Trend of Annual One-Day Maximum Rainfall Series over South ... · Theil and Sen’s Median Slope has been used to assess the practical significance of trend as a change that is statistically

Trend of Annual One-Day Maximum Rainfall Series over South IndiaDr. Ashoke Basistha

Hydrologist – Dam Rehabilitation & Improvement Project

1st National Dam Safety Conference, Chennai

Organization

Introduction

Objective & Methodology

Data

Results

Conclusion

24th March 2015Trend of Annual One-Day Maximum Rainfall Series over South India - Ashoke Basistha - 1st National Dam Safety Conference, Chennai

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Introduction

Design flood calculated using UH increases linearly with design rainfall

Out of the 5 States currently under DRIP, 3 are in South India

A lot of the dams in these states date back to pre-independent era of empirical hydrology

Earlier research reported significant increasing trend in one-day extreme rainfall over the south peninsular region (Guhathakurta et al. (2010, 2011)

It has been projected by that the end of this century, extreme precipitation events over wet tropical regions will very likely be more intense (IPCC, 2013)


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Objective & Methodology

This study explores the presence of long-term trend in annual maximum rainfall series over South India

Pre-whitened Mann-Kendall test has been used to detect presence of trend

Theil and Sen’s Median Slope has been used to assess the practical significance of trend as a change that is statistically significant may not have practical significance (Yue and Hashino 2003)


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Data

This study used gridded Indian daily rainfall data set over 1051-2008 prepared by the Indian Institute of Tropical Meteorology, Pune

Annual one-day maximum rainfall series for the South Indian land region (Kanyakumari to 19° north, 69 grid points) was derived therefrom


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Results: Pre-whitened Mann-Kendall Test

At 10% level of significance Out of 69 grid points over South India, significant rising trends shown at 20 points

None of the grid points showed significant falling trend

Out of the 29 grid points over the DRIP states, only 5 points show rising trend


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Out of the 29 grid points over the DRIP states, only 2 points show rising trend


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Out of the 29 grid points over the DRIP states, none show rising trend


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Practical Significance of Change

Assessed as percentage change over the data period

Obtained by multiplying Theil and Sen’s median slope with data period and calculating percentage over highest one-day rainfall at the point

A change of 10% or more considered to be of practical significance


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Results: Practical Significance of Change

At 23 grid points (out of 69) the increase over 58 year period exceeded 10%

At 33 points the increase was of magnitude less than 10%

At 12 points the absolute magnitude of decrease was less than 10%

At 1 point the absolute magnitude of decrease was greater than 10%

The maximum estimated increase was 19.55%

The maximum decrease was -10.24%

The average change over the region is 6.69%, increasing


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Results: Practical Significance of Change

At 2 grid points within the DRIP states (out of 29) the increase over 58 year period exceeded 10%

At 18 points the increase was of magnitude less than 10%

At 8 points the absolute magnitude of decrease was less than 10%

At 1 point the absolute magnitude of decrease was greater than 10%

The maximum increase in the DRIP states is 16.94%

The maximum decrease was -10.24%

The average change in the DRIP states is 3.04%, increasing


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Results: Combined

At 14 grid points (out of 69) the rising trend is significant both statistically and practically

Within the DRIP states rising trend is significant statistically and practically at 1 point only


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Conclusion

One-day maximum rainfall over South India shows increasing trend, reaffirming results of earlier studies

Though the increase at individual points warrants attention, average increase over the region does not appear to be alarming

The effect is less pronounced over the DRIP states

Further studies are required for attribution


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References

Guhathakurta, P., Menon, P., Mazumdar, A.B., and. Sreejith, O. P, 2010. Changes in Extreme Rainfall Events and Flood Risk in India During the Last Century, National Climate Centre Research Report No: 3/2010.

Guhathakurta, P., Sreejith, O.P. and Menon, P.A., 2011, Impact of Climate Change on Extreme Rainfall Events and Flood Risk in India, J. Earth Syst. Sci.120, No. 3, pp. 359–373.

IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex V., and Midgley P.M. (eds.)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Yue, S. and Hashino, M., 2003, Long Term Trends of Annual and Monthly Precipitation in Japan, Journal of the American Water Resources Association, 587-596.


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Documents

Trend of Annual One-Day Maximum Rainfall Series over South ... · Theil and Sen’s Median Slope has been used to assess the practical significance of trend as a change that is statistically