Runoff and losses of soil and nutrients from small watersheds under. shifting cultivation (Jhum) in the Chittagong Hill Tracts of Bangladesh (vol 274, pg 30, 2003)

摘要

The effect of traditional shifting cultivation practices using slash and burn (locally known as Jhum) on runoff and losses of soil and nutrients was investigated over two years in three apparently similar small neighboring watersheds of approx. I ha each in the Chittagong Hill Tracts (CHT) of Bangladesh. The experiment used a paired catchment approach, comparing the effect of a one-year clearing/cultivation with long-term fallow and mixed perennial vegetation. In the year of cultivation, peak discharges increased considerably and the loss of soil and nutrients were significantly accelerated. The median peak discharge increased by a factor 7 and annual runoff increased by approx. 16%. The direct runoff from small watersheds under long-term fallow and/or perennial vegetation may typically vary between 15 and 24% of the annual rainfall. The loss of soil material from watersheds under mixed perennial vegetation or fallow is typically about 3 Mg ha(-1) y(-1) whereas the loss during the year of cultivation was about 6 times higher. But as indicated by the sediment delivery ratio of 0.57, a sizeable part of the soil lost from the uplands was deposited in the lower part of the main drainage line within the watershed. The soil loss from the upland part of the watershed was therefore approx. 30 Mg ha(-1) yr(-1), while the regional average sediment yield associated with shifting cultivation in the CHT is estimated to be 1.2 Mg ha(-1) y(-1), considerably lower than previous estimates. The depletion of soil organic matter and nutrients in the upland area was considerable, with sediment enrichment ratios in excess of unity. It is conservatively estimated that the net depletion of soil organic matter in the upland part of the watershed, resulting from the combined effects of slash and burn and erosion during the one year of cultivation, is at least 7%. Shifting cultivation practice in the CHT is associated with a high degree of hydraulic resilience, as indicated by the return of the flow regime and the rate of soil loss to near 'normal' levels already in the first year of fallow after one year of cultivation. The implications of the findings for the interpretation of previous soil erosion and soil loss estimates in the region and for development planning are discussed. (C) 2002 Elsevier B.V. All rights reserved. [References: 47]