Getting the most out of the Effluent Calculator
Choosing a new or upgraded effluent system often involves a considerable capital spend, so being involved in the decisions about what will work best for you and your farm is crucial. This is particularly important if you've got someone doing your effluent storage calculation for you.
There is not a specific volume of storage your farm needs as it will vary depending on your farm's location, soils, size, infrastructure and your management. Take ownership of the decisions being made to ensure the final outcome is a good fit for you and your farm. And it's not all about getting the smallest storage volume, it's about the 'right' storage volume that suits your farm best.
Most New Zealand regional councils require storage volumes to be calculated using the Dairy Effluent Storage Calculator (DESC). The DESC has a library of more than 30 years of daily climate data (rainfall and evapotranspiration) for most dairying areas throughout the country, and uses this information to model soil moisture levels.
The DESC calculates how much effluent is generated each day, and determines whether soil conditions are suitable to irrigate effluent. If irrigation can take place, it calculates how much effluent can be removed from the pond. If effluent irrigation can't take place it calculates how much added effluent will enter the pond. From these calculations it can determine how much storage you will need.
There are many inputs to the DESC that will determine how much storage your farm needs.
These include soil risk, water use in the shed, effluent application depths, pump rates, milking season length, herd size, climate, catchment areas, storm water diversions to name a few. Some of these will be set on your farm, such as climate, catchment areas, herd size and season length. Others can be variable and up for consideration as a mitigation in reducing storage requirements.
You must be involved and talking to whoever is running the DESC for you - make them earn their money! Ask them for a variety of calculations or scenarios showing the impact of options. Where do you want to spend your money; on mitigations or extra storage? Get them to run a number of scenarios that help you decide which options you want to pursue.
You can cost all the different options to get an idea of what you want to pay for. These are some useful questions to ask yourself as you work through the DESC:
• Do I have any low risk soil on my farm that is not currently in my effluent block? • What would a low depth irrigation system look like on my farm? • How easy or hard would it be to manage? • What would a storm water diversion cost to install? • What reduction in storage volume would it give me? • How much water does my dairy shed currently use? • Did they measure/calculate my water use or just use an "industry average"? Using industry average will not result in an accurate calculation for your farm• What would water reduction look like on my farm? • Green water recycling can have huge savings- did my consultant consider this option?• How much storage would I need if I chose not to irrigate over calving?
When you start the cost benefit of all the options - don't forget to include the time it would take to manage your system as it's pointless to choose options that give you the smallest possible storage requirement, but are a nightmare to manage. Choose a system that fits you and your farm- both from a labour/time point of view and cost.
I have talked to many farmers who have built ponds and wished they had built them a little larger to give better flexibility around effluent irrigation.
Remember the person running the DESC is in your team and are- there to help you choose the calculator inputs that best reflect your farm and your desired mitigations. They won't be running the effluent system once built- you will, which makes your input to sizing the pond or tank crucial to the success of a large capital project which will be your everyday system for the next 20 to 30 years. Working together you will come up with storage requirements that fit your farm like a glove.
- Logan Bowler is the environment extension specialist for DairyNZ