Toward understanding the cumulative impacts of roads in upland agricultural watersheds of northern Thailand

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Abstract

This paper describes the interactions of various physical processes that allow unpaved roads to contribute disproportionately to basin-wide runoff and stream sediment in the 93.7 ha Pang Khum Experimental Watershed (PKEW) in northern Thailand. Many road sections in PKEW are constant sources of sediment entering the stream during most rain events because: (1) Horton overland flow is generated on the compacted surfaces after small depths of rainfall; (2) surface preparation processes, including vehicle detachment and maintenance activities, renew the supply of easily transportable surface sediment on inter- and intra-storm time scales; (3) erosion of the road surface is accelerated in locations where slopes are steep, overland flow distances are long, and/or vehicle usage is high; (4) surface runoff typically exits from the road directly into the stream. Owing to these collective processes, sediment delivery rate on PKEW roads is more than an order of magnitude higher than that on adjacent fields (≈120 Mg ha−1 per year versus 9 Mg ha−1 per year). Thus, unpaved roads appear to be on the same order of importance as agricultural lands in contributing sediment to the stream network, despite occupying a fraction of the total surface area in the basin (≈0.5% versus 12%). A more thorough assessment of linkages between all hillslope runoff/sediment sources and the stream network, however, is still needed to fully evaluate the relative impacts of roads versus those of agriculture practices in PKEW.

Introduction

Extensive forest removal and intensification of agriculture on steep slopes in the highlands of northern Thailand during the last several decades have been linked to changes in hydrologic phenomena and sedimentation; however, the detection of these impacts across varying scales has not always been conclusive (cf. Tangtham and Sutthipibul, 1989, Anecksamphant and Boonchee, 1992, Alford, 1992, Alford, 1994, Wilk et al., 2001). Concern about the long-term implications of unsustainable activities in the Thai highlands fostered the initiation of both domestic and international conservation projects (e.g. Hurni, 1982, IBSRAM, 2000; www.forestandpeople.org, 2002). Much research has focused on improving the agriculture practices of ethnic minorities, whose traditional short-term shifting systems have been evolving perforce to more permanent systems—but implemented frequently on the same steep slopes (Schmidt-Vogt, 1999). While unsustainable cultivation likely contributes to downstream environmental impacts, the expansion of the rural road network almost certainly plays a substantial part as well.

Mountain roads in northern Thailand stand out at almost any scale as sources of sediment entering streams (Fig. 1a). Road runoff empties frequently into stream channels at low-water bridges or via culverts/gullies (Fig. 1d). During prior research in Thailand, we determined that the contribution of basin-wide excess rainfall during frequently occurring, small rainfall events was disproportionate to their area extent, compared with other common land surfaces (Ziegler and Giambelluca, 1997). Rijsdijk and Bruijnzeel (1991) showed similarly that roads in east Java (Indonesia), comprising roughly 3% of the catchment, contributed a disproportionate sediment volume to the basin yield. Studies in upland temperate basins have also shown road-related impacts can be greater than or equal to those of other disruptive activities (e.g. Megahan and Kidd, 1972, Grayson et al., 1993, Megahan and Ketcheson, 1996). One study, conducted in Chiang Mai Province of Thailand, determined forest ‘destruction’ to be less significant than road construction in increasing annual runoff yields (Pransutjarit, 1983).

Rural roads are important for the development of the northern highlands of Thailand. For example, they provide access to agricultural watersheds, allowing the transport of crops to markets. Mountain roads are vital to national security, especially in border areas; and they facilitate policing of drug trafficking. Because of the necessity of accessing remote mountain basins, road-associated environmental impacts are typically accepted by local inhabitants and governing officials alike. Lack of attention to road impacts may also reflect the paradigm of forest removal and unstable agricultural practices as the major disruptive activities in the region. On the other hand, it may also represent a lack of understanding, both in the nature of, and solution to, the problem. Another critical issue is determining who pays for improved road design and routine maintenance.

