بررسی تاثیر تغییر اقلیم بر تغذیه ناشی از بارش در حوضه آبریز شیرامین

Assessment of the effect of climate change on recharge resulted from precipitation in the Shiramin basin

Extended abstract
Abstract
Extended Abstract
Introduction
Increases of CO2 concentrations and other greenhouse gases will have a significant effect on global climate which in turn influence recharge to the aquifer. Groundwater recharge plays a vital role in replenishing aquifers, which impacts groundwater resources availability. A large amount of water needed in different parts of arid and semi-arid areas is provided through groundwater resources. Understanding the process of groundwater recharge is fundamental to the management of groundwater resources. The quantity and quality of water resources have been reduced by unprotected exploitation in the recent decades. In addition, climate change and global warming will increase the severity of problem. Therefore, predicting the effects of climate change on groundwater recharge play an important role in the future management of these resources. While climate change affects surface water resources directly through changes in the major long-term climate variables such as air temperature, precipitation, and evapotranspiration, the relationship between the changing climate variables and groundwater is more complicated and difficult to quantify. In this paper, the climate of Shiramin basin in Iran is studied with respect to changes in the precipitation and temperature data. As another aim of this study, we use the Hydrologic Evaluation of Landfill Performance (HELP) to simulate the recharge, runoff and evapotranspiration in Shiramin aquifer.
Materials and methods
The Shiramin basin is located in the northwest region of Iran. This basin is a sub basin of the Urmia lake basin. The study is based on the data from Tabriz station, and a statistical weather generating tool, SDSM is used to downscale the climate change based on the HADCM3 model under A2 and B1 scenarios. To evaluate the performance of different models and to draw a comparison, were used evaluation indices including standard Error (SE) and Root mean square error (RMSE).
In this study, the physically based hydrologic model HELP3 is used to estimate regional groundwater recharge via a water budget approach. Hydrologic Evaluation of Landfill Performance (HELP) computer program is a quasi-two-dimensional hydrologic model of water movement across, into, through and out of landfills. HELP3 requires various climatic, soil, and design data to generate daily estimates of water movement through a soil column
In general, HELP3 simulates all of the important processes in the hydrologic cycle including surface runoff, evapotranspiration, vegetative growth, soil moisture storage, and vertical unsaturated drainage for each discrete layer soil column. Darcy’s law models the vertical water movement for each soil layer using unsaturated hydraulic conductivity computed by Campbell hydraulic equation along with Brooks-Corey parameters. Daily infiltration is determined indirectly from a surface water balance by assuming that infiltration is equal to the sum of rainfall and snowmelt, minus the sum of runoff, surface storage, surface evaporation, and plant transpiration. Vertical percolation leaving the bottom of the deepest model layer is assumed to reach water table and eventually become groundwater recharge.

Results and discussion
According to the simulation of the HadCM3 model under the A2 and B1 scenarios during the simulated period, the average monthly temperature in all months will increase in the study area.
In both scenarios, maximum increase in average temperature and maximum decrease in precipitation respectively will occur in July and August compared to the base period. The highest percentage of precipitation in the Shiramin basin will be consumed for evapotranspiration due to global warming. The highest percentage of evapotranspiration for A2 and B1 will be occur respectively in 2039 and 2040 years. In other words, in forecast years over 80% of the precipitation will evaporate. In A2 scenario year of 2038, with minimum average annual precipitation and year of 2027 with maximum average annual precipitation are respectively as the driest and wettest years in terms of rainfall. Despite more precipitation in year of 2027, large amount of precipitation will be consumed for evaporation. Warmer winter temperatures will reduce the extent of ground frost and shift the spring melt from spring toward winter, therefore in both 2027 and 2037 years the maximum runoff and recharge will occur in winter. Under B1scenario, 2038 will be the driest and 2033 the wettest in terms of precipitation. Recharge and evapotranspiration rate in both low and high precipitation conditions under B1 scenario is higher than A2 scenario. Study of seasonal recharge indicates that in the study area, the maximum groundwater recharge will be done in a very short period of time and in winter due to the existence of more water in the system and the inactivity of evapotranspiration. In the summer months, due to the lack of precipitation, runoff and groundwater recharge will not change compared to the base period and their amount will be zero.

Conclusion
Groundwater recharge is a fundamental component in the water balance of any watershed. However, because it is nearly impossible to measure recharge, and in some cases, base flow has been used as an approximation of recharge. In this research, the global atmospheric circulation model, HadCm3, under A2 and B1 scenarios were used to study the effect of climate change on evapotranspiration, runoff, and recharge via precipitation in Shiramin basin for period 2020-2040. SDSM model used for downscaling. The result of downscaling model showed that under both scenarios, in the study area, the average annual temperature will increase and average annual precipitation will decrease compared to the base period. The HELP model was used to simulate the amount of recharge, runoff and evapotranspiration in the future. The results showed that due to increase temperature, significant amounts of precipitation would be evaporated. In addition to reducing precipitation and increasing temperature, hydraulic properties and soil moisture play an important role in recharge. During the period of 2040-2020, the maximum annual recharge in A2 and B1 scenarios are respectively 12.5 and 15.93 percent of the total annual precipitation. Minimum recharge for A2 scenario is 3.7 and for B1 scenario is equal to 6.6 percent of the total annual precipitation. The results of this study could advise designers and managers of this region to take suitable actions in securing the water supply.
Keywords: Groundwater recharge, Runoff, HELP3 model, Spring melting, Shiramin basin.