1. Economic Cooperation Organization Training Course on “Drought and Desertification” Alanya Facilities, Antalya, TURKEY presented by Ertan TURGU from Turkish State Meteorological Service 14 - 16 November 2006 Standardized Precipitation Index (SPI) eturgu@meteor.gov.tr

2. What is Standardized Precipitation Index (SPI)?  A statistical method for assessing rainfall  More representative than using the average rainfall as a representation of what is “normal”.  it is designed to quantify the precipitation deficit for multiple time scales such as 1,3,6,9,12,24 months  The SPI is a dimensionless index where negative values indicate drought, but positive values show wet conditions. Advantages:  minimal data requirements (only monthly precipitation data)  simple and quick  can be calculated for varying time scales  can provide early warning of drought  can help assess drought severity  can answer such questions as; when, how long, and how severe a drought is. Disadvantages (from practical applications):  more suitable theoretical probability distribution can be found to model the raw precipitation data  limitation from the standardization process of index itself.  drought measured by the SPI can occur with the same frequency at all locations  misleadingly large positive or negative SPI values may result when the index is applied at short time steps to regions of low seasonal precipitation.

3. A deficit of precipitation has different impacts on the ground water, reservoir storage, soil moisture, snowpack, and streamflow. Soil moisture conditions respond to precipitation anomalies on a relatively short scale, while ground water, streamflow, and reservoir storage reflect the longer-term precipitation anomalies. Thus, McKee et al (1993) originally calculated the SPI for 3,6,12,24 and 48 month time scales. This long term record is fitted to a gamma probability distribution, which is then transformed into a normal distribution. Positive SPI values indicate greater than median precipitation, while negative values indicate less than median precipitation. Sequence of Drought Impacts: When drought begins, the agricultural sector is usually the first to be affected because of its heavy dependence on stored soil- water. Those who rely on surface water (i.e, reservoirs and lakes) and subsurface water (i.e,ground water) are usually the last to be affected.,river,lake,reservoir

4. METHODOLOGY: The SPI computation is based on the long term precipitation data for the desired time step. It is simply calculated by taking the difference of the precipitation from the mean for a particular time scale, and then dividing it by the standard deviation. McKee et al.(1993) defined the criteria for a “drought event” for any of the time steps and classified the SPI to define various drought intensities. SPI Values Drought Category 2.00 + extremely wet 1.50 to 1.99 very wet 1.00 to 1.49 moderately wet - 0.99 to 0.99 near normal - 1.00 to - 1.49 moderate drought - 1.50 to - 1.99 severe drought - 2.00 and less extreme drought

5. Calculation of the SPI: Δ The SPI computation is based on the long term precipitation data for the desired time step. Thom (1958) found the gamma distribution to fit precipitation time series well. The gamma distribution is defined by its frequency or probability density function: where α >0 is a shape parameter, β >0 is a scale parameter and x >0 is amount of precipitation. Δ Fitting the distribution to data requires α and β to be estimated. They are estimated for each station, for each time step of interest (3,6,9,12,48 months, etc) and for each month of the year. Integrating the probability density function with respect to x and inserting the estimates of α and β yields an expression for the cumulative probability G(x) of an observed amount of precipitation occurring for a given month and time step: Δ Since the gamma distribution is undefined for x=0 and q=P(x=0) >0 where P(x=0) is the probability of zero precipitation, the cumulative probability becomes as follow: Δ The cumulative probability distribution is then transformed into the standard normal distribution to yield the SPI.

6. By using this software, spatial and temporal dimensions of meteorological droughts can be investigated from vulnerability concept based on frequency and severity of drought events at multiple time steps.

11. Drought Occurences and Analysis: ▲ The index has been applied to long-term precipitation data at 101 stations for 1951-2001 period. ▲ Here, our aim is to identify some areas vulnerable to drought at comparable time steps based on their occurence frequencies. The resulting SPI values at corresponding drought categories were mapped using Surfer which is a grid based contour software.

12. Drought Occurences and Analysis:

13. Very Severe Drought Occurences (%) at 3 Month Time Scales: Very Severe Drought Occurences (%) at 6 Month Time Scales:

14. Critical Treshold Rainfall Analysis: ▲ In general, rainfall amounts required for non-drought conditions decrease from the coastal parts toward the interiors with increasing time steps. Critical Treshold Rainfall Values for Severe Drought at 6 Month (March-April-May) Time Scales:

15. Results: ▲ In this study, frequency and severity of meteorological droughts in Turkey have been investigated from a hazard concept and a detailed analysis of geographical variations in the drought vulnerability using the Standardized Precipitation Index (SPI) is presented. Frequency of drought events at different severity categories and critical (threshold) rainfall data are computed at different time scales to identify drought vulnerability. ▲ Information on regional drought vulnerability could aid decision makers in identifying appropriate mitigation actions for future drought events and minimize its impacts. With a map of drought vulnerability, decision makers can conceptually visualize the hazard risk and convey the vulnerability information to other sectors to make sure that they will act timely and effectively to tackle with drought conditions. ▲ While the South-eastern and Eastern parts of the country are more vulnerable to moderate droughts at short time scales, the impact would be expected less at the coastal parts where the drought is only effective at longer durations and occur at moderate drought levels. ▲ At longer time scales hydrologic drought is likely to occur at the coastal parts while the interior parts will suffer from agricultural drought under severe drought conditions.

16. Thank you...