6. An Overview of Water and Related Hazard Issues
in Southeast Asia
Roy C. SIDLE1

Introduction
Headwater streams emanating from forests and agricultural lands supply much of the
potable water for tropical regions of Southeast Asia. The quality, quantity, and timing ofwater from these headwater catchments are strongly influenced by land use (e.g., Baharuddin
and Abdul Rahim 1994; Douglas et al. 1992; Ziegler et al. 2000). In turn, the distribution andtiming of water routing in these headwaters greatly influences natural hazards such as riverflooding, severe surface erosion and landslides (Sidle et al., 1985; Inamura and Dang 1997;
Sidle et al., 2003).
The interactions of land use with water pathways and storm runoff, as well as with natural
hazards, have not been extensively studied in tropical ecosystems of Southeast Asia. Relative
anthropogenic impacts may also be strongly associated with the spatial and temporal
attributes of land uses (Sidle and Hornbeck, 1991; Maita et al., 1998). Without understanding
the controls on materials transported through and stored in headwaters, it is difficult todevelop prudent long-term management plans for larger catchments (Walling, 1983). The
issue is further polarized by effective but largely unsubstantiated opinions tabled by certain
environmental groups that view any industrial management practice (particularly forestry) as
unacceptable and unsustainable. Land management agencies and water users are therefore left
to decide on acceptable activities based on poor scientific information and the biased
perspectives of both environmental groups and the industrial sector. While planning anddecision processes related to integrated catchment management must consider socioeconomic and political objectives as well as public opinion, it must be remembered thathydrologic response is the primary driver in these systems.
Land Use Issues that Affect Water and Related Resources
Land use activities in the tropics of Southeast Asia can have a substantial impact on theoccurrence and severity of floods, landslides, debris flows, and severe soil erosion. In
addition to generating widespread sedimentation and damage to streams and riparian habitat,
these hazards pose significant risks to humans, infrastructures, and property in the runout
1

Professor and Head of the Slope Conservation Section, Division of Geodisaster, Disaster Prevention ResearchInstitute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan; e-mail: sidle@slope.dpri.kyoto-u.ac.jp

1

zone (Swanston and Schuster, 1989; ESCAP, 1999). The probability and severity of thesenatural hazards can increase related to timber harvesting, road building, vegetation
conversion, urbanization, and recreation (e.g., Sidle et al., 1985). While the media has beenquick to blame ‘so-called’ deforestation for the recent flood disasters in Southeast Asia, amore pragmatic look at the actual factors contributing to the flooding process is needed – e.g.,
spatial distribution of soil compaction, alteration of flow paths, sealing of soil surfaces, andchanges in evapotranspiration. These and other factors (e.g., overloaded slopes, reduced root
strength) must be considered in assessing the effects of land use on landslide initiation andthe onset of severe surface erosion.
Nowhere in the world are natural resources more at risk than in developing rural regionsof Southeast Asia. Government policies in the region that promote high cash value crop
production and transmigration together with the apparent needs of subsistence farmers togenerate additional sources of income from the land have placed increasing stresses on soil
and water resources. Additionally, export of timber and associated forest products from thisregion to developed nations worldwide has exacerbated these stresses. Along with thedevelopment of extensive agricultural and forest land are the methods employed for land
clearance (e.g., fire, logging, mechanical clearing) and the transportation corridors (ranging
from foot paths to roads).
The effects of roads and trails on hydrologic processes and sediment transport are
significant in tropical catchments (Baharuddin et al., 1995; Ziegler et al., 2000; Sidle et al.,2003). All roads and trails modify site hydrology by decreasing the hydraulic conductivity
and infiltration capacity of the traveling surface, redirecting incoming rainfall and water as
Hortonian overland flow, and concentrating this runoff into various parts of the catchment. In
many regions of Southeast Asia, secondary roads and trails (including forest roads and skid
trails) are not built to suitable engineering standards, are not surfaced with rock, and drainage
systems often do not exist; thus, surface runoff simply follows the natural contour of the roadand discharges onto the slopes or into channels at topographic low points or discharge nodes(Sidle et al., 2003). Where the roadbed is erodible, the lack of drainage systems can havecatastrophic consequences for sediment production, particularly in steep terrain. Baharuddin
et al. (1995) measured surface erosion losses in the range of 10-13 t ha-1yr-1 from plots on
logging roads and skid trails in a Peninsular Malaysian rainforest during the first year after
logging (Table 1). In the second year after logging, losses declined to 2-3 t ha-1yr-1. Using
field evidence of cumulative soil loss, Sidle et al. (2003) estimated that surface erosion from
logging roads and skid trails was about 275 t ha-1yr-1 in a selectively logged catchment in
Malaysia (Table 1). Erosion from road surfaces as well as headcutting of channels contributeto sedimentation of streams. Roads cut into hillsides may intercept subsurface water andreroute it into drainage systems or along the road surface, thereby enhancing peak flows(Sidle et al., 2003). Mass wasting from roads occurs episodically, with little soil loss in many
years and much erosion during years with large storms; while such episodic inputs can be2

