Calculator

The Calculator finds the impact of a given set of artificial influences on scenario flows. These scenario flows are then used to identify surpluses/deficits and compliance relative to flow targets.

The set of artificial influences used in the Calculator could take a couple of forms:

  • Artificial influences could be taken directly from the “base” WRGIS.

  • A modified, alternative or new set of artificial influences could be used. These influences might include user-prescribed changes/overrides compared with the “base” WRGIS.

It is anticipated that the Calculator will primarily be used to evaluate the latter of these two cases. Scenario flows, surpluses/deficits and compliance classifications in the former case are an output from the “upstream” WRGIS toolset and available in the WRGIS database.

The Calculator is based on a programmatic implementation of the “Waterbody Abstraction Tool” (WBAT) Excel workbook. The WBAT is a faster way to examine the effects of artificial influence changes compared with rerunning the full WRGIS toolset code. See below for further details of the relationship between the core logic of the WRGIS toolset, the WBAT and the SACO Calculator.

Note

The Calculator uses WRGIS abstraction impact fields that already account for (local) consumptiveness. This should be remembered if abstraction impacts are modified.

Terminology

We follow WRGIS use of “sub” and “ups” terminology. “sub” refers to just the current waterbody, whereas “ups” refers to sums for the current waterbody and all waterbodies upstream of it. So, taking natural flow as an example, “sub” represents runoff generated within the waterbody itself. Conversely, “ups” represents the “actual” outflow from the waterbody (including all of the area upstream of the waterbody outlet).

Logic

The starting point for the Calculator is a dataset that contains the tables described in Data. As noted above, the artificial influences tables may contain changes relative to the “base” WRGIS tables.

From this point, the Calculator follows a few steps to arrive at scenario flows, surpluses/deficits and compliance classifications:

  1. Aggregate the updated artificial influences (point) tables to waterbody-level (first “sub” and then “ups”, using the routing matrix).

  2. Apply the “ups” impacts to the natural (“ups”) flows to derive updated scenario flows (also “ups”) for each waterbody. The tool also identifies the consistent “sub” scenario flows for reference.

  3. Calculate updated surplus/deficit numbers for each waterbody as the new scenario flow minus the reference flow (typically EFI). If an alternative flow target table/column has been specified then surplus/deficit relative to this target can also be calculated at the same time.

  4. Identify the compliance band associated with the new scenario flow. The band is obtained by dividing the surplus/deficit (relative to the reference flow) by the natural flow (“ups”) and then comparing this quantity with the band definitions in the table below.

Compliance band definitions based on deficit (D) and natural flow (Qn)

Band

Definition

Compliant

D / Qn >= 0

1

-0.25 <= D / Qn < 0

2

-0.5 <= D / Qn < -0.25

3

D / Qn < -0.5

Note

Waterbodies with natural flows of zero are set as “compliant”. Selected waterbody types are not evaluated for compliance: ‘Seaward Transitional’ and ‘Saline Lagoon’.

The core of the Calculator thus follows the simple routed (instantaneous) water balance approach of WRGIS noted in Data. I.e. scenario flows are essentially the result of subtracting abstractions and adding discharges to natural flows, while ensuring that impacts upstream are propagated to downstream waterbodies. The primary output from the Calculator is a “Master” table - indexed by waterbody - that provides all water balance terms, surpluses/deficits and compliance bands.

Capping

One question relevant to the WRGIS toolset, the WBAT and the SACO Calculator concerns the impacts on flows downstream of a waterbody in which the “prescribed” impact (according to the artificial influences tables) cannot be met. For example, say a waterbody has a scenario flow of 10 Ml/d coming in from upstream (i.e. after all artificial influences upstream of the waterbody are taken into account). Say then that the impact of artificial influences inside that waterbody (“sub”) in a given scenario is prescribed as -15 Ml/d. Say also that the waterbody does not generate any runoff inside its area. The net impact (abstraction) therefore exceeds the available flow by 5 Ml/d.

In this example, the WBAT would effectively take the available 10 Ml/d, giving a scenario flow of zero for the waterbody. However, it would also then propagate a -15 Ml/d impact to downstream waterbodies, which would be subtracted from available flows if possible (i.e. up to the limit of the available flow in the downstream waterbodies). In contrast, the WRGIS toolset would limit the net impact propagated to downstream waterbodies to -10 Ml/d. I.e. the impact would be “capped” to what can actually be taken in the waterbody in question.

The WBAT works in this way partly because “capping” would be harder to implement in Excel. It might be expected that the WBAT provides slightly more conservative results than WRGIS where cases of “capping” arise in WRGIS. This is a situation that does occur with non-negligible frequency in the base WRGIS (and higher frequency in the fully licensed abstraction scenario).

In the SACO Calculator we have provided options to follow both the WBAT approach and a WRGIS-like approach to this situation. The latter approach is the default and involves capping net impacts at the waterbody scale, rather than adjusting individual artificial influences at the point scale. An upstream-to-downstream loop adjusts net impacts by the minimum amount needed to ensure that scenario flows do not become negative. We adopt this approach to emulate the WRGIS toolset calculations as far as possible, but an alternative could involve using a network flow model to solve for all flows and feasible impacts directly.

Note

If capping has been applied to avoid propagation of unfeasible impacts, scenario flows output from the Calculator may be larger than initially expected from performing a simple “ups” water balance calculation for some waterbodies. This is because capping reduced net impacts upstream to retain physical plausibility.