Chapter 2 Drip irrigation planning factors.pptx

Understand the effect of different factors such as the wetting pattern of drippers and lateral configuration on the design of drip irrigation system

1. Pressurized (Drip) Irrigation I EiT -M, School of Civil Engineerin g, IEC October, 202 4

2. 2 . Drip irrigation planning factors Contents Introduction CWR Wetting pattern of drippers Lateral configuration Dripper spacing

3. Objectives At the end of the chapter, students will be able to: Understand the effect of different factors such as the wetting pattern of drippers and lateral configuration on the design of drip irrigation system

4. Introduction Before starting to design drip irrigation system, many factors such as the pump characteristics, soil characteristics and plant characteristics have to be studied. A discussion on the study of pump characteristics which is needed for sprinkler and drip irrigation is available in Chapter 5 . Peak crop water requirement data is useful in deciding the maximum irrigable area from any water source. In the process of selection of a suitable plant row-lateral-dripper configuration for adoption, various alternatives are usually evaluated. In order to evaluate the alternatives, the root spread of crops both in depth direction and sidewards direction needs to be studied. Then suitable plant row-lateral-dripper configuration can be decided by using standard procedures with the aid of analytical equations.

5. Crop Water Requirement Crop water requirement is usually calculated by estimating evapotranspiration based on the meteorological data available for any specific area, and it is usually expressed in terms of depth of water required per day. Peak water requirement data is needed at the design stage because if the drip irrigation system is designed for peak water requirement, then the drip irrigation system would safely function during the other periods.

6. Maximum Area of the Land Irrigated After evaluating the suitability of the pump available in the farmer’s land, the next step is to ascertain the maximum extent of the land that can be irrigated. This depends on the discharge rate available from the pump during lean water availability period, peak crop water requirement and maximum duration of pumping possible. A relationship between the water availability and water demand on daily basis is useful to arrive at maximum irrigable area. The maximum area that can be irrigated can be found out by balancing the water supply and demand of water which is as follows: Where, = pump discharge rate (l/s), T = number of possible hours of irrigation per day; = uniformity coefficient (usually 90% for drip irrigation); = the peak reference crop water requirement in mm; = crop coefficient.  

7. Maximum Area of One Subunit One subunit means the area corresponding to one submain and its laterals with a control valve to control discharge into the submain . If the irrigation interval is n days, the total area is divided into (n - 1) parts, and for each part, n days water requirement can be given. The maximum area of one subunit can also be obtained by dividing the total area into n parts. The division of the total area into (n - 1) parts is to have a factor of safety.

8. Wetting pattern of drippers Fig. 18: Wetting profiles for different soil types When water is applied by a dripper on land surface, the typical wetted sectional elevation profiles for each soil type is shown in Fig. 18. For clayey soils, the diameter of wetting is higher and depth of wetting is smaller. For loamy soils, depth and diameter of wetting are approximately equal. For sandy soils, the diameter of wetting is smaller than the depth of wetting.

9. Wetting pattern of drippers It is always better to pay attention to the relationship between wetting pattern and dripper discharge rates. Digging a small trench and operating a dripper and observing the wetting pattern on the walls of the trench is very useful. A very simple assumption for predicting the wetted dimension for a point source is to assume the wetted zone to be a hemisphere (i.e. diameter of wetting (d) = 2 × depth of wetting). For this assumption following equation can be written as Where q is the application rate (m 3 /s), t is the duration of application ( sec ), and is the average change in volumetric water content in the wetted zone. In the absence of field data, it is recommended to substitute with a value of 50% of saturated water content if the initial soil moisture content is wetter which is usually the situation in drip irrigation  

10. Wetting pattern of drippers The preceding equation can be rearranged as Usually, wetted bulb due to a point source is approximated as a semi-ellipsoid (Fig. 19). Volume of semi-ellipsoid is given by the following formula: , where z is maximum depth of wetting and d is the diameter of wetting on the surface. The relationship between d and z depends on factors like discharge rate of dripper , volumetric soil water content before the irrigation , saturated hydraulic conductivity of soil and bulk density of soil.   Fig. 19: Semi-ellipsoid assumption of wetted bulb

11. Wetting pattern of drippers Empirical Equation for Wetting Pattern One popular equation used to get the dimensions of d and z is Amin and Ekhmaj (2006) equation which is as follows: * ** Where is the average change in volumetric water content in the wetted zone; is the volumetric water content in the wetted zone (m 3 ); q is the application rate (m 3 /s); and K s is saturated hydraulic conductivity (m/s). Equations * and ** can be combined to get following Eq.: (a)  

12. Wetting pattern of drippers Semi-Empirical Equation for Wetting Pattern Schwartzman and Zur (1985) proposed the following empirical equations correlating depth and width of the wetted soil volume to emitter discharge , saturated hydraulic conductivity of soil and volume of water in the soil volume. *** *@ Equations *** and *@ can be combined and approximated to get following Eq.: (b) Where is the volumetric water content in the wetted zone (m 3 ), q is the application rate (m 3 /s), and is saturated hydraulic conductivity (m/s).  

