[GRASS-user] largest number of consecutive days where rainfall >1 mm

[Ivan Marchesini]

Dear Veronica

thank you for your answer

I understand your point but I still believe that the code is not working 
properly for my aims

I try to explain myself with an example.

According to the example in the tutorial:

the first command creates a weekly mask based on my data (daily 
precipitation in 2021):

t.rast.aggregate input=test2021 output=weekly_mask basename=mask_week 
granularity="1 week" method=count nprocs=10

The second command add to the weekly value (7) the value of the 
consecutive days in the week period

t.rast.algebra base=test_rr_maggiore_1mm 
expression="test_consecutive_days_rr_maggiore_1mm = weekly_mask 
{+,contains,l} if(test2021 < 1 && test2021[-1] < 1 || test2021[1] < 1 && 
test2021 < 1, 1, 0)" nprocs=10

In the attached file

https://drive.google.com/file/d/1eusufL8JLysac0wLfWC6oub0IM79e-KO/view?usp=sharing

I show an extract of the the values of a given pixel in the study area.

In the first week the value of the map 
"test_consecutive_days_rr_maggiore_1mm is 10  (7 + 3). 7 is the weekly 
base. 3 is the number of times in which a rainy day is preceded [-1] or 
followed [+1] by another rainy day. This happen in 2021-01-05, 
2021-01-09, 2021-01-10 (in fact 3 days).

What is not fitting my needs here is the fact that between the 
2021-01-05 and 2021-01-09  there are other days without rainfall. 
Actually we have not 3 consecutive days in this week!! We have 2 
consecutive days: 2021-01-09, 2021-01-10 and another rainfall day at the 
start of the week.

I would like to have, in this first week, the value of 9 (7+2) not 10 
(7+3), because I'm interested in the annual maximum length of wet spell, 
in days.

I hope I was able to explain myself

Thank you again

Ivan









On 18/08/22 15:36, Veronica Andreo wrote:
> Hi Ivan,
>
> It does indeed count consecutive days meeting a condition per week. 
> That's what the [-1] means in the t.rast.algebra command, i.e., it is 
> the temporal neighbourhood modifier. If you then want to get the 
> largest weekly consecutive days meeting the condition within a month 
> you can aggregate the "weekly_consecutive" time series with 
> t.rast.aggregate granularity="1 month" method="maximum"
>
> HTH,
> Vero
>
> El jue, 18 ago 2022 a las 14:42, Ivan Marchesini 
> (<ivan.marchesini at gmail.com>) escribió:
>
>     Hi Markus
>
>     thank you
>
>     Yes I saw that algorithm but, if I'm not wrong, it does not fit
>     with my
>     needs. The calculation made returns the number of days, in a given
>     interval (here week), that meets a certain condition. Not sure
>     they are
>     consecutive. As an example: if negative temperature occur in
>     monday and
>     thuesday and then in friday, saturday and sunday then the code
>     count 5
>     for that week. In my opinion what we need is 3.
>
>     I hope I was enough clear
>
>     In any case it would be good to find a solution for this type of
>     problem
>     because these type of indexes are  used for climate data analysis
>     (e.g.:
>     https://www.ecad.eu/download/millennium/millennium.php
>     https://www.climdex.org/learn/indices/ )
>
>     thank you
>
>     Ivan
>
>
>
>     On 17/08/22 20:04, Markus Neteler wrote:
>     > Hi Ivan,
>     >
>     > On Wed, Aug 17, 2022 at 9:07 AM Ivan Marchesini
>     > <ivan.marchesini at gmail.com> wrote:
>     >> Dear grass user
>     >>
>     >> I have daily rainfall strds
>     >>
>     >> I would like to obtain a layer of the largest number of
>     consecutive days
>     >> where rainfall >1 mm
>     > May this script code help?
>     >
>     https://grasswiki.osgeo.org/wiki/Temporal_data_processing#How_to_count_consecutive_days_that_meet_a_certain_condition?
>     >
>     > Markus
>     >
>     >> Do you have any suggestions on how to calculate this climate
>     index using
>     >> grass's time modules?
>     >>
>     >> thank you very much
>     >>
>     >> Ivan
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>
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