Investigating the trend of changes in maximum rainfall in Iran

Dissertation for Master Degree

Civil Engineering" M. Sc"

Tendency: Water

Abstract

The aim of this thesis is to investigate the trend of annual rainfall changes in 37 selected stations in Iran using non-parametric method. The two Mann-Kendall tests and the age estimator, which are among the most common non-parametric methods, were used for the trend analysis of rainfall and temperature data in annual scales. 37 synoptic stations located in the area of ??Iran that had statistics between 1965 and 2008 were selected, and the above two tests were applied to their data, and the results of these two methods were compared. The results showed that the effectiveness of the above two methods in analyzing the trend of rainfall and temperature is similar in most cases. And it shows the presence of a significant decreasing trend for the total annual precipitation and a significant positive trend for the average annual temperature.

Keywords:  Trend analysis, precipitation, non-parametric method, Mann Kendall, Gerson estimation 1- Generalities

Introduction

One of the methods to analyze common hydrometeorological time series is to check the presence or absence of trends in them using statistical tests. Basically, the presence of trends in hydrometeorological time series may be caused by gradual natural changes and climate change or the effect of human activities (Brock and Kartz 1953, 414). Proving the existence of a significant trend in a time series of rainfall alone cannot be a conclusive proof of the occurrence of climate change in a region, but strengthens the assumption of its occurrence (Cyrano et al. 1999, 86).

Like many natural phenomena, weather and climate are also variable. The climate has a direct effect on the intensity and distribution of rainfall, which in turn has a great effect on soil erosion, fluctuations in the level of underground water tables and the occurrence of floods. It is very necessary to conduct research related to climate change in order to prepare as much as possible to deal with the costs of heavy damage caused by this change.

Iran with an area of ??about 1648000 square kilometers is located in Southwest Asia and approximately from 25 to 40 degrees North and extends from 24 to 64 degrees east.

The main mountain ranges of Iran are Alborz and Zagros, which stretch from west to east and from north to southeast, respectively. These mountain ranges play a very important role in the heterogeneous temporal and spatial distribution of rainfall in the entire country.

Knowing the trend of rainfall changes in a place can help many managers and those involved in water related to their future decisions related to the implementation of construction projects. Iran's climate is dry and semi-arid, with the exception of the northern coasts and the western mountainous region of the country.

The location of many parts of Iran in the arid and semi-arid belt of the world on the one hand and the decisive role that atmospheric precipitation plays in the country's water supply on the other hand, has made it important to be more aware of the changes in Iran's rainfall.

Iran's climate is hot and dry summers and cold winters, especially in parts The interior of the country is infinitely continental. The range of air temperature changes in Iran is around 22 to 26 degrees Celsius. The rainy season in major parts of Iran lasts from November to May, followed by a dry and hot period that lasts from May to the end of October. The average rainfall in Iran is around 240 mm, the highest of which occurs in the plains along the Caspian Sea and the slopes of Alborz and Zagros with 1800 and 480 mm, respectively. By entering the interior plains of eastern and central Iran, the amount of rainfall decreases to less than 100 mm according to the topography of the place.

In terms of synoptic, the climate of most parts of Iran, especially in the warm period of the year, is under the influence of the high pressure system of the subtropical region. This phenomenon causes very hot and dry summers in these regions.

Most of the rainfall in the area of ??Iran is produced by the Mediterranean systems that enter Iran from the west to the east in the direction of the western winds. The changes in synoptic systems and year-to-year changes in the number of systems that enter Iran are the main factors of the annual magnitude of changes in the country.

The systems of the Mediterranean front that enter the country in connection with the western air currents bring the main part of the country's rainfall in late autumn and winter.

Mediterranean front systems that enter the country in connection with the western air currents, provide the main part of the country's rainfall in late autumn and winter. In addition to the Mediterranean frontal systems, the Sudanese rain systems, which enter Iran from the southwest, form a major part of the rainfall in the west and southwest of Iran. In the mountainous region of the northwest of the country, thunderstorms and convection are the main atmospheric processes that cause rainfall in spring and early summer. These rain systems are only active in the western parts of the country and when they reach the center and east of the country, they do not have enough moisture to produce clouds and precipitation, therefore the central and eastern region of the country is the most prone to drought. Because the intra-annual and inter-annual variability of rainfall in this region is very high and the amount of annual and monthly rainfall has a large coefficient of variation. This region, which covers more than half of the country, is surrounded by Alborz and Zagros mountains.

The Zagros mountains take the moisture of the rain systems like a wall; As a result, the air that reaches this area does not have the necessary potential to cause rainfall. This phenomenon causes a lot of irregularity in the rainfall of this region. Lack of rainfall from May to October along with high temperature causes severe evaporation and transpiration and as a result severe water shortage.

Rainfall fluctuations generally have and will have the greatest impact on the economy of rich and poor countries. The importance and scope of these changes is such that it can cause political and social crises and even cause the fall or survival of governments.

Most of the energy consumed by humans is fossil fuels such as oil, gasoline and similar things. The consumption of these fuels causes the concentration of carbon dioxide gas, which is one of the main greenhouse gases, to increase in the atmosphere. These gases have made the atmosphere and the space around the earth into a chamber so that the long wavelengths reflected from the earth cannot escape from it, and this life causes the surface layer of the earth and the lower layers of the atmosphere to heat up and finally the earth and the atmosphere to heat up. Change in the time and amount of effective precipitation, in such a way that it rains more in winter and less in summer, change in the maximum, minimum and average amount of precipitation, change in maximum and minimum temperature, so that summers become hotter and winters colder, which causes the climate of the region to move towards a dry climate under the influence of this phenomenon. Change in the time of snow melting, change in the length of the growth period and harvest time of plants, increase in height above the water level of the seas and oceans, thinning and decrease in the area of ??ice in the Arctic, change in the amount and quality of water resources can be mentioned as important effects of this phenomenon. Changes in the temperature pattern and noticeable fluctuations in the amount of rainfall, which are significant symptoms of the climate change phenomenon, will result in adverse effects such as the reduction of water resources, sea level rise, destruction of forests, frequency and intensification of droughts, and threats to human health, and this will indirectly lead to economic damage to countries as a result of dealing with these effects.

Two types of vulnerability due to the increase in rainfall are considered: 1- Changes in the average rainfall rate In areas with dry farming. 2- Changes in the frequency of unexpected rain events in areas that are prone to floods and storms. In the first case, an increase in rainfall in areas dependent on rainfall can be beneficial.

While a decrease in rainfall can have a negative effect in arid and semi-arid areas where providing drinking water is a major problem, climate change will cause serious problems in providing drinking water in these areas. Also, the increase in floods and storms in the areas that are prone to these phenomena will increase the sufferings of the people. The implementation of flood control plans in these areas will reduce the vulnerability of the society. Since such projects require spending a huge budget, it is necessary to know about climate change in the future in the target areas. It is unlikely that all hydrological changes caused by climate change will be beneficial. For different regions, the impact of climate change and the extent of its damage and benefits are different.

Studies conducted in recent years, which are described in the third chapter, showed that the El Nino Southern Oscillation [1] (ENSO) phenomenon has been used as an important factor to justify the variance of rainfall in different regions of the world, including Iran.