Validation of Four Meteorological Drought Indices Based on the Growth Rings Beech, Poplar, and Willow in the Mashhad Area

Water resources management and its planning are increasingly impact by the climate change، especially in the arid and semi arid area. As the tree rings integrate the effects of internal water stress، their thickness maybe correlated with the amount and distribution of precipitation during the growth period. Therefore، based on the recommendations of the world Meteorological Organization، 20 years of seasonal and annual precipitation were collected from the Mashhad Synoptic Station، and correlated with the growth rings of beech (Fagus Orientalis Lipsky)، poplar (Popolus) and willow (Salix). These trees were irrigated solely by precipitation. Four indices، namely، rainfall anomaly index (RAI)، Standard Precipitation index (SPI)، percent of normal precipitation index (PNPI)، and Z score were evaluated to find the best fit between the growth ring thickness and the seasonal and annual precipitation. On the seasonal scale، the number of drought indicators was relatively the same for all of them except the RAI; however، they were different using the PNPI. Apparently، the PNPI is not a good indicator in the annual estimations; lengthening the precipitation duration decreases its prediction rigorousness. The result was a high correlation between drought indices and the growth ring thickness during spring and winter than in other seasons. Willow’s response to the amount of precipitation was higher than the other two trees. It should be so، as it is a phreatophyte. The results indicate clearly that there are sufficient correlations between the thickness of the growth rings and the chosen indices. All of the indices indicated the occurrence of about eight drought periods with various degrees of sensitivity. The SPI proved to be the weakest drought indicator in the group، as it showed the least sensitivity to the occurrence of droughts; tree rings increased their thickness when the SPI indicated the presence of a drought.