Deforestation and Erosion in the Nepalese Himalaya - Is the Link Myth Or Reality?

Forest Hydrology and Watershed Management - Hydrologie Forestiere et Amenagement des Bassins Hydrologiques (Proceedings of the Vancouver Symposium, August 1987; Actes du Co11oque de Vancouver, Aout 1987):IAHS-AISH Publ.no.167,1987.

Deforestation and erosion in the Nepalese Himalaya - is the link myth or reality?

W. J. H. RAMSAY Land Capability Consultants Ltd., Times House, Willingham, Cambridge CB4 5LH, UK

ABSTRACT Despite numerous popular accounts, data on erosion processes in the Himalaya are scarce and unre- liable. Quantitative studies of mass wasting and surface erosion in Nepal are reviewed. The literature reveals a consensus that in Nepal erosion rates are naturally very high, mass wasting is the dominant hillslope process, and geological factors are the most important determinants of slope stability. Deforestation is linked to surface erosion, gullying and shallow (~3 m deep) landslides. Locally this soil loss is important agriculturally. Currently there is no evidence to link deforestation to the large, complex slope failures which mobilize most sediment.

Deboisement et erosion dans les Himalayas du Nepal: lien fictif au reel? RESUME En depit de nombreux recits sur Ie sujet, les donnees sur les processus erosifs dans les Himalayas sont rares et peu fiables. Les etudes quantitatives sur les mouvements de masse et l'erosion de surface au Nepal sont passees en revue. Ces etudes s'accordent sur Ie fait que Ie processus des mouvements de masse domine la forte erosion naturelle du Nepal, et que la stabilite des pentes est determinee par des facteurs geologiques. Le deboisement est relie a l'erosion de surface, au ravinage et a des glissements de terrain peu profonds (~3 m). Ces pertes de sol sont importantes localement du point de vue agricole. II n'y a actuellement aucune evidence reliant Ie deboisement aux destabilisations complexes de pentes qui fournissent la masse des sediments.

INTRODUCTION

Deforestation and erosion are widespread in the Himalaya. This environmental deterioration is widely seen as the principal cause of severe flooding and sedimentation downstream in the Gangetic Plain. The perceived cause and effect relationship between upstream ac- tivities and downstream damages has had major impacts on both the decision-making process and on development-project design (lves & Messerli, 1984). However, this relationship is an assumption based on extremely poor data, and it may well reflect political and in- 239 240 W.J.H.Ramsay

stitutional influences rather than prevailing bio-physical processes (Thompson & Warburton, 1985). The Himalaya-Ganges-Brahmaputra system is one of the world's largest highland-lowland interactive systems (Ives & Messerli, 1984), and any analysis of environmental problems in the region must recognize both the dynamic nature of the geomorphological environment and the extreme heterogeneity of the area. Periods of record of geophysical events in the Himalaya are short. Statistics derived from such records may yield biased results due to intermittency, i.e. the variable geomorphological response to forcing events (Church, 1980). To avoid the hazard of generalizing locally-derived data over wider physical and social environments, what data there are should be treated with caution, and original sources referred to in order to clarify the context and methodology of the original study. This paper reviews existing studies on current slope processes in Nepal, and discusses these in relation to the effects of deforestation.

EROSION IN NEPAL

Mass wasting

Despite a wealth of anecdotal information, quantitative studies on current rates of mass wasting in Nepal are scarce. Bansode & Pradhan (1975) carried out a reconnaissance survey of landslides along part of the channels of the Sun Kosi and Tamur rivers above Tribeni in 1963. Their impression was of high levels of mass movement activity contributing to high sediment loads in the rivers, "mostly due to heavy precipitation, deep weathering, steep dip- slopes of the valley walls, under-cutting of the banks due to high velocity of these rivers, unstable nature of the rocks due to their structural disposition, high seismicity of the area, unplanned deforestation, etc." (Bansode & Pradhan, 1975; p.253). Failure surfaces were 30-70° (n = 19). Prasad (1975) reported on 10 years' observations of seismicity, rainfall and landslide occurrence in the Durbasha watershed near Chatra in eastern Nepal. Overall, landslide incidence corresponded with high levels of both precipitation and seismic activity in July and August.l However, slides also occurred in years of low earth- quake activity, and so Prasad concluded that "seismic shocks by themselves are not the main cause of occurrence of landslides in regions away from the epicentre" (Prasad, 1975; p.79). Hydrological conditions, i.e. high groundwater levels and intense precipitation, were considered to be more important. Williams (1977), investigating the east to west shift of the Kosi river, used satellite imagery to identify all slides larger than approximately 20 ha in the catchment of the Sapta Kosi in eastern

lThe author does not comment on the apparent seasonality of earthquakes. Deforestation and erosion in the Himalaya 241

