often hits the uncovered soil surfaces of arable land, land which has dried during the
antecedent dry season (Sect. 3.2). These high rainfall intensities often occur on
steep slopes, those which supply water and matter at high energy levels, leading to
significant down-slope movement. As a consequence, soils in these areas are
naturally prone to erosion (see Chap. 2). Furthermore, fast mineralization rates
cause nutrients to be mainly stored in living plants and in the thin and highly
erodible litter layer covering the topsoil (Sidle et al. 2006). Referring mainly to
rainfed arable sloping land, soil organic matter (SOM) stabilizes soil aggregates and
enables the quick infiltration of rainfall; however, such soils are subject to rapid
mineralization due to high temperatures and prolonged and high soil moisture
content during the wet season. This situation causes a breakdown of soil aggregates
and increases surface run-off, eroding the topsoil through rill and gully erosion.
Schultze (1995) discussed tropical wet-dry climates such as those in the study areas,
saying that they display the highest geomorphic process intensities among the
world’s ecosystems. In effect, this situation offers the potential for a wide range
of human induced as well as natural disturbances to become an integral part of such
ecosystems, and this needs to be considered when defining environmental
sustainability. And so, Chang (1993) details these environments in Asia’s humid
tropics with soil erosion rates exceeding that of any other region in the world.
Environmental sustainability and unbalanced competitive relationships: To stay
with the example of soil erosion on steep and permanent arable land, the problem is
that soil erosion rates exceed soil formation through bedrock weathering. This
unbalanced situation leads to an unsteady state in terms of resource stocks and
the local material balance, as the resulting incoming and outgoing fluxes differ. So,
as soon as human activities initialize considerable erosion processes, the systems
will change towards a new steady state. A good example is the intensification of
maize (Zea mays) and cassava (Manihot esculenta) monoculture on steep slopes in
north-western Vietnam, which has resulted in an irreversible loss of soil resources
and associated nutrients in the uplands, and a subsequent inability to generate an
income. Estimates from a short-term monitoring experiment performed by the
Uplands Program revealed erosion rates in mountainous north-western Vietnam
of up to 130 tons per hectare per year (see Chap. 2), meaning soils were truncated
through surface erosion by a maximum of about 1.1 cm per year, given a bulk
density of 1.2 g per cm
3 of soil. Consequently, the ultimate abandoning of agricultural land is likely to take place in a matter of a few decades, without even
considering the more extreme storm events that occur over the long-term and that
were not captured by this study.
Requirements of biophysical knowledge for resource management: Given this
example, the need to conserve upland resources over the long-term calls for land
and water management strategies to be introduced at both the political and farm
levels. These levels typically meet at the watershed scale, a scale where solutions to
sustainable resource use are sought and; therefore, where matter and water related
processes are often investigated (Neef and Thomas 2009). One key question posed
by many studies driven by the issue of appropriate resource care is: How do human
activities impact ecosystems? Answers to this question require three linked
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
111
antecedent dry season (Sect. 3.2). These high rainfall intensities often occur on
steep slopes, those which supply water and matter at high energy levels, leading to
significant down-slope movement. As a consequence, soils in these areas are
naturally prone to erosion (see Chap. 2). Furthermore, fast mineralization rates
cause nutrients to be mainly stored in living plants and in the thin and highly
erodible litter layer covering the topsoil (Sidle et al. 2006). Referring mainly to
rainfed arable sloping land, soil organic matter (SOM) stabilizes soil aggregates and
enables the quick infiltration of rainfall; however, such soils are subject to rapid
mineralization due to high temperatures and prolonged and high soil moisture
content during the wet season. This situation causes a breakdown of soil aggregates
and increases surface run-off, eroding the topsoil through rill and gully erosion.
Schultze (1995) discussed tropical wet-dry climates such as those in the study areas,
saying that they display the highest geomorphic process intensities among the
world’s ecosystems. In effect, this situation offers the potential for a wide range
of human induced as well as natural disturbances to become an integral part of such
ecosystems, and this needs to be considered when defining environmental
sustainability. And so, Chang (1993) details these environments in Asia’s humid
tropics with soil erosion rates exceeding that of any other region in the world.
Environmental sustainability and unbalanced competitive relationships: To stay
with the example of soil erosion on steep and permanent arable land, the problem is
that soil erosion rates exceed soil formation through bedrock weathering. This
unbalanced situation leads to an unsteady state in terms of resource stocks and
the local material balance, as the resulting incoming and outgoing fluxes differ. So,
as soon as human activities initialize considerable erosion processes, the systems
will change towards a new steady state. A good example is the intensification of
maize (Zea mays) and cassava (Manihot esculenta) monoculture on steep slopes in
north-western Vietnam, which has resulted in an irreversible loss of soil resources
and associated nutrients in the uplands, and a subsequent inability to generate an
income. Estimates from a short-term monitoring experiment performed by the
Uplands Program revealed erosion rates in mountainous north-western Vietnam
of up to 130 tons per hectare per year (see Chap. 2), meaning soils were truncated
through surface erosion by a maximum of about 1.1 cm per year, given a bulk
density of 1.2 g per cm
3 of soil. Consequently, the ultimate abandoning of agricultural land is likely to take place in a matter of a few decades, without even
considering the more extreme storm events that occur over the long-term and that
were not captured by this study.
Requirements of biophysical knowledge for resource management: Given this
example, the need to conserve upland resources over the long-term calls for land
and water management strategies to be introduced at both the political and farm
levels. These levels typically meet at the watershed scale, a scale where solutions to
sustainable resource use are sought and; therefore, where matter and water related
processes are often investigated (Neef and Thomas 2009). One key question posed
by many studies driven by the issue of appropriate resource care is: How do human
activities impact ecosystems? Answers to this question require three linked
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
111