International Journal of Science, Technology, Engineering and Management- A VTU Publication 2020; Vol: 2, Issue: 2, pp: 27 - 36 ISSN: 2582-5844 (Online) SPATIAL DISTRIBUTION OF DAILY RAINFALL IN KARNATAKA - DEVIATIONS FROM THE WORLD TROPICAL PATTERNS N.M.Thipperudrappa, a* a Professor, Department of Civil Engineering, Siddaganga Institute of Technology, Tumkur, India Abstract: The dependence of monthly rainfall on the number of rainy days and the mean daily intensity is studied in the case of two geographical regions of Karnataka, situated in the tropics. The pattern of rainfall distribution in the state, represented by the relationships, is A R T I C L E H I S T O R Y compared with that commonly found in tropical regions, situated elsewhere in the world. The study reveals that the pattern in the plains of Karnataka is somewhat comparable to that Received: 2020-06-14 generally found over the globe, while that in the case of mountainous Western Ghat region Revised: 2020-06-26 deviates much. The deviations are attributed to local factors, including the source of moisture Accepted: 2020-06-26 and the mechanism of precipitation. It is concluded that generalizations concerning rainfall pattern, based on geographical location, alone are dangerous and that general guidelines for hydrological designs must be adopted only after a study of the locally available rainfall records. Keywords: daily rainfall, tropical region, world patterns, Southwest monsoon, local factors, deviations from the general pattern. 1. INTRODUCTION in the state can be expected to furnish important information Studies on spatial and temporal distribution of rainfall, both on local factors such as location and topography in The study on regional and on global scales, have been of research is also expected to bring to light the deviations that exist in interest in hydrology for a very long time. They continue to the pattern of rainfall in this region from the general pattern be so, since generalization regarding the pattern of rainfall that is often believed to characterize the tropical rainfall distribution, even within similar climatic zones is ever (Jackson, 1977). elusive. Among all the climatic zones into which the globe is often divided (Strahler and Strahler, 1992, WMO, 1983 and 2. STUDY AREA AND THE DATA USED Jackson, 1977), the tropics are the most widely studied. The reasons are obvious - tropics experience much higher and Karnataka (Figure 1), with a total geographical area of much more intense rainfall than other areas and host a very 191,757 km2, is situated in the tropics between north large part of the world’s population. The work being latitudes 11○50’ and 18○05’ and east longitudes 74○00’and presented in this paper concerns the rainfall distribution over 78○05’. It is characterized by a coast line of length the state of Karnataka in South India, situated in the tropics exceeding 260 km, the Western Ghats (locally called and influenced by all the rainfall types. Karnataka forms an Sahyadri range of hills) running parallel to the coast and exceptional example of a case, where all the different extensive areas of plains situated in the plateau of South mechanisms of adiabatic precipitation (Strahler and Strahler, India. The coast line and the Sahyadri range of hills are 1992, Putty, 2009) are active and the range of rainfall oriented almost normal to the direction of Southwest magnitude is very wide – normal annual rainfall in the state monsoon winds and hence the state is bestowed with ranges between 450 mm and 7800 mm, within an area less bountiful rainfall, amounting to an average of about 135 cm than 200,000 km2. Hence, the studies on rainfall distribution per annum. However, the distribution of rainfall over the state is highly uneven (Figure 2), the rainfall magnitudes being markedly influenced by location with respect to the **Professor, Department of Civil Engineering, coast and the Sahyadri range of hills. The region influenced Siddaganga Institute of Technology, Tumkur, India by the orography of the Sahyadri range of hills receives exceptionally high rainfall, while the plains, beyond these ([email protected]) ranges, are characterized by low to moderate amounts of rainfall. The chief mechanism of rainfall in the coastal and © 2020 VTU Page No. 27 28 IJSTEM – VTU, 2020, Vol. 2, Issue. 