Remote Sensing (RS) and GIS play an important role in drought risk evaluation (both metrological as well as agricultural drought) in conjunction with SPI. Interpolation of SPI In the GIS may indicate zones prone to drought. The correlation and regression analysis between NDVI and SPI with reference to previous NDVI equivalent threshold based on trends in NDVI and foodgrain yields may specify agricultural drought. Similarly relationship established rainfall deficient values from normal and NDVI with respect to national or regional threshold may define metrological drought. Naturally, a temporal series of RS imageries are required to work out NDVI. The initial step in calculating the Standardized Precipitation Index (SPI) is to determine a probability density function that describes the precipitation series under analysis. Once this probability density function is determined, the cumulative probability of an observed precipitation amount is computed. The inverse normal function is then applied to the cumulative probability. However, it should be kept in mind that SPI is based on probability (chance) and several researchers have reported that SPI underestimates the severity of drought.

Sir,the information u provied is very useful for me,i have to first calculate the spi with the rainfall data.then relate this with MODIS(NDVI) images.to calculate spi m using formula dat is (monthly rainfall-rainfall mean)/standard deviation.Is this right procedure??

I think you are closer to solve your problem. However, you have to make literature review so that the problem becomes clearer to you. If you need some reference, I can do it.

You might look at studying tree rings in semi-arid areas of you country. The width

of the rings is an indicator of both winter and summer precipitation. Since some species of trees live for hundreds of years, they can help determine long term patterns

in precipitation, including long term cycles with low precipitation.

In the United States, the Dendro-Chronological Laborator at the University of Northern

Arizona, Flagstaff, Arizona maintains and studies tree rings for various areas of the

Western United States, as drought cycles vary from region to region.

In addition China is now studying cave formations in semi-arid areas as well. The thickness of the individual layers deposited each year also varies with changes in

precipitation.

Cave formations thicken at about 5000 years to 1 inch of new formation, while

trees add 1 inch in 10 years to 100 years depending on the species and its

access to water and nutrients.

In studying climate change and drought cycles, you will need very long term data sources. Nature tend to provide these resources if you look in the right places.

SPI can also be calculated from the USGS website SPI calculator

I have a raster data set of precipitation how can I calculate SPI please help

Hi,

For calculating SPI, it is better to use Gamma distribution function for precipitation data, also the length of data is so important. We should consider more than 30 years for accurate result.

Regards,

Nasrin

www.agrimetsoft.com