a drip irrigation system consists of the system layout and the determination of the number of laterals to be operated during an irrigation. The emitter specifications are decided and laterals, submains and mainlines are designed. Equipment for filtration and chemical injection is selected as required. Pumping plant specifications are finally worked out.
The friction loss for mains and submains can be computed from the Hazen-Williams or Darcy- Weisbach equation.
Darcy-Weisbach equation for smooth pipes in micro-irrigation systems when combined with the Blasius equation for the friction factor is given by –
Equation (15.64) applies for continuous sections of plastic pipe. For in-line emitters, on-line emitters, and other connectors the head loss should be increased. Such losses may be expressed as equivalent length of lateral pipe.
This increase in length Le can be estimated as follows:
(1) Le = 1.0 to 3.0 m for each in-line emitter.
(2) Le = 0.1 to 0.6 m for each on-line emitter.
(3) Le = 0.3 to 1.0 m for a solvent-welded Tee connector.
Selection of Emitters:
The type of emitter to be selected depends on factors like availability, precision required, type of crop to be irrigated etc. Different arrangements like double lateral, zig-zag, pigtail, multi-exit, and other configurations are used to enlarge the irrigated area in soils with poor lateral transmission properties and where crops with widely areal root distributions are to be irrigated.
Chemical Injection Equipment:
Fertilizers and chemicals for controlling clogging of emitters are sometimes directly injected into the drip systems as such application is very effective. Chemical injection requires that the pressure acting on the chemical is greater than the operating pressure in the system. Chemicals are to be injected upstream of the filters so that any precipitates are removed before they could enter the main system.
Example 2:
Sprinkler irrigation system design
