Irrigator Maintenance – Sand

A little while ago I wrote about Sand Traps and how they should be inspected and cleaned regularly. This week we’ll look at the effect sand has on your irrigation system.

In general terms, sand inside an irrigation system = bad!

Sprinkler spray pattern

Just like sand-paper or sand blasting, sand suspended in water inside your irrigator is abrasive and will cause expedited wear on your system. This wear occurs anywhere exposed to the sand. It will start at the pump, causing impellors to wear faster, go through your mainline and headworks. If you have a mechanical flow meter, this will suffer. The sand then gets into the riser and makes its way along your span-pipes causing wear on the galvanising all the way along. Some of the sand will make its way up into the goosenecks and then down into your regulators and sprinkler nozzles, then onto the sprinkler plates. And guess what – it’s still abrasive and causing wear on these components too!

The main two problems caused by sand wear are

1.       Decreased life expectancy

2.       Decreased performance

As components like regulators and nozzles experience wear, they will start letting more and more water through, the more water that is going through, the greater the wear and the cycle continues. Your nicely calibrated system at the beginning of its life will soon start to over-apply water and as it is doing this it is also causing more wear on the sprinkler plate which will distort the sprinkler’s application pattern.

If you have a lot of sand in your system, other than emptying the sand trap frequently, you should also be aware that your system will need checking and re-calibrating more frequently to account for the wearing effect.

In order to mitigate some of the bad effects caused, you might want to consider a sand separation system such as the lakos sand separator filter pictured below. This type of filter will remove 98% of sand-like particle matter down to 200 mesh (74 micron) with low and steady pressure loss of only 0.137 to 0.413 bar.

Lakos sand separation system

For more information about sand filtration systems, contact your local Lindsay dealer.

My Soil Moisture Sensors Are Spot On – Yeah Right!

There is a misconception that all soil (moisture) sensors are precise and tell you the exact soil moisture content.  Not so.  HydroServices runs a trial site where a number of soil moisture sensors are installed for comparison – for both the “calibrated” soil moisture content and any long term trends.  A boring looking trial site because the sensors are installed toward the small gap in the trees.  In all there are 8 sensors – neutron probe, Decagon 5TM and GS1, Acclima, AquaCheck and two (2) Aquaflex.  The Decagon 5TM, GS1 and Acclima sensors are installed at 10cm, the shallow Aquaflex on a slope from 10-25cm, and the neutron probe and AquaCheck can measure at 15 and 10cm respectively.

Sensors are provided with a factory calibration, usually one for silt loam, clay loam and sandy loam soil types.  These are generic and may or may not truly measure the soil moisture content at your location because (for example):

a)    Your soil is unlikely to be the same as the generic soil type;

b)    The sensor is poorly installed (especially if there is not perfect contact between the sensor and the soil); and

c)    The soil is loose (cultivated) and perfect contact is not possible

While the traces of soil moisture content are sort of similar, none (with their generic calibration) read the same soil moisture content, as shown in the plot of all sensors.  (Note the GS1 Sensor is a recent addition and no data is available for the dates compared).

The only sensor that has been calibrated against true soil moisture content (gravimetric laboratory analysis) is the neutron probe.  Knowing that Field Capacity at this location and soil type should be about 40%; only the neutron probe and Aquaflex measure soil moisture content at this level.  The other three sensors measure field capacity 10% less than the true field capacity. Disconcertedly two sensors measure soil moisture content between irrigation events (the vertical rise in the traces) at or very close to wilting point – approximately 17-18%.  This is not the case; the pasture never died nor showed any sign of being close to wilting point.

What is to be taken home from the comparison?  If you want sensible and realistic soil moisture measurements the sensors must be field calibrated.  The simplest and easiest way to field calibrate is by neutron probe – click on http://www.hydroservices.co.nz/index.php?option=com_content&view=featured&Itemid=308 for more details.

Irrigator Maintenance - Sand Trap

Have you ever removed your sand trap or flushed your pivot?

It may surprise you, but this is a question we often ask when people are having trouble with sprinklers on overhangs and we are questioned in return “What is the sand trap?”

Sand trap location shown by red arrow

Every centre pivot and lateral irrigator should have a sand trap just after the last tower and immediately before the overhang. You’ll often find it directly below the boost pump for the end gun.

During normal operation, any sand and fine particles that make their way into the irrigator will travel through the spans and then settle in the sand trap. For this reason, the sand trap is designed with a ring-lock mechanism so that it can be quickly and easily removed, inspected, emptied and replaced.

Components that make up the sand trap

How much sand and fine debris is pumped through your irrigator will determine how often you will need to empty the sand trap. How quickly the sand trap fills is something you will learn from experience. Note that the amount of sand pumped out of a well can change over time and may be affected by events such as earthquakes. You should never let your sand trap fill completely as this will mean sand will go past your sand trap and continue to go down into the overhang.

