The spreadsheets
Calculate a pollutant load in kg from kg/Ha/yr export rates (spreadsheet)
Calculate a pollutant load in lbs from lbs/acre/yr export rates (spreadsheet)
Convert an event mean pollutant concentration into an export rate (spreadsheet)
Compare the cost-effectiveness of investments in alternative treatment technologies (spreadsheet)
for beyond 'back of the envelope' assessment, see Stormwater Modelling
Explanations
Two simple calculations provide useful orientation on catchment management decisions:
- a calculation of the relative contributions to total pollutant loads of different land uses, and
- a calculation of the relative cost-effectiveness of investments in different devices within a catchment.
Usually the data available for each of these calculations is very imprecise - more indicative than predictive. However even with very imprecise estimates, coarse comparisons can be informative. They can differentiate between approaches that are likely and unlikely to be helpful.
1. Working out which land uses to focus on
Looking at a catchment from the perspective of a receiving water body, one can ask: what is the relative importance of each land use in shaping dynamics within that waterbody?
It is an important question to ask, because many policies and programs apply to a land use, as such (e.g. planning approval processes commonly have aspects that are land use specific, pollution control programs commonly target particular land uses). Surprisingly often management priorities don't derive from assessments of catchment needs.
A back of the envelope calculation involves two questions:
- what pollutants are having significant impacts on the water body?, and then
- for each of these pollutants, what does each land use contribute?
Ecological significance of pollutants
Judgments about what pollutants are having most impact on particular water bodies obviously need to be grounded in an understanding of the ecological dynamics of each waterbody. Some rules of thumb are possible, however: see Ecological Contexts.
Calculating relative contributions to annual pollutant loads
A rough assessment of each land use's contribution can be calculated using an estimate of a pollutant export rate for each land use and the land use's area.
This calculation is provided in the following spreadsheets:
Calculate a pollutant load in kg from kg/Ha/yr export rates (spreadsheet)
Calculate a pollutant load in lbs from lbs/acre/yr export rates (spreadsheet)
It is often useful to compare current land use with historic land use and with future land use. A rough sense of how significant land use change is - and land management, as one can model future land management under good or poor practice by varying the export rates down or up - can be derived by comparing scenarios using these spreadsheets.
Land use areas
Current land uses can be derived from catchment maps. (Maps of NSW, Australia, are available via Urbanwater.info.) Often zoning maps are available, but actual usage maps are not: zoning maps are a rough guide to land use.
Export rates
Estimating long term export rates is an imprecise art - particularly when (as is usually the case) local data is not available.
In practice the usual approach is to:
- find an export rate (a range of possible export rates ...) for land uses present in a catchment that are from catchments that are similar in some ways to the catchment being studied; then
- make an informed assessment of how the actual export rates in this catchment might differ (an expert judgment) - taking into account differences in rainfall, slope, soils, vegetative cover, etc.
Export rate estimates from which one can begin are available online:
- Australia
- Draft export rates for many pollutants (University of Newcastle)
- Nutrient export rates for CMSS (CSIRO)
- Tailored Google Search
- International
Given the uncertainties, it makes sense to do some sensitivity tests - to see if using an upper or lower estimate for an export rate leads to different conclusions about which land uses matter most, from the perspective of a given receiving water body. When using outputs from these spreadsheets:
- be explicit about assumptions and uncertainties when discussing your conclusions, and
- use the calculations to inform your thinking about the relative importance of given land uses, and not as sources of estimates of the 'absolute' total pollutant load on a system (i.e. don't use these calculations to assert 'on average this water body receives x tonnes of y per annum').
The critical issue here is: The spreadsheets are helpful if they are approached as a support for 'back of the enevelope' assessments of issues (which it is worth doing when you are scoping issues, getting oriented). They are unhelpful if the uncertainty of the underlying data and the limitations of their simplifying assumptions (e.g. that expert rates are uniform, that average annual loads measure what's ecologically important for a water body) are forgotten.
