Bucket Tests

Bucket Tests

Bucket testing is a method to test irrigator performance by filling buckets with the irrigator to determine application depth and uniformity for a given setting. This page is intended as a digital handout for training sessions. 


Bucket Test Outcomes

The aim is to ensure:

How to do a Bucket Test

Use a template. Your options are:

Check it isn't too windy - anything over five knots is too much. Rain is also going to affect the test unless you have a rain gauge set up outside the wetted area.

Ideally have pivots on the flatter ground to have a more even distribution of pressure along the spans.

Lay out the buckets as per the template although not as densely as pictured. You may put a rock or other impervious weight in the bucket to stop them blowing over. 

To make it easier, use a number of buckets that can be divided evenly into quarters, i.e. eight, twelve, sixteen or twenty. 

There should be at least one bucket per pivot span and one for the end gun. Skip the first three spans (unless it's a very short, fast pivot) as they move too slowly and the other buckets might evaporate by the time the first three spans have passed.

Note that Rotorainers and Hard Hose guns are tested the same as the travelling irrigator example. Use eight buckets across the wetted width. 

Fixed Grid is tested the same as K-Line with four buckets in a square with the sprinkler in the middle. Put the buckets at  half the wetted radius of the sprinkler or more.

Note the start time (if you want instantaneous application for a pivot, or if you are testing K-Line or Fixed Grid) and pass the irrigator over the pivot set at the application depth or speed normally used. Run K-Line tests for at least 30 minutes but ideally 45 minutes or an hour, then extrapolate out to a twelve hour shift. Run Fixed Grid tests for 15 minutes to an hour and extrapolate or interpolate to the actual run time (usually 18 to 30 minutes). 

If you have time you might test other pivot speeds (100% is a useful number to know for effluent in wet years).

While the irrigator is filling the buckets you will likely have time to lay out buckets for another couple of irrigators elsewhere to run at the same time. 

Come back when the pivot has nearly passed over and note the finish time. While waiting you can prepare your spreadsheet and start measuring buckets at the fast end of the pivot. Record each bucket in order on the sheet.

Any buckets that have blown over or have a tiny amount of water can be disregarded unless the sprinkler is visually faulty.

Once all buckets are filled you can follow the calculation on the sheet to find your results.

Make any notes on the sheet regarding breeze, pivot direction etc.


Results: Application Depth

Application depth is measured in millimeters (mm) similar to rain. This result is compared to the capacity of the soil at the time to accept that water without becoming too wet (soil water deficit, also measured in mm). If the grass becomes too wet it would result in lower grass growth, loss of nutrients and inefficient water and energy use putting on water that is damaging and unproductive. 

During the irrigation season your decision making is usually most relevant after rain when deciding whether to turn the irrigation back on. First, you need to measure the soil moisture to determine the soil moisture deficit. This is on a scale which you will have already calculated using the data for your particular soil type from SMap. If you have a fixed soil moisture probe it will already be set up to display the scale and you don't have to make one manually. 

If you have a handheld soil moisture probe you can make a scale like the one below. Find your farm and the soil type under that irrigator, download the "Classic Fact Sheet" and look for the Profile Available Water (PAW) for 300mm depth (rooting depth for grass) which is the total amount of water the soil can hold and determines how big your scale is. Measure the soil when it's bone dry and then saturate with water and measure. This gives the highest and lowest soil moisture reading on your particular soil moisture probe, and you can interpolate the figures in between.

The scale you have made runs from wilting point (when it's so dry the plants are dying), to stress point below which the plants aren't growing well, to field capacity (where water has displaced too much oxygen in the soil and plants aren't growing well) and then to saturation when the soil can't take any more water.

The goal is to apply water to keep it below field capacity but well above the stress point. To do this you need to know what your soil moisture probe is telling you, and how much each irrigator is applying. 

