The annual potential evaporation far exceeds the annual rainfall (Figure 2) with a mean
annual amount of approximately 1505 mm (1975–2010). The area has complicated land use
characteristics, mainly consisting of agricultural and urban/residential areas. Groundwater
has been used for various purposes, such as drinking, agricultural, domestic and industrial
needs. The most important economic activity of the area is agriculture, the chief crops are
garlic (Allium sativum), potato (Solanum tuberosum L.) and wheat (Triticum aestivum L.),
with actual irrigation being lower than total theoretical demand, as there is a considerable
deficit in relation to the amount of irrigated land.
Data Processing. For this research 29 years (1973-2002) precipitation data of 13
meteorological stations were studied. The data of adjacent stations were also taken into
consideration to reduce errors in data interpolations. The run test was used to confirm the
homogeneity of the data. Next, the missing data were added or supplemented and
erroneous data were corrected by using 29 years common data-period.
SPSS Software was applied to carry out the analysis. Regression analysis was done
between the stations and the missing data were estimated using data of different stations
with higher correlation coefficient and SPI values. Later, the computed SPI values were
utilized for further studies to analyze the beginning and terminating intensity, covering area,
frequency or return period, duration and the probability of occurrence of drought.
The following equation was used to calculate the probability of occurrences and the
corresponding risk values: