Introduction
Daily meteorological data from a number of weather stations covering the
area of interest constitutes the most important set of input data for the
system. This data is interpolated to grid level to provide input for
crop growth simulation. The interpolation is described in
Some of the meteorological parameters needed for the model (i.e. global
radiation and potential evaporation) are not generally available, or not
available on a daily basis, and are therefore estimated by the system. Not all
weather stations provide all data, and the system takes account of this by
labelling each station according to the sort of data it can provide on a
regular basis. The stations can be labelled as RAIN,
TEMP and REST,
meaning that the station provides rainfall data, temperature data or global
radiation and potential evaporation data. The interpolation algorithm selects
the best stations for each grid from the set of available stations.
Information about the stations is stored in table
WEATHER_STATION, which contains the following columns:
WMO_NO | (possibly WMO) reference number of the station |
WMO_NAME | station name (only for identification, not used) |
LATITUDE | latitude in decimal degrees |
LONGITUDE | longitude in decimal degrees |
ALTITUDE | altitude (m) |
CLIMATE_BARRIER_NO | a code to indicate the presence of a climate barrier |
COUNTRY_CODE | country code (only for identification, not used) |
DISTANCE_TO_COAST | the distance to the nearest coast (km) |
PERCENT_COMPLETE | the availability threshold |
Additional weather calculation
Global radiation and potential evaporation as required by CGMS
are calculated at station level and stored in the table CALCULATED_WEATHER
which contains the following fields:
WMO_NO | the (possibly WMO) reference number of the station | DAY | day | E0 | potential evaporation from a free water surface |
ES0 | potential evaporation from a moist bare soil surface |
ET0 | potential transpiration from a crop canopy |
CALCULATED_RADIATION | daily global radiation in KJ/m2/day |
The columns E0, ES0 and ET0 are expressed in mm/day. In order to calculate these values, and specifically the global radiation, the program needs a set of regression constants for each weather station. The program uses one of three different regression formulae for the global radiation calculation, depending on the availability of the measured parameters for the station. The three formulae, Angstrom, Supit and Hargreaves have their own set of regression constants for each weather station. These constants are a function of the location of the station. The constants are stored in the table SUPIT_CONSTANTS, which has the following fields:
WMO_NO | the (possibly WMO) reference number of the station |
ANGSTROM_A | interpolated Angstrom A |
ANGSTROM_B | interpolated Angstrom B |
SUPIT_A | interpolated Supit A |
SUPIT_B | interpolated Supit B |
SUPIT_C | interpolated Supit C |
HARGREAVES_A | interpolated Hargreaves A |
HARGREAVES_B | interpolated Hargreaves B |
This table is filled by a utility program called SupitConstants. This program estimates the regression constants for an arbitrary location by interpolating constants from a known location. The constants have been established for a number of weather stations covering a large part of Europe, and are stored in table SUPIT_REFERENCE_STATIONS. This table contains the following information:
STATION_NAME | station name for identification, not actually used |
LATITUDE | latitude in decimal degrees |
LONGITUDE | longitude in decimal degrees |
ALTITUDE | altitude (m) |
ANGSTROM_A | calculated Angstrom A |
ANGSTROM_B | calculated Angstrom B |
SUPIT_A | calculated Supit A |
SUPIT_B | calculated Supit B |
SUPIT_C | calculated Supit C |
HARGREAVES_A | calculated Hargreaves A |
HARGREAVES_B | calculated Hargreaves B |
Information in table SUPIT_REFERENCE_STATIONS represents a significant body of data and should not normally be changed by the user. The regression constants have been calculated with up to twenty years of meteorological data for some stations (Supit and Van Kappel, 1998).
