Irrigation patterns Archive - Irrigation Blog for Do-it-yourselfer

How to calculate the precipitation rate?

Johann Kodnar — Sun, 05 Mar 2023 20:05:16 +0000

Plants need a certain amount of water to thrive. For lawns, calculate about 1 inch of water per week that each patch of lawn needs to receive. In very hot periods even about 20 percent more. The precipitation rate indicates how much water is poured in one hour. It is therefore the basis for determining the correct duration of irrigation. In the following blog post you can find out how to calculate the precipitation rate yourself or how to recognize the assumption under which it was calculated.

Calculation for an already active irrigation

If the irrigation is already in operation, the easiest way to determine the precipitation rate is to compare the amount of water used and the area irrigated. This should be done separately for each irrigated sector, as the results can differ significantly. You can either simply read the amount of water from an already installed water meter, or you can get a water meter and install it after the water source. You can let the watering run for a whole hour and then compare the meter reading with the one at the beginning of the watering, or you can just let it run for a shorter time and then extrapolate to one hour. The area is either known or can otherwise be determined with a measuring tape.

Calculation formula Precipitation rate = gallon output per hour/area in square foot

In order for the calculation to work, the denominator and counter must be brought to the same units. The following applies:

1 gallon water = 231 inch³
1 foot² = 144 inch²

So if you water, for example, an area of 20 by 20 feet with 300 gallons water for an hour, then it would apply:

Precipitation rate = 300 * 231 inch³/20 * 20 * 144 inch²= 69,300 inch³/57,600 inch² = 1.20 inches

Attention: This value is an average value and says nothing about how evenly this precipitation is distributed over the area. It may be that some spots receive significantly more water than others, depending on how professionally you have planned your watering (see “square pattern” and “triangular pattern” in the next point).

The rest of the considerations are easy: how much water does the irrigated area need per week and how do you divide up the watering? If you pour the amount of water collected in one irrigation run per week, then you simply divide the required amount of water by the capacity of the irrigation system per hour and thus know the required running time. In the example above, the irrigation system is pouring 300 gallons per hour over a 400 square foot area, which is 0.75 gallons per square foot. The lawn is to receive 0.6 gallons per square foot per week, so watering must run for 1.25 hours (0.75/0.6). So an hour and 15 minutes. If you water twice a week, that would be two times 37 minutes.

Water flow meters at Amazon:

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Calculation for a planned irrigation

The sprinklers are positioned in a professional planned irrigation either in the square pattern or in the triangular pattern. The calculation of the precipitation rate also depends on the selected system, whereby the precipitation rate of the triangular pattern is about 10 to 15 percent higher in comparison due to the somewhat higher overlap.

For most sprinkler suppliers, the precipitation rate for both patterns is included in the product descriptions (the triangle symbol indicates the triangular pattern, the square symbol indicates the square pattern). The precipitation rate is given for different water pressure values, which affect the radius and flow rate of the sprinkler and thus also cause a different precipitation rate. An overview of this performance data for numerous sprinklers can be found on the sprinkler performance data page.

Calculating the rate of precipitation in the square pattern

As in the triangular pattern, the following values are required for the calculation:

The radius of the sprinkler
The flow rate of the sprinkler

Formula to calculate the flow rate in the square formation = flow rate/(radius*radius)

This formula is best understood using a single square of the square pattern:

Area irrigated in square pattern

The height and width of the square corresponds to the throw distance (radius) of the sprinkler. It receives water from 4 sprinklers in total (marked as black dots in the corners). And each sprinkler waters a quarter circle of the square.

Let’s take the Gardena T200 as an example: This has a flow rate of 150 gallons/hour and a throw distance of 26 feet in 360 degree operation at 29 psi pressure. Before you can calculate, these values have to be converted to the common unit of inches. Again the following applies:

1 gallon water = 231 inch³
1 foot² = 144 inch²

The flow rate in the square pattern would thus be 150*231/(26*26*144) = 0.36 inches.

