Trend Analysis and Change Points in Time Series of Water Level of Shallow and Deep Wells in Gorganrood Watershed

A quantitative study of groundwater resources and accurate monitoring of changes over time, especially in areas facing limited water resources, is considered essential for proper management and sustainable exploitation of these resources. Golestan province, one of the semi-arid provinces of Iran has faced a drop in the groundwater level and an increase in the salinity of the groundwater due to the excessive withdrawals from the groundwater table and the reduction of atmospheric precipitation in the past few years. Gorgan Plain with an area of about 4727 square kilometers is one of the largest plains in Iran and the most important plain of Golestan province in terms of water supply for agricultural and drinking purposes. In this plain, there is a network of piezometers and observation wells that include continuous monthly measurements for more than 30 years. The objective of this research was to investigate the changes in the groundwater level of shallow (30 years (1989-2018)) and deep (22 years (1997-2018)) wells. The Man-Kendall method was used to reveal the trend and Pettitt, Normal Standard, and Buishand methods were used to identify sudden change points in a time series of groundwater levels in 49 shallow wells and 12 deep wells. The results of this research showed that the groundwater level in most of the studied wells had a significantly decreasing trend at a significant level of 5%. Also, the largest amount of groundwater loss was in the southern and southwestern parts of the plain, which can be attributed to a large amount of water taken from the wells due to their proximity to urban areas and some local conditions such as the proximity of the wells of this area are located in altitudes and at the entrance border of the aquifer. In the same way, as it rises, the fall decreases in the middle of the plain, and the amount of fall decreases in the northern areas and the edge of the Caspian Sea. It can be related to the proximity to the Caspian Sea and the high water table, and as a result, the inappropriate quality of water and land (high salinity and low fertility), which has caused the water withdrawal from this area to be less.