Monitoring of Agricultural Drought in Markazi Province using VHI and PDSI Indices

One of the natural disasters of all is drought. Drought has long been one of the problems of Iran and has always been mentioned as a serious threat to the country. Drought is one of the natural and repetitive features of the climate and part of the climate that cannot be observed without a specific limit of occurrence and impact using high-speed ground station information. Drought rate is divided into severe, mild, moderate drought and long-term drought and short-term drought based on time components. Drought has had effects on land degradation, forest fires, reduced air and water quality, and reduced agricultural production. Frequent droughts have been a concern for many years, and on a global scale, in recent decades, especially in arid and semi-arid regions, the frequency, severity and duration of droughts have increased significantly. Drought according to different definitions in each area is divided into four main categories, including meteorological drought, agricultural drought, hydrological drought and socio-economic drought. In meteorology, drought, or in other words dry period, is the reduction of loss in a period of time compared to the average amount of rainfall in that period or the amount of long-term rainfall in the same period. Of course, if this amount of rainfall does not meet the needs such as economic and social needs. The duration or period of drought varies according to the climate of each region and depends on the location and time. In Iran, the drought period is approximately equal to the crop year. The severity of drought also varies in each climate and country. The same amount of rainfall in one country may be considered drought, while in another country, the same amount of rainfall does not indicate drought. Agricultural drought damages the economy, social conditions, agricultural products and consequently food security, so monitoring it is essential. For this purpose, agricultural drought in Markazi province has been studied. One of the factors affecting agricultural drought, soil moisture and plant evapotranspiration as a cause of water loss is the difference between the available water and its wastage. Therefore, to measure drought, an indicator that shows evapotranspiration is needed. In this research, RWDI index is used as an indicator of drought based on evapotranspiration. To obtain this index, one must first obtain the actual evapotranspiration and potential. Due to the advantages of remote sensing methods compared to traditional methods, remote sensing methods based on energy balance and SEBAL2 image processing model as well as Landsat 8 images, in dry and wet seasons, along with meteorological data to obtain heat flux Hidden, which depends on the parameters of soil heat flux, tangible heat flux and net radiation flux, is used. Using latent heat, instantaneous evapotranspiration is calculated daily and using the Penman 3 method, reference evapotranspiration is calculated. Potential method of Taylor 4, evapotranspiration and potential transpiration are obtained, and as a result, after calculating the actual evapotranspiration, the RWDI index is prepared, which shows the water shortage and the severity of agricultural drought. The results show that the values ​​of evapotranspiration are obtained with acceptable accuracy. Meteorological stations are obtained in REF-ET software. In addition, based on the results of the water deficit index, the values ​​of this index are expected to be higher in dry seasons than in wet seasons.