Watering a slope

Tips, Tricks, and Instructions for Planning Sloping Sprinkler Systems

Watering effectively, even on level ground, can be difficult; watering sloping areas can be considerably more challenging. Not only do you have to find the perfect watering level for specific plants, you have to combat gravity as well. There are some key factors that are important to consider when designing a sprinkler system for sloping ground. These are: the angle of the slope, the area of ground to be covered, and soil type and constitution.

Before beginning your site plan for the slope, it is critical that you calculate the angle of the slope and the exact dimensions of the area to be watered. Once you have calculated the area, you should consider the application rates of different types of sprinklers, and select the type which is most compatible with your landscape. Generally, rotors have lower precipitation rates than sprays and are better suited to slopes. If the area is too small for a rotor, drip irrigation may be the best option.

Here are some tips and tricks to help ensure efficient, productive watering.

Sprinkler Timers and Water Management

Selecting a timer with good water applications will help reduce run-off. The best options are either a controller with at least four start times per program, or one with advanced watering options like Cycle+Soak. If you opt for a timer with four or more start times, you will need to perform some simple math to calculate how many run times you need.

  1. Find out how long you can run a station before the water starts to run off instead of soaking.
  2. Determine the optimum amount of watering time your plant needs.
  3. Divide the plant watering time (step two) by the maximum run-off-free time (step one). This number is the minimum number of run times you will need per program.

For example, let’s say that I can run my sprinklers for only 15 minutes before the water starts to run off, but my plants need to be watered for an hour (60 minutes). I would divide 60 by 15, which is four. This means I need at least four run times per program- I would then set each run time with enough time between them to allow the water to soak in sufficiently.
If you want a simpler solution, opt for a timer with programs like Cycle+Soak, which automatically allow for soak time during the cycle.

Proper Lateral Line Layout

Pressure Compensation

When installing lateral lines, it is important to lay then horizontally across the hill, rather than vertically down the hill. If they are installed wrong, the elevation change will affect the pressure levels and can create very different pressures at the individual nozzles. Uneven nozzle pressure will cause irregular water distribution and could decrease the operating life of pipes and sprinkler heads.

Slope Compensation

On ground with a 2:1 slope, sprinklers that are adjusted properly with throw roughly 40% of their radius above the head and 60% below (because of gravity). When planning your sprinkler system, it is important to allow for this, particularly at the base of the hill. It can be hard to visualise this, since site plans can’t show a slope, but when planning your sprinkler system, the spacing between the top, middle, and bottom lateral lines can’t be the same. The bottom and middle lines should be closer together (have less distance between them) than the top and middle lines. The middle and bottom lines should also be shifted up the hill a little, so that the bottom lateral line is not right at the base of the slope. This will create even, thorough head-to-head coverage.

Sprinkler Heads and Nozzles

Sprinkler Head Spacing

When planning your system, it is a good idea to limit the number of sprinkler heads you place in each zone, for two reasons. With fewer sprinkler heads in each zone, the system will perform better and have a lower risk of damage. The more sprinkler heads you place in a zone, the larger the components and infrastructure. Larger components means larger quantities of water in the system, and in the event of a system breakage, the larger the quantity of water, the greater the damage. If you do still opt for larger zones, it is important to incorporate safeguards against potential problems.

Another benefit of smaller zones is that you can plan each zone based on the type of plant, sun exposure, and wind. This will promote plant health and increase watering efficiency.

Fixed or Pop-Up Sprinkler Heads

When planning a sprinkler system for sloping ground, most people choose a non-retractable spray mounted on a riser. However, for a more aesthetically pleasing option, consider installing a 6- or 12-inch pop-up sprinkler. Not only do they look better, they are also less prone to being run over, bumped, or damaged.

Sprinkler Heads with Check Valves

Another great erosion-prevention option is to select sprinkler heads with built-in check valves. When installed correctly, the check valves will prevent any remaining water in the lateral lines from leaking out of the heads. After a run cycle, there will inevitably be some residual water in the lateral lines. On sloping ground, the slope can cause the water to build up enough pressure to seep out of sprinkler heads at the bottom of the slope. Opting for sprinkler heads with built-in check valves simplifies installation, as you do not have to install separate check valves beneath each sprinkler head.

Matched Precipitation

Uniform water distribution is crucial to having an effective and efficient slope irrigation system. One way to achieve this is by placing only one type of spray pattern sprinkler in each zone – only full-circle sprays in zone A, only half-circle sprays in zone B, etc. – and then adjusting station run times accordingly. An easier alternative is to opt for matched-precipitation rate nozzles – these nozzles are design specifically to match precipitation rates, regardless of variations in arc and radii.

Designing with matched precipitation safeguards against uneven watering in the same zone – in systems with unmatched precipitation, it is common for the system to allow overwatering in one part of a zone, while another area of the same zone will receive too little water. Not only is this bad for the plants, it also dramatically increases the risk of erosion.

Sprinkler Head Angling

When installing the sprinkler heads at the top of the slope, it is best to use low-angle trajectory nozzles, if possible. These have a smaller throw radius, which reduces wind dispersal. Since they have a smaller radius, they need to be installed closer together than standard sprinklers, and should be installed vertical to the slope. If you opt for standard sprinklers at the top of the hill, place them slightly below the crest (highest point) of the slope, and angle them towards the bottom of the slope to reduce wind drift.

Mid-slope sprinkler heads should be installed at a 45 degree angle to the slope, and sprinkler heads at the base should tilt at a slight angle away from the slope.

With careful planning and using these tips and tricks, it is possible to have an efficient sprinkler system and beautiful landscape on sloping ground.