missed in short-term monitoring, they may be dominant sediment inputs (Douglas et al.,1999).
Issues related to urbanization are more spatially restricted and constitute a major concern
related to water supplies, pollution and flooding because of the high density of inhabitants
concentrated in the ever growing cities of Southeast Asia. Water-related hazards are aparticular concern due to the demographics. A recent example is the extensive flooding that
occurred in Jakarta in early 2002, partly exacerbated by poor urban drainage systems. Healthissues arise during flooding in many Southeast Asian cities because storm drains are notseparated from sewage lines; thus, when these systems are overloaded, contaminated water iswidely distributed overland. High temperatures in the region promote the spread of infectious
water-transmitted diseases.
The conversion of tropical forests to oil palm plantations has been undoubtedly the mostwidespread land use change in Malaysia and Indonesia during the past several decades (e.g.,
McMorrow and Abdul Talip, 2001). At the turn of last century, more than 55% of Malaysia’sforest was reported as oil palm plantations. The recent rate of conversion of native forests tooil palm plantations in Indonesia is even greater and this, when added to the impacts of illegal
logging, are drastically reducing tropical forests. Few studies have been published in thescientific literature on the effects of conversion of tropical forests to oil palm plantations;based on field observations, the impacts on soil and water resources are much greater interrain steeper than 20%. Most of the impacts appear to occur during the land clearing process
and in the initial 2-3 years following plantation establishment, with much rapid storm runoff
and sediment produced on roads and connected disturbed areas (e.g., terrace cuts) within theplantation. Additionally, the clearing of native forests by fire, a common practice in Indonesia,
has contributed substantially to the recent haze problems in equatorial Southeast Asia.
The conversion of native forests to permanent agriculture and other plantations (e.g.,coffee, rubber, fruit trees) has also created problems for soil and water conservation (Table 1).
Forest conversion to coffee plantations is occurring throughout much of Southeast Asia,
particularly in Vietnam and Indonesia, while tea and fruit plantations, as well as a variety ofhigh cash value crops are grown on steep hillsides throughout the region. Much of theforested landscape in northern Thailand was converted to agricultural land between 1954 and1976; more recently, forest cover has been relatively stable, but dynamic cultivation changes
have occurred on the converted land related to a shift from subsistence to cash crops (Fox etal., 1995). Where crops are grown as monoculture, soils are typically more susceptible toerosion compared to forest cover.
An apparent compromise to such land conversions is the application of agroforestry.
These practices are believed to offer a combination of both short-term economic returns tofarmers (i.e., crop production) together with longer-term investments and soil and water
conservation benefits (i.e., trees) (e.g., Fischer and Vasser, 2000). The effectiveness ofagroforestry depends strongly on the adaptation of the practice with the local environmentand culture as well as the effectiveness of restoring some of the water, sediment, and nutrient3

