Precipitation trends and suitable drought index in the arid/semi-arid southeastern Mediterranean region

The Mann-Kendall test was applied on homogeneous Jordanian and Israeli precipitation series to test for trends. Overall, 37 precipitation series with continuous data for the time period 1961-1990 were identified as homogeneous. During this time period a non-significant negative precipitation trend could be observed on annual sum. The same analysis was thereafter applied at the same stations, but with longer time series. The data availability differed from station to station, but the mean time period with available data was 1950-1997. The new result showed a stronger negative precipitation trend. This demonstrates how time series of different length can influence the result of trend analysis. As a step to identify a suitable drought index for the region, the correlation between precipitation and the Normalized Difference Vegetation Index (NDVI) received from remote sensing was investigated. A correlation would indicate that a precipitation based drought index could be suitable to assess the impact of possible precipitation trends. A high correlation was generally observed on natural vegetated areas; the correlation on irrigated land was lower. This suggests that a drought index based on precipitation alone may be applicable to the region, but the performance on non-rainfed agricultural areas may increase with consideration to irrigation.