[GRASS-user] Rainfall interpolation for a time series

GRASS GRASS user
1

Hi,

I have rainfall measurements from 6 gauges that I want to interpolate
to an areal value (a 'surface'), so that I can compare the the
interpolated gauge values to a satellite rainfall estimate that covers
a grid cell of 28kmx28km. Two of the gauges are outside, but close to
the border of the grid cell. Therefore, I also need to clip the
interpolated surface to the grid cell and to get the average of the
surface value in this clipped surface.

A search on Google revealed that v.vol.rst is probably a good choice
for interpolating the rainfall because it takes elevation into
account.

However, for each rain gauge I have 730 values representing a daily
measurement over two years. As output from GRASS, I need a text file
with the interpolated rainfall values for each day in my time series.
So, I was wondering if there is an 'easy' way to get my output without
creating 730 vectors and rasters?

Regards
Hanlie

2

Hi Hanlie,

I’ve this before with 11 gauge stations, also using TRMM data. You’ll probably want to write a script and use r.mask to mask the interpolation map to your grid cell, then do r.sum on your masked area. After that just divide by the area to get the average rainfall. Given the small area and limited number of gauges this shouldn’t take very long to compute for 2 years of data. You should be able to fairly easily output your desired time series to a text file.

Best,
Nick

On Mon, Aug 15, 2011 at 3:00 PM, Hanlie Pretorius <hanlie.pretorius@gmail.com> wrote:

Hi,

I have rainfall measurements from 6 gauges that I want to interpolate
to an areal value (a ‘surface’), so that I can compare the the
interpolated gauge values to a satellite rainfall estimate that covers
a grid cell of 28kmx28km. Two of the gauges are outside, but close to
the border of the grid cell. Therefore, I also need to clip the
interpolated surface to the grid cell and to get the average of the
surface value in this clipped surface.

A search on Google revealed that v.vol.rst is probably a good choice
for interpolating the rainfall because it takes elevation into
account.

However, for each rain gauge I have 730 values representing a daily
measurement over two years. As output from GRASS, I need a text file
with the interpolated rainfall values for each day in my time series.
So, I was wondering if there is an ‘easy’ way to get my output without
creating 730 vectors and rasters?

Regards
Hanlie

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3

Hi Nick,

Thanks for the reply.

Did you do your comparison with a time series as well? If so, do you
perhaps have the start of a script that you would be willing to share?

Thanks
Hanlie

2011/8/15, Nick Jachowski <njachowski@gmail.com>:

Hi Hanlie,

I've this before with 11 gauge stations, also using TRMM data. You'll
probably want to write a script and use r.mask to mask the interpolation map
to your grid cell, then do r.sum on your masked area. After that just divide
by the area to get the average rainfall. Given the small area and limited
number of gauges this shouldn't take very long to compute for 2 years of
data. You should be able to fairly easily output your desired time series to
a text file.

Best,
Nick

On Mon, Aug 15, 2011 at 3:00 PM, Hanlie Pretorius <
hanlie.pretorius@gmail.com> wrote:

Hi,

I have rainfall measurements from 6 gauges that I want to interpolate
to an areal value (a 'surface'), so that I can compare the the
interpolated gauge values to a satellite rainfall estimate that covers
a grid cell of 28kmx28km. Two of the gauges are outside, but close to
the border of the grid cell. Therefore, I also need to clip the
interpolated surface to the grid cell and to get the average of the
surface value in this clipped surface.

A search on Google revealed that v.vol.rst is probably a good choice
for interpolating the rainfall because it takes elevation into
account.

However, for each rain gauge I have 730 values representing a daily
measurement over two years. As output from GRASS, I need a text file
with the interpolated rainfall values for each day in my time series.
So, I was wondering if there is an 'easy' way to get my output without
creating 730 vectors and rasters?

Regards
Hanlie
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