Step 3b is divided into two tasks

• Task 8 - Apply modeling approach for the basin

  Having recorded your total baseline for the basin, it is time to use the model you selected in Step 3a to understand the hydrology of the region.

  In the case of water quantity targets, there are three data points that are required: present-day stream flows, natural stream flows, and environmental flow requirements. Depending on the basin and the model you are using, the model may have already included all this information.

  • Present-day stream flows reflect the observed volumes of water in the basin. In other words, they indicate how much water is carried by a river or held in an aquifer or lake.
  • Natural stream flows are modeled to display theoretical unaltered flow regimes. They reflect the flow of water that would be found in the basin in an average year and in a natural state where no water was withdrawn for human use.
  • From the two points above, it is possible to infer the amount of water that is removed from the basin by human use.
  • Environmental flow requirements represent the amount of water that should be in the stream to maintain adequate ecological conditions.

  In the case of water quality targets, there are only two data points that you need to gather for your basin: maximum allowable nutrient loads and current nutrient loads (for the limiting nutrient in the basin).

  Maximum allowable nutrient loads describe the maximum amount of nutrient concentration, for nitrogen and phosphorus, that could be present in the ecosystem before they cause eutrophication problems.

  • • If you are using the McDowell (2020) model, these concentrations are 0.80 mg/L for nitrogen and 0.046 mg/L for phosphorus.
    • If you are using local models, they may consider different concentrations (on account of local ecosystem conditions).

  Current nutrient loads refer to the observed concentration of nitrogen and phosphorus in the ecosystem. The methods only require identifying the concentration of one of the two nutrients, whichever is scarcer than the other. This is then called the “limiting nutrient.”

  • • The global model for water quality by McDowell, for example, considers that algae and cyanobacteria normally require these nutrients at constant ratios (e.g., seven parts of nitrogen for every part of phosphorus). The McDowell (2020) model will indicate, for each basin, which is the limiting nutrient.
    • Local models may use different ratios on account of local conditions.

  Take note: You will only set targets to reduce the nutrient loading of the limiting nutrient, as the remaining amounts of the other nutrient will not lead to eutrophication in the ecosystem.

  These different data points should be available in the model you are using. Note that there is an interface that allows you to access the global water quantity model by Hogeboom (2020) available here. This tool will quickly let you access all three parameters for your basin. For the global water quality model by McDowell (2020) you can use this temporary app to access the data.

  If you are using local models, you may need to rely on hydrology experts to get this data. Note that if the model you are using is unable to provide all of this data, the model might not be appropriate for target setting. You should consult with hydrology experts to see if the data can be found with additional tools or if a different model should be used instead. You may also reach out to the SBTN network and partners, or the validation team, to assess how to overcome any potential data gaps.

  Output(s) of this task:

  • • Hydrological data for the basin.
• Task 9 - Calculate required basin-wide pressure reductions

  Having collected all the key parameters for the basin, in your next task, you will calculate how much reduction in water withdrawals or nutrient pollutant loading is required to bring the basin back to a healthy state. The methods refer to these quantities as required basin-wide reductions and they are defined as a percentage of the current basin-wide pressures.

  Take note: In this case, we refer to the total, or basin-wide pressures, which are different from the pressures that your company exerts on the ecosystem. Basin-wide pressures include not only your company’s own pressures, but also those of all other stakeholders (including other companies and local communities).

  For water quantity targets, if you are using the global quantity model by Hogeboom (2020) and have accessed the online tool, the required basin-wide reductions in water withdrawals will be automatically provided.

  If you are not using the global model, first, calculate the excess withdrawals in the basin as the environmental flow requirements minus the present-day stream flows. Then, calculate the present-day withdrawals as the natural stream flows minus the present-day stream flows. Finally, calculate the required basin-wide reduction as the excess withdrawals divided by the present-day withdrawals (expressed as a percentage).

  Excess withdrawals = Environmental flow requirements − Present day stream flows

  Present day withdrawals = Natural stream flows − Present day stream flows

  Required basinwide reduction = Excess withdrawals % Present day withdrawals

  For water quality targets, calculate first the excess nutrient concentration as the current nutrient concentration minus the maximum allowable nutrient concentration. Then, calculate the required basin-wide reduction by dividing the excess nutrient concentration by the current nutrient concentration (and express this as a percentage). Remember that you will only do this for the limiting nutrient.

  Excess nutrient concentration = Current nutrient conc. − max allowable nutrient conc.

  Required basinwide reduction = Excess nutrient concentration % Current nutrient concentration

  Output(s) of this task:

  • Record of the required, basin-wide pressure reduction.