2.1. General site description The CHT lies within 21o81'10" –23o84'50" N and 88o80'10" – 91o84'50" E. The region is surrounded by the Chittagong and Cox’s Bazar districts stretching along the Bay of Bengal to the west, by the foothills of the Indian states of Tripura and Mizoram to the north and east and by the Akyab District of Myanmar (Burma) to the south and southeast. Of the region, 90% is hilly, 4% is villages, rivers and lakes, and only 6% is considered suitable for intensive agriculture.
The CHT is part of the Hindu Kush Himalayan mountain ecosystem, folded into a succession of pitching anticlines and synclines aligned almost north–northwest to south–southeast and consisting of mainly unconsolidated beds of sandstone, shale and some conglomerates. Most of these lands are located within 1000 m altitude and are closely dissected and sharp-edged with steep and very steep slopes. As a result of the high silica and low base content, the soils of this region are inherently acidic with low retention capacity and nutrient status (except K, Fe and Mn), and the mineral fractions are mostly dominated by clay minerals like kaolonite and illite (Gafur et al.; reporting in progress).
The CHT has a subtropical monsoon climate characterized by heavy seasonal rainfall with periodic local flooding and drought, tropical cyclones, high humidity, hot rainy summers and a pronounced dry season in the cooler months. The average annual rainfall at the experimental site (Bandarban) is 2682 mm (1941–2000; c.v. 18%) with distinct wet and dry seasons. The rainy season starts in May and continues up to October, with a sharp increase in the rainfall from May to June and the July rainfall accounting for 24% of the annual rainfall. The annual reference evapotranspiration is approx. 1350 mm leaving a potential rainfall excess of 1350 mm y-1
2.2. Experimental watersheds Three small neighboring watersheds of approx. 1 ha each were demarcated within the research farm of The Soil Conservation and Watershed Management Center (SCWMC) at Banderban. Contour maps of the watersheds at the scale of 1:940 with 4 feet vertical interval were used when delineating the boundary of the catchments at the beginning of the research program in 1997.
The watersheds are located within 153–181 m altitude with the characteristics listed. The watersheds represented two different land use situations in 1998: (i) 5th year of fallow Jhum (FJ and CJ), and (ii) a stand of 10–15 years old mixed trees of horticulture and forest species planted within fallow vegetation (MP). The fallowed Jhum area was naturally divided into two sub-watersheds (FJ and CJ), of which the CJ subwatershed was slashed and burned and cultivated in 1999 followed by fallowing in 2000, while leaving the FJ sub-watershed fallowed during the study period
FJ: The FJ watershed was dominated by secondary growth of shrubs and vines, thickets and grasses, with some horticultural and forestry trees on and along the crest of the watershed. Unweathered rocks were exposed along the main waterways and gullies. At the very steep and extreme end of the main channel, practically no soil and vegetative cover was left on the exposed shale, other than the dense bamboo canopy. Very little or no undergrowth was present along the very steep side-slopes of the channel. The FJ watershed remained in undisturbed fallow condition during 1999–2000.
CJ: The vegetation cover in the CJ watershed before slash and burn was quite similar to that of the FJ watershed. In March 1999, a tribal family of three members was engaged to cultivate the site according to their normal and traditional Jhum cultivation practices. Thus, the site (Fig. 3) was cleared for the luxuriant secondary vegetation of mostly shrubs and creepers in mid-March 1999, cutting-off the branches of the bigger trees while leaving the trunks standing. The vegetation was burned after one month of drying in mid-April, and the burning satisfactorily converted most of the slashed vegetation into ashes. Some remaining unburned trunk materials were piled up and burned again.
The first rain of the season came on 5th May 1999, 20 days after the burning. After some additional rain, mixed cereal and vegetable seeds were broadcasted or planted without major topsoil disturbance. Weeding by hand was done once before and 2, 5 and 10 weeks after sowing. Weeds grow fast in the area and may cover the seedlings a few weeks after germination if no weeding is done. Between June and December 1999, the CJ watershed was thus covered with mixed Jhum crop. Harvesting is a nearly continuous process, starting about 4 weeks after sowing with vegetables and ending with cotton and tuber crops in the middle of December 1999. By the end of 1999, the CJ watershed was in agreement with the local practice reverted to fallow and was completely covered by natural re-growth at the end of the rainy season 2000.
MP: In the MP watershed the vegetation consisted of horticultural trees (jackfruit (Artocarpus heterophylus), mango (Mangifera indica), banana (Mesua spp), kamranga (Averrhoa carambola), and orange (Citrus aurantium)) and forestry trees (Gamar (Gmelina arborea), Teak (Tectona grandis), and Korai (Albizia procera)) and some bamboo (Bambusa vulgaris; B. tulda; Teinostachyum dullooa). The MP watershed was generally left undisturbed, except for soil surveys and sampling, the construction of the gauging station, and occasional harvesting of the fruits by the farm authorities. Pruning and fertilization were not allowed, whereas farmers’ normal practice would also involve pruning the trees in winter for firewood and for making space for other undergrowth to flourish.
2.3. Measurements and data analyses Climate: An automatic raingauge (ARG100; 0.2 mm per tip) and a standard manual gauge were installed at the summit and midpoint between the CJ and FJ watersheds (Fig. 1), and additional manual gauges were installed in the area for cross-checking. Rainfall events, separated by a minimum of one hour without rain, were identified from the record totaling 318 and 328 events in 1999 and 2000, respectively. Rainfall erosivity (R) was estimated using the kinetic energy equation of Brown and Foster (1987), as recommended for the RUSLE by Renard et al. (1997). (Note: the erosivity is estimated here only to indicate the distribution of erosivity over the year 2.3. Measurements and data analyses Climate: An automatic raingauge (ARG100; 0.2 mm per tip) and a standard manual gauge were installed at the summit and midpoint between the CJ and FJ watersheds, and additional manual gauges were installed in the area for cross-checking. Rainfall events, separated by a minimum of one hour without rain, were identified from the record totaling 318 and 328 events in 1999 and 2000, respectively. Rainfall erosivity (R) was estimated using the kinetic energy equation of Brown and Foster (1987), as recommended for the RUSLE by Renard et al. (1997). (Note: the erosivity is estimated here only to indicate the distribution of erosivity over the year.