| Miêu tả |
Two years research has been conducted in Tam Duong district, Vinh Phuc province, upstream
in the Red River Delta in northern Vietnam, comprising three geographical regions, i.e.
mountainous areas, terraces in hilly land and flat land. The extent of soil degradation in the
district was delineated, using the Red/Near-Infrared band ratio of satellite images, calibrated
on the basis of soil maps and field checks. Results showed strong dynamics in soil degradation
with the largest area of degraded soil of 3280 ha in 1992, associated with a substantial
reduction in forest cover and expansion of the agricultural area from the mid-1980s.
Subsequently (1996), re-forestation, particularly planting of eucalyptus plantations, led to a
reduction, followed (2000) by expansion again, as a consequence of harvesting of production
forests.
In the mountainous and hilly areas, soil erosion is the dominant degradation process, very
severe at individual plot scale, but far less at sub-watershed and watershed scales (i.e.
measured soil losses over eight events, with the same total rainfall, were 1360 and 773 kg ha–1
in a cassava and an eucalyptus plot, while it was 45 in the sub-watershed and 125 in the main
watershed). Performance of the soil erosion model LISEM was evaluated in the study area;
results showed differences in performance at different spatial scales. In the main watershed,
simulated total runoff and soil loss were underestimated, because of storage and release in
terraces and rice fields. In the upland sub-watershed, total soil loss was overestimated due to
overestimation of sediment concentration, as a result of high detachment and transport
capacity.
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