If you’ve got an exceptionally sandy water-source, or you’ve neglected to empty your sand trap for some time, your overhang may be filling with sand. This adds weight to your irrigator increasing mechanical stress and in extreme cases blocks entirely and stops water from coming out of the sprinklers on the overhang. If your overhang has a build-up of sand, you can remove the plate on the end of the overhang and pump a small amount of water through your irrigator to flush the sand out.

Of course, proper maintenance and emptying of the sand trap will avoid having to flush your machine. Sand also does all sorts of other damage to irrigators. We’ll look at what sort of damage it does and how to mitigate this in a future post.

Stu Bradbury

Lindsay NZ

Good Old Spring Weather

In the past seven days we have had a bit of everything, 10 to 29 degree days, 100km per hour NW winds, rain through to hail and bitterly cold southerly winds reminding us that spring is upon us!

Whilst forecasters don’t always get the rainfall amounts correct, they are pretty accurate when it comes to predicting strong winds.  After the wind on Sunday, it only took two minutes after leaving home to see some damage caused by the strong NW winds.

With a long dry irrigation season predicted, a broken irrigator can take a long time to fix leading to reduced yields in crops and lost dry matter in pastures. This can be avoided by having a simple plan in place for windy conditions, whether it be securing pivots to a heavy roller or parking a Rotorainer in a sheltered position on the farm.

Rainfall

Measuring and recording rainfall through the season is also key, rain gauges are cheap and may help save valuable water, especially in the shoulder seasons.  Turning irrigators off for a couple of days here and there may not seem like it making a big difference at the time, but those days all count at the end of the season if water allocations are getting tight.

Weather forecasts

The forecasters are predicting cold and wet weather on Tuesday this week, perhaps an opportunity to save a round or two with the pivot if they are correct.

http://www.yr.no/place/New_Zealand/Canterbury/Ashburton/

Happy Irrigating!

Mark Fitzgibbon

Hydroservices

Variation in Electro-magnetic (EM) Readings

When talking to clients about Electromagnetic (EM) surveying of soil’s conductivity, I often get asked what a low or high EM variability is. This is quite a hard question to answer, as there are so many factors that you need to take into account. These include:

1.       The region of the country where the land to be surveyed is located.

For example, as a general rule there is a big difference between the readings we get in Otago compared to the Canterbury Plains. However within each of those regions we can also get big variations from the stony soils to the areas of heavy clay. So it is definitely site dependant within a certain range of values. In areas like Seddon and Ranfurly we have also come across salinity issues that take the EM readings to a different level all together. 

Figure 1: A vineyard in Otago on sloping ground.

Figure 2: The relatively flat, stony river bed soils on the Canterbury plains.

Figure 3: Salinity issues near Ranfurly can be visibly seen to effect crop growth and can dramatically increase EM readings.

2.       Geography and topography features.

These features can also influence factors affecting the soil depositions and therefore the EM readings. We often find that weathering and water movement over many years can create areas of higher EM readings. Topography data from an EM survey also backs up this finding. We use the VA Gateway software platform to analyse these different layers side by side, bringing all the different data together.

3.       The time of year the survey is conducted.

We take this in to consideration in our EM reports. At Agri Optics we don’t start our surveying season until the Autumn and then not until the ground has had enough rain to bring it nearer to field capacity, eliminating the effects from that summer’s irrigation. In previous blogs we have shown why surveying in Summer does not provide good data and I would be extremely cautious of the quality of data provided by anyone offering to conduct a survey for you in the summertime. The EM readings in Autumn are slightly less than you would get in the Winter and early Spring. Winter generally is the season that gives the higher readings followed by Spring. Our main concern in Winter is that the ground can be travelled on safely without damaging the crop and without getting our light weight EM buggy stuck. We continue to get very good data Spring, but as we get into further into Spring the EM season is draws to an end as farmers start  up their irrigators. At this stage we stop EM surveying, as we start to pick up the influence of the irrigation applications on the soil maps. So if anyone is still thinking about having a EM survey conducted this year, you should get on to it straight away, as time is slipping away.

When we carry out an EM survey the sensor is measuring the soils conductivity at two depths simultaneously, the shallow EM and deep EM. In a pastoral situation we recommend using the shallow EM layer to base decisions off as the crops roots are predominantly within this shallower profile. In an arable scenario where you have crops roots going deeper into the soil profile we would recommend using the deep EM layer to make your management decisions from. These readings are generally higher than the shallow EM readings as they measure deeper into the soil profile.

In the next few blogs I will go into more detail on how historic management practices and historic boundaries can sometimes come through on an EM survey and how the EM survey can be used to identify salinity issues. For more information on EM surveying or if you have any questions please contact us at Agri Optics.

~ Chris Smith