Complying with Victorian guidelines on using recycled water

Major amendments to Victoria’s Environment Protection Act are expected to be passed in 2021. The amended legislation is expected to place greater emphasis on risk assessment and community involvement than is presently the case.

This paper is intended to provide professionals and others with a standard method for calculating infrastructure requirements in agricultural water recycling proposals.

With public debt burgeoning, this method can reduce compliance costs greatly, thus enabling more efficient use of increasingly scarce finance.

Compliance with policies and guidelines for the use of recycled water in Victoria requires special design criteria. In conventional irrigation, the aim is to supply enough water to meet the peak demand in dry years, whereas for recycled water, used in accordance with the EPA guidelines, there is likely to be a water shortage in all but the wettest years.

To prevent the discharge of waste 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.

This paper describes a method for optimising the irrigation area and water storage requirements using a spreadsheet. The method is unique in that it resolves the problem of monthly rainfall distribution which has a marked effect on the storage volume and irrigation area requirements, and hence the cost of regulatory compliance.

1 Introduction

The State Environment Protection Policy (Waters of Victoria) requires that facilities for reclaimed water storage and disposal by land irrigation should be designed and constructed to contain all wastes in at least the 90th percentile wet year.

EPA Guidelines for Use of Recycled Water (Ref 1) explains that, “in order for a treatment plant to be recognised as not discharging to surface waters, that plant needs to have a management framework enabling the handling of all effluent in a 90th percentile wet year.

For a ‘normal’ reuse site, this is likely to involve storage facilities, and/or reserve land, to cater for the excess of reclaimed water caused by reduced demand during particularly wet periods.”

1.1 An important effect of this ‘90th percentile rule’ is that, contrary to conventional irrigation design, the water supply is not necessarily intended to meet the peak irrigation demand in a dry year.

However, 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. This affects many aspects of recycled water irrigation, including irrigation system design, fencing, cropping and livestock management.

1.2 Known values for the relevant variables including wastewater flows, rainfall, evaporation and soil characteristics can be used to meet EPA criteria using an Excel spreadsheet. The spreadsheet is an advanced version of the water budgeting technique described in EPA Guidelines for Wastewater Irrigation (Ref 2).

1.3 Historic ‘wet years’ with rainfall exceeding the 90th percentile can be used to calculate irrigation area and storage volume requirements. However, selection of random ‘wet years’ overlooks seasonal rainfall distribution which drives major irrigation area and storage requirement variations, even if the annual rainfall total in different years is the same.

A wet summer, for example, will reduce the irrigation requirement. In such a case, either the land area or the storage volume must be increased to balance the water budget. On the other hand, a wet winter may increase the water storage requirement but will not have a major effect on the irrigation requirement.

For example, rainfall in East Gippsland is notoriously ‘peaky’. Heavy rainfall events associated with easterly low pressure systems can occur at any time and may render compliance impossible.

2 Features and functions of the water budget model

2.1 The spreadsheet calculates the mean irrigation area and water storage volumes from rainfall and evaporation data collected at the nearest available weather station. The mean irrigation area and storage volume are adopted as the starting point for a series of 20 consecutive years.

2.2 A coefficient of 0.8 is applied to “Class A” pan evaporation to relate pan evaporation to lake or pond evaporation. Coefficients for lakes in Australia range from 0.7 to 0.8 (Ref 3). Sewage ponds, being shallow and constantly supplied with water at or above ambient temperature, probably correspond to the upper end of the range of pan coefficients.

2.3 Rainfall is then subtracted to give net evaporation, before multiplying by the pond surface area. The formula will return negative values in wet months, i.e. a negative water loss is a gain.

2.4 The first year in the series uses water carried over from the last month in the ‘average’ (start) year. Inputs and outputs are treated as a connected series with water carried over from one year to the next. The cumulative net waste volume is added to the following month.

The irrigation area and storage volume are adjusted so that water inputs and outputs are balanced in each year. In reality, the available storage volume cannot readily be changed. However, the model can be run so that storage volumes for each year in the series are calculated for a specified (fixed) irrigation area. Specifications and constraints can be added using the ‘Solver’ tool in Excel.

It is the 90th percentile of the irrigation area and storage volume requirements calculated from the 20-year series, and not a randomly selected ‘wet year’ that provides a rational basis for compliance with the EPA 90th percentile criterion.

Running the spreadsheet with the irrigation area ‘fixed’ at the 90th percentile of the series enables selection of the matching 90th percentile storage requirement. The volume will obviously vary, year to year, with the irrigation area fixed. This is due to water being carried over to the next irrigation season, especially following a wet summer.

2.5 The model is most sensitive to water inputs from the wastewater and rainfall. If evaporation data are not available for the location, a multiple of data for the nearest station can be applied in the model to match the mean irrigation requirement to the appropriate contour on the map shown on page 90 of the irrigation guidelines (Ref 2)

Mean evaporation for the nearest station is used due to the lack of data for many locations and its variability with elevation and other local conditions.

2.6 Monthly waste inflows or rainfall can be adjusted by entering a multiple. This allows future growth and seasonal scenarios to be readily tested.

2.7 A ‘best fit’ relationship derived from Table 7A of the irrigation guidelines is used to estimate the amount of rainfall that will remain in the root zone of plants for a range of monthly rainfall and evaporation totals. Rainfall that remains in the root zone is said to be ‘effective’. The effectiveness of rainfall is adjusted according to the nominal water holding capacity of various soil texture classes.

2.8 ‘Class A’ pan evaporation is related in the model to potential evapotranspiration by growing plants.

2.9 Effective rainfall is subtracted from potential evapotranspiration to estimate the irrigation requirement.

2.10 Weighted crop coefficients are calculated for each month, depending on the percentage of the land under each type of irrigated vegetation (Ref 4)

2.11 If runoff from the irrigation area is to be collected for re-use, the model calculates the area from which runoff will be received, allowing for the percentage of the land draining to a collection dam.

In some cases, both spray and flood irrigation might be used and runoff collection may not be required for the whole area. Alternatively, it may only be necessary to collect runoff from the most recently irrigated area.

2.12 Allowance is made for differences in the moisture status of soils. Essentially, it is assumed that approximately 50% of monthly rainfall that does not remain in the root zone is lost as leachate or surface runoff. This will be an underestimate for some rainfall events, given that antecedent soil moisture and other conditions vary, and an overestimate for others.

2.13 The model calculates exfiltration from saturated treatment and/or storage pond linings.

2.14 Provision is made for leaching of salt from the root zone by entering data related to rainfall, irrigation water salinity and crop salt-sensitivity.

3 Applications

The water budgeting techniques described above can be used to reduce the risk of a water shortage at the end of a dry irrigation season or, on the other hand, having to discharge or carry water over in storage between irrigation seasons.

The ability to estimate the volume of recycled water remaining in storage at the end of each month of the irrigation season is therefore valuable to recycled water irrigation operators and also to those concerned with planning and environmental risk management.

The model can also be used to estimate the least cost mix of irrigation area and storage requirements.

The water budget model is available from Max Thomas at maxthomas4@bigpond.com

References

1. Guidelines for Environmental Management: Use of reclaimed water, EPA Publication 464.2
2. Guidelines for Wastewater Irrigation, EPA Publication 168
3. Hydrology for Engineers. Ed. Linsley, Kohler and Paulhus. 3rd ed. (1982) p.150. Mcgraw-hill pub.
4. Irrigation and Drainage Practice, RWC. VIC. (1988). pp.28-33.