2, pp. 27 – 36 Thipperudrappa et al. hilly region is orographic, while it is convective in the plains. rainfall. However, the state of Karnataka, despite being The tropical cyclones originating off the eastern coast of located well within the tropics, has two clearly distinct South India also influence rainfall in southern part of the divisions, influenced by different rain state, although to a small extent only. Hence, for the sake of present study, the state of Karnataka can be considered to be made up of two distinct parts (Figure causing factors as discussed in the preceding section. Hence, 3), experiencing different patterns of rainfall - (i) the highly a study of daily rainfall in the state may be expected to help wet coast and the Sahyadri range of hills, characterized by in understanding more about the tropical rainfall and the orographic type of rainfall occurring during Southwest influence of local factors, morphology in particular, on monsoon and (ii) the semi-dry to sub-humid plains, rainfall distribution. The present work is an attempt in this characterized mainly by convective type of rainfall and also direction and is being reported to highlight the differences in by cyclones during the Northeast monsoon. This study on the rainfall distribution pattern that exist in two subdivisions daily rainfall is carried out by analyzing records of 20 and the unique nature of distribution in the Sahyadri range of stations from each of the two regions (Figure 1). The stations hills. have been so chosen that they represent almost the complete range of rainfall magnitude in the two parts of the state. The 4.METHODOLOGY details such as location and the normal annual rainfall, concerning these stations are shown in Table 1. The rainfall The aim of this study is to review the relationships that exist records for a period of twenty years, extending from 1997 to between the total monthly rainfall (TRF) with each of the 2016, made available by the Karnataka Directorate of two variables - the number of rainy days (N) in the month Economics and Statistics, Bengaluru, have been used in the and the mean daily intensity (r), in various cases. The basic present study. data needed for this purpose are the long term averages of TRF, N and r. These parameters are estimated by taking the 3. OBJECTIVE average values from the daily rainfall records for a period of 20 years (1997-2016) for each of the 20 stations. This data According to Jackson (1977), the two relatively simple set is presented in Table 2, in order to provide a general idea parameters that provide a fairly good picture of the tropical regarding the temporal variation of rainfall in the two rainfall characteristics are the number of rainy days and the regions of the study area. The methodology used in involves mean daily intensity. A number of studies concerning the the following steps: distribution pattern of these two parameters, including their relationship with monthly rainfall, have been reported. Developing the best fitting regression equations between Jackson (1977) has carried out studies in Tanzania and found TRF and each of the two parameters N and r, separately for that in this area, the total monthly rainfall is directly the two regions; associated with the number of rainy days rather than mean daily values, and that the relationship is nonlinear. Comparing the form of the best-fit relationships in the two regions mutually and with those derived by investigators Harrison (1983) has carried out similar studies in Eastern elsewhere; and, Orange Free State in the USA. He has found that different kind of relationships exist in different regions - the number Examining the applicability of the relationships derived in of rainy days in a few cases, the mean daily intensity in other regions to the stations in Karnataka and inferring on some, and both the parameters in others, explain well the the deviations. variation in total monthly rainfall. Using data from a large number of stations in tropical areas all over the globe, In step (i) above, various forms of regression equations – Jackson (1986) has shown that the pattern of rainfall in the linear, logarithmic, polynomial, power and the exponential tropics is almost uniform. He has presented what he calls the are validated and the most suitable models are chosen for World equations for the association of these two parameters further analysis. The relationships referred to above in Step with total monthly rainfall. However, in a later study, (i) are developed for each of the rainy months individually, Jackson (1987) has found that the pattern of distribution in and also by combining all the months together. The first set Western Australia deviated from the World pattern and of relationships is called the ‘Monthly relationships’ and the concluded that there are dangers in generalizations. As far as latter are termed the ‘Intra-annual regional relationships’. India is concerned, studies seem to be not many, since only a The results are presented and discussed below. part of the country is in the tropics and even here, a large area is cyclone influenced. Agashe and Padagalwar (2005) 5.RESULTS AND DISCUSSION have discussed the characteristic features of daily rainfall in the central Maharastra region. They find that while the total Monthly Relationships monthly rainfall is influenced to some extent by the number of rainy days; mean daily intensity has little influence on it. A few sample scatter diagrams showing the relationship between the values of TRF and N and between TRF and r are The studies discussed above have been carried out in regions shown in Figure 4 and in Figure 5 for Maidana region.