University of Moratuwa, Katubedda, Moratuwa, Sri Lanka
Abstract: The climate is one of the conspicuous topics in the world at the moment because of the changes that have occurred in it and the deviations that it is showing right now. Changes in precipitation are one of the prominent ways to identify the climate changes because those variations in the rainfall are the basic symptoms of disastrous end results of the climate change. So the prediction of rainfall is so crucial from that point of view and another set that massively affected by this are the countries which have rain-fed agriculture-based economic system because they might have to their adaptation strategies against upcoming changes in precipitation. So the world has moved to analyse the precipitation changes and lots of research work have been done locally and internationally considering various analysing methods using hourly, monthly and annual rainfall intensities and depth data. Analysis of data over a series of years at least 30 years back considering time trends has shown prominent effects on design rainfall events previously. The results obtained from these research work show that there are actually identifiable time trends in rainfall patterns and another thing that has revealed is that these trends can only be identified for some areas and the trends are varying from one region to the other. Most of the results all around the world highlight that the rainfall intensities have been increased while the duration has been decreased. So likewise this research is focussed on the identification of time trends in Rathnapura rainfall pattern and determination on the impacts on design rain events of that area.
Keywords: climate; precipitation; trends; Rathnapura; events; patterns
Water is one of the basic needs of all living beings. Rain is one of the main sources that the world gets fresh water. So the existence of the world massively depends on the rainfall. There are lots of factors which affect the precipitation throughout the world. Amount of the tropical rainforests that are present in the world is one of the main factors among them. Though the rainfall is unpredictable, with the continuous uncontrolled deforestation that has been happening all around the world and because of the other governing factors the rainfall or the precipitation seems to be showing a bit different behaviour which was not seen in the past. This kind of differences in rainfall could be the symptoms of huge climatic changes and these changes could affect the world extremely. So for the developing countries like Sri Lanka, it would be better to have rather accurate and dependable forecasting systems that can tackle even small changes and patterns in rainfall spatially and temporally.
Sri Lanka is an island in the Indian Ocean which is located at the southern tip of the Indian subcontinent extend between 5°55′ and 9°50′ N and between 79°41′ and 81°53′ E. Because of this location, the rainfall climate of the country is being governed by the Indian Ocean monsoon system. Mainly the country receives rainfalls under two principal monsoon seasons (south-west and northeast) and two inter-monsoon seasons (First inter-monsoon and second inter-monsoon) in different time periods throughout the year (Burt & Weerasinghe, 2014). According to the spatial annual precipitation, two main climatic zones can be identified in the country which extends to the west side and the east side from the central hills as wet zone and dry zone respectively (Malmgren, Hulugalla, Hayashi, & Mikam, 2003).
Rathnapura is one of the cities in the wet zone in Sri Lanka which receives rainfall throughout the year with an annual average precipitation of 3922.6mm. Mainly Rathnapura gets heavy showers due to south-west monsoon between May to September and due to second inter-monsoon between October and November. Normally maximum rainfall is recorded in May and minimum rainfall is recorded in February (Ampitiyawatta & Guo, 2010). Hence the rainfall plays a significant role in day to day life in this part of the country it’s very important to design rain events for the area rather accurately because the life will depend on a lot and most importantly the Rathnapura area is situated in a floodplain of Kalu Ganga. Since up to now in Sri Lanka the rain forecasting was done without considering trends in time in the rainfall pattern now it’s the time to use an advanced method to analyse the rainfall data with the impact of temporal trends and it will lead to better designs of rainfall events.
2.1 Climate change
Climate change is the most pressing issue at the moment in the world. The reason why this issue becomes this much important is the adverse impacts that have been affecting the world since the beginning of the 21st century. For an example; between 1998 and 2001 because of the floods in European rivers 700 deaths, half a million displacements and a waste of 25 billion worth economic losses were reported (Thielen, Bartholmes, Ramos, & de roo, 2009). From the research, it has found that the main reason for the climate change is the global warming. The emission of greenhouse gases which has been increased with the industrial development is the main source of the greenhouse effect and it is the main reason for the increment of the temperature. The other main thing which is related to this issue is the hydrological cycle with extreme rainfalls and drought, melting of glaziers ETC. With the current extreme events that are happening, we can easily identify that the climate change has already effected on it.
In 1999 Trenberth found that the ultraviolet rays produced by the greenhouse effect can increase the evaporation of water and this helps to increase the capacity of the atmosphere to hold more water vapour in hot days. This is one of the main reasons for the intensification of the hydrological cycle and it increases the frequency of extreme rain events (Trenberth, Weather and climate extremes, 1999). From a research conducted in 2007 it was found that the surface temperature has increased by 0.7 C over the last century with a significant warming in several regions and another thing that found by the was the rate of the temperature increment in the land areas is higher than that in oceans (Trenberth, et al., 2007).
