studying temporal-spatial changes of frost in zayanderoud basin

zayanderoud basin is one of the important agricultural areas in country level because of climatic variety and resuscitative water of zayanderoud river. since frost phenomenon especially late spring frost incurs great damages on fruit trees and agricultural products of this region, it is necfessary to study frost in this basin.in this research, the intensity of frost during cold period and spring season in zayanderoud basin in five thresholds: mild (0 to -5 0c), moderate (-1.5 to -10 0c), strong (-1.10 to -150c), very strong (-1.15 to -20 0c) and extraordinarily strong (lower than -200c) was studied using daily minimum temperature statistics of 13 meteorological stations during 16-years statistical period (1992-1993 to 2007-2008). frequency zoning map of different frost intensities was also drawn in arc map environment. the results of research show that all studied stations of zayanderoud basin experience mild to very strong frost during cold period, but the extraordinarily strong frost only is occurred within elevated regions (above 2000 m). in spring season, mild frost is occurred in until late farvardin month and within elevated stations until third decade of ordibehesht and moderate frost is only occurred within 2000 m stations and above in farvardin month. strong frost is only occurred in the studied north-western region. according to zoning maps, north-western zayanderoud basin is hazardous in terms of the frequency of occurrence of different frost intensities.