This calculation is correct if one assumes that the area is irrigated with sprinklers that irrigate in a full circle. And it is also true for sprinklers with built-in MPR principle such as the Hunter MP-Rotator and for sprinklers that are operated with MPR nozzles. For all of these, the flow rate of a sprinkler is used in the full circle principle. It is therefore assumed that the irrigation looks like this:

Area irrigated in square pattern (within the 4 black dots). The sprinklers irrigate full circles

So each sprinkler that acts on the square runs a full circle. Why is this important? For sprinklers that do not have the MPR principle built in and are not used with MPR nozzles, the precipitation rate is directly related to the circle sector that the sprinklers irrigate. It’s easy to explain why: These sprinklers always release a certain amount of water in a certain amount of time, say 150 gallons per hour. If the sprinkler runs a full circle, it will spread that 150 gallons over the full circle. If, on the other hand, it only runs half a circle, then it runs twice its half circle in the time in which the sprinkler previously ran the whole circle and the lawn in this area receives twice as much water in the same time. With a quarter circle it would even be four times as much. Therefore, with the exception of the MPR sprinklers mentioned above, it is relevant when planning how the irrigation is planned, whether sprinklers irrigate full circles or sprinklers irrigate semicircles. If you plan to use full-circle sprinklers, then the above formula is correct. If you plan your sprinklers to irrigate semicircles or quarter circles, then the result must be multiplied by 2 or 4. So the correct formula is:

Formula to calculate the square pattern for operation in a semi-circle (180 degree sector) = 2*flow rate/(radius*radius)

Formula to calculate the square pattern for operation in a quarter-circle (90 degree sector) = 4*flow rate/(radius*radius)

For this reason, the sprinkler manufacturers indicate in their description for which operation it was calculated. This is mostly for operation in the 180 degree sector, but with some sprinklers also in the 360 degree sector. It is therefore very important to bear this in mind, otherwise the actual precipitation rate may be half or double what is stated in the description.

Derive missing manufacturer information yourself

Sometimes it happens that this information, whether the precipitation rate was calculated with 180 degrees or 360 degrees, is missing for one or the other sprinkler type. This is no problem for you with the formulas given here: Simply take the flow rate and the throw distance and enter them once in the formula for full-circle sprinklers and once in the formula for half-circle sprinklers. Compare the result with the information provided by the manufacturer and you will know which of the two variants was used to calculate it. The same applies to the calculation in the triangle pattern (see next point).

Calculating the rate of precipitation in the triangular pattern

In principle, the same applies here as described above. The two values throw distance (radius) and flow rate are also required, only the formula for the calculation is slightly different:

Formula to calculate flow rate in triangular pattern = radius/(throw*(throw*0.866))

Let’s take again the example of the Gardena T200 with a flow rate of 150 gallons/hour and a radius of 26 feet and again the following applies:

1 gallon water = 231 inch³
1 foot² = 144 inch²

The flow rate in the triangular pattern would thus be 150*231/(26*(26*0.866)*144) = 0.41 inches

How can the formula be understood? In the triangular pattern, a pattern of many equilateral triangles emerges:

Area irrigated in triangular pattern

In the triangle pattern, each triangle receives water from 6 different sprinklers. The triangle is equilateral, meaning that each of the sides is the same length and the triangle is perfectly symmetrical. And it means that the height of the triangle is 86.6 percent of the side’s width. That’s the 0.866 in the formula above. The area is calculated by multiplying the length of the side by the height.

As before, it is also relevant here whether the sprinklers are operated in the 360 degree circle or in a 180 degree or 90 degree circle. For the 180 degree or 90 degree circle, multiply by 2 or 4 as before:

Formula to calculate the triangular pattern for operation in a semi-circle (180 degree sector) = 2*flow rate/(radius*(radius*0.866)

Formula to calculate the triangular pattern for operation in a quarter-circle (90 degree sector) = 4*flow rate/(radius*(radius*0.866)

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Place the sprinkler correctly: Triangular pattern

Johann Kodnar — Wed, 02 Feb 2022 20:10:50 +0000

The correct positioning of the sprinklers on the area to be irrigated is one of the most important points when implementing automatic irrigation. A distinction is made between two different principles, the square pattern and the triangular pattern. I present the square pattern in a separate blog post. I explain in detail why a system is used to position the sprinklers in the planning pages. This post is about the triangular pattern: how it works, how to construct it and where it has advantages over the square pattern.