pathways inherent in the former natural forest environment. The incorporation of a covercrop within a plantation of trees, or between rows of trees or shrubs, has been shown to be aneffective control on surface erosion in Peninsular Malaysia (Hashim et al., 1995), Philippines(Tacio, 1993), and Sumarta (Rosadi et al., 1999) (Table 1). Coffee and cocao trees grown on
moderate slopes experience high rates of erosion (11-70 t ha-1yr-1), but rates declined to 0.153.4 t ha-1yr-1 when cover crops are incorporated (Hashim et al., 1995; Rosadi et al., 1999)(Table 1). The sustainability of various agroforestry practices in Southeast Asia has yet to beevaluated at the watershed scale.
Soil loss from conventional hillslope agriculture practices in Southeast Asia are generally
high, although rates are have typically been assessed only for surface erosion at the field plotscale (Table 1). Such rates are generally higher than at the catchment scale because there islittle opportunity for deposition and storage of sediment on uniform gradient plots. For
vegetable crops grown on moderate to steep hillsides, the highest levels of soil loss (38-140 t
ha-1yr-1) occurred when cultivation was oriented up and down the hillslope, a typical practicein the region (Paningbatan et al., 1995; Presbitero et al., 1995; Sombatpunit et al., 1995;
Poudel et al., 1999). Cultivation along the hillslope contour together with the use of
agricultural hedgerows reduced erosion by half or more (Table 1). Modest erosion rates werereported from rice paddy fields in northwest Thailand (Nishimura et al., 1997). Midmore et al.
(1996) note that preparation and maintenance of terraces cut into hillsides are major sourcesof erosion in tea plantations in the Cameron Highlands, Malaysia (Table 1).
No studies have quantitatively addressed the impacts of forest conversion on landslides intropical Southeast Asia. Terraces built on steep slopes can increase landslide potential byboth concentrating water on the terraces (especially if back-sloped) and by oversteepening
the terrace face. Aside from terracing, the major effect of agricultural conversion on slope
stability can be attributed to a more or less permanent loss in rooting strength compared toforest or brush vegetation. Shallow rooted agronomic species have negligible rooting strengthcompared to deeper-rooted trees and shrubs (Sidle and Dhakal, 2002). This long-term
reduction in root strength can be translated into in increasing probability of land sliding (e.g.,
Sidle 1992).
Shifting cultivation is still common in the region and involves leaving intensively
cropped land fallow to restore soil fertility, however, it can increase both surface andlandslide erosion if practiced on steep hillsides. Intensive cultivation may decrease theinfiltration capacity of soils, thereby increasing runoff and surface erosion during storms (e.g.,
Roose and Ndayizigiye, 1997). Additionally, mass wasting may increase due reductions inrooting strength (Sidle et al., 1985). Because shifting cultivation clears forested hillsides forlong periods and because reforestation on abandoned, nutrient-depleted agricultural fields isvery slow, the impact of this practice is longer-lived compared to timber harvesting withregeneration. When fire is used to clear forested sites (i.e., slash and burn), additional erosion
may occur.

4

Few studies have assessed the direct effects of recreation on hydrologic and erosion
processes in tropical Asia, although inferences have been made to such impacts. In Pulau
Penang, Malaysia, a mountainous island that has undergone rapid urbanization, results from a
one-year catchment study provide some insights into the magnitudes of cumulative effects ofrecreational pressures on sediment losses (Wan Ruslan, 1997) (Table 1). Erosion loss fromthe upper catchment that was cleared for crops and orchards (with roads) was 9.1 t ha-1yr-1

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Table 1. Soil erosion from various land uses in Southeast Asia and similar environments
Area

