Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: Buffering of Hortonian overland flow (original) (raw)
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Journal of Hydrology, 2004
Measurements of saturated hydraulic conductivity ðK s Þ and indices of Horton overland flow (HOF) generation are used to assess the influence of landscape fragmentation on near-surface hydrologic response in two upland watersheds in northern Vietnam. The fragmented landscape, which results from timber extraction and swidden agriculture, is a mosaic of surfaces having distinct infiltration characteristics. In general, human activity has reduced infiltration and altered near-surface flow paths on all disturbed land covers. Compacted roads, paths, and dwelling sites, for example, have the propensity to generate HOF for small rainfall depths. Although these surfaces occupy a small fraction of a basin land area (estimated at ,1%), they contribute disproportionately to overland flow response during typical rainfall events. Recently abandoned fields have the lowest K s of all non-consolidated, post-cultivation surfaces tested. Beginning 1 -2 years following abandonment, diminished K s recovers over time with the succession to more advanced types of secondary regrowth. If a grassland emerges on the abandoned site, rather than a bamboo-dominated cover, K s recovers more rapidly. The decrease in K s with depth below disturbed surfaces is more acute than that found at undisturbed sites. This enhanced anisotropy in near-surface K s increases the likelihood of the development of a lateral subsurface flow component during large storms of the monsoon rain season. Subsequently, the likelihood of return flow generation is increased. Because the recovery time of subsurface K s is greater than that for the surface K s ; the impact human activity has on hydrologic response in the fragmented basin may linger long after the surface vegetation has evolved to a mature forested association. q
Forest Ecology and Management, 2006
We use field observations and diagnostic computer simulations (KINEROS2) to estimate the effective slope lengths (ESL) for buffers on disturbed hillslopes in two fragmented basins in northern Vietnam. Grassland, disturbed forest, and intermediate forms of secondary vegetation are the most effective buffering vegetation in the study area because these surfaces tend to have the highest saturated hydraulic conductivity. The ESL (m) is described by the following function of slope (m m À1 ): ESL = 98 + 15 ln(slope). This non-linear relationship predicts comparatively longer buffer lengths at gentle slope gradients than guidelines/practices currently in use. The predicted buffer lengths range from roughly 30 to 100 m for slope gradients ranging from 0.01 to 1.0 m m À1 . However, for large storms, steeper slopes, and/or more degraded conditions, buffer lengths greater than those predicted by the ESL criteria may be needed to minimize impacts from overland flow. On slopes with particularly large contributing areas, multiple or staggered buffers may be required. For the occurrence of concentrated overland flow, no practical buffer length may be sufficient. The ESL estimations provide a starting point for determining appropriate buffer dimensions needed to infiltrate upslope surface runoff in disturbed montane watersheds at the study site. Final determination of buffer dimensions should consider the physical characteristics of contributing hillslopes, the nature of the material to be filtered (e.g., water, sediment, chemicals, nutrients), and the likelihood of adoption of any buffering practice. Finally, buffers should be regarded as complementary practices to other hillslope conservation activities. Recognizing that the use of long buffer lengths may not be feasible for steep terrain in intensely managed tropical watersheds, we derive a second equation to predict the minimal effective slope length (MESL) for buffers: MESL = 32 + 4 ln(slope). MESL values range from approximately 15 to 30 m over the same slope gradients, but they are less effective at reducing HOF than ESL buffers, particularly for large storms when erosion risk is highest. #
International Journal of River Basin Management, 2011
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Land Degradation on Barren Hills: A Case Study in Northeast Vietnam
Environmental Management, 2008
The term ''barren hills'' has been a keyword for land degradation in the uplands of Vietnam for over a decade. Nevertheless, the ''barren'' land is still not adequately ecologically characterized. In this work, we analyze land use-induced changes in vegetation and soil properties along a sequence of barren hills types formed on one physiotope. The study is undertaken in the Bac Kan province, one of the poorest upland regions where livestock plays an important role. A transition from an old-growth laurel forest to a sparse manmade grassland is characterized by a total of 177 species, rapid species turnover, and discrete dominants, and an overwhelming effect of disturbance history on both soil and vegetation patterning. Land degradation is most apparent in land use-induced maintenance of arrested successions, and the regeneration course is shifted towards drier formations. We hypothesize a conceptual model as an aid to understanding the process of early fallow differentiation in response to the patterned, fine-scale disturbances. The larger-scale implications of the observed trends in regeneration potentials deviation, and, in particular, the effect of water buffaloes in halting fallow successions, are discussed.
