An analysis of drought events for central plains of Iran through an employment of NOAA-AVHRR data

Drought is a major problematic phenomenon for the mostly semi-arid country of Iran. The north centralregions of Iran (north of Esfahan and Ghom province) have suffered from severe droughts several times duringthe last three decades. The frequent occurrence of drought in these regions is due to low and inconsistentprecipitation, abnormally high temperatures, increases in surface albedo and evapotranspiration; especiallyduring spring. The surface characteristics of Kashan and Ghom regions consist of salty flats and sandy hillscovered by sparse vegetation. The average elevation of the area is 1987 mASL. In the present work, NOAAAVHRRdata have been employed to assess vegetation indices and environmental conditions in the study regionfor the years 1998 – 2004; which are then compared to actual ground data such as rainfall, temperature andrelative humidity for a detailed drought analysis. Spatial and temporal variations of meteorological droughts inKashan have been analyzed using Standardized Precipitation Index (SPI) at annual and seasonal scales, and havebeen generated through GIS based interpolation. Vegetative and thermal drought indices have been calculatedusing NDVI, VCI, and TCI values derived from NOAA-AVHRR data. Results from applying remotely senseddata show that this area is generally of low vegetation index values. The artificial forests and farmlands at thefoothills of the mountains northwest of Kashan region showed relatively high vegetation index values. TCI andVCI generally show a good relationship with meteorological observations. According to the output of utilizationNDVI and VCI, 2000 and 2001 years were characteristic of drought conditions, while 2002 and 2004 did nonrepresent drought years. Since TCI index is completely dependent on surface temperature, the combined analysisof the May and April results were influenced by the low land surface temperatures experienced in April, andtherefore it was very different from the other indices i.e. NDVI and VCI. Therefore thermal IR channels can beemployed used to monitor drought conditions in the semi-arid and arid regions of Iran to assess these regions,environmental conditions.