Monday, May 10, 2010

Rainfall variation in Windward Vs Leeward sides of the Cardamom Hills (Western Ghats)

The data from Elappara (Windward side) and Bodinayakkanur (Leeward side) were used in this study. The first is instrumental data from the Elappara during 110 years (1896-2005), representing high altitude humid tropics in the Elappara range of cardamom hills (part of Periyar basin in Kerala facing the Arabian Sea). The station is one of the few hill stations in southern India, which has a record of more than 110 years of rainfall data. The station is located in high altitude (1500 msl) mixed tropical forest, which is a major type of forest in Western Ghats, where there is no clear seasonality in environmental factors such as rainfall and temperature. The other data set is from the Bodinayakkanur station located in Tamil Nadu, on the eastern slopes of the cardamom hills facing the Bay of Bengal (falling under semi-arid tropics) and provided 100 years of rainfall data (1906-2005). This rain gauge station is attached with TWAD (Tamilnadu Water and Drainage Board) board, Bodinayakkanur, Tamil Nadu. The Elappara range forms the western boundary of the cardamom hills' reserves (CHR). The Bodinayakanur range forms the eastern boundary of CHR in Tamil Nadu. The Chokkanadu and Kolukku Malai form the northern boundary. To the west of Elappara range, tropical rainy climate prevails. As one moves eastwards, the climate gets more arid, till it is regarded as completely arid in the region between Saptur and Usilampetti ranges. The CHR thus constitutes an area of 334 square miles, with Bodinayakanur and the Elappara are the representative stations for the two extreme ecosystems the humid and semi-arid tropics (Fig 1) respectively.


Semi-arid slopes and the plains of Bodinayakkanur are characterized by a high temporal variability of seasonal and annual rainfall. The data show a drastic change in the rainfall pattern from seasonal to decadal scales (Fig 2), but no significant periodicity appears to be present. Over the last hundred years, the contributions of the ISM, NEM and BR (Blossom Rainfall) were 21.3%, 48.1% and 30.6% respectively. The highest rainfall of 2270.4mm was recorded in the year 1958 and the lowest rainfall of 328.5mm was reported in 1983. Mean annual rainfall for entire period was 767.1mm with a coefficient of variation (CV) of 36.9% and standard deviation of 282.8mm. Highest CV was reported for ISM (108.6%) followed by BR (44.30%) and NEM (40.3%). ISM rainfall ranged from19.82 to 1327.8mm while NEM rainfall varied from 83.8 to 773.3mm. BR also varied greatly from 32.9 to 665.4mm. Since CV was higher than 30% for all the seasons, the region experienced more frequent and severe droughts during the period. Higher Precipitation Concentration Index (PCI) (>20) was recorded only in the recent past three decades, which implies very long dry periods for up to 3-5 months. The PCI denotes uniformity of monthly distribution in a year over a particular station or the inter-annual variability. Therefore year-to-year variation in rainfall was considerable and showed definite decreasing trend during the last three decades. The index value was always below 20 for the previous decades.

The mean rainfall of the recent past three decades was well below the centennial average rainfall, clearly indicating decreased monsoon activity. This could be related to increase in tropical deforestation and land use change occurring in the areas surrounding this semi-arid ecosystem, like the Montane cloud forest high waives (Mehamalai) and the cardamom hills during the report period. No alternating pattern of increasing and decreasing rainfall in the decades is seen and no epochal trends in ISM as observed for the rest of the country. The 20, 30, 40 and 50 year running means showed declining trends (2.7 to -3.4 mm per year, with R2 values ranging from 0.42 to 0.85, significant at 95%). This means the future rainfall in the rain shadow slopes of the cardamom hills is likely to decrease. Though historical records say that the eastern slopes have enjoyed good rainfall, in the last century the slopes had lost their thick evergreen forest (90% of primary forest). Whether this has triggered or will trigger changes in rainfall pattern, questionable and sensitivity analysis performed elsewhere showed some teleconnected effects of deforestation in the precipitation change of south East Asia. El Nino and La Nina events have profound influence on the rainfall of this region. The most intense El Nino year in the last century was 1997, when the rainfall was well above the average. Nearly 58% of the El Nino years had below average rainfall. On the other hand 42% of the El Nino years accounted above normal rainfall. Amazingly the lowest rainfall of the last century was in 1983(328.5mm), which happened to be a typical La Nina year, and the effect was spectacular. 70% of the 17 La Nina years brought about below average rainfall and only 30% of the La Nina years experienced more than average rainfall. This shows a changing relationship between semi-arid rainfall climatology and ENSO in this region. Up to many decades ago, the entire area was under cardamom cultivation. For the past 8-10 years, the actual area under cardamom cultivation has shrunk to about 40,000 hectares. Relatively drought resistant crops like coffee and silk cotton are cultivated in the remaining areas. This is indicate the insufficiency of rainfall on the eastern and northern borders of CHR, and the severity of aridity is advancing towards up in the cardamom hills' reserves into the state of Kerala.


