Dairy Effluent Storage Calculator (DESC)

The DESC is now accessed online. The original calculator which required the programme to be downloaded onto a computer has expired, and the online version is now used exclusively. The upgrade was made as the software the old version used is out of date.

The calculation is not exact – some parts of the original calculator were written with the coding and functionality available, some things like calculating the 90^th percentile used different calculations as there were no standard functions to calculate this available – now a simple, standard function is in use, and the end result approximately the same, but not exact.

The soil risk framework was also developed after the calculator had been developed, and the inclusion of a limit on drainage above an agreed 10mm for low risk soils has been included to align this with the FDE code of practice.

We recommend that final calculations are done by Dairy Effluent WOF assessors, Accredited Effluent System Designers or other suitably experienced individual. There are a number of critical data that should be checked to ensure the calculation is valid, also there are scenarios that can be run that if implemented could reduce storage requirements, and enough knowledge is needed to do these appropriately.

Changes to the DESC in the new online version 29/08/2021

In most areas of the calculator, the coding of the original calculator remained fit-for-purpose and was transferred through to the new version. Users should expect similar results when comparing the original and online calculator reports. Provided below is a summary of improvements in the new online version:

• An upgrade to the DESC has resulted in a significant change to how storage is calculated on low-risk soils. See below for further details of this change.
• Multiple dates or periods can now be selected for milking days; stormwater diversions (yard, feed pad and animal shelter); and for non-irrigation days.
• Multiple irrigation depths and associated pump rates can now be entered. Simply enter an application depth and the daily volume that can be irrigated.
• A bladder option is now available under storage options
• The emergency storage calculation has changed slightly. The old version used several fixed values as part of this calculation. Most of these are now farm-specific in the upgrade, providing better representation for each farm.
• 90th percentile calculation has been corrected. Due to limitations at the time, the old version used a calculation to closely approximate the 90th percentile.
• Farm files will no longer be able to be saved to a user’s computer. They are now saved online within the user’s account.
• Files can be shared online with other users. When sharing files the recipient can be given either read or write access. As the name suggests for these sharing functions, the file writer maintains access to the file
• Files can be assigned to another user. If this is done the file is removed from the file writer's account and shifts to the recipient's account. If access is still required a copy needs to be made before assigning it to another user. Click copy and give the file a new name and save, then assign the copy.
• Company logos can be uploaded to the DESC for printing on the PDF reports
• Likewise, if a user is WOF certified or the company FDE design accredited these logos can be printed on the PDF reports

Over the past three years the Dairy Effluent Storage calculator (DESC) has been in development to produce a web-based version. Through this change process it has become apparent that the existing calculations in the DESC could be changed to better reflect industry guidelines when irrigating to low risk soils, and meet the requirements of the soil risk framework.

The changes implemented to meet the soil risk framework will reduce storage requirements on calculations on low risk soils at application depths less than 10mm, and increase storage requirements on calculations on low risk soils at application depths greater than 10mm.

Fundamentals of the DESC

High Risk soil irrigation

Irrigation to high risk soils is restricted by the available soil water deficit (SWD). For irrigation to take place, SWD needs to be greater than application depth. With only small SWD’s generally available during wetter colder months, options for irrigation are few and far between unless very low application depths are used. The tradeoff is, low application depths give more opportunity to irrigate, but the ability to irrigate reasonable volumes becomes difficult. As an example, an application depth of 2mm requires twice the area to be covered in one day as an application depth of 4mm, so either significant investment into low depth irrigation infrastructure to cover large areas on a daily basis, or a significant requirement of labour to shift the irrigation equipment multiple times per day is required. By nature of the calculations, the user was encouraged to apply shallower depths than previously being achieved.

Low Risk soil irrigation

Low risk soils can be irrigated any day following a drainage day i.e. any time the soil is at or drier than field capacity. Therefore, many irrigation days become available- there is no need to wait for a suitable SWD to become available, we just need to wait 24 hours post a drainage event. The Massey University version of the DESC has never had a depth restriction on low risk soils, so regardless of application depth entered into the DESC the storage requirements will be the same.

Since the release of the low risk option in the Massey University version of the DESC (approx. 2011), the soil risk framework has been released (see Figure 1 below).  Included in the soil risk framework is a restriction on application depths of 10mm on low risk soils when they are at field capacity. A 10mm application onto a low risk soil at field capacity will induce a drainage event of approximately 10mm. As application depths and subsequent drainage depths increase, so does the risk of bypass flow. The 10mm depth was deemed acceptable on low risk soils due to their drainage characteristics “treating” the effluent as the majority of the applied effluent passes through the entire matrix as opposed to bypass flow.

The change in calculation

As mentioned above, a 10mm drainage event was deemed acceptable on low risk soils, so the user of the DESC now has an input to the makeup of the 10mm drainage. The combination of rainfall and effluent irrigation added together cannot produce drainage greater than 10mm. So, as an example, if a user decides the farm can have irrigation infrastructure that can irrigate 4mm of FDE per irrigation event, this can be done any day post a field capacity day that no more than 6mm of rain falls.

Multiple application depths and associated pump rates can now be entered. The result of this is more irrigation days become available and subsequently reduced storage requirements.

The 10mm maximum drainage is now also incorporated into depths above 10mm (previously no impact on storage requirements with the Massey University version), so, if a 11mm depth is entered a 1mm SWD is needed before irrigation can take place, 15mm depth requires a 5mm SWD etc.

The result of this change on low risk soils is less storage requirements than the Massey University version when using depths less than 10mm, and greater storage requirements when using depths greater than 10mm.