2009
L'histoire recente du Vietnam a ete marquee par vingt dernieres annees de transition, passant d'une economie socialiste a une economie de marche. Dans le domaine agricole, les politiques recentes ont causes d'importants changements jusque dans des zones plus reculees comme les regions montagneuses du Nord du pays. Concernant l'irrigation, une diversification des modes de gestion des systemes irrigues a ete constatee dans le district de Van Chan, province de Yen Bai, situe dans ces zones de montagne. C'est l'histoire, mais aussi les contraintes du milieu naturel dans lequel evoluent les paysans qui determinent la diversite des situations observees aujourd'hui. Une typologie des systemes irrigues de montagne mettant en evidence cette diversite a ete etablie, elaboree en synthetisant l'etude de plusieurs systemes irrigues du district de Van Chan :- Type 1 : Tres petits systemes irrigues de montagne geres par un ou quelques foyers d'usagers ;- Type 2 ...
The Song Gianh is a small-sized (~3500 km2), monsoon-dominated river in northern central Vietnam that can be used to understand how topography and climate control continental erosion. We present major element concentrations, together with Sr and Nd isotopic compositions, of siliciclastic bulk sediments to define sediment provenance and chemical weathering intensity. These data indicate preferential sediment generation in the steep, wetter upper reaches of the Song Gianh. In contrast, detrital zircon U-Pb ages argue for significant flux from the drier, northern Rao Tro tributary. We propose that this mismatch represents disequilibrium in basin erosion patterns driven by changing monsoon strength and the onset of agriculture across the region. Detrital apatite fission track and 10Be data from modern sediment support slowing of regional bedrock exhumation rates through the Cenozoic. If the Song Gianh is representative of coastal Vietnam then the coastal mountains may have produced around 132,000–158,000 km3 of the sediment now preserved in the Song Hong-Yinggehai Basin (17–21% of the total), the primary depocenter of the Red River. This flux does not negate the need for drainage capture in the Red River to explain the large Cenozoic sediment volumes in that basin but does partly account for the discrepancy between preserved and eroded sediment volumes. OSL ages from terraces cluster in the Early Holocene (7.4–8.5 ka), Pre-Industrial (550–320 yr BP) and in the recent past (~150 yr BP). The older terraces reflect high sediment production driven by a strong monsoon, while the younger are the product of anthropogenic impact on the landscape caused by farming. Modern river sediment is consistently more weathered than terrace sediment consistent with reworking of old weathered soils by agricultural disruption.
Journal of Environmental Management, 2010
Recent catastrophic floods in Viet Nam have been increasingly linked to land use and forest cover change in the uplands. Despite the doubts that many scientists have expressed on such nexus, this common view prompted both positive forest protection/reforestation programs and often-unwarranted blame on upland communities for their forest management practices. This study discusses the disparity between public perceptions and scientific evidences relating the causes of catastrophic floods. The former was drawn on the results of a questionnaire and focus groups discussions with key informants of different mountainous communities, whereas the latter was based on GIS and remote sensing analysis of land cover change, including a statistical analysis of hydro-meteorological data of the Huong river basin in Viet Nam. Results indicate that there is a gap between the common beliefs and the actual relationship between the forest cover change and catastrophic floods. Undeniably, the studied areas showed significant changes in land cover over the period 1989e2008, yet, 71% of the variance of catastrophic flood level in the downstream areas appeared related to variance in rainfall. Evidences from this study showed that the overall increasing trends of catastrophic flooding in the Huong river basin was mainly due to climate variability and to the development of main roads and dyke infrastructures in the lowlands. Forest management policies and programs, shaped on the common assumption that forest degradation in the upland is the main cause of catastrophic flood in the downstream areas, should be reassessed to avoid unnecessary strain on upland people. (R. Shaw). 1 Tel.: þ84 914 051 223. 2 Tel./fax: þ81 75 753 5708.
As Vietnam embraces the market economy, and a number of state policies promote reforestation and rural market integration, land use and land cover (LULC) changes are occurring in the country's northern uplands in increasingly complex and fragmented ways. Yet understandings of the degree and consequences of LULC changes in this diverse agro-ecological region are incomplete. We conduct a systematic literature review of research reported in academic articles tracing and analysing LULC change in Vietnam's northern regions. We find that these studies have tended to take place away from the most mountainous, northern borderlands. The studies nonetheless highlight a diversity of land use land cover changes caused by numerous causes, making the distinction of overall trends difficult. To complement and extend this body of research, we introduce recent LULC change research we have completed in the mountainous border districts of Lào Cai province, on the Sino-Vietnamese border. The heterogeneity of causes of LULC change in both the review articles and our case study points to the importance of adapting land use policies to local agro-ecological and socio-economic conditions and ethnic diversity, taking into account state–farmer relations, household livelihood decision-making, and policy implementation at the commune and district levels.