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Water Balance for Land Application of Domestic Effluent
 

On-site wastewater planning tools

1.   Water balance

What is a water balance?  Where is it used?


A water balance is  a very simple calculation to balance all the inputs affecting a wastewater system and all the outputs returning the hydraulic load (the water) to the hydrologic cycle.  Perhaps the easiest way to visualise what is meant by water balance is to think of our wastewater system as a bucket as shown in Figure WB1.

The wastewater from the house includes all those contributions shown in Figure S1 under the 'septic tanks' page.  The volume of wastewater generated by the occupants will depend upon many factors, over which they have some control.

The rainfall is the use of one of the rainfall statistics for the local area.  These data can been downloaded from the Bureau of Meteorology's website (www.bom.gov.au) and some simple statistical analysis conducted - we'll deal with this later.

Another term we need to define is "retained rainfall".  When rain falls on the ground at reasonably high intensity (rate), some water will enter the ground (absorption rate) and some water will runoff (runoff rate).  The water that is absorbed by the soil is simply the difference between rainfall and runoff. The runoff rate depends upon rainfall intensity and the nature of the topsoil, slope and vegetation.

So, our inputs are simply   Wastewater (Ww)  +  Retained rainfall (Prf)

The evapotranspiration (Et) is the water that is returned to the atmosphere as humidity from the transpiration of the vegetation and the evaporation from the soil.  The combination is slightly less than the 'pan evaporation' as reported by the BOM because of shading of vegetation and the complex surface conditions compared with an open water surface.  We will discuss the use of a 'crop factor' to apply to the pan evaporation.

Drainage is the downward movement of water, out of the root zone that replenishes the soil moisture and drains towards the groundwater, or deep within the soil profile. The drainage rate is dependent upon soil type and soil depth.

Thus, the outputs are:   Evapotranspiration (Et) +  Drainage (D)

The simple arithmetic is:   Monthly water = (Ww + Pr) -  (Et + D).

If we measure all these variables as millimetres depth, the result is simply in millimetres.

2. CONVERSION OF VOLUME TO DEPTH

The metric systems allows us to do some very simple conversions.  One litre of water is the same as one millimetre of water over one square metre.  So when it rains, say 10 mm over your roof area of 300 m2, then 3000 L of rainwater will be added to your tanks. If your house generates 600 L/day of wastewater, and you live in a relatively dry region,  and get winter evapotranspiration and drainage of 6 mm/day, then you need 100 m2 of irrigation area to adequately dispose of all the wastewater.  I have used the winter values otherwise we would design to a hot summer and the system would fail in the winter when evapotranspiration is significantly less.

From now on, we can use litres and millimetres to describe our inputs and outputs, and square metres to describe the land application area.

3.   WATER BALANCE VARIABLES
The example used above showing that wastewater generation of 600 L/day required an irrigation area of 100 m2  when we had combined drainage and evapotranspiration of 6 mm/day in winter was just too simplistic.  That calculation assumes that there is no added rainfall during that period, and that the calculation is identical for Alice Springs, Cairns and Hobart.  That is not the case and we need to modify our equation for each and every month (or day) of the year based upon historic rainfall and evaporation for our specific locality.  We are going to modify our water balance to adjust for typical local conditions.

Measurement of evaporation

Weather stations provide a range of measuring devices at a particular locality.  This photograph shows a weather station at a dam in southern Queensland.  The Class A evaporation pan to the left is manually operated daily to allow the calculation of the depth of daily evaporation from an open water source.  While the wire screen may impart some imperfection to the measurement, access by birds and kangaroos has to be prevented.  A raingauge is seen in the right foreground, a large diameter catch surface. Often rain gauges are fitted internally with tipping buckets to measure the intensity of the rainfall - millimetres per hour.  The white box is a Stevenson Screen - a special white painted, slatted box, sited 1.2 m above the ground in which temperature and humidity are measured, now mostly electronically, but previously as a daily manual reading of a thermometer and a wet/dry bulb thermometer.

