G.R. NMMI

Necessity of irrigation in India

i) Uncertainity of monsoon rainfall:

80% of rainfall in India is received during monsoon period. Monsoon rainfall is very uncertain. So irrigation is very important to supply water to plants also and when needed.

ii) Uneven distribution of rainfall:

To compensate the uneven distribution in an area, supplemental irrigation is needed.

iii) Effect of winter rainfall (N India)/ Effect of SWM in S. India:

Supplemental irrigation is inevitable in the regions due to poor rainfall.

iv) Cultivation of high yielding crops:

High yielding crops produce heavy biomass and economic yield. Higher biomass need more water for its production. Hence supplementation of water as irrigation is essential.

v) Difference in water holding capacity of the soil:

Sandy soil - low WHC – frequent irrigation. Clay soil - high WHC - frequency is less.

 

Drip irrigation is the most efficient method of irrigating. While sprinkler systems are around 75-85% efficient, drip systems typically are 90% or higher. What that means is much less wasted water! For this reason drip is the preferred method of irrigation in the desert regions of the United States. But drip irrigation has other benefits which make it useful almost anywhere. It is easy to install, easy to design, can be very inexpensive, and can reduce disease problems associated with high levels of moisture on some plants. If you want to grow a rain forest however, drip irrigation will work but might not be the best choice!

Drip irrigation works by applying water slowly, directly to the soil, bloop, bleep, bloop, bleep. The high efficiency of drip irrigation results from two primary factors. The first is that the water soaks into the soil before it can evaporate or run off. The second is that the water is only applied where it is needed, (at the plant’s roots) rather than sprayed everywhere. While drip systems are simple and pretty forgiving of errors in design and installation, there are some guidelines that if followed, will make for a much better drip system.

 

Parts of a Drip system:

If you don’t know a lateral from a pressure regulator start by learning about the basic parts of a typical drip irrigation system. I strongly suggest that even if you are familiar with drip irrigation you start be reading through The Basic Parts of a Drip System page now. It contains a lot of tips and recommendations.

A simple drip system.

 

Types of Drip Irrigation Emitters

Emitters are classified into groups based on how their design type and the method they use to regulate pressure. You can create a very simple emitter by drilling a very small hole in a pipe. However, a hole alone does not work well. Unless the hole is extremely small, the water tends to forcefully shoot out of it like a tiny fire nozzle and way too much water will come out. More importantly, there is little uniformity of flow when using a simple hole. If you have a long pipe with holes drilled in it the holes on the end nearest the water source will have a large water flow from them, while those at the far end will have a very small flow.

Since using a simple hole in a pipe does not work very well, the early pioneers of drip irrigation started playing around with mechanical devices that would better regulate the flow. These devices have been given the name “emitters” (or sometimes “drippers” is used.) The emitters are installed on the pipe and act as small throttles, assuring that a uniform rate of flow is emitted. Some are built into the pipe or tubing, others attach to it using a barb or threads. The emitter reduces and regulates the amount of water discharged.

Long-Path Emitters

There are many different methods used by emitters to create and maintain this uniform, low, flow rate. Some emitters route the water through a very long, narrow passage or tube. The small diameter and great length of this path reduces the water pressure and creates a more uniform flow. These are called long-path emitters. A typical long-path emitter has a long water path that circles around and around a barrel shaped core. Long path emitters tend to be fairly large in size due to the need to fit that long tube in!

Soaker hose, porous pipe, drip tape, laser tubing

Soaker hose, porous pipe, drip tape, and laser tubing are various adaptations of the “extremely small hole in a pipe” type of drip system. They just have very small holes drilled (usually using a laser) into a tube, or are made from materials that create porous tubing walls that the water can slowly leak out of. The advantage of these is obviously very low cost. The disadvantage is that the tiny holes are very easily clogged, especially with hard water containing lots of minerals, and for some products watering uniformity can be uneven. These types of systems are most often used in landscapes for portable irrigation (moving the tubes around the yard between irrigations tends to break the mineral deposits loose so they don’t build up. These products are also widely used in agriculture, where the tubes are removed and thrown away or recycled at the end of each growing season. My experience with permanent installations of these products has been that they have a fairly limited lifespan when compared to other drip irrigation types. They work best with water that has very low mineral levels.

