Spatial and temporal patterns of dry spells in western Turkey

Abstract

An analysis of spatial and temporal variations of dry spells was carried out for western Turkey using daily precipitation data of 28 stations from 1966 to 2011. Three indices and four levels of precipitation-per-day threshold were considered. Indices are number (NDS), mean length (MDS) and maximum length (MxDS) of dry spells, and the thresholds are 0.1, 1, 5 and 10 mm/day. The results showed that a general decreasing gradient from north-east to south-west is evident for NDS with exceptionally higher values at south-west at 10 mm/day threshold. Mean MDS depicts an opposite pattern to that of NDS particularly for 0.1 and 1 mm/day. Longer mean MDS values are observed at western and eastern parts for 5 and 10 mm/day. For all threshold levels, there is a clear increasing gradient from north-east to south-west for mean MxDS. Trend analysis showed that the majority of trends of the indices at all thresholds are statistically non-significant at 95% level. It can be concluded that the study area as a whole has experienced a general and slight (statistically non-significant) decreasing tendency for NDS but a general and slight (statistically non-significant) increasing tendency for MDS and MxDS. Over the same period, total annual precipitation has shown statistically non-significant decline at almost all stations. It can be concluded that the slightly decreasing tendency in total annual precipitation has been accompanied by slightly decreasing NDS and by slightly increasing MDS and MxDS. This suggests that the study area has received less precipitation and experienced less frequent but longer duration of dry spells over the period 1966–2011. The droughts in the study area have become relatively worse in terms of not only the total amount of precipitation but also duration of dry periods, although dry periods have occurred less frequently.