Evaluation of Drought Severity-Duration Effects on Watertable Elevation at Different Time Scales (Case Study: Neyshabur Plain)

The Neyshabur Plain has encountered increasing recession in watertable (WT) elevation in recent years. Therefore, prudent water resources management dictates implementation of a detailed study on groundwater accessibility in different locations of the plain. As rainfall distribution and geological settings of the various parts of the plain differ significantly, we tried to study the relationships of drought severity and duration (DSD) on the WT fluctuation in the plain. To correlate DSD with the WT elevation, we employed the Ward clustering technique to select representative piezometers at the following locations in the plain: 1. Sultan Abad Cluster, 2. Filkhaneh Cluster, 3. Amman Abad Cluster, 4. Arazie Mohandes Cluster, 5. Amir Abad Cluster, and 6. Hussein Abad Jangal Cluster. The mean annual precipitation of each cluster was calculated for the 1993-2006 duration using the inverse distance weight. Results of the Man-Kendall test showed that the slope of WT in the selected six piezometers had negative trends for different months. Therefore, the WT level was de-trended using the trend extrapolation method. Results of the correlation between the rainfall amount and "no trend" WT level indicated that the effects of rainfall on WT levels were repeated annually. Therefore, the standardized precipitation index (SPI) was devised to find the drought and wet period effects on the WT elevation. The analytical results showed that for clusters 1 through 6, the highest R2 between no trend WT elevation and the SPI with the time scales of 54, 42, 36, 48, 36 and 36 months equaled 0.405, 0.434, 0.063, 0.248, 0.362 and 0.211, respectively. Moreover, it was revealed the Aman Abad Cluster was affected by the groundwater movement and natural recharge taking place in the eastern part of the plain and Binaloud Ranges.