The triangular pattern has an advantage over the square pattern when it comes to watering non-rectangular areas. Especially when it comes to circular areas, it is much easier to use the triangular pattern. In the case of rectangular areas, on the other hand, the square pattern is easier to use.

Another advantage of the triangular pattern is that this formation allows the sprinklers to be spaced slightly further apart than is the case with the square pattern. This is of great importance, especially for the professional sector, in which huge areas are irrigated. In the square pattern the distance between the sprinklers is 50% of the sprinkler throw diameter, in the triangular pattern you can place the sprinklers with a distance of up to 60% of the sprinkler throw diameter and still have a sufficiently good overlap. If the maximum possible distance is exhausted, the triangular pattern manages with slightly fewer sprinklers on the same area than the square pattern. I then first show a triangular pattern with a sprinkler spacing of 50% of the sprinkler throw diameter (= throw) and then, for comparison, the same formation with a sprinkler spacing of 60% of the sprinkler throw diameter.

The sprinklers positioned in the triangular pattern on the irrigation area result in the image of equilateral triangles, which are placed offset in each row:

The distance between the rows in the triangular pattern is 86% of the radius width. The triangle pattern works particularly well for corners that are around 60 or 120 degrees (as in the image above).

The single triangle will be watered in the triangle pattern with a pattern as below (at 50% sprinkler spacing):

A sprinkler is placed in each corner of the triangle. Unlike the square formation, which only receives water from the 4 sprinklers in the square formation, the triangle also receives water from points outside the triangle. Two sprinklers from the neighboring triangles that are to the left and right of the upper corner point and one sprinkler from the triangle below rain into the triangle. This can be understood in the following sketch, in which you can see that the triangle framed in blue receives water from a total of 6 sprinklers:

This applies at least to those triangles that are not on the edge of the irrigation area. The triangles on the edge lack a neighboring sprinkler on the edge side and are therefore watered a little less. This even applies to the optimal basic shape with 60 or 120 degree corners.

For example, over an entire irrigation area, the triangular pattern looks like this. It can be seen that the same pattern is repeated in each triangle in the same way, only rotated 180 degrees for the inverted triangles. And that the triangles on the edge are missing part of the pattern.

In practice, the reduced watering at the edges would have to be compensated for by installing additional sprinklers. For this reason, the triangular pattern is particularly suitable for large connected areas, where this fact plays a comparatively small role.

In any case, the best alternative is the triangular pattern for circular areas. The reason for this is that a circle can be divided into numerous pie slices, i.e. triangles. Exactly how this works is explained in a separate blog post on watering circular areas.

Reference to triangle formation in sprinkler operating data

The sprinkler manufacturers indicate the amount of precipitation for their sprinklers in their product descriptions. This is done for a particular water pressure and nozzle used, with reference to either the square or triangular pattern. For more on this topic, see the square formation blog post.

Adjusting the amount of precipitation – tuning the nozzles

When placing the sprinklers, make sure that the amount of precipitation from the sprinklers has to be adjusted depending on the size of the circular sector. Why? Most sprinkler models always release the same amount of water in a certain amount of time, e.g. 200 gallons/hour at 30 psi pressure. A quarter-circle sprinkler would therefore distribute four times the amount of water as a full-circle sprinkler on its sprinkling area in the same amount of time. A more detailed explanation of this and how to compensate for this effect or avoid it entirely by using sprinklers with an integrated MPR function can also be found in the explanation of the square formation.

Triangular pattern with a sprinkler spacing of 60% of the sprinkler throw diameter

Below is an example of a triangular pattern in which the maximum possible sprinkler spacing has been exhausted. The sprinklers were not placed within the sprinkler throw distance from each other, but a little further apart, at 60% of the sprinkler throw diameter. In the previous example, the sprinklers were placed 26 feet apart, now 31.20 feet apart. Since the sprinkler throw remains the same, this means there is slightly less overlap. Each triangle only receives water from 3 sprinklers instead of 6 before. This can be easily understood from the following sketch, which shows a positioning with 60% of the sprinkler throw diameter:

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Place the sprinkler correctly: Square pattern

Johann Kodnar — Sun, 30 Jan 2022 20:31:08 +0000

The correct placement of the sprinklers on the area to be irrigated is one of the most important points when planning an irrigation system. Firstly, this is crucial for watering the garden as evenly as possible and consequently saves water. And secondly, incorrect positioning can often only be corrected later with great effort.