Since the evaporation (Eo) from an open pan is consistent with evaporation when there is no hinderance from available water, the conversion to evapotranspiration (Et) is calculated by applying a 'crop factor' (Cf) to the open pan value. 

Evapotranspiration (Et) is the combined transpiration from plants and the evaporation from the exposed soil.

Thus, Et = Eo x Cf. 

For most lawns and gardens, the crop factor is less than one, higher in summer when there is more atmospheric heat to drive the evaporation, and lower in winter when low temperatures reduce evaporation.

For most on-site wastewater modelling, average daily pan evaporation is used (available from Bureau of Meteorology) multiplied by a summer crop factor of 0.8 and a winter crop factor 0f 0.6.

Evaporation from plants

We know that plants transpire water, moving water from their roots, through their stems to the leaves from where small cells within the leaves (stomates) provide an interface with the atmosphere.  When the atmosphere is drier than the water vapour in the stomates, that water vapour evaporates (moves from the plant into the atmosphere).  This movement then 'sucks' up water through the plant.  The process removes water from the soil around the roots.  While the actual process is more complicated than that, the simple explanation will suffice for this discussion.

We can measure the evaporation from plants, although the exercise has a few drawbacks.   As shown in the photograph, you place a plastic bag over a bunch of leaves and secure with a string tie.  After some time, depending upon whether the bag is in the sun or not, you will observe water droplets on the inside of the bag and perhaps some water accumulating in the bottom of the bag.  The sunlight is the driving force, 'sucking' water from the soil up through the plant. 

Because the water cannot evaporate through the plastic bag, the humidity increases which in turn slows down the evaporation.  You know that feeling of 'sweating' on a very humid day - the same is occurring in the plastic bag. You could weigh the water, estimate the proportion of the plant that you had captured in the plastic bag and estimate the amount of water that the plant would transpire over a period. 

 

 

4.   RAINFALL STATISTICS

Which rainfall statistic we use depends upon the level of risk the authorities will allow us to have.  It is not possible to have a system that will NEVER fail because we are relying upon the natural return of the wastewater to the hydrologic cycle and a series of wet and low temperature periods is likely to thwart all efforts to avoid a saturated soil profile.  Thus, we need to implement measure that will minimise the risk of failure while avoiding negative impacts on human and environmental health.  No mean issue!

Rainfall:   The Bureau of Meteorology website provides daily and monthly summaries of rainfall for a large number of towns.  A quick look at "Climate Data Online" (http://www.bom.gov.au/climate/data/)    and you will see you can choose 'daily rainfall' or 'daily weather observations'.  Your choice will depend upon the type of model you are running.  For most purposes a monthly model will suffice, providing a reasonably conservative model for on-site wastewater estimation.  You can download the data and open into a spreadsheet.  Some statistics are already provided in the 'weather and climate - monthly statistics'.

The simple terms you need to understand are:

mean or average    calculated by dividing the total of all observations by the number of observation
median                    rank the observations from highest to lowest, and find the mid point value (half the number of observations).
percentile rank        rank the observations from the highest to the lowest and find each 1/100 value.   So the 90th percentile is 10% down the number of observations from the highest.  It is also the value that is either equalled or exceeded only 10% of the time.   So the 50th percentile is the same as the median because it is the mid point in the number of ranked observations.  The 50th percentile is the value that is equalled or exceeded 50% of the time - thus a failure rate of 50%, or one time for every two.

The rainfall that we record today is independent of the rain that fell yesterday, or the rain that will fall today. While there may be some seasonal patterns, the chance event of a particular amount of rain is random as far as the water balance modelling is concerned.  Just because there was 124 mm in December last year, doesn't mean anything about the rain we will get in December this year.  Hence the modelling is done with rainfall statistics that have some risk attached.  Above, we said that median rainfall occurs 50% of the time - maybe.  But median is not the same as average, since medium is simply the midpoint in a number of observations, while average is the arithmetical calculation of the total amount divided by the number of observations - definitely not always the same, but could be.

