# Calculate water yield per hectare

Hello, I have certain doubts and problems that I want to clarify. In the work that I am doing, I need to make a cartography of the water yield expressed in M3/ha, it is still not clear to me how I can obtain this raster layer, I have thought of using the water yield tiff and dividing it by the number of millimeters in a cubic meter, but I only get express in M3 but I need to express it in hectare, I don’t know how to get a map expressed in M3/ha, I would like you to help me get the water yield map but by M3/ha. …Thanks a lot

Hi @felipeskull -

The model output is in millimeters per pixel. You can get a volume by calculating the number of millimeters times the pixel size in square meters. And you can convert the area of the pixel from square meters to hectares.

~ Stacie

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Hello, to know if I understood the calculation process correctly, I will explain it to you as I understood it, ok; First, the operations are carried out on the raster.

The first thing that occurred to me was to first divide by 1,000,000, since 1 cubic meter is a million millimeters, then it would be to convert the meters to hectares, it would be to multiply the raster by the value of the hectare, which is 10,000 square meters.

the raster is expressed in millimeters per pixel, I have a pixel of 900 m2, all the operations are carried out in Arcgis, when dividing by the number of millimeters in a cubic meter I already obtain the raster in cubic meters, then I multiply it with the number of square meters in one hectare and I get the final raster… am I wrong?

First, a reminder that we don’t recommend using the per-pixel absolute values of water yield for this model. The equations used in this model are made to be interpreted at the (sub-)watershed scale (and the model calculates volume per (sub-)watershed for us).

That said, volume per pixel is just (water yield depth x area of the pixel)
or (water yield depth in mm x 900m2) - this gives a volume in m3 per 900m pixel.

Then you’d convert to volume/hectare by multiplying by (900 / 10000).

~ Stacie

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Hello, I was doing some tests with the water yield model, I changed the resolution of the rasters, the resolution is 12x12, and I changed the shape of the basin, a little bigger, my question is, should I Do the same procedure in the previous point, the same steps, the only change is to multiply the yield in mm by 144 and then to pass it to m3/hec it must be multiplied by (144/10000), that would be the procedure if I’m not wrong …

Yes, that sounds like the right unit conversion to me.

Hello, good morning, I have a problem with the water yield model, I am running the model with the same data and now that I do the calculation again to go to m3/ha, it does not give me the same results, I do not understand what happened, if I apply the same formula to pass the units, the raster has a spatial resolution of 30x30, I am going to include the results of the new processing, I deleted the old data because I wanted to change the resolution and I didn’t care either,…and now When I do the calculations again, the results are not the same… the calculations now vary by one decimal…

the new results:

the old results:

rendimiento.zip (4.4 MB)

Hi @felipeskull,

Could you say more about which input rasters you changed the resolution of and how you did that downscaling? How did you downscale the precipitation raster?

I’m not exactly sure why your updated results would be off by a factor of 10 or 1 decimal place.

When converting water yield from mm/pixel to volume we need to be careful in what we are interpreting.

Let’s say we have a 12m and 30m pixel each with 500mm of precipitation.

500mm * 144m^2 / 1000 = 72m^3 / pixel = {72m^3 \over pixel} * {1 pixel \over 144m^2} * {10000m^2 \over 1ha} = 5000m^3 / ha
500mm * 900m^2 / 1000 = 450m^3 / pixel = {450m^3 \over pixel} * {1 pixel \over 900m^2} * {10000m^2 \over 1ha} = 5000m^3 / ha

Where 144m^2 pixel = 0.0144ha and 900m^2 pixel = 0.09ha. Note that we need to divide by 1000 to convert mm to meters. @swolny, could you confirm that makes sense?

Hopefully this is helpful and not more confusing!

~ Doug

Hello, I’m sorry, I didn’t express myself well, when I did the calculations again they told me that I did exactly the same calculations with the 30x30 rasters, that is, I took the same information from the first time, as can be seen in the map presentation. I did the calculations again with the same data with the same calculations and the result varied, it is the image with the arcgis program, it is the second attempt with the same data and the same calculations and the result is different, it is the image with the program arcgis…

and what you explained was more confusing than enlightening…

and the 12x12 rasters were obtained at the time of interpolation, one can choose the resolution with which the rasters will be generated

Hi @felipeskull,

Sorry that was more confusing than helpful. I’m not sure why you would have different results from a raster calculator call given the same equation and data inputs. The only thing I can think of is a typo in the raster calculator equation.

I’ll try to be clear with what I think is the proper conversion.

wyield * pixelarea / 1000 = m^3/pixel

We divide by 1000 to go from millimeters to meters. So, with some sample values

300mm * 900m^2 / 1000 = 270m^3 \text{ per pixel} which is equivalent to 270m^3 \text{ per }0.09ha

It would be great to hear about the results you are trying to present. Like was mentioned earlier in this thread, usually with the water yield model we would present watershed or sub waterhsed results. I’m not sure how to think about a pixel output of volume per hectare because the 30m x 30m pixels only represent 0.09 hectares. So if you are showing a pixel to have 5000m^3/ha of water yield, that might be true, but really that pixel is only contributing 450m^3 of volume. Maybe keeping results in per pixel terms but having the legend indicate that 1 pixel = 0.09ha would be useful?

Sorry for any confusion, my background is in computer science and software development! So take my input with a grain of salt!

~ Doug