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Evapotranspiration values (ET) are crucial for agriculture where estimates of water reserves available for crops are the basis for scheduling the time and intensity of irrigation, yield prognoses, etc. Detailed evapotranspiration data are, therefore, of essential value. However, stations performing direct measurements of evapotranspiration are very scarcely distributed in Poland, and for this reason the interpolation of data is necessarily biased. Hence, evapotranspiration values are calculated using indirect methods (usually empirical formulas). Data from geostationary meteorological satellites are used operationally for the determination of evapotranspiration with good spatial and temporal resolution (e.g. Land-SA F product). The study of the relation between evapotranspiration values determined with the use of satellite data and those calculated using the Penman-Monteith formula was performed for the study area in Poland. Daily values and cumulated (i.e. decadal, monthly and yearly) values were analysed to determine the quality and possible added value of the satellite product. The relation between the reference ET and actual ET in two consecutive years was discussed, both for the whole test area and for individual stations, taking into account land use and possible water deficit in the root zone, represented by H-SA F (EUMETSA T Satellite Application facility supporting Operational Hydrology and Water Management) soil wetness index product. The differences are presented and discussed.
During last two years in Satellite Data Receiving and Processing Centre in Krakow, the method of snow cover detection was prepared and tested. This method is based on information from visible and infrared channels of the AVHRR instrument. Detection of the snow cover consist of: real time receiving of daytime AVHRR transmission, selection of appropriate area in all AVHRR channels, calibration of visible and infrared channels, snow cover detection and cloud masking, transformation of classified image to map projection and insertion of the coast lines to distinguish land snow and sea ice. Most important part of this method concern separation of snow and clouds especially on images with low Sun elevation. Use of the all five AVHRR channels and correction of the Sun elevation and Sun to sensor position are necessary. For good snow detection in forest area, classified image with types of ground cover is used as mask. This image were prepared using temporal profiles of vegetation indices calculated from AVHRR data on the area of Poland. In case of partial cloudiness, composition of several classified and geometrically corrected images is available.
The method and software for HRPT/NOAA satellite data acquisition, AVHRR thermal channels calibration and surface temperature determination was prepared in IMWM in Kra­ków. Using prepared software, satellite data from vegetation season of 1993 were processed. As a result, maps of surface temperature on the area of Poland were printed. The medium surface temperature for morning and afternoon passes of NOAA satellites for each decade were prepared, starting from beginning of April and finishing on the end of October. Such compositions of the satellite images, allow to avoid cloudiness for most of the decades. Application of satellite derived surface temperature images on the area of Poland was discussed.
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