The NSW Environment & Health Protection Guidelines On-site Sewage Management for Single Households (DLG et al., 1998) suggest that for a monthly water balance model, the median monthly rainfall values are sufficient.  Obviously, the author/s of that document failed to understand that the summation of median monthly rainfall may be entirely different to the median annual rainfall.  Calculations performed by Dr Robert Patterson, Lanfax Labs, indicate that for many towns in NSW, the summation of the median monthly rainfall amounts to less than the 30th percentile annual total.  That means the when the median monthly values are applied to a water balance model, the model has a risk that the system will fail seven years in every ten - hardly a rational target (see Rainfall Statistics page)

The various statistics that may be used for such a water balance are set out in a Technical Note prepared by Dr Patterson, available through this website at TECHNICAL DOCUMENT -WATER BALANCE.   There is a limit to what statistic is used.  To use too high a percentile simply increases the cost without providing much in the way of reduced risk.  It is necessary to consider that failure of a land application area may be dependent on the prior treatment of the wastewater. Subsurface disposal of wastewater may be the preferred option when one considers long wet periods since the final treatment still occurs subsurface, whereas surface applied effluent has the potential to runoff the site with the surface runoff. 

5.   WATER BALANCE MODELLING

Most modelling is done using monthly data as a conservative approach to assessing the return of water to the hydrologic cycle.  The aim of the modelling is to mimic all the variables shown in Figure RS-2, allowing for rise and fall in soil moisture within the acceptable levels for the soil texture class, avoiding overflowing the system into the wider environment - that's a fail.

  The land application area has to accept both rainfall and wastewater, although some rainfall will result in runoff when rainfall intensity exceeds infliltration rate.  The losses from the land application area are evapotranspiration, interception store (water evaporating from leaves without reaching the soil), and deep drainage.

If these inputs and outputs do not cancel out (equal zero) then either some water has to be stored in  'wet weather storage' or carted off-site.  Dealing with the return of the effluent in the wet weather storage to the land application area is possibly a more difficult operation than dealing with the original effluent.

In accordance with Figure WB-2, rainfall may never reach the soil - evaporating from leaves of grasses, shrubs and trees. This process happens when we have small recorded rainfall events of less than 2 mm, but also happens to a small proportion of rain in higher falls.

As water infiltrates into the ground, it moves by drainage in the large pores and by capillary action in the minute pores.  This movement of water is referred to as percolation and can be measured with special devices and assigned a percolation rate.  As the water moves downward through the soil, plant roots are able to access that water and return that water to the atmosphere as evaporative losses. 

Further downward movement of this drainage water deeper into the soil profile becomes out of reach of the plant roots and is called 'deep drainage'.

At other times, water in the soil, within about a metre of the surface, may return to the surface by capillary flow and be lost from the soil surface, or again be absorbed by the plant roots.

Now that we know what variable are involved we can decide upon the calculations that we need to perform for each month to determine the land application area (m2)

The inputs are:   monthly wastewater volume (L), monthly retained rainfall depth (mm) (rainfall minus any calculated runoff - usually as a proportion of rainfall), and the unknown land application area (m2).

The outputs are evapotranspiration (open water evaporation multiplied by a crop factor), deep drainage (losses beyond the root zone and capillary fringe), and an allowable storage component within the soil profile.

The outcome is a matter of juggling varying land application areas until the monthly calculation results in zero.  Easier said than done, although spreadsheets make the calculations easier.

Now we know what variables are required, we need to decide which monthly statistic we will use.  Unfortunately, the lack of readily available data means that open pan evaporation (Eo) has to be average monthly data as daily data has to be sourced directly from Bureau of Meteorology rather than through the website.  The rainfall data are available from the website (http://www.bom.gov.au/climate/data/) for some prepared statistical values, or as raw daily or monthly data for the period the station has been recording.  We will examine these latter values from which we can derive percentile values that meet our risk requirement.

6.   SENSITIVITY TO INPUT VARIABILITY

So let us assume that we had a reliable mathematic equation to undertake monthly water balance modelling, which takes us back to Section 1.

