Runoff and sediment losses from 27 upland catchments in Southeast Asia: Impact of rapid land use changes and conservation practices

Abstract

Rapid changes in upland farming systems in Southeast Asia generated predominantly by increased population pressure and ‘market forces’ have resulted in widespread land degradation that has been well documented at the plot scale. Yet, the links between agricultural activities in the uplands and downstream off-site effects remain largely unknown because of the difficulties in transferring results from plots to a larger scale. Many authors have thus pointed out the need for long-term catchment studies. The objective of this paper is to summarize the results obtained by the Management of Soil Erosion Consortium (MSEC) over the last 5 years from 27 catchments in five countries (Indonesia, Laos, Philippines, Thailand, and Vietnam). The purpose of the study was to assess the impacts of cultivation practices on annual runoff and erosion rates. Initial surveys in each catchment included topography, soils and land use. Monitoring included climatic, hydrologic and erosion (total sediment yield including bed load and suspended sediment load) data, land use and crop yields, and farmers’ income. In addition, new land management options were introduced through consultations with farmers and evaluated in terms of runoff and erosion. These included tree plantations, fruit trees, improved fallow with legumes, maize intercropped with legumes, planted fodder, native grass strips and agro-ecological practices (direct sowing and mulch-based conservation agriculture). Regressions analyses showed that runoff during the rainy season, and normalized runoff flow coefficient based on erosive rainfall during the rainy season (rainfall with intensity exceeding 25 mm h⁻¹) increase with the percentage of the catchment covered by maize. Both variables decrease with increasing soil depth, standard deviation of catchment slope (that reflects terrain roughness), and the percentages of the catchment covered by fallow (regular and improved), tree plantations and planted fodder. The best predictors of sediment yield were the surface percentages of maize, Job's tears, cassava and footpaths. The main conclusions generated from this study were: (i) soil erosion is predominantly influenced by land use rather than environmental characteristics not only at the plot scale but also at the catchment scale; (ii) slash-and-burn shifting cultivation with sufficiently long rotations (1 year of cultivation, 8 years of fallow) is too often unjustly blamed for degradation; (iii) in its place, continuous cropping of maize and cassava promotes high rates of soil erosion at the catchment scale; (iv) conservation technologies are efficient in reducing runoff and total sediment yield at the catchment scale; (v) the adoption of improved soil management technologies by upland farmers is not a function of the degree of intensification of their farming system and/or of their incomes. The results suggest that if expansion of maize and cassava into already degraded upland systems were to occur due to increased demand for biofuels, there is a risk of higher runoff and sediment generation. A failure to adopt appropriate land use management strategies will result in further rapid resource degradation with negative impacts to downstream communities.

Introduction

In the tropical uplands of Southeast Asia, rapid changes in land use practices are taking place, the sustainability of which is in question. Current uncertainty on the amplitude of the impact of these changes on land degradation and environmental services weakens the message of the scientific community, facilitates controversies, and therefore delays the decision-making process. Three main forces are currently recognised as driving change: population pressure, government policy and market demand. In addition, two emerging driving forces have appeared: climate change and land degradation. It is well recognized that these five factors are interlinked and the relative importance of their role in influencing land use changes depends on the region in question.

Continuing increases in population pressure result not only from natural growth but also from migration of adjacent lowland farmers. Declining crop productivity and urbanization in lowland areas are forcing the continued expansion of cultivation to increasingly steep slopes, often involving the clearance of native upland vegetation. In addition, government policies favouring the resettlement of remote and scattered villages, to provide improved access to education, health and markets, is leading to high local population densities in some regions (Lestrelin and Giordano, 2007). Very high rates of economic growth and escalating market demands for agricultural products are encouraging upland farmers to replace food crops for cash crops, which in turn may lead to their increased dependence on market forces (Burgers et al., 2005). Thus, even if market access conditions have greatly improved in recent decades, this has also meant that subsistence farmers increasingly need to generate additional sources of short-term income (Sidle et al., 2006b).

