Question about runoff conversion

[JKALanger]

Description

In convert.py, one comment in the code says that the hydrological parameters are in "m of water per day" and that they need to be multiplied by 1,000 to get to m3/hour.

However, the ERA5 documentation for runoff says that the runoff is aggregated per hour (see attachment), so is the division by 24 really necessary? Also, why is it multiplied by 1,000? The conversion from "m per day" to m3/hour implies that the unit of the coefficient is m2, so do you assume an area of 1,000 m2 and if so, why?

Thanks for clarification and your continued work on atlite.

[coroa]

You are right in both points that the unit conversion is inconsistent.

There was quite some iteration in the used datasets and projections in the beginning. When this was conceived it used a different runoff dataset which reported runoff in kg / m^2 / day which is about equivalent to mm / day and had shapes with areas that were given in km^2 so that you had to do:

mm / day * km^2 = 1 / 1000 * m / 24 / h * 1000^2 m^2 = 1000 / 24 * m / h

But none of this is valid anymore with the current era5 dataset and the cea projection, which uses metres directly.

But it also does not matter too much, since this function is mostly intended to generate a profile rather than a full inflow estimation for each power plant. ie. the output of this function should be rescaled according to historical output at measurement points in the river or historical annual/monthly generation of plants; so that whatever global factor is put in, will normalize out anyway.

You're invited to submit a quick PR to fix it here now.

[JKALanger]

Thanks for the fast and clear response.

Sorry, but I am rather new to GitHub, so I don't know how PR work. But if I understand correctly, you would just need to remove the (1000.0 / 24.0 ) term from the code.

[coroa]

Indeed

[lukasschirren]

Hello together,

I came across the same issue, which I believe is not solved yet. I used the runoff in my code to get a theoretical estimate of hourly hydropower potential.
In atlite/convert.py line 1038:

    # The hydrological parameters are in units of "m of water per day" and so
    # they should be multiplied by 1000 and the basin area to convert to m3
    # d-1 = m3 h-1 / 24
    runoff *= (1000.0 / 24.0) * xr.DataArray(
        basins.shapes.to_crs(dict(proj="cea")).area
    )

    return hydrom.shift_and_aggregate_runoff_for_plants(
        basins, runoff, flowspeed, show_progress
    )

The ERA5 Reanalysis dataset is still per hour, not day, so removing it still holds true. I would open a PR to solve this if there are no objections.