Designing irrigation for hill country

Designing an irrigation system for hill country – no matter what part of the world you are in, requires a different set of considerations due to changes in elevation, slopes and topography.

G-set hill irrigation

As slopes increase on any given property there are a number of considerations that have to be taken into account:

1. Cost

Because often we need to lift water from rivers or other catchment areas, the cost in energy terms can be as much as double for the same amount of water. Compared to flat land areas, the cost of running hillside irrigation can be substantially more expensive.

2. Water Pressure and Pipe Size

With the change in elevation and the requirement for constant delivery pressure to sprinklers, it’s also important to fit pressure regulators and select the appropriate pressure class of pipe e.g. 6 bar pipe is roughly 60m rise and 9 bar pipe is roughly 90m rise.

3. Worker Safety

If the system elsewhere on the farm requires manual shifting like K-Line™ then we need to consider the safety of staff as they perform their tasks. There will be areas that are too steep for 4WD access and as a result, irrigation is unlikely to be used on those slopes.

Irrigation systems on hill country should always consider Health and Safety as a prime factor.

4. Aim for an Irrigation Deficit, not full Field Capacity

Management of productive soil should focus not on bringing the soil to field capacity, but to have a deficit suitable to receive any rainfall that naturally (and unpredictably) occurs.

When irrigating, water should always be applied to a soil in deficit enough to receive all that is applied without wastage in the form of runoff.

To illustrate this concept, think of a sponge. When the sponge is dry it will accept a large amount of water. If the sponge is damp, it will take less water before it starts to leak. If the sponge is fully saturated, it won’t take any more water and the water will simply overflow as waste.

Any system that is used on hill country should also be as low on instantaneous application rate as possible.

5. Distribution of Sprinkler Units

Distribution of irrigation sprinklers should also change as the land rises and falls around a sprinkler and consideration should be taken as to the effect that this has on the pattern or uniformity.

Our observation with the Senninger 80 series sprinklers in the K-Line G-Set™ product is that the pattern is simply moved down the hillside by a few metres.

6. Water Runoff

There are a number of factors that influence runoff:

  • as the steepness of the slope increases, the amount of runoff potential will also increase
  • As the soil approaches field capacity the infiltration rate of the soil will approach nil and further application of water will runoff
  • As the instantaneous application rate increases, the soil will tend to runoff.

K-Line™ or K-Line G-Set™?

RX Plastics manufactures two main systems:

K-Line™ irrigation system, a movable irrigation system based on small irrigation sprinklers in black pods linked by a small and flexible polyethylene pipe, and

K-Line G-Set (G-Set) irrigation that is a solid set (fixed) irrigation system based on larger irrigation sprinklers surrounded by blue protective pods and placed in a triangular spacing arrangement at 40m apart.

So how do K-Line and K-Line G-Set compare in delivering on the two key factors of safety and appropriate water delivery?

K-Line™

  • Health and Safety – Because K-Lines are shifted often by 4 wheeler motorbike, consideration must be given to areas that should not be irrigated and many farms will have these places where risks to worker safety are too high.
  • Soil management – It is important when designing K-Line systems that they are designed to the soil conditions. Practically speaking, some soils will not hold what K-Line will apply over 24 hours.
    K-Line application depth over 24 hours can be 50mm- >100mm so soils must have a deficit of this amount available. This then means that systems need to be designed with attention to these issues.
    Solutions to avoid over watering may include 2 shifts per day, time clocks or separating areas for irrigation through a day by zoning.
  • With K-Line’s instantaneous application rates of 2-5mm per hour there should be little issue with soils absorbing this water. If runoff occurs, consider if the soil is already at field capacity or is it damaged due to pugging or other issues.
  • Operating pressure can be regulated either at the sprinkler or at the start of the pipe line or both. This ensures the pressure does not exceed the required level for optimum irrigation.
  • The sprinklers employed are typically plastic and don’t have differential rotations due to slopes. This assists to maintain the sprinkler pattern.

K-Line G-Set™

  • Health & Safety – Because the G-Set is solid set and is therefore not moved around paddocks, as long as installation is successful (and this has its risks) the system will have minimal health and safety issues.
  • Soil management – As G-Set is a solid set system, return periods are not significant as it is possible to completely cover all areas in 24 hours (or less) and apply a small application depth or to extend that return out to days in length and apply a correspondingly greater application depth in each application.
  • This is also variable rate irrigation as each individual unit can potentially apply a greater or lesser amount of application depth from the average by using the controllers.
  • With G-Set’s instantaneous application rates of 4-6mm per hour, there should be minimal issues with soils absorbing this water, assuming that there is a soil moisture deficit. Applying water to soils already saturated will of course lead to runoff, especially on slopes.
  • The operating pressure of the G-Set can be managed by pressure reducing valves in each unit or the laying of the ring main to the same elevation around hill areas or to pressure reduce the ring main or all of these solutions together.
  • Sprinklers employed are typically plastic and don’t have differential rotations due to slopes. This assists to maintain the sprinkler pattern.

Read more about G-Set systems and K-Line options here on the site.

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