# Seasonal Water Yield - Rain events vs Rain days

JessieWells
Member

Hi,

I'm just trying to make sure I understand each calculation in the model, and I'm slightly confused by the descriptions accompanying the first equation (not by the equation itself)

I'm looking at the documentation for the latest nightly build

Under 'How it Works'

The first variables are described as:

ai,m= [Pi,m/ni,m] /25.4 and

ai,m (mm) is the mean rain depth on a rainy day at pixel i on month m [mm].

ni,m is the number of events at pixel i in month m [-]

Pi,m is the monthly precipitation for pixel i at month m [mm].

But isn't ai,m just converting from P/n (in mm per event), into inches?

If so, then wouldn't ai,m be in inches, and be defined as "the mean rain depth per event, in inches" ?

(The results for QFi,m in the next equation convert back into mm, by multiplying the calculations by 25.4 mm/inch)

Rain events vs Rain days:

For it to also be the "mean rain depth on a rainy day", there can only be 1 event per day. This would mean that ni,m is the number of events, but also the number of rainy days.

Am I correct that the model makes this assumption? (1 rain event = 1 day on which rain occurred)

For places such as the wet tropics where rain events can happen more than once per day, it would be helpful to make this explicit, so we know to convert from N events to N days containing events. (otherwise subdaily data such as TRMM could give multiple events in one day)

Similarly, in the Data needs section, the 'Rain events' table, you could change from "A rain event is defined as >0.1mm." to something like "A rain event is defined as a day with >0.1 mm rain". (So ni,m = the number of days on which >0.1mm fell, for pixel i in month m).

Best wishes

- Jessie

ai,m= [Pi,m/ni,m] /25.4 and

ai,m (mm) is the mean rain depth on a rainy day at pixel i on month m [mm].

ni,m is the number of events at pixel i in month m [-]

Pi,m is the monthly precipitation for pixel i at month m [mm].

But isn't ai,m just converting from P/n (in mm per event), into inches?

If so, then wouldn't ai,m be in inches, and be defined as "the mean rain depth per event, in inches" ?

(The results for QFi,m in the next equation convert back into mm, by multiplying the calculations by 25.4 mm/inch)

Rain events vs Rain days:

For it to also be the "mean rain depth on a rainy day", there can only be 1 event per day. This would mean that ni,m is the number of events, but also the number of rainy days.

Am I correct that the model makes this assumption? (1 rain event = 1 day on which rain occurred)

For places such as the wet tropics where rain events can happen more than once per day, it would be helpful to make this explicit, so we know to convert from N events to N days containing events. (otherwise subdaily data such as TRMM could give multiple events in one day)

Similarly, in the Data needs section, the 'Rain events' table, you could change from "A rain event is defined as >0.1mm." to something like "A rain event is defined as a day with >0.1 mm rain". (So ni,m = the number of days on which >0.1mm fell, for pixel i in month m).

Best wishes

- Jessie

Post edited by JessieWells on

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