Short-Path Emitters

Short-path emitters are similar to the long path emitters. They just have a shorter and smaller water path. Advantages: they are very cheap and will work on very low-pressure systems where other types will not work at all. They are the best emitters for very low pressure systems, such as gravity flow drip systems fed by water from rain barrels. Disadvantages: They clog up easily, especially if the water is hard with lots of minerals in it. They have poor water distribution uniformity compared to other types of emitter. They work good on small systems, where cost is a critical issue and uniformity of water distribution is not critical. By far the most common of these short-path emitters is a very inexpensive generic emitter called a “flag emitter” or a “take-apart emitter”. This emitter is made under numerous brands and names. It is easily recognized by the little flag shaped handle on it, you can disassemble it by twisting and pulling on the flag. The photo below shows two flag emitters, the one on the right is disassembled. You can see spirals that form the short, narrow water path on the male part of the disassembled emitter.

Water, Soil, and Plants

You’re out in the desert (on a horse with no name?) and you’re really hot and thirsty. So you open your canteen and pour all the water over your head. It feels really great, but you’re still thirsty. Why? Because you don’t drink water through your hair follicles! As most people are aware, most plants “drink” through their roots. But not all of the roots are for drinking. The roots that most plants use for drinking (and eating too) are found in the top 15 cm (6 inches) of the soil. (I often see water conservation articles that say the roots in the top 45 cm uptake water, but in my practical experience most common garden plants seem to have great difficulty utilizing water deeper than 15 cm. Desert plants and chaparral plants are an exception, they do tap into water far below the soil surface.) The deeper roots are primarily for holding the plant in place. Watering these lower roots is a waste of water, just as pouring water over a thirsty man’s head is a waste of water!

Now lets take that same thirsty man and give him a large cup of water. But we’ll force him to drink it through one of those tiny plastic straws used to stir coffee. That doesn’t help much either, does it? The point is that, like the man, the plant can only drink water if it is applied in the proper place, in the proper amount. The plant can only take up a limited amount of water through a single root, so we have to get the water to as many “feeder roots” (the roots the plant uses to obtain water and nutrients) as possible or the plant won’t be able to get enough water. How do we do this? Stupid question?

As a general rule, the feeder roots of most common garden plants are primarily located in the top 15 cm (6 inches) of soil throughout the area called the drip zone. The “drip zone” is the area of soil located directly under the leaves of the plant. If you draw a circle around the plant on the ground at the outer edge of the plant’s leaves, the area within that circle is the drip zone. (The line at the edge of the leaves is called the “drip line“.) So we need to concentrate on watering that area under the leaves in order to make the most efficient use of our water. That also makes for a healthy plant. Again, desert plants and those adapted to very dry climates have wider ranging feeder roots that allow them to adapt to a limited water supply. These plants typically only need supplemental irrigation water (often only for the first few years to get them established), so it is still OK if we only concern ourselves with irrigating the drip zone for them as well.

When we drip water onto the ground at the optimum slow rate the water will almost immediately soak into the soil. Once in the soil the water moves both downward and sideways through the soil. The water moves between the grains of soil by a combination of water pressure, gravity, and capillary action. How fast and far the water moves horizontally (sideways) from the point it is applied depends on the texture of the soil. In fine textured soils, such as clay, the water will move the farthest, but it also moves at the slowest speed. In a very “heavy” clay soil it might take days for the moisture to move the length of your arm. In “light” coarse-textured soils like sand or silt it will not move nearly as far, but it will move much faster! It might move the length of your arm in a few minutes in a silty soil.

Emitter Quantity and Spacing

The number of drip emitters needed and the distance between them is determined by the size of the drip zone and the type of soil.

Size of drip zone: If the plant has a large drip zone, like a tree, you will need more emitters than you would for a small shrub. Obviously the size of the drip zone will be smaller when the plant is young and will increase in size as the plant grows. So you need to plan for enough emitters to water the drip zone of the plant when it is mature. You can start out with just one or two emitters when the plant is a seedling, and add more emitters as the plant grows. Just be sure to plan enough water capacity in the system to supply those future emitters. So how do you figure out what size the drip zone of the mature plant will be? Just type the name of almost any plant into an Internet search engine and you will find a number of websites that will tell you what the expected diameter of that species will be when mature.

