Revealing changes in precipitation in Kermanshah province using statistical models

Dissertation for obtaining a master degree

in the field of natural geography

Climatic change trend

Abstract:

With the process of global warming and water resources becoming more valuable due to population growth and the needs of industries and agriculture, numerous surveys have been conducted in line with water conservation and protection programs in order to investigate changes in precipitation and planning in the field of facing the consequences of positive and negative changes. The purpose of this research is to study the possible deviation of the climatic element of precipitation in Kermanshah province from the normal state.  In order to investigate climate changes and precipitation change, statistical tests of Man Kendal and Arima were used in the synoptic stations of Kermanshah province (Kermanshah, Islamabad Gharb, Kangavar, Ravansar and Sar Pol Zahab) during the statistical period (1988 to 2012) based on rainfall indicators. The results of this research show that: the Mann-Kendall test was carried out at NORMAL-Z level and P-VALUE level in Kermanshah, Islamabad, Kangavar, Ravansar and Sarpol Zahab stations, and this test shows and confirms the existence of rainfall changes during the 25-year statistical period.  Based on the amount of T statistic obtained from the table as well as the significance levels and estimated error, it can be concluded that based on four ARIMA models, SARIMA(1,1,1)(1,1,1)12 and SARIMA(1,1,0)(0,1,1)12 models with the lowest significance level close to the error level of 0.01 and the amount of T statistic at the most acceptable level can provide the best fit. As a result, in Kermanshah province, the total annual rainfall has the most changes.

Keywords: Kermanshah province - precipitation - trend - Man Kendal - ARIMA

Recently, one of the most important topics in the discussion of climate in different regions is the topic of climate and its change as an irreversible situation, and many researchers have addressed the various dimensions of this issue. One of the manifestations and consequences of this phenomenon is the change in climatic elements, especially the temperature and precipitation of different regions. A slight disruption of the global climate balance has caused the average global temperature to show a tendency to increase (Hejazizadeh, 2018: 43)

In Iran, rainfall is one of the basic variables for evaluating potential water resources, but its temporal and spatial distribution is very heterogeneous, and for this reason, the distribution of water resources in the country is not uniform. Maintenance and management of water resources is a function of received rainfall and rainfall variability. The smaller the spatial changes of rainfall, the more stable the water sources will be, and the permanent supply of water will be more possible. For this reason, the temporal variability of precipitation is very important in the evaluation of available water resources of watersheds and the relative study of available water resources on a local and regional scale. (Massoudian, 2010: 121)

Controlling and guiding the amount of precipitation changes as the main element in climatic drought is of great importance. Since the location of Iran is between 25 and 39 degrees north latitude, the most important factors controlling its climate should be sought both in the subtropical and extratropical regions. Iran is located in the presence of two regimes, Hadley and Rozbay, and due to the wave nature of the air orbital movements in this region, based on the strength and weakness of the Rozbay regime in spring, it is subject to cold and warm air waves alternately. Since the synoptic systems responsible for Iran's spring frosts are repeated every year with alternating frequency and intensity, they should be considered as part of the definite characteristics of Iran's climate. Therefore, due to the fact that Iran is exposed to the invasion of these systems, most of the country's regions (including Kermanshah province) have a rainy regime. Therefore, long-term forecasting and monitoring of climatic conditions in each region can be a good solution to deal with the adverse effects of climate change (freezing, drought, floods, etc.). (Shakiba, 51:1389)

1-2- Statement of the problem:

Climate change can be studied by tracking characteristics such as quantity and time-spatial pattern of climatic elements. Meanwhile, the study of precipitation is very popular. Precipitation, as one of the fundamental elements of climate, has increased by about 1% during the 20th century especially in the middle and upper latitudes of the Northern Hemisphere. On the other hand, in the tropical drylands (latitude 10-30 degrees north), the rainfall trend has decreased by about 3%, and in the tropical areas, the relative increase in rainfall has been around 2-3%. (Masman[1] et al.(Masman[1] et al., 2004:43) Looking for the effects of the climate change phenomenon on the climatic elements of the world, Iranian thinkers have also been trying to investigate the effects of the phenomenon on the climatic elements of Iran in recent decades. In this regard, some researchers have focused their attention on long-term changes in rainfall. (Asakreh et al., 2011: 62) In Iran, precipitation is one of the most important vital parameters and important factors in climate structure. Although rainfall in Iran has a periodic nature, its spatial and temporal distribution is extremely uneven. (Masoudian, 1390:132) For this reason, open or hidden dependence on rainfall has caused uneven distribution in water resources, animal life, etc. The unevenness in the spatial and temporal distribution of rainfall is related to the unevenness and their arrangement and the geographical location of Iran, respectively. It is on this basis that the geographical arrangement of the rainfall areas in Iran shows the dependence of the amount of rainfall on the unevenness and the time of rainfall on the latitude. (Same source: 79) In general, based on simulation models and several scenarios, the average water evaporation density in the world and the amount of precipitation will increase during the 21st century in the middle and high latitudes of the Earth in winter, but on the other hand, we will expect an increase and a decrease in the low latitudes. Also, in areas where the average increase in rainfall is expected, the annual rainfall changes will be greater. (Yildirim Yildirim [2] et al., 43:4004) in every year, there is a certain relationship and stable order between the rainfall of that year and other phenomena. However, the state of dispersion of phenomena varies from year to year, and as a result, the amount of precipitation or the intensity of the relationship also changes. Therefore, statistical models and methods and the law of probabilities are used to reach a stable and dominant result. For example, statistical and mathematical models are used for the zoning of Iran's rainfall, to know the prevailing order in the spatial slope of rainfall between east and west of Iran. (Alijani, 1371: 12) Analyzing Mann-Kendall's statistical-graphical test and predicting the behavior of a variable using the ARIMA model are among the most important statistical methods used in the study of rainfall changes. Based on this, this study is carried out in order to fill the void of studies with the title of revealing changes in rainfall in Kermanshah province. 1-3- The main questions of the research: 1- What pattern does the trend of rainfall changes in Kermanshah province follow?

1-4- Research assumptions:

1- It seems that changes in precipitation in Kermanshah province have slow increasing and decreasing trends.

2- In Kermanshah province, the total annual rainfall has the most changes.

1-5- The purpose of the research:

The general purpose of this study is to reveal the trend of changes. Precipitation and also the changing behavior of precipitation in Kermanshah province, which will be discussed and investigated using Kendall's statistical-graphical test as well as the time series method. 1-6- Necessity and importance of the research: Rain is one of the most variable elements of the climate, whose changes can have negative environmental, social, economic and even cultural consequences for human societies. It is, especially the increase of change and variability is of considerable importance, it can play an important role in climate forecasts and as a result in water resource management and optimal environmental, economic and agricultural planning. The assumption of precipitation changes in the global climate has occupied the minds of researchers and human societies. And the changes in precipitation, even in a small way, will have significant and significant effects on all the elements of human life and natural life. The scientific approach to this phenomenon is often carried out by earth science researchers, especially water and meteorology, meteorology, hydrology, environment, physics and agriculture. Each of them has investigated the issue with different methods such as statistical analysis, synoptic analysis, remote sensing and with laboratory equipment and complex scientific models. Climatologists, through a special research method, often statistically or synoptically, using the measured data of weather stations and maps of the earth's surface and the upper levels of the atmosphere, analyze the changing behavior patterns of precipitation.