SPATIAL ANALYSIS OF DROUGHT IN GROBOGAN REGENCY USING STANDARDIZED PRECIPITATION INDEX ( SPI )

Drought is a natural disaster caused by the phenomenon of weather irregularities. In certain conditions, the drought is not so dangerous, but in a severe drought, it could threaten human life in many sectors such as in agriculture, economic and social sectors. Grobogan is an area in Central Java province which always suffers from drought during the dry season. This study aims to determine the spatial distribution of drought in Grobogan using the Standardized Precipitation Index (SPI). The data used is secondary data of Grobogan rainfall for 21 years (1994-2014) at 21 rainfall stations. The results showed that the incidence of droughts lasted for five months, which is in May, June, July, August and September. Early drought occurred in May with a value of SPI -1.01 to -1.15 with the drought level of moderate in an area of 123494.97 ha or 62.50% of the area. Drought in June with SPI value of SPI -1.11 to -1.49 occures in the entire region with the drought level of moderate. Drought in July with SPI values between -1.29 to -1.77, consists of 71313.85 ha or 36.09% area in a moderate drought level and 126272.1 ha or 63.91% area in a severe drought level. The peak of the drought occurred in August with SPI value of -1.53 to -2.51, a region with extreme drought level covering 74394.37 hectares or 37.65% area and severe drought level covers 123,191.6 hectares or 62.35% area. In September drought was reducing with SPI value of -1.06 to -1.56 which consists of 123 495 ha or 62.50% area was in moderate drought level and 74091.03 ha or 37.50% area was in severe drought level.


A. Introduction
Drought is a deficiency of precipitation over an extended period of time, usually a season or more, which results in a water shortage for some activity, group, or environmental sectors.However, in terms of typologies, droughts are commonly classified as meteorological, agricultural, hydrological, and socio-economic.
(UN/ISDR, 2007).Drought is one of the most complicated and least understood natural hazards, affecting more people than any other hazards (Wilhite, 2003).
Unlike earthquakes, floods and cyclones, drought occurrence is a gradual process with long-lasting effect.The drought from 1980 to 2000 posed a threat to the food security and human mortality all round the world.On average, 28 % of the geographical area of India is vulnerable to drought.Meteorologically, ± 19 % deviation of rainfall from long-term mean is considered normal in India.Deficiency in the range 0f 20 to 59 % represents "moderate" drought and more than 60% "severe" drought (Samra, 2004).

C. Results and Discussions
Based on drought index calculations by the SPI method in Grobogan at 21 stations using rainfall data for 21 years  can be seen that the drought lasted for five months from May to September world's vulnerability to drought is increasing within the last 25 years, and climate projections show will get worse in the future.Based on the data IRBI (Indonesia Disaster Risk Index), Grobogan including one district that is prone to drought (BNPB, 2014).Droughts are disasters that have broad impact on the community because it involves the need for water for domestic and agricultural.Drought is one of the natural disasters that occur slowly (slow-onset disaster), lasting until the rainy season arrives, wide impact, and cross-sector.An example of the drought that hit California in 2015 (and has been for 4 years) has caused negative impacts in the agricultural sector causing huge economic losses, totaled $ 2.7 billion (et.allHowitt, 2015).According to Partridge (2002), rainfall in Indonesia affected by the El Nino Southern Oscillation (ENSO).But the size of the effect varied from one place to another.Based on the results research of Manik, et.all (2014), a real decline in rainfall in Lampung province occurred in 1994 and 1996 in conjunction with the El-Nino phenomenon that in many studies are known to be closely related to the dry season in Indonesia.It is also similar to research results of Lilik Supriatin and Martono (2016) in Cilacap, it was recorded that in 1984 -2013 there were four time occurrences of El Nino Limitations (https://climatedataguide.ucar.edu/climatedata/standardized-precipitation-index-spi):• As a measure of water supply only, the SPI does not account for evapotranspiration, and this limits its ability to capture the effect of increased temperatures (associated with climate change) on moisture demand and availability • Sensitive to the quantity and reliability of the data used to fit the distribution; 30-50 years recommended • Does not consider the intensity of precipitation and its potential impacts on runoff, streamflow, and water availability within the system of interest B. Research Method Standardized Precipitation Index (SPI) is a method developed by McKee et al (1993).This method is a model for measuring the shortage / rain deficiency at various periods based on normal conditions.For analysis of drought using Standardized Precipitation Index (SPI) required monthly rainfall data with a longer period.For this study, analysis used rainfall data for 21 years (1994-2014) at 21 rain stations SPI value calculation based on the number of gamma distribution is defined as a function of frequency or probability of the following events.Values α and β are calculated for each rainfall station using the formula: For x = 0, then the value of G (x) be: Note: q = number of rain events = 0 (m) / amount of data (n).SPI value is the transformation of the gamma distribution (G (x)) into a normal standard with an average (mean) of 0 and a difference of 1, or using the following formulaSPI calculation are classified according to criteria of the sharpness of the drought index according to WMO.

Picture 5 .
Maps of SPI drought index at Grobogan Regency in SeptemberD.Conclusion and RecommendationThe results of drought index calculation in Grobogan Regency with SPI method based on rainfall data for 21 years at 21 rainfall stations showed the drought for five months, starting from May until October, and the peak of the drought occurred in August, with 14 rainfall stations (123191.6 ha or 62.35%) belongs

Table 1 .
Sharpness Drought Index SPI

Table 3 .
SPI Drought Index at Grobogan Regency in May Picture 4: SPI drought index at Grobogan Regency in August 5.