Analysis and regionalization of dry and wet spells in Western Anatolia

Objective: The objective of the study is to analyse spatial and temporal variations of various dry/wet spell indices in western Anatolia, and to regionalize extreme ones. Material and Methods: Using daily rainfall data recorded at 47 weather stations located in Western Anatolia, thirteen dry/wet spell indices, under three categories named normal, extreme and structure, were produced. L-moments method was used for regional frequency analysis of extreme dry/wet spell indices. IDW (Inverse Distance Weighted) technique was employed to produce spatial variations of averages of normal and extreme indices, and Mann-Kendall test and Sen's Slope estimator to estimate temporal variations of their annual values. For series having significant serial correlation at %5 level and high coefficient of variation Trend-Free Prewhitening procedure were applied. Results: Normal and extreme indices show a general increasing or decreasing pattern along southwestern-northeastern direction. Temporal trends are insignificant at 5 % level at vast majority of stations. Short duration dry and wet spells (≤10 days and ≤2 days, respectively) prevails. Total annual rainfall is most contributed by wet spells with 2 to 4 days duration. The most of annual total dry days comes from dry spells with less than or equal to 10 days and greater than 40 days. In regional frequency analysis for extreme indices, three homogeneous subregions are identified, and the fitted distributions are mainly Pearson type III (PE3) for subregion 1, generalized extreme value (GEV) for subregion 2, and generalized logistic (GLO) for subregion 3. Probable values for extreme indices are estimated for various return periods (2, 5, 10, 20, 50, 100 and 1000 years). Conclusion: The southwestern part of the study area is characterized by extreme dry and wet spells. Over the study period, there is no significant temporal change for dry/wet spell indices. For extreme dry/wet spell indices, the distributions Pearson type III (PE3), generalized extreme value (GEV), generalized normal (GNO) and generalized logistic (GLO) are fitted. Keywords: Dry Spell, Wet Spell, Regional Frequency Analysis, L-moment, Spatial and Temporal Variation