When positioning sprinklers, a distinction is made between a square pattern and a triangular pattern. These two are the dominant principles around the world by which most irrigation is planned. The square pattern described below is the technique that is somewhat easier for the user to implement and is particularly suitable for rectangular areas. In the following blog post you can find out how it works, what its advantages are and how the square pattern is reflected in the performance data of the sprinkler manufacturers.

The basic principle of the square pattern is the principle of double overlap: each sprinkler must be reached by the throwing circles of (at least) two other sprinklers. For example, this can look like this for a square basic shape:

A sprinkler is placed in each corner. This is reached by the throwing circle of its left and right neighbor sprinkler (= double overlap). On the planning page you will find further examples for the placement of sprinklers in different basic shapes.

In the square pattern, 4 sprinklers are placed in such a way that – if you mentally connect the sprinklers – a square results. These 4 sprinklers together correctly cover the square area they enclose – according to the rules of the square pattern. The distance between the sprinklers in the square pattern is approx. 50% of the sprinkler throw diameter, i.e. approx. the throw range (= radius) of the sprinkler. For example, if you have a 30 foot throw, the sprinklers will be positioned 30 feet apart.

Before starting, the ground plan of the garden – if it does not already consist of a square or a rectangle – is divided into rectangular parts. You can find an example of this in the planning section. When drawing in, start with the trickiest spots – these are the 4 corners of the area to be watered. A sprinkler is placed in each of these, throwing a quarter circle:

Next, proceed to the second most delicate area: the edges. The sprinklers are set on the edges with half-circles in such a way that they correctly overlap their right and left neighbor sprinklers. Your throwing circle must therefore reach to the neighboring sprinklers. It then looks like this:

Before drawing in the other sprinklers, you can make auxiliary lines. To do this, connect each of the sprinklers on the edge, with the exception of the corner sprinklers, to the sprinkler on the opposite side. On the plan, this results in a checkered pattern:

Full-circle sprinklers are now drawn in at the intersections of the auxiliary lines. After the first circumnavigation it looks like this (I have removed the auxiliary lines for the purpose of a clearer representation):

An unirrigated spot remains in the middle. This is now also filled with full circles. This ultimately leads to the following result:

You can see that there is a continuous pattern over the entire irrigation area, which always looks exactly the same within 4 sprinklers. This consistent pattern means the lawn is watered evenly across the entire area. This means that all areas of the lawn receive the same amount of water.

In my example, it is exactly the case that the set full and semicircles result in exactly the length and width of the area to be irrigated. In practice, this often does not turn out so well, so that, for example, a horizontal and/or vertical area remains that is only half the diameter of the circle, for example. Here you can then either strictly follow the principle further and thus accept a larger than planned overlap at this point. Or you try to cushion this effect of the additional overlap by reducing the throwing circle or choosing smaller nozzles for the sprinklers in question in order to achieve the same amount of precipitation as possible as in the other squares. A third possibility is the use of MP Rotator sprinklers, which independently keep the amount of precipitation constant (more on this below in the chapter “Sprinklers with integrated MPR function”).

Reference to square and triangle pattern in sprinkler operating data

The sprinkler manufacturers indicate the precipitation rate for their sprinklers in their product descriptions. This is done for a specific water pressure and a specific nozzle used with reference to the square or triangle pattern. Following is an example:

Rain Bird 3500 Series

Nozzle	Water Pressure	Radius	Flow	Precipitation inches/hour ∎	Precipitation inches/hour ▲
2,0	25 psi	27 feet	84 gallons/hour	0.37 inches/hour	0.43 inches/hour
2,0	35 psi	27 feet	101 gallons/hour	0.45 inches/hour	0.52 inches/hour
2,0	45 psi	27 feet	116 gallons/hour	0.51 inches/hour	0.59 inches/hour

If you irrigate according to the principle of the square pattern, then the lawn receives 0.37 inches of precipitation per hour at 25 psi. This corresponds to 0.23 gallons per square foot.

