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# Water Balance

Hi,

I am playing around a bit with the Groundwater Overlay. Therefore I've created a empty map (size 1000x1000m). In the overlay I've created a weather event. It will rain for 2 hours and the total rainfall is 10 mm. Calculation Preferences is set to *Accuracy. *Grid cell size is 0.5 m (Grid accuracy Threshold is also 0.5 m).

I would expect the water balance will show me the total amount of water is (1000 m * 1000m * 0.01 m) 10000 m³. However the Water Balance tells me the total volume of water *In *is 9960 m³.

If I change the grid cell size to 1 m (Grid Accuracy Threshold also to 1 m) the Water Balance changes to 9920 m³ (see screenshot below).

Why isn't the total amount of rainwater not 10000 m³. I guess it has something to do with the grid cell size, but I don't see how it is related.

## Comments

Hello Ward,

We have caluculated this for you and the explanation is that the edge cells are not taken into account for the volume.

Therefore your conclusion about the size of the gridcell is correct. That influences the ammount of m3 volume.

If you have cells of 1m in a 1km2 project the ammount of cells per row is 1000 but 998 cells per row are taken into account for the calculation of the volume. So the calulation will be 998

9980.01= 9960m3.This is corresponding with your results. I hope this gives you enough insight in how this volume is calculated.

Kind regards,

Hansje

Tygron support team

Let me see. So if the row is 1000 meters, it uses 998 cells. The cells by themself are 1 meter, so it doensn't use the first and the last row. And the same is true for the columns. Not sure about your statament that "9989980.01= 9960m3". I think it should be 998 times 998 is 996004 (times 10 mm is the 99600m³).

But yes, thank you!

Yes you are correct about the calculation, the 'times' symbool has dissapeared! But I think you got the point anyway ;)

Kind regards,

Hansje

Tygron support team

For more information, check out the Wiki page on the border model: https://previewsupport.tygron.com/wiki/Border_model_(Water_Overlay). The Water Module assumes by default an edge of 2 cells wide.

Tygron support team