2.2 Rainfall patterns
Rainfall is one of the most unpredictable climatic variables. Though it varies in an unpredictable way if we analyse rainfall data over a period of time it shows some patterns. These patterns can be varied according to the area we obtained the data and the methods we use (Douka, 2017). So the rainfall patterns can be identified spatially. For an example, in Sri Lanka, we can identify three main parts in the island according to the precipitation. Those are the wet, intermediate and dry zone (Malmgren, Hulugalla, Hayashi, & Mikam, 2003).
Figure 1: Climatic zones in Sri Lanka
Not only spatial patterns but also the temporal patterns of rainfall can also be identified. These temporal variations in rainfall occur according to how the winds are behaving in the region, location and the geographical properties of the country. For Sri Lanka, the rainfall is governed by the monsoon system and according to that system, Sri Lanka gets rainfall under four monsoons in four time periods every year. So because of these patterns, it has been easier to predict rainfall events.
But with the climate change these patterns have started to show slight deviations from their usual paths and because of that, the design rain events is being deviated from the actual rainfall events. This makes the world consider the time trends in rainfall patterns to predict or design rain events more accurately. For analysing rainfall data to identify time trends specialized methods should be used such as Mann-Kendall test, linear regression line method ETC. For the analysis, continuous series of data should be collected from the gauging stations at least up to 30 years and data should be sorted according to our objective. As an example, if we want to identify trends in annual maximums we have to sort the data and take the annual maximum rainfall values. Then the data should be represented and it will be a help for the task
2.3 Data representation
Generally, precipitation data representation is done using tables and graphs. Data can be present according to various gauging station to check trends individually.
Figure 2: Column charts
Figure 3: Tables
Other than this various types of graphs can be used to represent variation of rainfall properties such as intensities, depth ETC and trends with the time.
Figure 4: Other tables
2.4 Analysing methods
Various parametric and nonparametric methods have been used to identify the trends in time series of rainfall in research work. Standard linear regression analysis and Mann Kendall test are the methods used mostly to analyse the hydro meteorological time series among them. Apart from that various other methods also have been used in order to find various relationships. As examples Shapiro-Wilk test for identifying temporal classes and climatic risks, Spearman’s rank correlation analyses to find the relationship between the events ETC.
2.4.1 Mann-Kendall test
This test is widely used in different fields of research for the detection of trends in time series because of its simplicity and its ability to deal with missing values and values below a detection limit. Mann-Kendall test equations are as follows. (Ampitiyawatta & Guo, 2010)
Where Zmk Mann-Kendall statistics, n is the length of the data set, Xj and Xi are sequential data values, m is the number of tied groups (a tied group is a set of sample data with the same value), and t is the number of data points in the mth group. A positive value of Zmk indicates an increasing trend and negative value of Zmk indicate a decreasing trend.
2.4.1 Standard linear regression analysis
Following parameters are used to calculate the trend of the rainfall pattern.
Is the slope of the trend line
Positive value means that increasing trend & negative value means that decreasing trend.
Significant results on temporal trends in rainfall patterns have been obtained up to now by various research all around the world in their respective study areas. (Ampitiyawatta & Guo, 2010)Have done a research in Kalu Ganga basin to identify time trends in that basin using eight gauging stations and identified following trends in those gauging stations
Table 1: Mann-Kendall statistic values in gauging stations
They have also obtained the obtained temporal distribution of monthly rainfall trends from January to December. The results imply that the Kalu Ganga basin shows a decreasing rainfall except for January.
Table 2: Magnitude of positive/negative and average trends for different months
By considering above mentioned we can conclude that past researchers have been able to find the time trends in rainfall patterns in respective study areas. In Sri Lanka also a considerable number of researches have been conducted in limited areas towards the same course. But when considering the application of these results and identifying impacts of these trends on the design of rain events is not up to the standards in Sri Lanka. Since the research is focussed on Rathnapura rainfall pattern and still there are no proper identified temporal trends in the area with the ongoing climate changes in the world, the same procedures will be able to use for the data analysis. And for the impact assessment of these trends on design rain events, previously used methods and the new methods which are found through the previous researches will have to be checked and then a proper method will be concluded later.
The author is grateful to the research supervisor Dr.T.M.N.Wijerathna for the guidance and to the colleagues for the help.