Evaluating the effects of climate change on groundwater level in the varamin plain

In this research, a number of paired three-dimensional Atmosphere-Ocean General Circulation Model (AOGCM) from CMIP (Climate Model Inter Comparison Project) 5 group with the base period of 1989 – 2005 have been evaluated and the output of these models was micro-scaled and calibrated by LARS-WG software. The appropriate model was selected to simulate temperature and rainfall data under the emission scenarios of RCP (Representative Concentration Pathway) 2.6, RCP4.5 and RCP8.5 for the futureperiodof2020 – 2050,andthentomodelthegroundwaterleveloftheregion,GMSsoftwareforboth stableandtransientstatesforonewater yearwascalibrated and thenwasvalidated byobservationdata. The results in the future periods showed an increase of 1 – 1.5 degrees in temperature and an increase in rainfall in the early months of the year to late spring season and a decrease in rainfall in autumn season. Generally, the RCP4.5 scenario showed slightly more annual rainfall increase over the next 30 years comparedtothebaseperiodthantheothertwoscenarios.Thetimeseriesinvestigationoftheaverageof groundwaterlevelshowsthat theimplementationof RCP 2.6,RCP 4.5 and RCP 8.5 scenarios respectively leads to an average monthly increase of 4.2, 4.3 and 4.6 cm of the groundwater level.


INTRODUCTION
Global temperature changes and its rising trend are known as climate change regarding the average weather conditions around the world. Climate change has a considerable impact on surface and groundwater resources (Hashmi et al. ). Considering that the impact of climate changes on groundwater resources is indirect and slower than surface water resources, monitoring the status of these resources and maintaining their sustainability under the influence of these changes is of great importance (Shakiba & Cheshmi ).
The first step in investigating the effects of climate changes is to examine the impact of this phenomenon on climate parameters. Therefore, in order to investigate the effects of climate change in future periods, the amount of climate variables in the future must first be simulated (Node Farahani et al. ). One of the most reliable tools for investigating the effects of climate change is the use of climatic variables simulated by the downscaling models of climatic parameters such as LARS-W that can predict climatic parameters in local scale. In this regard, numerous studies have been conducted, that are mentioned in the following. With regard to the importance of the effects of climate change on water resources, especially valuable groundwater resources, research in this field is necessary. The present research with the aim of investigating climate change and its effects on groundwater level due to rainfall and temperature has been conducted in a case study in the semi-arid climate region of Iran. Regarding the position of the region, increasing removals for agricultural water consumption has caused a drop in water level in the plain, that by conducting this research and determining the status of the aquifer under climate change scenarios in the future period, a principled planning to control removal and aquifer development plans in the region can be presented.

Study area
Varamin plain, a strategic region in terms of agriculture, has been located at distance of 40-45 km south to southeast of Tehran province. The climatic situation of this plain is in many ways similar to the climate of the central plateau of Iran and is located in arid to semi-arid climate. Varamin plain catchment basin with an area of 1,720 square kilometers is one of the sub-basins of Namak Lake, which has been located in the geographical area of 35 0 0 0″ to 36 0 0 0″N latitude and 51 0 0 0″ to 52 0 0 0″E longitude ( Figure 1). The Jajrood, Kondrood, Galanduāk and Damavand Rivers have been located in the study catchment basin, the most important of which is the Jajrood River, on which the Latian Dam has been constructed in the upstream. Latian Dam is one of the factors in the hydrological cycle of the basin and also downstream agriculture development itself has a great impact.