Methods/
Slope

Cover/treatment

Erosiont ha-1yr-1
0.01 – 3.12

Southeast Asia: Pen.
Malaysia, Sabah,Java, & Thailand

Catchment& plot exp.
Variousslopes

Undisturbed natural and secondary rainforest

Sabah

Catchment
study

Logged lowland dipterocarp rainforest:
1. During and immediately after logging
2. 5 – 6 years after logging, after regeneration
Logged tropical rainforest:
1. selective logging
2. logging road (1st year after logging)
3. skid trail (1st year after logging)
4. logging road (first 16 months after logging)
5. skid trail (first 16 months after logging)
1. Bare2. Up/down slope cultivation: corn, sweet potato
3. same crops + contour cultivation with hedges(no intercropping)
4. same as 3 w/ rotation/intercropping, groundnut
Corn & mungbean with:
1. Up/down slope cultivation
2. Hedgerow alley cropping; contour till
3. same as 2 but with mulch residue
4. Alley cropping + no till + mulching
Cabbage, tomato & corn) with:
1. Up/down slope cultivation
2. Contour planting
3. Strip cropping
4. Contour hedgerows of asparagus, pineapple,
pigeon-pea, & lemongrass
Mixed vegetables with tea on prepared terraces(large area)
Rozelle (fiber crop) with:
1. Up/down slope cultivation
2. Hedgerow alley cropping; contour till
3. same as 2; residues mulched
4. same as 3 + no till
5. bare soil; no fiber crop
Rice paddy

1. 2.82. 3.03. 1.5
4. 0.3
5. 48.4
3.9

Oil palm plantations
1. orchards, agricultural crops + roads
2. urbanization/recreation

7.7 – 8.9
1. 9.1
2. 30.9

Peninsular Malaysia

Baybay, Leyete,
Philippines

Laguna, Philippines

Catchment
Plot experiments
erosionfeatures
Plots; 50%
slopesPlots; 1421% slopes

Northern Mindanao,
Philippines

Plots; 3544% slopes

Camaron Highlands
Malaysia
Khon Kaen, Thailand

Air photoassessment
Plots; 3.6%
slopesUpper Mae TangRiver Basin, NW
Thailand
Malaysia
Pulau Pinang,
Malaysia
Sunmberjaya,
SumatraIndonesia

Small plot
exp. 21.4%
slopesVarious
Catchment;
VariousslopesPlots; 27%
slopesSouthern Minanao,
Philippines

Plots; steep
slopesEast Coast ofPeninsular Malaysia,
Kemaman (south
Terengganu)

Large plots;
17.6%
slopes“Sun” coffee plantation with:
1. No ground cover
2. Paspalum ground cover
3. Natural weed ground cover
Agroforestry: vegetable crops interplanted
between double contoured rows of N-fixing
shrubs & trees
Agroforestry: Cocoa, shade trees (Gliricidia) and1. Intercropped w/ banana; bare ground2. same as 1 w/ legume ground cover
3. Monocropping (no banana); bare soil
4. Monocropping; legumes
5. Bare soil; no crops

6

References
Morgan & Finney 1982;
Douglas et al. 1992; Sinun etal. 1992; Baharuddin & A.
Rahim 1994; Wan Ruslan
1997
Chappell et al. (in press)

1. 16
2. 5.9
1. 0.3-0.4
2. 13.3
3. 10.1
4. 272
5. 275
1. 68.7
2. 38.1
3. 18.6

Baharuddin & A. Rahim 1994
Baharuddin et al. 1995
Sidle et al. in press
Presbitero et al. 1995

4. 2.7
Paningbatan et al. 1995
1. 140.3
2. 23.0
3. 2.84. 1.8Poudel et al. 1999
1. 65.3
2. 37.8
3. 43.7
4. 45.4
24

