This function downloads rainfall remote sensing data of TRMM and IMERG from NASA GSFC servers, extracts data from grids within a specified watershed shapefile, and then generates tables in a format that SWAT requires for rainfall data input. The function also generates the rainfall stations file input (file with columns: ID, File NAME, LAT, LONG, and ELEVATION) for those selected grids that fall within the specified watershed.
Usage
GPMswat(
Dir = "./SWAT_INPUT/",
watershed = "LowerMekong.shp",
DEM = "LowerMekong_dem.tif",
start = "2015-12-1",
end = "2015-12-3"
)Arguments
- Dir
A directory name to store gridded rainfall and rain stations files.
- watershed
A study watershed shapefile spatially describing polygon(s) in a geographic projection crs='+proj=longlat +datum=WGS84'.
- DEM
A study watershed digital elevation model raster in a geographic projection crs='+proj=longlat +datum=WGS84'.
- start
Beginning date for gridded rainfall data.
- end
Ending date for gridded rainfall data.
Value
A table that includes points ID, Point file name, Lat, Long, and Elevation information formatted to be read with SWAT, and
a scalar of rainfall gridded data values at each point within the study watershed in ascii format needed by SWAT model weather inputs will be stored at Dir.
Details
A user should visit https://disc.gsfc.nasa.gov/information/documents Data Access document to register with the Earth Observing System Data and Information System (NASA Earthdata) and then authorize NASA GESDISC Data Access to successfully work with this function. The function accesses NASA Goddard Space Flight Center server address for IMERG remote sensing data products at (https://gpm1.gesdisc.eosdis.nasa.gov/data/GPM_L3/GPM_3IMERGDF.07/), and NASA Goddard Space Flight Center server address for TRMM remote sensing data products (https://disc2.gesdisc.eosdis.nasa.gov/data/TRMM_RT/TRMM_3B42RT_Daily.7/). The function uses variable name ('precipitation') for rainfall in IMERG data products and variable name ('precipitation') for TRMM rainfall data products. Units for gridded rainfall data are 'mm'.
IMERG dataset is the GPM Level 3 IMERG *Final* Daily 0.1 x 0.1 deg (GPM_3IMERGDF) derived from the half-hourly GPM_3IMERGHH. The derived result represents the final estimate of the daily accumulated precipitation. The dataset is produced at the NASA Goddard Earth Sciences (GES) Data and Information Services Center (DISC) by simply summing the valid precipitation retrievals for the day in GPM_3IMERGHH and giving the result in (mm) https://gpm.nasa.gov/data/directory.
TRMM dataset is a daily 0.25 x 0.25 deg accumulated precipitation product that is generated from the Near Real-Time 3-hourly TMPA (3B42RT). It is produced at the NASA GES DISC, as a value added product. Simple summation of valid retrievals in a grid cell is applied for the data day. The result is given in (mm) https://gpm.nasa.gov/data/directory.
Since IMERG data products are only available from 2000-June-1 to present, then this function uses TRMM data products for time periods earlier than 2000-June-1. Keep in mind that TRMM data products that are compatible with IMERG data products are only available from 2000-March-01. The function outputs table and gridded data files that match grid points resolution of IMERG data products (i.e., resolution of 0.1 deg). Since TRMM and IMERG data products do not have a similar spatial resolution (i.e., 0.25 and 0.1 deg respectively), the function assigns the nearest TRMM grid point to any missing IMERG data point as an approximate (i.e. during 2000-March-01 to 2014-March-11 time period).
The GPMswat function relies on 'curl' tool to transfer data from NASA servers to a user machine, using HTTPS supported protocol. The 'curl' command embedded in this function to fetch precipitation IMERG/TRMM netcdf daily global files is designed to work seamlessly given that appropriate logging information are stored in the ".netrc" file and the cookies file ".urs_cookies" as explained in registering with the Earth Observing System Data and Information System. It is imperative to say here that a user machine should have 'curl' installed as a prerequisite to run GPMswat.
Author
Ibrahim Mohammed, ibrahim.mohammed@ku.ac.ae