Spatial and Temporal Monitoring of Climate Drought using SPI and VCI Drought Indices

Drought as a creeping natural disaster is one of the main hazards that affects agriculture, food security, and the general livelihood of the population worldwide. Temporal-spatial variations of drought in a watershed can have numerous effects on the engineering, management, and planning of water resources. Drought is a complex phenomenon with different effects. Drought is a chronic, potential natural disaster characterized by a prolonged, abnormal water shortage. However, determination of drought onset, duration, and recovery is often difficult due to the differences in hydro-meteorological variables, socioeconomic issues, and the complex nature of water demands in different areas over the world. Drought occurrences cause serious problems to different parts of society, such as agriculture, energy generation, recreation, and ecosystems Therefore, drought indices are used to determine the severity and extent of drought. Most of these indices are based on meteorological criteria and take into account variables such as such as soil moisture, temperature, and precipitation. Precipitation is one of the most important parameters used in the calculation of drought indices. Accordingly, drought occurs when rainfall falls below normal for a period of time. In 1993, McKay et al. proposed a new definition of drought called the Standardized Precipitation Index (SPI), which believed that it was dimensionless and applicable at any time and place. Because rainfall measuring stations are scattered and access to rainfall measurement is often do with a time delay, other methods for drought monitoring are essential. In this regard, satellite information and remote sensing data can be used. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation, especially the Earth. Remote sensing is used in numerous fields, including geography, land surveying, and most Earth science disciplines (for example, hydrology, ecology, meteorology, oceanography, glaciology, geology); it also has military, intelligence, commercial, economic, planning, and humanitarian applications. In current usage, the term "remote sensing" generally refers to the use of satellite or aircraft-based sensor technologies to detect and classify objects on Earth. It includes the surface and the atmosphere and oceans, based on propagated signals (e.g. electromagnetic radiation). With the provision of different satellite data and the widespread use of them, it is possible to study drought using this technology. The advantages of satellite images include increasing the sampling points, wider coverage area, higher resolution and lower cost. The purpose of this study is the spatial and temporal drought monitoring in East Azerbaijan province using ground stations and satellite data. Therefore, indices (VCI and SPI) were used during the years 1993-2016. Rainfall and vegetation conditions were two variables used. These two variables were evaluated in two periods of dry and wet time. The SPI and VCI indexes were calculated using ground stations and satellite data. The SPI index has statistical consistency and is capable of showing both short and long-term drought effects in variable timescales of precipitation anomalies. The use of different time scales allows the effects of a precipitation deficit on different water resource components (groundwater, reservoir storage, soil moisture, streamflow) to be investigated. Due to SPI's probabilistic nature, its comparison in various regions is possible. The Vegetation Condition Index (VCI) compares the current NDVI to the range of values observed in the same period in previous years. The VCI is expressed in percent and gives an idea of where the observed value is situated between the extreme values (minimum and maximum) in the previous years. These two variables were evaluated in both dry and wet years. Also, the trend of drought variations in the selected period and the efficiency of the mentioned indices were investigated.The results showed that during the statistical period, the study area was faced with different classes of drought, but most stations had slight to moderate drought conditions. Dry and wet years were obtained based on SPI and VCI indices and it was observed that 2007 is the driest year and 2009 is the wettest year. The results of regional zoning showed that drought is one of the climatic characteristics of the region and the southern parts of the province often had more critical conditions. It was observed that the frequency of mild drought in the northern regions of the province and the occurrence of severe drought in the southern regions of the province were more likely. Also, the trend of variations in the SPI index in terms of Shen line slope and Menkendal values was positive. The results showed that the VCI index was a very suitable method for estimating drought through remote sensing techniques. There was a good agreement between the results obtained from ground stations and satellite data. Therefore, in areas where meteorological stations are scattered or non-existent, satellite data can be used to estimate drought.