Missing data
A station can be found suitable for use in the interpolation even
if meteorological data are not available for the whole period for which the
interpolation is to take place. In this case, the missing data will be
substituted by data from table REFERENCE_WEATHER. This table, filled
by the utility program ReferenceWeather, contains an entry for each station
for every possible day of the year (i.e. 366 days). The data in this table
represents the daily long-term average data for this station. The day field in
the table contains dates from 1-1-1956 until 31-12-1956, where the year 1956
was chosen solely for the fact that it is a leap year. The table contains the
following fields:
WMO_NO | the (possibly WMO) reference number of the station |
DAY | the day (year is 1956) |
MINIMUM_TEMPERATURE | minimum temperature (°C) |
MAXIMUM_TEMPERATURE | maximum temperature (°C) |
CALCULATED_RADIATION | daily global radiation in KJ/m2/day |
WINDSPEED | mean daily windspeed at 10m (m/s) |
RAINFALL | mean daily rainfall (mm) |
VAPOUR_PRESSURE | mean daily vapour pressure (hPa) |
E0 | pot. evap. from a free water surface (mm/day) |
ES0 | pot. evap. from a moist bare soil surface (mm/day) |
ET0 | pot. transpiration from a crop canopy (mm/day) |
Some of these fields can contain NULL data, e.g. the rainfall field will not contain data for a station that only supplies temperature data.
Weather data availability
The availability of a weather station and its role in the interpolation is
determined by the weather data availability calculation. For all of the
stations in the WEATHER_STATION table, the program will assess if
the amount of data available for the station in the requested period equals
or exceeds the threshold value PERCENT_COMPLETE.
This assessment is done for each of the three ‘labels’ RAIN,
TEMP and REST,
and checks the relevant data subsets, e.g., the check for
REST only involves the CALCULATED_WEATHER data. Whilst checking
the availability for a station, the program will make sure that for those
days that data is missing the REFERENCE_WEATHER table can provide
a substitute value. If this is not the case, the station will not be available.
results of the availability check are stored in the WEATHER_DATA_AVAILABILITY table,
which contains the following fields:
YEAR | year for which the station is labelled |
WMO_NO | station identifier |
AVAILABILITY_TYPE | "RAIN", "TEMP", or "REST" |
It is clear that a station that can provide a full set of data will have three entries in this table for a given year.
If the period for which the availability check is being performed is in the
current year (as defined in the SYSCON table), then the check will only
be performed for the period from Jan 1st until the last day of the
period requested. Otherwise the check is being performed using the data for
the whole calendar year.
Grid weather
Once the additional data for the individual station has been calculated
and the availability of the stations has been established, the interpolation
can start. The first part of this process involves the ‘scoring’ of the
weather stations for each grid, in order to establish exactly which stations
to use for the interpolation. The second step is the actual interpolation.
The results of the interpolation are written into the table GRID_WEATHER
which contains the following fields:
GRID_NO | grid number |
DAY | day |
MAXIMUM_TEMPERATURE | maximum temperature (°C) |
MINIMUM_TEMPERATURE | minimum temperature (°C) |
VAPOUR_PRESSURE | mean daily vapour pressure (hPa) |
WINDSPEED | mean daily windspeed at 10m (m/s) |
RAINFALL | mean daily rainfall (mm) |
E0 | pot. evap. from a free water surface (mm/day) |
ES0 | pot. evap. from a moist bare soil surface (mm/day) |
ET0 | pot. transpiration from a crop canopy (mm/day) |
CALCULATED_RADIATION | daily global radiation in KJ/m2/day |
The grid weather table cannot contain NULL data. The maximum distance
between a weather station and the grid centroid is set in the SYSCON
table. Stations beyond this distance are not considered for interpolation.
For control purposes, the information about the use of the stations for
interpolation for a particular grid is written to the table STATIONS_PER_GRID
containing the following fields:
GRID_NO | grid number |
YEAR | year |
RAIN_STATION | the station selected for the rainfall data |
TEMP_STATION_1 | up to four stations used for temperature |
TEMP_STATION_2 | |
TEMP_STATION_3 | |
TEMP_STATION_4 | |
REST_STATION_1 | up to four stations used for the rest |
REST_STATION_2 | |
REST_STATION_3 | |
REST_STATION_4 |
Note that this information is only stored for the year in which the interpolation is taking place. If the process is run again for a different period in the same year and the weather data availability has changed, then these values will change.