Accurate estimation of reference evapotranspiration (ETo) is needed for water resources management, farm irrigation scheduling, and environmental assessment. A large number of methods have been developed for assessing ETo from meteorological data. However, most weather stations around the globe are located in nonagricultural settlings of dry, bare soil surface and/or concrete surfaces. Using these weather data may cause serious errors. Satellite images on the other hand cover large cultivated areas and fields. Throughout the present study, Penman–Monteith equation was converted to a simple equation of three components for each component of which there was a linear regression presented with satellite input data. To create and test regression equations, 297 NOAAAVHRR satellite images for duration of ten years were acquired. The present investigation has been carried out in Amirkabir irrigated unit in Khuzestan province. The results indicate that the simplified equation can estimate ETo with an R2 of 0.92 and an error of 8 percent.

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