Soil type: In sandy soil your emitters will need to be closer together because the water does not move as far horizontally in a sandy soil. In a clay soil, where the water moves farther sideways, the emitters may be farther apart. Unfortunately determining what type of soil you have, and translating that into a spacing for your emitters, is difficult for the average person. Most people can’t really tell if a soil is silt or clay simply by looking at it. Even after numerous college courses in soil science and many years of experience I still get fooled now and then by a soil that doesn’t test out as I think it will. Embarrassingly, my own yard is an example of one I mis-guessed on! There are a couple of quick visual tests. One is that clay soil often cracks and splits when it dries. Another test is to take a handful of wet soil and ball it up in your hand, if it will not hold together well in a ball it is sandy or silty. Clay soil feels like… well, surely you’ve made something out of modeling clay at some point in your life and know what it feels like! It’s sticky and pliable. But the best method to find out our emitter spacing is to actually test the water movement in the soil. So the link below will provide you with instructions for a simple method of testing water movement. It involves consuming 2 liters of your favorite drink, so it can’t be too bad!

Why not just use an emitter with a higher flow rate for larger plants? This is a common misconception. The reason using a higher flow rate emitter doesn’t work is that the higher flow emitter does not wet more feeder root area. The additional water just goes down deeper into the soil or runs off on the surface, and the extra water is useless to the plant and wasted. So a larger emitter is of no help at all to a larger plant as it does not wet any more of the feeder root soil area. Going back to our thirsty man illustration, a larger emitter would be like pouring a large pitcher of water in our thirsty man’s mouth all at once. He could swallow a cup of so of it but the rest would just spill out onto the floor. So all the emitters on your drip system should have the same flow rate. The exception: Yep, there often is one! The exception is when watering potted plants. Each potted plant may have a different size pot, a different type of soil in it, a different type of plant, and each pot may have a different sun exposure that causes the soil in the pot to dry faster. For these reasons there will be major changes in water needs between pots. When watering pots I like to use the emitters that have an adjustable flow. That way I can adjust the emitter in each pot to get the right flow rate for that specific pot.

 

Test to Determine Horizontal Water Movement

To figure out how far horizontally the water will move in your soil you can perform a simple test which I have described on a separate page. Simple test for determining the horizontal water movement in soil.

Calculating Emitter Spacing

Once you know how far the water will soak horizontally in the soil you can determine an optimal emitter spacing. Just multiple the distance the water moved by 1.9 to get the spacing distance. Using 1.9 rather than 2 allows a slight overlap of the wet areas. So if you find the water moves 525mm in the soil you would multiply 525 x 1.9 to give a optimal spacing of 1000mm or 1 meter (36 inches).

If you didn’t test the actual soil you can estimate the spacing based on the soil type.

Typical spacing of 4 lph (1 gph) emitters:

Typical spacing of 2 lph (0.5 gph) emitters:

Watering Large Landscape Trees

It is pretty obvious that due to the huge diameter of a large shade tree it would take a lot of emitters to fully water the area within the tree’s drip zone. Fortunately some things help you out here. The first is that most large trees have aggressive root systems that are able to seek out water from deeper below ground and beyond the drip zone. This means that for a mature tree you can often put emitters a bit farther apart and you can even leave a few small areas of the drip zone dry. (There are always exceptions to the rules. Water loving trees like willows and cypress are going to want all the emitters and water you are willing to give them. Hopefully these water-loving trees are planted near a natural water source that they can grow roots into, like a creek or pond.) Another factor in tree irrigation is that most landscape trees are not planted alone. Typically a tree will have a lawn under part of it’s canopy, or perhaps a combination of ground cover and shrubs. Remember we are discussing large, established trees. If you are planting a new container or bareroot tree you will want to place at least two emitters per tree, one on each side of the rootball. Most newly planted trees need lots of water to get established and grow.

