How much recycled water storage is enough in Victoria?

The Environment Protection Act (2017) introduced a General Environmental Duty (GED) which requires anyone in Victoria who is engaging in an activity that may give rise to risks of harm to human health or the environment from pollution or waste must minimise those risks, so far as reasonably practicable.

Recycled water irrigation schemes must therefore have the necessary infrastructure e.g., water storage volume and irrigation area, to comply with the GED and the applicable EPA guidelines.

EPA Victoria Publication 1911.2 Technical Information for the Victorian Guideline for Water Recycling (March 2021) requires that recycled water storage and irrigation systems should be designed and constructed to contain all wastes up to the 90th percentile wet year. An important effect of this policy is that, contrary to conventional irrigation design which aims to supply growing plants with sufficient water in a dry season, there is likely to be a shortage of water in years drier than the 90th percentile wet year if no supplementary water supply is available.

In effect, the 90th percentile policy ‘builds-in’ a water shortage so that the irrigation water demand will exceed the available supply in most years. This affects many opportunities for the beneficial use of recycled water. Regulatory and agronomic risks are important considerations for recycled water irrigators. Water balance modelling is one of their decision-making tools. For example, it feeds into irrigation system design and operations, cropping, fencing and livestock management.

EPA Publication 1911.2 outlines a preferred water balance model (EPA Model) to determine the size of the irrigation area and water storage components of recycled water irrigation schemes. The EPA Model is used to calculate the storage requirement from a random, isolated ‘wet’ calendar year. Sometimes inapt rainfall statistics such as monthly 90^th percentiles are selected.

In any case, a calendar year splits the irrigation season . Variations in seasonal rainfall distribution and excess water or storage overflows are not considered by the EPA Model. These features of the EPA Model create uncertainty about compliance from the outset. However, this paper describes an alternative model that addresses these issues using a series of linked financial years.

It can be shown that the water balance modelling recommended in EPA Publication 1911.2 does not guarantee containment of recycled water ‘up to the 90th percentile wet year’. Referring to Section 156 of the Environment Protection Act (2017), it would appear that infrastructure managers are expected to meet specified obligations that can only be determined with hindsight e.g., an extreme weather event.

Equally, Section 157 of the Act would place EPA in an invidious position should the manager contend that the non-compliance resulted from adherence to EPA guidelines.

Irrigation can be regarded as merely a substitute for rain. Every gardener knows that watering isn’t necessary in a wet season. Obviously, a ‘wet’ year cannot be accurately predicted but it is also important to realise that above average or 90th percentile rainfall in a given year does not necessarily reduce irrigation water demand.

The seasonal distribution of rainfall is even less predictable than the annual total but it is a critical determinant of the irrigation water demand. The storage requirement depends largely upon the volume of inflowing water and the seasonality of rainfall. The result is that operational compliance with the EPA 90th percentile wet year policy can only be determined retrospectively which is not useful for irrigation system design purposes.

Models which adopt an arbitrary wet year or a multiple of mean rainfall can lead to a costly and operationally difficult overestimate of the storage requirement. On the other hand, underestimating the storage requirement may lead to unlicensed releases of effluent, regulatory action and remedial costs. To prevent the release of recycled water to the environment in all years drier than the 90th percentile wet year, it is necessary to find the optimum balance between water storage capacity and the area of land to be irrigated.

In reality, because additional opportunities for irrigation are not usually available nearby, excess water is released to the environment instead of being held until the following irrigation season. Applications to the EPA under Section 30A of the Environment Protection Act 1970 for emergency releases of recycled water from irrigation storages to the environment have become almost routine.

An alternative water model

The aim of this paper is to describe a method for optimising the irrigation area and water storage volume requirements.

The ‘Advanced Water Balance Model’ described herein is a modified and extended version of the original EPA model. It is unique in that it resolves the problem of monthly rainfall distribution, which has a marked effect on the water storage volume and irrigation area requirements.

The correlation between rainfall and the storage requirement is covered in the Advanced Model by water being carried forward in storage when irrigation demand falls in a wet irrigation season. This feature provides a more realistic indication of potential exceedances of the 90th percentile ‘wet’ year criterion than the original EPA Model.

