An Evaluation of the Impact of Climate Change on Climatic Parameters and Dry and Wet Spells in the next 100 years using Combining IDW and Change Factor Methods (A Case Study: in Tehran-Karaj Subbasin)

1- Introduction To investigate climate change impact on different sources in the future periods, at first climatic parameters should be simulated under climate change effect. Various methods simulate the parameters, but the most valid method is application of AOGCM (Atmosphere-Ocean General Circulation Model). A general circulation model is a three-dimension mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources (radiation, latent heat). These equations are the basis for complex computer programs commonly used for simulating the atmosphere or ocean of the Earth. Atmospheric and oceanic GCMs (AGCM and OGCM) are key components of global climate models along with sea ice and land-surface components. AOGCMs are widely applied for weather forecasting, understanding the climate, and projecting climate change. Estimates of global warming are generally based on the application of general circulation models, which attempt to predict the impact of increased atmospheric CO2 concentrations on the weather variables. In this study, it is tried to investigate the effect of climate change on five climatic parameters including maximum, minimum, and average temperatures, rainfall and the relative humidity in Tehran-Karaj sub-basin using HadCM3 model under A2 emission scenario for three future periods containing 2010-2039, 2040-2069 and 2070-2099 as near, middle and far future periods, respectively. The years 1976-2005 is considered as the base period. 2- Theoretical framework Atmospheric carbon dioxide levels have been recorded continual increases since the 1950s, a phenomenon that may significantly alter the global and local climate characteristics, such as temperature and available water resources. IPCC (Intergovernmental Panel on Climate Change) predicted that the world mean temperature will be increased up to 1°C by the year 2050 and up to 3°C by the end of the next century. AOGCMs are the most suitable tools for studying the climate change phenomenon,; however, regarding large-scale spatial resolution of these models, regional scale is not possible. In other words, the models simulate climatic parameters in large scale. Whereas comparing the output for historical periods with observed data shows differences. Therefore, various methods of downscaling are created. 3- Methodology In this paper, IDW method is presented with eight grids and climate change scenarios are achieved using change factor method. Making time series of climate scenario for the future periods, the change range and its averages in different periods were calculated. In the first step, the monthly mentioned climatic parameters for the base and future periods for main box and the surrounding eight boxes were extracted from IPCC website. Then spatial and time downscaling processes were carried out using IDW and change factor methods, respectively. 4- Discussion It is predicted that the minimum temperature will increase up to 0.17-1.65, 1.38-3.61 and 2.28-6.4 degrees Celsius for the three periods, respectively. These values for maximum temperature are 0.43-2.23, 1.79-4.16 and 3.23-7.59 and they are 0.63-2, 1.74-3.92 and 2.93-7.02 for average temperature. Taking rainfall into consideration, all months except November and January have decreasing rainfall percentage which is more significant in warm months. Relative humidity has also decreased in all periods and months however, it is more severe in warm months than cold months. Most changes are related to 2070-2099 period. Rainfall moving average has a decreasing trend when approaching the future period indicating increasing dry spells compared to the base period. 5- Conclusion and Suggestions In all periods, the three parameters relating to temperature will increase with the far future period showing maximum differences. The reverse of this situation will happen for relative humidity parameter; especially it will have high severity in warm months. The changes for rainfall are not significant but a decreasing trend is predictable. Finally, the impact of climate change on dry and wet spells is assessed using a 5-year moving average manner. The results verify more effects approaching the far future period. The results also indicate increasing temperature parameters and decreasing relative humidity in the future periods in comparison to the base. For example it is predicted that the average temperature parameter will have increased between 2.9 to 7 degrees Celsius up to the end of future period. Moreover, a gradual decrease in rainfall approaching the far future period increases dry spells. The estimation of long period changes in climatic and hydrological variables requires a more careful management of water resources, taking into consideration the consequences of climate change, especially for the regions having arid and semi-arid climate like the studied region.