My design approach to drip irrigation for trees is to start by selecting the emitter locations for shrubs and groundcover as if there were not any trees. Then I add an emitter (or two) next to the rootball of each NEW tree to be planted, as well as any young existing trees. Finally, I look at both existing and future tree locations to see if there are any large unirrigated areas left under the tree canopy. Then I add emitters for those dry areas if I think they are needed. (You can look up the tree on-line to see what the water requirements are.) If I don’t think more emitters will be needed I still leave a little extra capacity in the design so I can add them later. That’s an advantage of drip irrigation; it is relatively easy to come back and add more emitters if it seems like the tree is in need of more water.

Native trees: Some established mature trees should not be irrigated. This is particularly true of some native species, especially oak trees. Regular irrigation of these trees can cause diseases that will damage or kill the tree. As a general rule if a tree is surviving well without any irrigation, it is best to not put any irrigation within the drip zone of that tree. If you are planting a new tree you may install irrigation under it in most cases. It is only mature, established trees, that have been living without irrigation for years, that have a problem with irrigation. Like a lot of older people, many old trees don’t like change! If you need to plant something under an existing native tree, most experts suggest that you plant shrubs or groundcover that can survive without any regular irrigation. Some careful hand-watering of the new plants to get them established after planting is usually OK, just keep it as minimal as possible.

Hedges, Hedge-Rows and Wind Breaks

Hedges, hedge-rows and wind breaks consist of plants placed tightly together in a row for various purposes. They are typically watered using dripperline or regular drip tube with evenly spaced emitters, similar to the description for agricultural drip systems below. Read the section below on Agricultural Drip Systems for more details. Avoid using the disposable laser-tube and drip-tape products unless you plan for the irrigation to be temporary.

 

Agricultural Drip Systems

In an agricultural situation most of the same rules for spacing emitters apply. The primary difference is that plants in an agricultural setting tend to be planted in rows. This means the emitters are most often placed in rows as well, and most often dripperline (also called dripline) is used. Dripperline is drip tubing with built-in emitters evenly spaced along the tubing. The advantages of dripperline are: it is easier and faster to install, the emitters are typically molded on the inside of the tube so they are less likely to be broken by field workers, and finally it is easier to move the tubes to allow the soil to be tilled, or to allow harvesting of the crop.

When watered with dripperline the roots of larger crops, such as vineyards and trees, will tend to grow in a row, following the wet soil along the length of the dripperline. This is not a problem, as in agriculture the plants are often pruned or trained into hedge-rows. So both the foliage and the irrigated roots are growing in a row.

Row Crops: For row crops emitters spaced at 30cm (12 inches) along the tube are most often used. Typically large spreading row crops (such as cucumbers and melons) use a single tube per row of plants. Most smaller row crops (strawberries, broccoli, etc.) use a wide berm with one tube down the center between two rows of plants. With row crops a lower cost disposable laser-tube or drip-tape is often used, this disposable tube/tape is intended to only last for one or two growing seasons. The disposable tube/tape is buried 75-150mm (3 to 6 inches) below ground and then is pulled up after harvest and is (hopefully) sent to a recycler. A careful gardener may get several seasons of use out of these tapes before they fill with roots and plug up. (Make sure you run the water at least weekly to help keep out roots.) For most home gardens I recommend using standard poly dripperlines, with built-in emitters spaced 30cm (12 inches) apart. This heavier poly tubing will last several years. Because the emitters are built into the tube, the tubing can be easily rolled up and stored between seasons. If you try to roll up tube with the punch-in emitters installed on it my experience is that a lot of the emitters will get broken off. I think you will find the heavier poly dripperline tube is also much more durable than the “drip tapes” which is helpful in home gardens where it is more likely to get stepped on and nicked by shovels and weeding tools.

Vineyards and Orchards: For vineyards a single dripperline is often hung above ground on the lowest vine wire. With tree crops typically two dripperlines are used, one running on each side of the row of trees, with the tubes about 1m to 1.5m apart (3-5 feet.) For larger trees like walnuts 3 or 4 rows of tubes may be used. Agricultural dripperline for vines and trees typically have emitters spaced 60cm (24 inches) apart on the tube. Remember that there is often a trade-off between water application and crop production. While using only 2 rows of tubes for trees, rather than 3-4 rows, may save money and produce a nice-looking tree, it might also cause a significant drop in crop production.