13. Wetting pattern of drippers Semi-Empirical Equation for Wetting Pattern Either Eq. (a) or Eq. (b) can be used for finding wetted diameter for any desired minimum depth of wetting. With 20% factor of safety, 80% of maximum diameter of wetting obtained using these equations may be taken as the dripper spacing to get a continuous wetted strip. Shallow rooted field crops may be wetted to a minimum depth of 0.3 m, medium rooted field crops may be wetted to a depth of 0.6 m, and tree crops may be wetted to a depth of 0.9 to 1.5 m.

14. Lateral Configurations Fig. 20: Lateral configurations for closely spaced crops Suitable lateral configuration is decided taking in to consideration factors such as crop spacing, crop water requirement, irrigation interval, rooting pattern, soil type and economics. Different lateral configurations that are commonly used for closely grown field crops are shown in Figure 20. One lateral per crop row is very common.

15. Lateral Configurations Sometimes, the wetted volume obtained by providing one lateral per crop row is not sufficient. In such cases two laterals per crop row are provided. If installation cost reduction is important, one lateral for two or more crop rows is also used. In Fig. 20, for the case of one pipe line every other crop row has uniform crop row spacing. But in case of one lateral line per two crop rows, the spacing between crop rows adjacent to lateral will be smaller than the spacing between the crop rows which do not have lateral in between. They are called as paired or dual crop rows (Fig. 21). Fig. 21: Lateral spacing and dripper spacing for paired or dual crop row

16. Lateral Configurations The spacing between emitters in Fig. 21 is denoted as x, whereas the spacing between crop rows is denoted as y . Usually, ‘ y ’ is preferably less than ‘ x ’. This is to ensure that the wetted front reaches both rows of plants sufficiently. For widely spaced tree crops, two laterals per row of plants are sometimes used to provide more emission points per tree. Sometimes drippers are looped around a tree or one micro sprinkler may also be provided. Layout types 1 and 2 in Fig. 22 are useful to roll the laterals easily, when intercultural operations are done in field. Fig. 22: Lateral configurations for widely spaced tree crops

17. Lateral Configurations Dripper spacing and discharge rate are the important design factors because the wetted zones and root zones must interact satisfactorily so that the applied water is optimally used by the plant with minimum wastage. Selection of a satisfactory configuration of drippers and number of laterals per crop row depends on crop and soil characteristics and climate condition. In Fig. 23, the wetted bulb and the root zone are shown for the case when one lateral is provided for two crop rows. It is inevitable to irrigate some unwanted soil volume. So obviously a better design and operation are to minimize the wetting of unwanted soil volume where the root zone does not exist or grow. Fig. 23: Wetted bulb and root zone for one lateral for two crop rows

18. Lateral Configurations Example Sugarcane crop is irrigated with drip irrigation. The crop rows are at a spacing of 1.5 m, and for each row, one lateral is laid. Discharge rate of each dripper is 2 l/h. The soil is clay loam. The clay loam soil has a saturated hydraulic conductivity of 0.000064 cm/s. The effective root depth on 30 th day and 60 th day is 20 cm and 30 cm, respectively. The effective root depth after 120 th day is 40 cm constant throughout. As per the irrigation program, during the period till 90 th day, irrigation is done when the soil moisture suction reaches -30 centibars or kPa. The soil moisture level at the -30 centibars has been found to be 20% (gravimetric). After 90 days, irrigation is done at the soil moisture suction of -50 centibars. The soil moisture level at the -50 centibars is 18% (gravimetric). Available soil moisture at the field capacity is 0.18 m per meter depth of soil. The permanent wilting point is 12.5% (gravimetric). The bulk density of the soil is 1.6 g/cm 3 . The minimum depth of wetting during irrigation is 20 cm. Three dripper spacings in the lateral have been proposed. They are 60, 75 and 90 cm. Assuming the cross section of wetted profile is rectangular, Recommend the dripper spacing which would suit well to this situation. Find out the duration of irrigation during the periods near 30 th and 45 th day and also during the period near 120 th day for the selected dripper spacing.