If you determine that there is a soil moisture deficit greater than what the particular irrigator will apply then you need to turn it on to provide the right amount of water at the right time. Here's an example of the scale with the Campbell Scientific HSII soil moisture probe and Ngapara Silt Loam soil: 


Irrigation Scheduling Decision:

If you measure a reading of 33% then you have a soil water deficit of 22mm which means you can apply up to 11mm before reaching field capacity. Most pivots and fixed grid will achieve this low rate and if you have bucket tested that a pivot applies 9mm you would turn it on if there isn't any other rain forecast within three days (the accurate range of forecasting), but K Lines mostly will apply more than this (albeit with a slower return) so they may over-apply water.

Alternatively, if you measure a reading on the soil moisture probe of 38% volumetric water content (VWC%) you can see the soil water deficit is 13mm and therefore you can only apply 2mm of water to reach field capacity. Therefore you would decide to wait a day or two to measure it again. The rate at which the soil dries out is measured at the Windsor Evapotranspiration Site which could be drying out the soil by up to 7mm per day. This would be worse on a windy hill as the Windsor site is down in a valley. In practice irrigation is usually constant in North Otago during the summer months unless it has rained, and more care is required during the shoulder seasons to ensure over watering doesn't occur.


Results: Distribution Uniformity (DU)



Distribution uniformity (how evenly the irrigator is watering) is an indicator of irrigator maintenance for some types of irrigator obtained by comparing the lowest quarter of results to the overall average and expressing it as a percentage. It should be over 80% and anything less indicates a problem with pressure or flow which needs investigated. This works well for pivots, and it's important to perform maintenance such as unblocking nozzles and ensuring full operating pressure before doing the bucket test. It's a waste of time bucket testing something you already know is faulty. 

DU for K-lines and fixed grid is determined entirely by sprinkler placement and wind, so it isn't a relevant measure. However, bucket tests can still measure application depth for these.

If the irrigator fails on DU it could be a number of causes which may or may not need your water company to check. To try resolve it yourself check for blockages, leaks, or failed restrictors (fast spinning sprinklers are a give away). Check your flows and pressures are normal, that the pump is working as it should and the screens aren't blocked. Check the nozzles are installed as per the design - it's common to find incorrect nozzles replacing the original which has been lost somewhere when the dropper blew off.


Instantaneous Application Rate

This is most relevant to effluent consent applications and therefore mostly to Otago farmers at the moment who are applying for consent to discharge effluent. 

Instantaneous application rate is defined by the Otago Regional Council (ORC) as the application depth if the irrigator ran in the same place for an hour. To calculate this we also need to time how long the irrigator takes to pass over the bucket and then extrapolate to an hour. 

For example, if the pivot took half an hour to pass over the buckets and applied 9mm it would have an instantaneous application rate of 18mm.

ORC planning staff are currently seeing red flags if an irrigator applying effluent applies more than 25mm per hour.


Testing Variable Rate Irrigation (VRI)

The first thing to consider when testing VRI pivots is to select an application depth at least three times the minimum application depth (100%  speed) available on that system to ensure that the pulsing of the sprinklers doesn't create an anomaly in the test. Generally this will need to be at least 9mm. If it's an effluent pivot subject to consent it might not be more than 12mm.

Check the VRI is working properly with the usual test.

A variable rate pivot will apply different rates along the length of the pivot so you will need to either pick a direction for the pivot where all application rates on the VRI plan are the same, or create a plan where it applies the same amount in all directions for the purpose of bucket testing.

Don't turn the VRI off during testing because some VRI pivots are nozzled larger and don't have good distribution uniformity when turned off, particularly those with swing arms.

Zimmatic New Zealand have offered to provide further advice for those who need it and will produce guidance shortly which I will post here.

Contacts

Feel free to contact me if you have questions, require clarification, spot an error, or would like to contract out your bucket testing.

Sven Thelning
sven.thelning@gmail.com
021 027 86087


Zimmatic contact for VRI advice:

Sarah Elliot
06 212 0550
021 811 266