At times you may wish to determine the difference in flow or discharge between an upstream and a downstream point in a river catchment. This requires the subtraction of the upstream flow from the flow at a downstream point in the river.

Determine the time of travel between sites

1. Identify the monitoring sites that you wish to review by selecting the Level under which all the sites can be found. This selection will become the All-series filter. You may wish to select a time period also to remove as many irrelevant sites as possible.
2. Drag the datetime variable to the bottom x-axis of your chart.
3. Drag and drop the flow or discharge variable onto your chart.
4. In the Filter Panel click on the Discharge Variable Tab.
5. Add filter conditions specific to your first site (e.g. Level2/Level 3 selections).
6. Drag and drop another flow or discharge variable onto your chart.
7. Right-click on the Chart and select Chart Editor.
8. Click on the series and rename the as the site name for convenience.
9. Select the next series tab, and change its filter conditions to specify the next site.
10. You would expect that the Upstream site will have a Lower flow, with an earlier peak in hydrograph and the downstream site will have a greater flow, later peak in hydrograph.
11. Determine the time difference in the hydrograph peaks in seconds by zooming into your chart. This number, in seconds, will be entered into a conversion formula, and is represented here as N.

Determine the difference in water volume between the two sites

1. Open Calculations from the Chart.
2. In Master Variable A select the downstream site, then pick up depth, and select the rating (conversion) table you want to use for that site.
3. Add a new Variable B, select the upstream site, then pick up depth, select the rating (conversion) table for that site.
4. Click in the formula bar and type TIMEDIFFA. This determines the time increment between consecutive points in the depth variable A at the downstream site. Click on the index button in the output area to see the result for each step. When using event based logging where data is not equidistant, the time diff function will pick up the difference in time between each record.
5. Continue to write the formula. Multiply timediffA by variable A;TIMEDIFFA * A        This part of the formula converts the flow into a volume of water rather than flow rate. For any indexed point, the formula gives the volume of water that has passed that individual point. You should note that the depth variable A has been converted to flow using the ratings table.
6. Subtract the upstream site. Continue to type into formula bar "minus TIMEDIFFB*(VALUEBYTIMEB(N)). This part of the formula gives the time difference between consecutive points for variable B (from the upstream site) multiplied by the flow for each of those records.
7. Check that the formula is correct. It should read TIMEDIFFA*A-TIMEDIFFB*(VALUEBYTIMEB(N))
8. The formula in total gives you the difference in volume between the upstream site and the downstream site - the additional volume of water on the downstream site.
9. Select an output variable e.g. Discharge (Difference).
10. Check the Database box in the output section.
11. Click Update. Enter comments about your processing, if any. Then click OK.
12. Once you have completed the calculation you can Save the Calculation Template (from the Quick Access Toolbar) if you wish to use this specific calculation again. The Calculation can be named.

Display the difference in discharge

13. Within the Chart, you can right-click onto properties and overlay the two flow measurements from the upstream and downstream sites (move series A to right hand axis, move series B to right hand axis leaving the newly calculated value of volume difference plotted on the left hand axis.
14. As get close to an event if the time of travel between the peaks is out, which can happen between high flow and low flow, you will get a drop in volume before the event starts and vice versa after an event sometimes you can see the flow has dropped off more quickly.
15. You would expect to get a large positive flow downstream compared to the upstream site.