Rainfall partitioning in a tropical forest of Manipur, North East India

Precipitation follows several pathways before arriving at the forest floor in a forested ecosystem (Rutter & Morton 1977). Incident rain passing through the forest canopy can be divided into three fluxes. One flux freely falling through the canopy and dripping down the leaves is called throughfall, whereas the other flux runs down the branches and trunks and is called stemflow. However, a part of precipitation never reaches the soil but is evaporated from the forest canopy and is known as interception. Numerous investigators have measured rainfall both in the open and beneath forest canopies (Joroen et al. 2008; Keim 2004; Link et al. 2004; Price et al. 2003; Pypker et al. 2005; Staelens et al. 2006). Since the data available from tropical regions are limited, the present paper aims to quantify the various pathways of rainwater reaching the soil in two stands of tropical forest from Manipur, northeast India. The forest stands presently studied happen to be sacred groves. Sacred groves are the forest patches protected by the local community for their beliefs and traditions. The study was conducted at Nambol (24.72oN latitude and 93.837o E longitude) and Wangoi (24.673oN latitude and 93.899oE longitude) sacred groves protected by the Meitei community. Nambol sacred grove is drained by Nambol river and the dominant species of the site is a deciduous tree Ficus glomerata. Wangoi sacred grove is drained by Imphal river and the site is dominated by evergreen species like Celtis australis, Eugenia praecox, Michelia champaca. The area is characterized by sub-tropical climate. The average temperature ranges from 0oC to 32oC, mean relative humidity from 57.18% to 83.98%. The average annual rainfall was recorded as 1635.5 mm. The characteristics of the two sites are given in Table 1, following Usharani (2004). Throughfall and stemflow were measured following Opakunle (1989). Three plots, each 20 m x 20 m in size, were selected for the hydrological study in each grove. Enumeration of all the trees was made in each grove. A total of 29 trees which were sampled for the hydrological studies from both the groves were selected. For measurement of throughfall 10 funnels, each of 100 mm diameter, in each plot, were used. The rainfall was measured by placing three such collecting funnels in an open area near the experimental sites. Net precipitation is the amount of rainfall which reaches the soil after penetrating the canopy and litter layer and was calculated by adding throughfall and stemflow. Gross precipitation in the studied plot was obtained by multiplying mm of rainfall by plot size. A total of 201 rainfall events in the hydrological year (1999-2000) were considered for calculating throughfall, stemflow, net precipitation and interception loss. The maximum number of rainfall events was 120 in shower class 0-10 mm which constitutes 59.70% of total events. The number of rainfall events was 28 under shower class 10-20 mm that accounted for 13.9% of the total events. Thirteen rainfall events were recorded under shower class 20-30 mm and between 30-50 mm shower class, nine events were recorded. There were only three rainfall events