The calculator was originally developed to identify the storage requirements to successfully practice deficit irrigation over the past 30+ years on high risk soils. Changes were later made to determine storage requirements for deferred irrigation on low risk soils.

The calculator assumes everyday there is sufficient effluent that effluent irrigation takes place. Failure to irrigate on every suitable occasion and apply the nominated volume of effluent in early or late lactation will result in greater storage requirement than the calculator predicts. No allowance for solids build-up in ponds has been allowed for in the calculations and should therefore be considered when choosing storage volumes or dimensions.

The Dairy Effluent Storage Calculator (DESC) runs a daily, soil water balance, over 30+ years of daily climate data. This soil water balance allows it to determine soil moisture levels and depending on soil characteristics determines when irrigation could have occurred.

To determine how much storage is needed requires the daily effluent generation (deposited dung and urine, wash water and rainfall on catchments) to be calculated, and any volume of effluent irrigated (removed from storage) each day, to calculate the days storage requirement. Yesterday’s pond volume plus today’s effluent generation, minus today’s effluent irrigation (if any), equals today’s end volume. The DESC simply calculates the daily pond volume using the above calculation and displays the seasons peak volume for every season that the DESC has climate data for.

Effluent irrigation days are determined by soil characteristics.

High risk soils require a soil water deficit equal or greater than effluent application depths to allow irrigation to occur. This means during times of the year when evapotranspiration rates are low, sufficient soil water deficits to allow effluent irrigation to take place can be few and far between. Using low depth irrigation will help a farm utilise some of the smaller soil water deficits, giving more irrigations days per season, thereby reducing required storage volumes.

On low risk soils, effluent irrigation could take place any day the soil is not draining (at field capacity or drier). This gives many more irrigation days per year than on high risk soils and subsequently greatly reduced storage requirements - often 80 to 90% reduction in required storage. Due to piston flow/matrix flow (where the effluent drains down through the entire soil matrix as opposed to finding faster drainage networks (preferential/bypass flow)) in these low risk soils, any irrigation event should be held in the soil profile until eventually being expressed down beyond the root zone. Irrigating in accordance with the above criteria should result in most effluent being retained for plant growth and therefore prevent most effluent from escaping below the root zone.

A change in this new web based version has also allowed the option of applying shallow depths to low risk soils which are actively draining (to a maximum of 10mm drainage). The user gets to choose how the 10mm excess is made up between rainfall and effluent application. For example, if the user could apply 3mm depth of FDE, they could do this on days where there was no mare than 7mm rainfall above field capacity.

As with all models, the outputs are only as good as the input data/information. Its imperative realistic input data is used and attempts are made to verify input data rather than just use industry averages. Unrealistically high pump rates and unrealistically low irrigation depths and water use only end up with dissatisfied farmers with undersized storage.