Hydroclimatology studies
Meteorological information including pluviometry, thermometry and climatology (relative humidity, frosty days, sunny hours, wind, evaporation and transpiration) have been collected from Varamin Synoptic Station, which is the plain station represention.The average annual temperature in this basin is 16.9 C and the warmest month of the year is July with an average temperature of 29.5 C and the coldest month of the year is January with an average

Groundwater balance in the study area
The general equation of groundwater balance is presented as Equation (1): where Q in is the sum of the effective factors in feeding and Q out is the sum of the effective factors in discharging the aquifer and ΔV is the changes in the volume of water storage in the aquifer during the balance period. On the other hand, the amount of changes in groundwater reservoir volume is calculated by Equation (2): The changes in the storage volume of the balance area in terms of million cubic meters is (ΔV), the area of the balance extent in terms of square kilometers is (A), the average annual changes in groundwater level in the balance period in terms of meter is (Δh) and the average storage coefficient of the balance area is (S y ) that based on making the amount of feeding and discharging balance, using Equations (1) and (2), has been determined equal to 6%.
The ratio of groundwater inflow and outflow after deter- position to determine the ratio of groundwater inflow and outflow from the groundwater aquifer is essential.
Also based on the results obtained from water balance parameters Underground in Varamin aquifer, The groundwater balance for 2014-2015 of Varamin aquifer is presented in Table 1.
As it is clear from the groundwater balance sheet table of the study area, the changes in inflow and outflow in the target year are negative, which can be a sign of uncontrolled extraction from exploitation wells. Therefore, the study of the effects of climate change on groundwater changes in the study area will be more important than ever.

Groundwater modeling by GMS software
In this research, GMS10.3 software was used to establish a relationship between GIS software and MODFLOW code which k is the hydraulic conductivity, h is the potential load, w indicates the volume flux per unit volume, which shows the feeding and discharge, s s is the specific storage for porous materials, and t is the time.

Conceptual and numerical model of varamin aquifer
The conceptual model of the aquifer is a three-dimensional view of structural, hydraulic, hydrodynamic, and so on characteristics, which has been obtained based on the analysis of the discharge data of exploitation wells and their return water ratio, observation wells, aqueducts and springs, the ratio of surface feeding (rainfall and runoff) to the aquifer, temperature and evapotranspiration potential and boundary conditions of the aquifer (Figure 3). The more data is obtained from the aquifer, the closer the conceptual model is to the real conditions. In general, in Varamin plain, there is practically a main free aquifer corresponding to the groundwater balance area between the clay layers in its southern parts, and the wells are also located in this main layer. The highest aquifer thickness is in the north of the plain with 280 meters and the lowest alluvial thickness is in the southwest of the region with 150 meters. The bedrock of the main aquifer has generally been composed of clay sediments, marl, congenital Miocene and myopliocene   Error Criterion were used.

Downscaling
Due to the low resolution power of AOGCM models, it is necessary and essential to make them micro scale. In this research, in order to downscale the data to generate climate change scenarios, the LARS-WG stochastic climate  Due to changes occurring in climatic parameters of temperature and rainfall under various RCP scenarios, the quantitative behavior of aquifer will also have fluctuations.
Based on these changes and comparing it with the existing conditions, the effects due to climate change of the region on groundwater can be realized.

Evaluating AOGCM models
The performance of AOGCM models in the base period of 1989-2005 was evaluated with observation data of the case study area. According to the results of Table 2, the EC-EARTH model has shown a high correlation coefficient in simulating temperature and rainfall compared to other models, The value of correlation coefficient (ρ), represents the linear relationship between the simulated data and the observations, the value of which is between zero and one.
The closer the value of ρ is to one, the stronger the linear relationship between the two values. so the EC-EARTH model was acceptable and was selected for further study.

Calibration results of LARS-WG model
To ensure the model's ability in generating data in the future, the data micro-scaled by the model were compared with the observation data at the Varamin synoptic station. the results of the Kolmogorov-Smirnov and T-test on the parameters of rainfall, minimum temperatures and maximum temperature and daily radiation in the study station showed that the p-values of rainfall parameters, minimum temperatures, maximum temperature and daily radiation in the study stations, in most months is reliable at the significance level of (90%), which indicates that the LARS-WG model, has the necessary ability to simulate rainfall, minimum    noted that in all zones, the increase in groundwater level in the next three decades under the RCP8.5 scenario is more than the other two scenarios. Also as mentioned before,

DATA AVAILABILITY STATEMENT
Data cannot be made publicly available; readers should contact the corresponding author for details.