Assessment of drought condition in Guilan Province using the Keetch–Byram Drought Index (KBDI) in accordance with the Percent of Normal Precipitation Index (PNPI)

Drought is a natural disaster that occurs over a long period of time, and affects human societies through negative effects on water resources and agriculture, and consequently the economy. Forests are an important part of the ecosystem that covered about a third of the land surface. Occurrence of wild fires has been degraded the quantity and quality of the forest. The phenomenon of fire is one of the drought complications that cause irreparable damage to the ecosystems. The drought duration and its repetition have been reported more than other natural disasters. The definition of drought has continually been a key issuefor drought monitoring and analysis. The drought is a condition of insufficient moisture caused by a deficit in precipitation over some period of time. Difficulties are primarily related to the time period over which deficits accumulate and to the connection of the deficit in precipitation to deficits in usable water resources and the underlying impacts.

There are several indicators for drought identification and interpretation. The calculation of Keetch–Byram Drought Index (KBDI) requires a long series of meteorological data. When the data are available its calculation is straightforward. However, it is strictly valid for the location of the meteorological station and its immediate vicinity. The temporal and spatial characteristics of drought were investigated using Percent of Normal Precipitation Index (PNPI) and KBDI, at 4 stations in Rasht, Bandar Anzali, Astara and Manjil in Guilan province during a 30-year time period (1987-2017).

According to the results, the drought index KBDI, the monthly time series of the available data were categorized as follows; normal drought class, 96.3%, medium drought, 3.6%, and severe drought, 0.5%. On the other hand, by analyzing the PNPI index, the percentages of normal, moderate, severe and very severe drought classes were 68.9% and 8.1% and 7.5% and 15.4%, respectively, which are almost same as the KBDI results. Taking into account the cumulative percentages of the drought classes of the area, can be deduced from the normal dryness of the area. On the other hand, the most severe dry year, according to the KBDI index was observed in 1988, while 2010 year was the most severe drough year based on the PNPI index. The longest dry period was roughly the same in both indicators, which was determined between 2004 and 2007 years.

The results showed that the probability of severe drought based on the analysis of the past time series is widespread in the months of June and July, which is important in the management of water resources and vegetation. Also, the drought intensity in this province is gradually increasing. Considering the decreasing trend of rainfall in the region, we can expect more droughts in the future. It is recommended that the managers and experts pay special attention to the prevention and management of water resources and vegetation in the two months of July and August during their executive activities.