Turn a concentration into an export rate
Two kinds of measure of pollution generation are commonly provided. As well as an 'average annual load' (e.g. in kg/Ha/yr), Event Mean Concentration (EMC), often expressed in mg/L, is commonly used. If the data you have is in mg/L, you can convert it to kg/Ha/yr.
Convert an event mean pollutant concentration into an export rate (spreadsheet)
for beyond 'back of the envelope' assessment, see Stormwater Modelling
2. Calculating Relative Cost-effectiveness
There are many technologies - and packages of technologies - in which one can invest to improve catchment performance (Built Environment provides an introduction to them).
When it comes to saying exactly what difference a set of devices will make in a catchment, there are many uncertainties. These include:
- available performance data will usually be for a small number of devices over a relatively short time period (whereas one may want to invest in many devices and be interested in their long term performance),
- available data is much more likely to relate to the performance of an individual device, than to their performance in series (as a 'treatment train'),
- the rainfall and catchment conditions under which performance was measured is likely to be quite different from those prevailing in the catchment you are managing, and
- there is likely to be variation in performance through time (this may be intrinsic to the device or practice, it may be a function of maintenance, ...).
However coarse comparisions of cost-effectiveness may still be helpful. Perhaps surprisingly, recommendations on what practices should be adopted are often grounded in a sense of current best practice uninformed by exploration of what is likely to be most cost-effective in a given catchment, given the water bodies (with their particular pollution issues) that catchment management intends to sustain.
A very simple comparison - of a 'back of the envelope' kind - can be made by designing programs (e.g. 'investment in constructed wetlands', 'investment in swales', 'investment in community education', 'investment in treatment trains [of xyz design]', 'rebates for rainwater tanks', etc) and estimating:
- their likely performance, and an upper and lower limit, and
- their likely cost, and an upper and lower limit,
and seeing how they compare when you contrast the likely cost-effectiveness of each program: comparing ranges from most expensive / least effective to least expensive / most effective. These ranges give a cost-effectiveness band in which a program is likely to fall, and a sense of where within that range the program might tend to fall.
When investment strategies have been designed simply by taking 'best practice' recommendations into account, this simple 'back of the envelope' calculation may point the way to improvements.
To use this spreadsheet you will need to:
- consider what pollutant(s) its most important to manage, given the ecosystems you wish to sustain
- identify some investment strategies of interest
- estimate their costs (anticipated, upper limit, lower limit)
- estimate their relative performance (anticipated, upper limit, lower limit)
- make a rough comparison using the spreadsheet.
Usually there are multiple pollutants that it is important to manage, from the perspective of a receiving water body. And usually there are multiple receiving water bodies that one wants to sustain. Depending on local priorities and the dynamics of aquatic ecosystems, it may be useful to do this calculation a number of times:
- contrasting what works best for one pollutant with what works best for another, and
- contrasting what is best management from one receiving water body's perspective with another's (e.g. a creek's and an estuary's).
Together these can provide a good initial feel for which strategies should be taken to the next stage, and explored with more care.
Export rates of pollutants are often roughly correlated (because volume and speed of surface water are major drivers of pollutant export). So management prioritisation can often be simplified, for 'back of the envelope' purposes (i.e. during initial exploration of alternative strategies), by choosing one pollutant that represents the greatest risks and exploring alternative ways of reducing its export, transport and delivery.
This exercise can usefully be done at two levels: (i) using very rough judgments of what might be possible, and (ii) using ballpark estimates of the effectiveness and costs of particular programs that you believe could actually be implemented.
Asking what investment options are actually available in a given catchment - given physical constraints, community willingness to pay, political feasibility, etc - is often a useful exercise in its own right. Considering these constraints often brings out the difficulty of actually managing a catchment / waterbody system in a sustainable way.
Compare the cost-effectiveness of investments in alternative treatment technologies (spreadsheet)
for beyond 'back of the envelope' assessment, see Stormwater Modelling