The literature reflects an increasing awareness of the need to address road-related impacts (e.g. Harr et al., 1975, Megahan and Clayton, 1983, King and Tennyson, 1984, Reid and Dunne, 1984, Anderson and Potts, 1987, Bilby et al., 1989, Fahey and Croker, 1989, Luce and Cundy, 1994, Montgomery, 1994, Jones and Grant, 1996, Wemple et al., 1996, Foltz and Elliot, 1997, Thomas and Megahan, 1998, Croke et al., 1999, Luce and Black, 1999, Croke and Mockler, 2001, La Marche and Lettenmaier, 2001). Only a few of these studies, however, have been conducted in SE Asia, where surface erosion on unpaved mountain roads can be severe during heavy rains of the annual monsoon rain seasons (e.g. Pransutjarit, 1983, Kamaruzaman and Nik, 1986, Malmer and Grip, 1990, Rijsdijk and Bruijnzeel, 1991, Van der Plas and Bruijnzeel, 1993, Ziegler et al., 2000b). In fairness, watershed managers in even the most developed countries in the world have yet to solve many of their road-associated environmental problems (cf. Gucinski et al., 2001).

In 1997, we began the Thailand Roads Project (TRP), a study of hydrological and geomorphologic impacts of unpaved mountain roads in northern Thailand. One object of TRP is ascertaining the relative impacts of roads versus agriculture practices in the highlands of northern Thailand. Although conducted in Thailand, results of the work contribute to understanding the environmental consequences of road-associated impacts in any geographical region. In this paper, we summarize the principal findings of TRP Phase I, which ended in May 2000.

Section snippets

Site description

The study area is near Pang Khum village (19°3′N, 98°39′E), which is located within the Samoeng district of Chiang Mai Province, approximately 60 km NNW of Chiang Mai in the eastern range of the Thanon Thongchai Mountains (Fig. 2a). Most work has been conducted in the adjacent 93.7 ha Pang Khum Experimental Watershed (PKEW; Fig. 2b), which is located in Mae Taeng District. PKEW is, therefore, part of the larger Mae Taeng River Basin, which drains into the Ping River, the major tributary to the

Conceptual framework: the road prism

In Fig. 2c, we conceptualize the three-dimensional road prism. One common source of road runoff (RO) is Horton overland flow (HOF, rainfall rate in excess of infiltration rate and ponding, Horton, 1933). Horton flow can be prevalent on road surfaces (HOFr) when compaction reduces the rate of infiltration (I). Saturated hydraulic conductivity (Ks) is often used as an index of infiltrability. In some instances, Horton flow entering from upslope surfaces (HORu) may increase total on-road overland

Field data

To gather information needed to quantify road- and agriculture-associated impacts in PKEW, we performed the following:

  • 1.

    Hydrological and physical measurements, such as saturated hydraulic conductivity (via disk permeameters), bulk density (BD), penetration resistance (PR), and texture at several locations on PKEW roads and all other important land covers.

  • 2.

    Bedload and suspended sediment measurements in Loei stream at the basin outlet (Station 405, Fig. 2b).

  • 3.

    Assessments of cross-sectional physical

Results

Unpaved roads in PKEW are on the same order of importance as agricultural surfaces in contributing surface runoff and sediment to the stream. The relative contributions of road and agricultural fields to basin-wide HOF during the largest observed storm can be seen in Fig. 5a. Total rainfall is about 58 mm, with most falling during the first 30 min after runoff initiation on the road. Mean 1 min rainfall intensity during this period is 89 mm h−1; the maximum 1 min intensity is 137 mm h−1. During the

Conclusion

Compared with other land surfaces, roads contribute disproportionately to the basin runoff and sediment yield in PKEW because (1) infiltration is reduced by compaction; therefore, roads have the propensity to generate HOF during most rain events; (2) sediment production on roads is high because easily-removed material is re-supplied by inter- and intra-storm phenomena (including vehicular detachment, road repairs, and cutbank failure); (3) runoff on most road sections drains directly into the

Acknowledgements

This work is funded by the National Science Foundation USA (Grant Nos. 9614259, 0000546). ADZ was supported by an Environmental Protection Agency Star Fellowship and a Horton Hydrology Research Award (Hydrological Section, American Geophysical Union). The research was approved by National Research Council Thailand (NRCT); conducted with permission of the Royal Forest Department (RFD), Mae Taeng District; and sponsored by the Geography Department, Faculty of Social Sciences, Chiangmai University.

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