Centennial rainfall variation in semi arid and tropical humid environments in the cardamom hill slopes, Southern Western Ghats, India

Abstract

Studies of rainfall variation generally focus on large areas. For example, in India, the area average monsoon rainfall series of the whole country or meteorological subdivisions are used. This would be of no use for local agriculture, particularly in places where rainfall is very high or very low, especially for crops like small cardamom and vanilla which are very sensitive to soil moisture and atmospheric air relative humidity. With this view, we present the data and analysis of the long term rainfall variations in semi arid as well as tropical humid environments in cardamom hill slopes in south-WesternGhats in India. In our analysis, the semi-arid cardamom hill slopes showed decreasing trend in rainfall over the recent past three decades ranging from 2.7 to 3.4 mm/yr with R2 values ranging from 0.42 to 0.85 (P= 0.05). Year-to-year variation in rainfall was considerable. No alternating pattern of increase and decrease in rainfall was seen in decads, epochal trends as accounted by ISM for the rest of the country were absent. If the present trend continues, agricultural production in this semi-arid, poor rain fed system will suffer, unless conservation and mitigation efforts are undertaken. In case of windward side humid tropics, the year- to- year variations in the monthly rainfall values were very large. For example, January and February rainfall values changed by a factor as high as two or more from one year to another. The lowest rainfall during a period was reported in 1987 which was one of the intense El-Nino years. 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 case of La-Nina years, fifty per cent of the years had enjoyed excess rainfall. However, the impact of El-Nino phenomenon was not severe during the first pentad of this century (2000-2005) in this mountain range. This indicates that the relationship seems to have weakened. The coefficient of variation of rainfall for the entire data 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 less than the annual mean (5000 mm) for the whole period (1896-2005). Only during the middle period (1926-1975) the decadal average rainfall exceeded the series annual mean.