Investigating Changes in the Intensity of Short-Term Rainfall in the Last Half Century in the Context of Data Scarcity (Case Study: Rasht City)

Due to the occurrence of drought periods and increased exploitation of the plain, alongside of the agriculture expansion and the rise in groundwater extraction, the level of the groundwater table in the Zanjan plain has decreased, which has resulted in a fall in the level of the groundwater table and a decrease in the efficiency of the wells. The purpose of this research is to investigate and Zoning of Temporal and Spatial of the depth, level and changes of the level of groundwater table of Zanjan plain, trends of depletion and annual changes and temporal monitoring of the drought (GRI) of groundwater resource of Zanjan plain based on the representative hydrograph.

In order to carry out this research, the statistics of 69 observation wells are analyzed during 3 time periods (2001-2008, 2008-2013 and 2013-2018). At first, statistical data is collected and after importing the data into the Geographical Information System (GIS), maps of iso-depth, iso-level and zoning changes of the groundwater table are prepared with the interpolation method. The plain hydrograph has been prepared to investigate the long-term changes and fluctuations of groundwater table, as well as to detect the periods of rise and fall of the water level, during the statistical period for the Zanjan plain. Long-term changes and fluctuations of groundwater table are also drawn in the Excel. The phenomenon of drought in the long term causes the reduction of water resources through the drying up of surface and underground streams. For this purpose, the GRI index has been used to temporal monitoring of drought of groundwater resources in Zanjan plain.

The results obtain from the maps of the iso-loss show that the highest level of groundwater in the first time period (2013-2018) in the northern part of the plain is 12.4 m, in the second time period (2008-2013) in the southern part and the eastern part of the plain has dropped by 9.4 m and in the third period (2001-2008) in the northwestern parts, it has dropped by 14.5 m. The hydrographs of the groundwater table illustrate that during the period of 20 years, the depletion of the Zanjan plain table is 12.9 m, which means that the water level has depleted by 0.65 m annually on average. This actually indicates the negative changes in the groundwater level in the studied area. The results of the changes in the volume of the Zanjan plain demonstrate that the volume deficit of the Zanjan plain is 705.8 MCM. Also, temporal monitoring of the drought in groundwater resources of the Zanjan plain with the GRI index shows a very descending trend towards drought during the twenty-year period.

Groundwater is the main source of agricultural needs, especially in arid and semi-arid areas; therefore, having a sustainable agriculture requires careful management and planning on how to use these resources, which itself requires sufficient knowledge about the spatial changes of the groundwater level in a certain period. In the present research, the investigation of the depletion of the groundwater table, especially in the piezometers of the northern and side parts of the Zanjan plain, shows that this water disaster (groundwater level depletion of about 13 m) is mainly due to the improper management of water resources and the increase in the issuance of permits for deep and semi-deep wells with 61% growth in the last two decades. The negative balance of the plain is so severe that even the good rains of the last few years have not been able to stop the process of lowering the groundwater table. It is recommended to prevent the continuation of the Zanjan plain volume decrease, to manage and control the exploitation of the wells and to prevent any over-exploitation, as well as to use the implementation and development of plain projects to supply the shortage of water resources and feed the plain.