N of annual water yield (N=the number of rain events per year)

Dear, community,

The mannual of AWY says that the Z parameter is positively correlated to N, the number of rain events per year. What do you consider as one rain event? Is it related to the volume of water? Is it related to the number of days with rain?
In some papers, though, researchers have suggested that Z can be either calculated as N/5 or calibrated. Is that correct?

What is the issue or question you have?

What do you expect to happen?

What have you tried so far?

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Hi @BiancaBendito -

The Seasonal Water Yield model has an explicit input for number of rain events, and says “A rain event is defined as >0.1mm precipitation.” I would expect this definition to also apply to this N parameter for Annual Water Yield.

~ Stacie

Hi Stacie, thank you for your response.

So I would like to continue and ask one more question.

For example, let’s assume the situation below shown in the table. As shown in green, these are the precipitation values (mm) for the first 10 days of January. What would I do to calculate the number of rain events in January (considering only these 10 days in the example)? the mode that is written in blue, the red mode or the mode that is written in purple?

  • the mode in blue considered an event unit such as the day it rained > 0.1mm and added it up (the rain event numbers area integers);
  • the mode in red considered the total rainfall on the day that was greater than >0.1mm and added it up;
  • the mode in purple considered the total precipitation on the day minus 0.1 and added it;

Previously I had done it the way it is in blue (the rain event numbers were integers). However, in the video on YouTube, on the Natural Capital Project (NatCap) profile, the values for rain events have decimal places after the decimal point, so they are not integers (as shown in the figure below).
WhatsApp Image 2024-04-17 at 11.04.03

video link: https://www.youtube.com/watch?v=TffrUBYMz6U&t=603s

@swolny please correct me if I’m wrong, but my understanding is that a “rain event” is a period of continuous rain (of at least 0.1mm). Thus the number of rain events in a given month should be an integer. I’m guessing that the rain event values shown in the video are not integers because they’re averaged over multiple years.

Therefore the amount of precipitation on a given day does not directly tell you the number of rain events that day; it could be one or more. Ideally you would have weather data that specifically reports the number of rain events.

@esoth I agree that ideally we would have an actual number of rain events, which may be several per day. Realistically, we’re much more likely to only have total rainfall per day (and lucky to have that!), and since those values are aggregated, all we can say is that it rained that day. So then the number of rain events would end up being the number of days where > 0.1mm of rain fell. This isn’t ideal, but it’s better than most situations have, and if it’s the best available data, that’s fine.

As for integer vs decimals, unless the model code requires integers, I don’t see a problem with providing values that aren’t whole numbers. Again, ideally we would have an actual count of number of rain events, which would indeed be integer. If we are simply counting the number of days in a year with > 0.1mm rain, then again we should have integer values. But if we’re doing something like averaging over multi-year data, then I can see where decimals would result. And it seems like the modeling results wouldn’t be significantly different if a value of 120.4 was provided instead of 120.

~ Stacie

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