Note: The 0.23 gallons per hour does not refer – as is often wrongly believed – to the one sprinkler listed on its own, but is only achieved if this sprinkler is operated together with others in a square pattern. So if there is a proper overlap. The one sprinkler on its own would have a much lower precipitation rate, which would be only a fraction of what it was advertised to be! You can calculate this relatively easily by calculating the area of the circle and then dividing the flow rate by the area. In the example, with a throw distance of 27 feet, we would have a full circle with an area of 2,289 square feet (27 x 27 x 3.14). If you divide the amount of water available per hour of 84 gallons by the number of square feet, you would arrive at a rainfall of just 0.037 gallons/hour per square feet. Even if you operate the sprinkler not as a full circle but as a quarter circle, only 0.15 gallons per hour would result.

Adjust precipitation – use of appropriate nozzles

When placing the sprinklers, you must ensure that the amount of precipitation from the sprinklers must be adjusted depending on the size of the sector of the circle. Why? Most sprinkler models always release the same amount of water in a certain amount of time, e.g. 200 gallons/hour at 30 psi pressure. If the sprinkler is set to full circle, it would distribute these 200 gallons in a full circle. The sprinkler starts at a certain point and then slowly rotates until it has completed the full circle. Then he does it all back in the opposite direction. If the sprinkler was set to a quarter circle, it would also pour 200 gallons per hour, but spread the water over an area that is only a quarter as large. The areas irrigated with a quarter circle would receive four times as much water in the same period as the areas irrigated with a full circle. And the areas irrigated with a semicircle would get twice as much as those irrigated with a full circle.

This effect is compensated for by the nozzle size of the sprinkler. So you change the nozzles of the sprinklers in order to have the same precipitation rate for everyone. The semi-circle sprinklers have nozzles with twice the flow rate as the quarter-circle sprinklers. And the full-circle sprinklers have nozzles with twice the flow rate of the half-circle sprinklers. In our example, the full circle sprinkler would pour 200 gallons per hour, the half circle sprinkler about 100 gallons and the quarter circle sprinkler about 50 gallons per hour. Each quadrant would therefore get the same amount of water of about 50 gallons per hour. The sprinkler manufacturers usually reflect this system in the classification of their nozzles: A No. 4 nozzle has twice the flow rate as a No. 2 nozzle a No. 8 nozzle has twice the flow rate as a No. 4 nozzle.

So you could think that the quadrant sprinklers could simply be equipped with No. 2 nozzles, those with a semi-circle with No. 4 nozzles and those with a full circle with No. 8 nozzles and thus achieve the same precipitation rate for all. Unfortunately, it’s not quite that simple! The problem is that using different nozzle sizes also affects the throw distance of the sprinklers. With the same water pressure, larger nozzles lead to greater throw distances, smaller nozzles to shorter throw distances. It is therefore not possible to depict a correct square or triangle pattern in this way, since the same throwing distances are required for these, regardless of the size of the sector of the circle!

The sprinkler manufacturers have solved this problem with the introduction of so-called MPR nozzles. MPR stands for “Matched Precipitation”, i.e. the principle of matching the precipitation rate to one another. An MPR nozzle set contains 4 nozzles for 90 degrees, 120 degrees, 180 degrees and 360 degrees. Depending on which section of a circle you want to sprinkle, you use the appropriate nozzle in the sprinkler. This not only adjusts the precipitation rate, but also results in an identical throw distance. All 4 included nozzles therefore have the same precipitation rate and throw distance at the same water pressure.

Important: When choosing a sprinkler, make sure that the sprinkler is MPR-capable! This only applies to the newer model generations, older sprinklers cannot handle MPR nozzles or MPR nozzles are not available for them at all.

A possible alternative to adjusting the precipitation rate using MPR nozzles would be to group together sprinklers with the same sized circular sections in one sector and to achieve the same amount of precipitation over different irrigation times. So let the sector with the full-circle sprinklers run 4 times as long as the one with the quarter-circle sprinklers.

Sprinklers with integrated MPR function

Hunter’s MP Rotator sprinklers go one step further. These already have the MPR function built in: If you adjust the radius or the circular section to be irrigated, the sprinkler automatically adjusts the water flow to the same extent, so that you always have the same precipitation rate. In addition, the precipitation rate of the MP-Rotator is always the same for all models, so that different models can be mixed in one sector without any problems.

Below is a link to MP Rotator sprinklers and the associated bodies on Amazon:

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