As the pace of economic and social change in this region accelerates, large tracts of forest may be converted to agricultural land with potentially critical environmental implications that include a higher frequency of floods and droughts leading to crop failure. Upland soils have been subjected to misuse and unsustainable farming practices, resulting in land degradation. Uplands are eroded and nutrients depleted resulting in lost soil stability and permanent damage. As the land resource base becomes less productive, food security is compromised and competition for dwindling resources increases. Thus a downward spiral is created. However, as mentioned by Scherr (2000), it is both avoidable and reversible in many circumstances. Farmers often spontaneously seek innovations to stabilise or improve the resource base, or to compensate for their welfare effects by depending less on the degrading resource. Thus land degradation can lead to farmers giving up cultivation all together or adopting new farming systems.

Land use changes have several on-site (i.e. at the scale of the cultivated fields) impacts such as the clearance and fragmentation of native vegetation, losses in biodiversity, changes to water regimes and soil degradation, all of which have been well documented (Gardner and Gerrard, 2003, Sidle et al., 2006b). These problems may greatly affect individual land owners/users but may not represent the broader off-site and downstream impacts. Runoff and soil erosion are often not only the primary consequences and symptoms of land mismanagement but contribute to negative downstream off-site impacts such as flooding, pollution and siltation of water bodies and reservoirs. It is generally hypothesized that increased exploitation of land resources in headwater catchment areas, even as small as <1 km², with associated fragmentation of native forest vegetation, can result in increased sediment discharge and elevated nutrient loads that act to reduce water quality and availability to downstream users (Bruijnzeel, 2004). Thus land degradation resulting from changes in land use and/or climatic conditions is a concern not only to upland farmers but also to the users of water resources downstream. Indeed, although land degradation is often associated with poverty (Pender et al., 2001, Penning de Vries et al., 2002) especially in mountainous regions dominated by people who are often politically disempowered and economically marginalized, public interest in the management of uplands emerged mainly from a realization that environmental degradation of these lands has an impact on the entire community (Lian, 1993).

Central questions for environmental policy have been raised: Is there firm evidence for causal links between agricultural activities and off-site impacts? Is it thus relevant to envisage some form of payment for watershed services? Can we expect that continued population growth and improved market access will result in positive adaptive responses in terms of sustainable land management? Several scientists have questioned the impact of deforestation on large scale flooding suggesting that these effects have been overestimated (Kiersch and Tognetti, 2002, Bruijnzeel, 2004) and that major sources of sediments can be roads, poorly constructed and maintained terraces, coffee plantations or bank erosion (Sidle et al., 2006b). Recent workshops on management strategies in the uplands of Southeast Asia underlined the need for long-term catchment studies (Tomich et al., 2004, Sidle et al., 2006a, Sidle et al., 2006b) to investigate the vulnerability of upland catchments especially in developing countries where investments often focus more on disaster recovery than on prevention (Mirza, 2003).

In order to address these issues and provide sound data at an appropriate scale, a regional network called ‘the Management of Soil Erosion Consortium’ (MSEC, http://msec.iwmi.org/) was established towards the end of the 1990s. Five countries (Indonesia, Laos, the Philippines, Thailand and Vietnam), the International Water Management Institute (IWMI) and the French Institut de Recherche pour le Développment (IRD) have been implementing a long term research program aimed at monitoring changes in farming practices and the resulting runoff and sediment yields at the catchment scale. This consortium also aims to assess various innovative land use practices to reduce soil losses and enhance the livelihood of affected communities (Maglinao et al., 2003).

This paper summarizes the main results obtained through the consortium over the past 5 years with the objectives (i) to assess the impacts of rapid land use changes on runoff during the rainy season (RUNW), normalized runoff coefficients (RC25) and total sediment yield (TSY) including bed load (BLD) and suspended sediment load (SL) from 27 catchments and sub-catchments in five countries in Southeast Asia, and (ii) to assess selective land use practices aimed at improving soil conservation in these catchments.