Vegetable Gardens:

Read the Agricultural Drip Systems section above as gardens are similar. For vegetable gardens I recommend using a good dripperline with emitters spaced at 30cm (12 inches) and not buried. Connect them together using garden thread style hose couplers, or with garden hose quick connect couplers so they can be easily disassembled and removed. Use stakes to hold the dripperline in place. Top quality dripperline will last for many years and is less likely to be accidentally damaged than the disposable tapes/tubes.

Potted Plants:

Often home landscapes will have potted plants. Potted plants are where I break a lot of the rules I’ve previously given you. As noted above in the Emitter Quantity & Spacing section I like to use adjustable flow emitters for plants in pots. I also use the small diameter distribution or “spaghetti” tubing from my larger drip tube up to the emitter in the pot. The small tubing is much less ugly. I try to hide it as much as possible. A metal stake is used to hold the emitter in the pot. Do not try to put more than 2 emitters on a single length of the small distribution tubing. The small tube size restricts the flow and 2 emitters is about the maximum you can use. So a typical drip system for pots would consist of a 16mm (1/2″) tube running along the ground between the pots, a 6mm (1/4″) distribution tube from the larger tube up into the pot, and an adjustable flow emitter staked in the pot.

Emergency Shut-Off Valve

An emergency shut-off valve should be installed at the closest point possible to your water source, that is, the location where you tap in for the irrigation system. Without this valve you will need to shut-off the water to the entire house if you have an irrigation breakdown and need to work on the mainline or irrigation valves. The most commonly used valves for this purpose are “gate valves” because they are inexpensive. Unfortunately the cheap gate valves you’re likely to use also tend to wear out quickly and start leaking. While a gate valve will get you by, I recommend that you use a “ball valve”, “disk valve”, or “butterfly valve”. These may cost a bit more (prices are becoming more reasonable as ball valves slowly are replacing gate valves for plumbing.) Ball valves are the least expensive of these and are much more reliable and will last several times longer than a gate valve. So if you pay twice as much for a ball valve it’s probably still the best deal! If you do use a gate valve make sure that it is a good quality one. There’s nothing worse than trying to work on a irrigation system when you can’t shut off the water completely. For some very small drip systems an emergency shut-off valve is simply not cost effective. For example; a manually operated drip system where an existing faucet or hose bib is used to turn the system on and off.

Zone Control Valves

Zone Control valves are the valves that turn on and off the water to the drip tubes. Often these are automated valves that are turned on and off by a irrigation controller/timer. For a small drip system there may be only one zone control valve. Bigger systems may have several zone control valves, for example they may have one the turns on the water to the front yard, another for the side yard, one for the vegetable garden and a final one for the back yard. There are two basic styles of zone control valves to choose from. Take a look at the image below, descriptions follow.

Globe Valve, Anti-Siphon Valve

Glove Valve on left, Anti-Siphon Valve on Right

Standard Globe Valve:

Glove valves are available in just about any size. They are often installed underground in a box or vault. Since a globe valve doesn’t incorporate a backflow preventer you must provide one separately. The globe style valve is the most commonly used valve on large commercial drip systems.

Anti-Siphon Valve:

Available only in 20mm (3/4 inch) and 25mm (1 inch) sizes. This is my recommendation for most homeowners. The anti-siphon valve incorporates a backflow preventer into the valve. This saves a considerable amount of money, as backflow preventers are very expensive. The anti-siphon valve MUST be installed above ground and MUST be at least 150mm (6″) higher than the highest drip emitter. This may prove a problem for some locations, since you would likely have to put the valves at the highest point in the yard. I have seen a anti-siphon valve installed on top of trellis in order to get it above the emitters for hanging baskets. On a slope the simplest solution is to run a mainline up the slope to the anti-siphon valve installed at the top of the slope. From there pipes run down to the emitters.