EPA Publication 1911.2 allows alternate water balance modelling to be used provided that:

• The modelling provides a better or equivalent solution.
• Better or equivalent environmental outcomes and factors of safety as those outlined in EPA Publication 168 are obtained.
• An EPA agreement should be sought for use of alternative water balance models that vary significantly from the one recommended in EPA Publication 168 or that are not widely used by irrigation practitioners.

The Advanced model functions cumulatively and calculates the storage requirements for 20 linked financial years. This redefines the ‘year’ placing the irrigation season conveniently so (hypothetical) water in excess of the irrigation requirement can be carried over in storage to the next irrigation season. The 90^th percentile storage requirement calculated by the Advanced Model does not necessarily correspond to a 90^th percentile ‘wet’ year due to water carried over between irrigation seasons.

The Advanced Water Balance Model treats rainfall as one of the variables that determine the storage requirement, regardless of the annual rainfall total or its statistical ranking in the series. Broadly speaking, the Advanced Model uses rainfall and evaporation data for 20 consecutive financial years to calculate the irrigation demand.

If the volume of water in storage exceeds the irrigation demand, it is carried forward in storage with continuously accumulating influent to the following irrigation season. The MS ‘Excel’ model interpolates but returns a 90th percentile storage requirement which is close to the third-ranked volume in the 20-year series.

The Advanced Model has some crucial advantages:

• Calculates the 90th percentile storage volume from data for a series of consecutive years.
• Accounts for variations in seasonal rainfall distribution.
• Includes a salt leaching fraction.
• Includes irrigation runoff returns, if required.
• Solves the storage requirement with either a fixed or variable irrigation area.
• It can help to estimate the cost of various storage and irrigation area combinations.

The Advanced Model is not new to EPA or the water industry. Many works approval applications were assessed and approved using earlier versions of the Advanced Model. Calibrations were done with an earlier version of the model for a working site in Gippsland. The modelled storage volumes at various times during the irrigation season were found to closely match irrigation records for the site. That exercise was repeated at another site with similar results which are reported herein.

In the absence of a standard EPA (advanced) water balance model, it would be relatively simple to under or overestimate the storage requirement, unintentionally or otherwise, to reduce or increase the cost of the works. Selecting a year with total rainfall close to the 90^th percentile but with low rainfall during the irrigation season would substantially reduce the storage requirement and/or the irrigation area.

Conversely, a 90th percentile ‘wet’ year with a wet irrigation season would require relatively more storage capacity. Incrementally increasing the irrigation area in the model identifies the minimum storage requirement, i.e. further increases do not reduce the storage volume. The model can be used to assist with finding the most cost-effective combination.

A core irrigation area may be used to manage the built-in water shortage that results from the EPA 90th percentile containment policy. The core irrigation area might consist of perennial irrigated pasture while a reserve or backup irrigation area might have salt-tolerant trees planted in rows or ‘alleys’ adjacent to the pasture.

The trees would be irrigated only in years when the storage lagoon water level is higher than average, especially later in the irrigation season. Trees between pasture alleys extract moisture and (potentially) nutrients from deeper in the soil profile. Growing trees may also qualify as a carbon offset against emissions to the atmosphere from the wastewater treatment process.

During or following a 90th percentile wet year, excess water must either be stored for future irrigation or released offsite, subject to EPA approval. Adopting the Advanced Model places irrigators in a better position to request approval to release water when the storage water level (or volume) reaches the approved 90th percentile, rather than waiting for the moving annual rainfall aggregate to exceed the 90th percentile. Alternatives to releasing excess water may become available before an emergency situation develops if the storage limit is clear and unambiguous.

Timing releases of excess water to coincide with high flows and relatively low water quality in the receiving environment would be consistent with section 3.2 of the EPA Technical Information Risk Assessment Framework.

Apart from reducing the potential for ecological disturbance, carefully timed and approved releases of excess water would avoid the regulatory risk associated with ’emergencies’ that might be deemed foreseeable. A standard method for determining the storage requirement would improve regulatory guidance while reducing risk to water recyclers and the environment.

The full paper with supporting tables and statistics is available here.