In our analysis for the period (1896-2005) (Fig 4) the highest rainfall of 7000 mm (1961) was reported. This could be the second highest rainfall in the leeward hill slope of south-Western Ghats in peninsular India after Shimoga in Karnataka where in 8000 mm in a year was received. Careful study of the long term climate record has shown that even seemingly modest fluctuations in rainfall can create havoc in vulnerable societies. Orographic lifting favors the windward to the leeward side ofmountains for convection. However, a close examination of other gauge stations data of the cardamom hills (10-20 years data) indicate that the constant high rainfall (>5000mm in a year) is received near Anaimudi peak (2694 msl). The putative reason for the rainfall maximum in these mountains could be attributed to the strong convection, which undergoes diurnal cycle in which these mesoscale mountains play important role. However, the studies on the reasons for local rainfall maximum need careful observation and analysis besides understanding the physics behind convection-circulation interaction in the mesoscale mountains. The inter annual variations in the monthly rainfall values are very large; for example the values for January and February changed by as much as a factor of two or more from one year to another. The lowest rainfall during the period was reported in 1987 which was one of the two most intense El-Nino years in the world.

There was a strong association of El-Nino phenomenon with below average rainfall (67% of the El-Nino years), indicating a good ENSO relationship. In the case of La-Nina years, fifty per cent of the years enjoyed excess rainfall and rest of the years were low rainfall years. However, the impact of El-Nino phenomenon was not severe during the first pentad of this century in this mountain range. This shows the relationship seems to have weakened in the recent past. Considering these, the scientists concerned should take precautions in forecasting local climate. The coefficient variation of rainfall (CV) for the entire series ranged from 9 to 27%. The decadal averages for the first three decades (1896-1925) and the last three decades (1976-2005) studied, reported values less than the annual mean (5000 mm) for the whole period (1896-2005). Only during the middle period (1926-1975), the decadal average rainfall did exceed the series annual mean. Interestingly the strongest El-Nino years (1997-1998) had received (5120 mm) fairly well above the annual mean. Since 1896, of the total El Nino years, only ten years each in El-Nino and La-Nina years had envisaged less than average. Otherwise all other El-Nino and La-Nina years found to be more than the annual average (5000 mm). These results are found to be for and contradictory to the results reported for many regions of varied environment in India Therefore, extreme rainfall anomaly can occur even in the absence of strong ENSO events, thus uncertainty in ENSO effects is quite large from one environment to another even that are close to each other. The production trend of major spices and plantation crops shows a negative trend for the first one and half a pentads (2000-2007) of this century. Cardamom reported a maximum percentage change of -12.5.1% followed by black pepper (-10.0%) in CHR. In particular, tea production in the cardamom hills has gone down significantly reporting a percentage change of -20% during the recent past seven years. Therefore, this unhealthy trend of production of specific crops is certainly due to the change in the climatic condition which affects both quantity and quality of crops.

Though at this point in time the cardamom hills receive more rainfall than the crops require (for the successful production of tea and cardamom), in the near future the cardamom and tea ecosystem hydrology may get affected as the demand for water is expected to raise many fold. Already many stream flows have been found to decrease and become short lived. Thereby, the ecosystem degradation could be more pronounced at than the present level because cardamom is very sensitive to drought conditions. The most semiarid eastern slopes of the cardamom hills are experiencing down ward march of rainfall. Under the decreasing rainfall pattern coupled with continuing atmospheric warming scenario, the near future could jeopardize the agriculture and sustainable development in this resource (water) poor semi-arid ecosystem, and the coming years could be highly challenging to climate and agriculture scientists as the demand for water would increase by 20-80% in this densely populated valley. Already half a million people from the eastern slopes have moved to western slopes in search of livelihood and the eastern slopes now look like a desert, with at least100 area (Thevaram- Bodi tract)which have been rendered into a desert like system. Immense ecological degradation and dangerous climatic variability of cardamom agriculture has already been an immutable truth elsewhere in the world. Therefore, understanding rainfall variability in these limited ecosystems is important.

Source:M.Murugan, V.Mukund, R.Ramesh, M.B.Hiremath, A.Josephraj kumar and P.K.Shetty for Caspian Journal of Environmental Sciences, University of Guilan, Iran
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1 comment:

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