Midmore et al. 1996
Sombatpunit et al. 1995

1. 10.7
2. 0.14
3. 1.3
3.4

Nishimura et al., 1997

Morgan & Finney 1982
Wan Ruslan 1997

Rosadi et al. 1999; Oki et al.
1999

Tacio 1993

Hashim et al. 1995
1. 69.7
2. 3.4
3. 11.2
4. 1.05. 120.7

compared to 30.9 t ha-1yr-1 in the rapidly urbanizing lower catchment. These losses are
particularly notable since they are from catchment studies, not plots. Erosion loss from anearby undisturbed catchment on the island was 1.02 t ha-1yr-1. Effects of such large-scale
recreation are difficult to assess because tourism includes a multitude of infrastructures, all ofwhich impact erosion and hydrologic processes to some degree, but which are difficult toseparate from other anthropogenic impacts. The Cameron Highlands in Peninsular Malaysia
is another example of where development pressures related to tourism (e.g., new roads, hotel
development, horticulture) have caused many landslides and severe surface erosion. The
scope of outdoor recreation in Southeast Asia has greatly expanded in the past two decades,
particularly coastal tourism, which has resulted in the destruction of many coastal and nearshore forests. Large coastal sediment plumes detected on satellite images attest to theregional scale of soil loss following land clearance and development (Gupta, 1993).
Recent Water-Related Issues in Southeast Asia
In the past few years, there have been a number of flood disasters in Southeast Asia. Most
severe were the floods in central Vietnam in 1999 that claimed more than 700 lives andcaused the evacuation of more than 25,000 households. Rainstorms in late October and earlyNovember of 1999 caused catastrophic flooding in the central Vietnam provinces and, just as
relief efforts were underway, new flooding from heavy rainfall in the first week of December
occurred. The total property damage likely exceeded 300 million USD.
During 2002, many water-related disasters have occurred in the region. In February,
major flooding and associated landslides in Java (including Jakarta) killed more than 125
people, and destroyed about 1200 km2 of rice fields. These flood losses were at least as highas those (≈ 200 million USD) during the devastating May 1998 floods in Java that caused
widespread looting and arson and helped topple the former President Suharto regime. A
prolonged drought during the usual rainy season affected many of the southern Cambodia
provinces in 2002, with more than half a million people facing food shortages.
Simultaneously, the Mekong River reached flood stages along the Cambodian-Laotian border,
killing at least 30 and affecting 1.5 million people. Widespread flooding also occurred in
Myanmar with more than 4000 km2 of farmland inundated by floodwaters. Throughout thePhilippines, monsoon rains and typhoons during the summer of 2002 caused extensive
flooding and landslides, killing at least 15 people and destroying more than 370 km2 offarmland. The death toll from widespread flooding in northern Thailand reached 170 by theend of August 2002; damage was estimated at ≈ 32 million USD. Although manyorganizations have earmarked ‘deforestation’ as the main culprit for these floods, better
causal links between spatially distributed on-site disturbances and hydrologic response need
to be developed to better evaluate flooding.
In a region of economically developing nations, Singapore is an enigma as a rather
wealthy island state. Most of Singapore’s water is piped in from neighboring Malaysia. A
lesser percentage of the water supply (unspecified by the Public Utilities Board) is captured
7

in reservoirs on the island. Thus, Singapore has a vested interest in the important regional
issues related to water resources because of its dependence on Malaysia for a sustainablepotable water supply (Ong, 1999). Seemingly, Singapore has a responsibility to minimize
environmental impacts and foster high standards for land and water stewardship in foreign
areas affected by their water withdrawals. Because of the scarcity of land and high population
density in Singapore, the current protected catchments in the central part of the island provide
a fixed but limited potable water supply (Ong, 1999). However, these reservoirs do notappear to be efficiently managed. During a recent drowning incident (3 people killed) in
April 2002 near the outlet of Kranji Reservoir, it was revealed that the Public Utilities Board(PUB) authorizes releases of water stored in the reservoirs to the sea during storms, rather
than pumping this water into alternative storage sites when reservoirs are near capacity.
While PUB denied any connection between the releases of water at Kranji Reservoir with theoff-shore drownings, the practice releasing stored fresh water to the sea and, at the same time,purchasing water from Malaysia is clearly an unsustainable use of this resource. During thepast two decades of rapid growth, Singapore has not elected to adopt water conservation
measures such as ‘gray water’ recycling systems in apartment and business complexes.
Further puzzling, is the fact that Singapore is considering expensive high-technology
solutions (e.g., desalinization, filtration) to their water crisis. It is apparent that Singapore
needs to embrace a broader regional perspective on water use to achieve sustainability of thisresource.

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