19. Dripper spacing Emission Point Layouts for Widely Spaced Crops Rather than deciding the lateral–dripper configuration based on volume of soil needed for accommodating peak crop water use, the lateral–dripper configuration selection is also done by arbitrarily fixing up the fraction area to be wetted by drippers. More the area is wetted , more the possibility of increase in yield due to improved drought resilience and also availability of more soil for nutrient extraction . But the cost of installation would increase if the fraction of area wetted is increased. Keller and Bliesner recommend a method (very much suited for widely spaced tree crops ) for choosing the lateral–dripper configuration . Table 5 provides values of estimated area wetted by a 4 l/h emitter operated for different wetted soil depth in different types of soils. They are based on the daily or alternate day irrigations that apply volumes of water sufficient to slightly exceed the crop water use rate.

20. Dripper spacing Emission Point Layouts for Widely Spaced Crops Table 5: Estimate of area wetted by a 4 l/h under different field conditions

21. Dripper spacing Emission Point Layouts for Widely Spaced Crops The second value is equal to the diameter of the wetted bulb The table can be used for selecting the emitter spacing to get a continuous wetted strip. For instance, if the wetted soil depth needed is 0.75 m and the soil type is coarse and stratified, then the emitter spacing may be chosen as 0.6 m. Stratified refers to relatively uniform texture but having some particle orientation or some compaction layering that gives higher horizontal than vertical permeability. Layered refers to changes in texture with depth as well as particle orientation and moderate compaction. Though the table values correspond to 4 l/h discharge rate, the same value can be used approximately for any other discharge rate, because the time of operation can be adjusted to apply the total volume required. 0.75 m depth wetting can be used for smaller trees and 1.5 m depth wetting for larger trees.

22. Dripper spacing Emission Point Layouts for Widely Spaced Crops Fig. 24: Emission layout with single lateral for widely spaced crops Figure 24 shows an emission point layouts with a single lateral for widely spaced crops. Definitions of the terms related to Fig. 24 are as follows:

23. Dripper spacing Computing Percentage Wetted Area The percentage of area or soil volume of potential root zone to be wetted is important in designing drip system. It can be determined using the following equations: For straight single lateral systems with Se ≤ S'e, the percentage wetted can be computed as follows: For double lateral systems and multi-exit layouts, if Se ≤ S’e Micro sprinkler wets a larger surface area of soil than drip emitters (Figure 25 ). They are often used on coarse textured soils where wetting a sufficiently large area would require a large number of drip emitters.  

24. Dripper spacing Computing Percentage Wetted Area Figure 25 shows wetting pattern of a micro sprinkler. Approximately the area wetted beyond the spray radius can be assumed as 50% of the S'e value from Table 5 for the homogeneous soils. Therefore the wetted surface area for spray emitter is as follows: Where and are the area and perimeter of spray respectively.   Fig. 25: Wetting pattern for micro sprinklers

25. Dripper spacing Table 6 : Wetted area (%) for different soils with respect to lateral spacing and discharge rates for applying 40 mm Effective spacing between laterals S l , m Emission point discharge Less than 1.5 LPH 2 LPH 4 LPH 8 LPH More than 12 LPH Recommended spacing of emission points along the lateral for coarse, medium and fine texture soils- Se, m C M F C M F C M F C M F C M F 0.2 0.5 0.9 0.3 0.7 1.0 0.6 1.0 1.3 1.0 1.3 1.7 1.3 1.6 2.0 Percentage of soil wetted 0.8 1.0 1.2 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 88 100 33 70 100 25 58 92 20 47 73 15 35 55 12 28 44 10 25 37 9 20 31 8 18 28 7 16 24 6 14 22 5 12 18 100 100 40 80 100 33 67 100 26 53 80 26 40 60 16 32 48 13 26 40 23 34 10 20 30 9 18 26 8 16 24 7 14 20 100 100 100 100 100 100 67 100 100 53 80 100 40 60 80 32 48 64 26 40 53 23 34 46 20 30 40 18 26 36 16 24 32 14 20 27 100 100 100 100 100 100 100 100 100 80 100 100 60 80 100 48 64 80 40 53 67 34 46 57 30 40 50 26 36 44 24 32 40 20 27 34 100 100 100 100 100 100 100 100 100 100 100 100 80 100 100 64 80 100 53 67 80 46 57 68 40 50 60 36 44 53 27 34 40 27 34 40

26. Dripper spacing Example An orchard with a tree spacing of 3.0 m × 5.0 m planted on a deep, medium textured homogeneous soil. Three emitter configurations are being considered. A single row of 4 l/h emitters A looped layout with 4 numbers of 4 l/h emitters/ tree A micro sprinkler wets a surface area with radius of 1.0 m . Find the percentage area wetted by each emitter configuration and recommend a configuration which provides the area of wetting more than 33%.