Indexing Valves (standard and anti-siphon):

Indexing valves are a single valve unit that controls several valve zones. The index valve has a water inlet and several water outlets. When the valve receives a signal from the control unit it opens the first water outlet, at the next signal it switches from the first to the second outlet. At each signal it switches to the next outlet until it gets back to the first outlet, at which point it shuts off. Indexing valves require a special controller to operate them. Indexing valves are usually available in models with or without a built in anti-siphon device. So an indexing valve may be also an anti-siphon valve. The anti-siphon indexing valve MUST be installed above ground and MUST be at least 150mm (6″) higher than the highest drip emitter. Indexing valves have never been widely popular and are generally only available in localized regions where a nearby manufacturer has heavily promoted them. Perhaps the best know indexing valve is made by the K-Rain company, they are popular in Florida where K-Rain is located.

Operation Method:

The control valves may be manually operated or they can be remotely controlled. Manual control is simple, the valve has a handle you use to turn it on. Remote control valves are either electric or hydraulic, but almost everyone uses electric solenoid type valves. The valves are turned on and off by a timer called an “irrigation controller” or often just called a “controller”. Anti-siphon, globe, and angle valves are all available as automatic valves. Most controllers and valves sold today are standardized, you don’t need to use the same brand of controller and valve. The standard is a normally closed valve that uses 24 volt alternating current to actuate the valve. When 24 volts of current is applied to the valve solenoid wires the valve opens, when the voltage is turned off the valve slowly closes. This way the valve will close during a power failure or if a wire breaks. There are some exceptions to this standard operation method. To save power, controllers that run on batteries or solar power often use a special type of solenoid on the valves called a “latching solenoid”. Latching solenoids work like a toggle switch, when a short burst of power is detected the valve switches open (if it was closed) or closed (if it was already open). Generally if latching solenoids are required there will be a warning and instructions on the controller. If the controller doesn’t plug into a power source, chances are it uses latching solenoids. There are a few specialty controllers and valves that use their own proprietory system and are not compatible with either the standard or latching solenoids, but these are rare and seldom used by homeowners. The most common are Indexing Valves (see above). Another common one is a small solar-powered controller and custom valve solenoid combination sold under the brand name LEIT®. While a little beyond the budget of most homeowners, LEIT controllers can operate on very low levels of light, they claim moonlight is sufficient. (If you see something that looks like a parking meter installed in the middle of a landscaped road median island, you’ve spotted a LEIT controller. They are very popular with highway departments.)

Valve Body Materials:

Valves are available with either brass or plastic bodies. Most used today are plastic, but brass is still widely available and preferred by some pros, especially when high water pressure is present. There is no doubt that a brass valve will last longer if installed in the sunlight. From an operational point of view, both are reliable, especially for automatic systems. For manual valves my experience is that brass will last much longer. For automatic control valves I almost always use plastic, my experience is that when buried or protected from sunlight it holds up as well as brass and is less expensive. If you use plastic valves above ground you may wish to consider building a cover for them to protect them from sunlight, which can destroy the plastic over time. My experience is that even when made using UV resistant plastic, the plastic valves will start to break down after a few years in the sunlight.

 

Backflow Preventer: A device that allows water to go through it in one direction, but prevents it from going backwards in the opposite direction.

A backflow preventer is like a one-way gate for water. Most backflow preventers are used to keep unsafe water from reversing flow and entering the clean water supply. Backflow preventers can be as simple as a single check valve that closes when water flow reverses. Using a simple check valve as a backflow preventer might be considered the equivalent of a turnstile at a store entrance, it is not very reliable, even a small amount of effort will overcome it. A more elaborate backflow preventer can be a complicated device that consists of multiple check valves, water release valves, air vents, and/or systems to allow it to be tested to assure it is working properly. This kind of backflow preventer might be the equivalent of an airport exit security checkpoint with one-way gates and a armed guard.

Here are links to the backflow preventer related topics below in case you come back and want to reread something. (Sorry this page is so long, it’s a complex topic and there are a lot of options available to you!)

 

Measure Your Supply Pipe Circumference:

Grab a piece of string about 6″(152mm) long. Strip away any insulation so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once. This is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe. But school’s out so let’s forget about doing geometry calculations! Based on the string length use the table below to find your pipe size.

For Copper or PEX Pipe

2.75″ (70mm) = 3/4″ pipe

3.53″ (90mm) = 1″ pipe

4.32″ (110mm) = 1 1/4″ pipe

5.10″ (130mm) = 1 1/2″ pipe