Effluent Storage Calculator NZ

This works out how much effluent storage a dairy shed actually needs, and the answer usually surprises people because the cows are not the biggest contributor. Wash water and rain both go into the same pond, and on a shed with an open yard the rain is often the larger of the two. Every square metre of uncovered yard collects whatever falls on it, and so does every square metre of roof that drains into the system rather than away from it, so a wet month can put more into the pond than the herd does. That is why roofing a yard or diverting clean roof water is frequently the cheapest way to solve a storage problem, and why a pond sized on cow numbers alone comes up short in exactly the season you most need it not to. The number of days you have to hold is the other half, and it is not a national figure: it depends on how long your soils stay too wet to irrigate onto without ponding or run-off, which is a very different number on free draining Canterbury country than on heavy Waikato clay. Your regional council sets that requirement, so this page takes the days from you rather than assuming them.

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L
L
m2
m2
mm
1,531,500 L (1,531.5 m3)
storage needed over the holding period
From the cows607,500 L
Wash water540,000 L
Rain384,000 L
Rain as a share25.1%

Diverting the roof water alone would take 96,000 L out of the requirement without touching the herd or the wash system. Storage volume only. A pond also needs freeboard above the working level, and your regional council sets both the holding period and the design requirements.

No default holding period, effluent volume or wash water figure is supplied as a standard. The holding period comes from your regional plan, and effluent and wash water are best measured on your own shed. The rainfall arithmetic here is exact: a millimetre of rain on a square metre is exactly one litre.

How it works

Effluent from the cows is the herd size multiplied by the litres each a day and the days. Wash water is the daily figure multiplied by the days. Rain is the catchment area multiplied by the rainfall, and that conversion is exact rather than approximate: one millimetre of rain falling on one square metre is exactly one litre, so 320 millimetres on 1,200 square metres of yard and roof is 384,000 litres. The three are added for the total, which is then shown in cubic metres as well, since ponds are specified that way and a cubic metre is a thousand litres.

Why the roof line matters

Roof water is clean until it enters the effluent system, at which point it becomes effluent and has to be stored and irrigated like everything else. Diverting it costs guttering and a downpipe. Storing it costs pond. The comparison is rarely close, which is why the roof area is a separate input here rather than being bundled into the yard: seeing what it contributes on its own is usually enough to make the decision.

Worked example

A 450 cow shed producing 15 litres of effluent a cow a day, using 6,000 litres of wash water a day, and holding for 90 days, generates 607,500 litres from the cows and 540,000 litres of wash water. With 900 square metres of uncovered yard and 300 square metres of roof draining in, and 320 millimetres of rain over the period, another 384,000 litres arrives from the sky. The total is 1,531,500 litres, or 1,531.5 cubic metres, and the rain is 25.1 percent of it. Diverting just the roof water would take 96,000 litres out of the requirement without touching the herd or the wash system.

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Data sources: no holding period, effluent volume or wash water standard is supplied, because the holding period is set by your regional council and the other two are best measured on your own shed. The rainfall to volume conversion is exact by definition: one millimetre on one square metre is one litre.

Frequently asked questions

What goes into an effluent pond besides effluent?

Wash water and rain, and on many sheds the rain is the larger of the two. Every square metre of uncovered yard and every square metre of roof that drains into the system collects rainfall, so a wet month can put more into the pond than the cows do. That is why roofing the yard or diverting clean roof water is often the cheapest way to reduce the storage a shed needs, and why a pond sized on cow numbers alone comes up short.

How many days of effluent storage do I need?

Enough to get through the longest stretch when the soil is too wet to irrigate onto, which depends on your soils, your climate and your regional rules. Deferred irrigation means holding effluent until the ground can take it without ponding or run-off, and the number of days that requires differs enormously between a free draining Canterbury soil and a heavy Waikato one. Your regional council sets the requirement, so check your plan rather than a national figure.

How much wash water does a dairy shed use?

It varies widely with plant size, whether the yard is washed with a hose or a flood wash, and how the system is set up, so no default is supplied here. Measuring it is straightforward: read the meter on the wash system over a few days and divide by the milkings. A figure taken from your own shed is worth far more than any average, because wash water is one of the largest and most controllable inputs to the pond.