Rainfall + Effluent = Drainage + (evaporation x crop factor) + Runoff + Change in storage in soil or trench    Equation 1.

Equation 1 is run for each month with relevant changes to the inputs.  Excess water may be stored within the soil (for irrigation) or the storage capacity within the trench. Perhaps the latter needs some explanation because excess monthly effluent must be stored somewhere.  The idea of a special wet-weather storage tank is just asking for public health and hygiene problems and serious management issues with regards to getting that stored effluent out into the land application area. Such management is well beyond the capacity of the household to perform; hence the public health issue.  The alternative is to have excess effluent stored in the soil for irrigation systems, or stored within trenches.

To test the sensitivity of the inputs, we accept that excess effluent can be stored in the soil up to a maximum of about 120 mm per metre depth of soil.  That value is approximately the same for loams and clays.  For a soil with a porosity of about 40%, we can stored about 50 mm of effluent without seriously overloading the soil. We therefore set our target 'in-soil' water storage in the modelling to 50 mm of effluent.

The same water balance model, as used under the heading "RAINFALL STATISTICS", Table WB-11, has been set out to show the sensitivity to the model by small changes to the input variables for a household generating 600 L of wastewater per day in Armidale NSW (BOM Station 56037).  The discussion under "Rainfall Statistics" address the use of different rainfall data.  For this exercise the average monthly rainfall data will be used and the average monthly evaporation rate. Only one variable was changed at a time to compute the values in Table WB-1 and reset to the standard before the next variable was reset.

You can see the changes across columns 2-5 with the change (m2) across the range shown in column 6.  So how sensitive is the modelling to changes in ONE variable? The runoff rate is slightly sensitive; the summer crop factor is insensitive; slightly sensitive for the winter crop factor; highly sensitive to the soil texture which determines the design irrigation rate; and very sensitive to daily wastewater flow.

There soil type and daily wastewater flow regimes are critical to this type of modelling for which an understanding of the soil profile is essential, while the household must understand that for their particular soil type, water conservation can plan a significant on-going performance factor.

What if we were now to develop a multi-variable change to the table above without it being too complex.  That's where the spreadsheet approach to a water balance modelling exercise is invaluable. 

7.   STORAGE OF EFFLUENT IN GROUND

The excess effluent can be safely and effectively stored in the soil or the trench, overcoming the health and environmental concerns about maintaining a wet-wet weather storage. In the irrigation area, there is a porosity of about 40% that can be used to store excess effluent during dry times and draw-down during dry times. In the pipe trench, the sum of the volume of the pipe and the gravel amounts to about 40% porosity so in a given length of trench there is the ability to store significant quantities of effluent.  In the tunnel trench, the sum of the gravel and tunnel amounts to a porosity of 70%, storing almost twice the effluent as a pipe trench.

While other factors may also be important, the ability to store water in the soil against the drainage capacity of the soil results in smaller disposal areas and avoids the problems of wet-weather storage

 

8.   CONCLUSION

An understanding of the components and their interaction in a water balance model is essential.  The old adage "garbage in is garbage out' certainly applies to wastewater modelling.  While a mathematical model may appear robust, until realistic data are selected, then the model is only window-dressing posing.  The poorer the data, the higher the risk of land application area failure.

Any water balance modelling must take into account the risk of failure from rainfall periods greater than the model uses.  Simply accepting median monthly values because that is all that is required to meet NSW regulations is really keeping one's head in the sand. In the calculations referred to above, most median values reflect the 30th percentile annual rainfall. Similarly, the mean or average rainfall generally has a 50% failure.  Some councils consider the 90th percentile monthly values are worthy rainfall figure, yet the summation of monthly 90th percentile rainfall figure often exceeds the annual total for the wettest year on record, sometimes for records longer than 100 years. Accepting such inputs verges on the unbelievable, that is, one can only assume the Council officers have poor understanding of statistics.  It is my view that the monthly 70th percentile monthly values generally equates to about the 80th percentile annual values, providing a reasonable degree of protection.

These conclusions are examined in detail, with further explanation in the pages on Rainfall Statistics.