Türkiye üzerindeki kuraklığın trend analizi

Turkey was divided into 323 grids with 0.5º ×0.5º lat/lon. The Standardized precipitation evapotranspiration index (SPEI) values studied are for 1, 3, 6, and 12 months' time-scales for the periods of 1901-2015 / 1901-1981 / 1982-2015 were brought from http://spei.csic.es/, and the monthly Vegetation Condition Index (VCI) values were studied for the period of 1982-2015 and calculated depending on Normalized Difference Vegetation Index (NDVI) data brought from (AVHRR) sensors onboard (NOAA) satellite. The data for both SPEI and VCI were subjected to the Mann-Kendall's test to find out their trends, and their significance. Then the trend slope value was extracted by the Theil-Sen test. Finally, the Pettitt-Whitney test was used to find out the year of change in the trend. GIS software was used for the spatial trend analyzation. SPEI trend results suggested that drought trend prevails in most parts of Turkey with some variations when the time-scale changes from 1 to 12, however some parts such as northern, western, and some central parts of the country showed a wetter trend, with varying MKZ significance. However, unlike SPEI the VCI MKZ results ware significant for most of Turkey with not only less tendency to drought but tend to be a wetter trend. Such difference between SPEI and VCI analysis results can be attributed to the source of raw data used to obtain each index (SPEI is a meteorological-based index, while VCI is a satellite reflective-based index), that is why coincidence is not to be expected for those two indices.