Nepal, correlated these with data from 1:63,360 topographic maps available for a small part of the basin, assumed a failure scar recovery period of 50 years, and used Simonett's (1967) empirical volume/area relationship derived for slides in New Guinea to give a "total slide volume" for the Sapta Kosi basin over 50 years of 0.91 x 109 m3. He estimated that these "large landslides" contributed 31% of the sediment load of the Sapta Kosi, with a further 64% coming from small slides, surface erosion and gullying. The assumptions concerning basin homogeneity and failure age, area, and volume required for this procedure leave the accuracy of Williams' figures open to question. Laban (1979) carried out a reconnaissance slide intensity survey of the whole of Nepal, expressing his data in terms of number of failures per linear km seen from one side of a light aircraft. He attributed 5% of all slides observed to road and trail construction. Wagner (1981, 1983) used a statistical analysis of geological and other characteristics of 100 landslides, mainly along roads in the Middle Mountains, to develop a site-specific landslide hazard as- sessment and mapping methodology. He concluded that geological factors were of overriding importance in determining debris and rock-slide hazard. Brunsden et al. (1981) made the observation that in the Low Himalaya of eastern Nepal mass movement phenomena were concentrated in two locations: low level undercut situations such as ravines and the outside of meander bends, and areas of structural discontinuity, suggesting an important role for "intensely shattered rock and preferred water movements". Modal angles for debris slides were 35- 43°, and they tentatively identified a slope angle of 30° as a lower limit for first-time shallow debris slides. "Mudslides" (elongate or lobate masses of weathered debris which move in well defined tracks bounded by steeply inclined lateral and basal shear surfaces (Brunsden et al., 1981)) occurred on slopes of 25-29°. They also described "mass movement catchments", steep rapidly eroding channels with active, expanding heads supplying material to the channels by debris slides, debris flows, rock-debris chutes, and gullies. Severe gullying in areas underlain by gneiss, which weathered to depths of 20 m, was attributed to man-accelerated soil erosion as a result of cultivation following forest clearance. Caine & Mool (1982) investigated mass movements in the Kathmandu- Kakani area as part of the United Nations University Mountain Hazard Mapping Project (see Ives & Messerli, 1981). Their analysis of slide morphometry and slope material properties led them to em- phasize the importance of material controls on the landslides in their study area, particularly the brittle behaviour of the weathered, untransported bedrock. The high incidence of catastrophic landsliding was further explained by relief, seasonally high water tables, and recent deforestation. Rainfall was thought to be of comparatively minor importance. They gave an estimated rate of surface lowering by landsliding of 12 mm year-l. Matsuura (1985) studied landslides in a small catchment northwest of Pokhara and found a close relationship between mineralogy, weathering, and the development of slip surfaces. Ramsay (1985, 1986, 1987) carried out a reconnaissance survey of sediment production and transfer mechanisms in the 122 km2 Phewa 242 W.J.H.Ramsay

Valley in the Middle Mountains of Nepal and identified a variety of mass movement processes. The commonest events were shallow translational failures on slopes of, typically, 36-45°, with volumes <1 x 103 m3, and with slope revegetation taking less than ten years. Larger slides occurred on slopes oversteepened by fluvial action. Flows developed in areas of weak rock and unfavorable structure, and were associated with groundwater discharge. Flow velocities accelerated during the monsoon. The highly fractured and deeply weathered zones around faults were the sites of mass movement catchments, and these complex failures were responsible for approximately 90% of all sediment production by mass wasting in the basin. A first estimate of surface lowering by mass movement processes in the Phewa Valley was 2-3 mm year-i. Locally, surface erosion on severely overgrazed pasture may be 5-6 mm year-i. No data were available on soil losses from cultivated areas, and, similarly, losses due to shallow creep, gullying and solution remain unknown. Ramsay also reported on landslide and fluvial morphometry in the area. With additional evidence from geomorphological mapping (Thouret 1981; Kienholz et al., 1983; Fort et al., 1984; Zimmerman et al., 1986) and studies of the late Quaternary history of the area (e.g. Mukerji, 1975; Yamanaka et al., 1982), it is reasonable to conclude that "mass wasting is the dominant process in the evolution of natural slopes throughout much of the Nepalese Himalaya" (Carson, 1985). In this evolution there is continued evidence of the importance of large-scale catastrophic events in mobilizing sediment, often with seismic triggers. The largest recorded landslide in Nepal occurred some 30,000 years ago in the Langtang Valley and involved some 15 km3 of material (Heuberger et al., 1984), and a 4 km3 event occurred some 4501100 i4C years ago in the Pokhara valley (Fort, 1987). Many large landslide scars are attributed by villagers to rainfall in 1934 (B. Carson, personal communication). Glacial lake and landslide-dam outbursts continue to cause major damage (Carson, 1985; Vuichard & Zimmerman, 1986).

Surface erosion and gullying

Measured rates of surface erosion in Nepal are only available in five publications2: Chatra Research Centre (1976), Laban (1978), Mulder (1978), Impat (1981), and Byers (1986) (Table 1). Currently all other figures for surface erosion in Nepal are estimates. These five sources report on runoff plot experiments, and Laban's paper also includes a literature review and original data on erosion determined by silt accumulation behind gully check dams. Byers (1986) presents preliminary results from studies in the Sagarmatha (Mt. Everest) National Park on a relative, between-plot basis. A further runoff plot installation exists on the Shivapuri watershed

2Haigh (1982) reports sediment trapped on deforested micro- catchments in the Garhwal Himalaya (India) as being 5-7 times greater than sediment trapped below forested catchments. TABLE 1 Surface erosion rates reported from runoff plot studies in Nepal

Location and Plot Data Land Use Erosion Rate Source

Siwaliks: Chatra, east Nepal; south aspect, Various, forest to grazing. 7.8-36.8 t ha-lyear-l Chatra Research sandstone; period of measurement and number of Centre in Laban t:;, plots not given. 1978 ro 8' Siwaliks: Gagretal, near Surkhet, west Nepal; Severely degraded heavily 200 t ha-lyear-l Sakya, pers. >-t south aspect, sandstone, average slope 60%; grazed forest on intensively corom., in Laban ro 1978 tn period of measurement and number of plots not gullied badlands. r1- given. i:U r1- Mulder 1978 .... Middle Mountains: Banpale, Phewa watershed, near Fenced pasture. 9.4 t ha-lyear-l 0 Pokhara, central Nepal; south aspect. elevation Unfenced grazing land. 34.7 t ha-lyear-l :J 1405 m; grey phyllitic schist; soils 40-70 c~ i:U clay loam, moderately well drained; one 10 m :J bounded plot on each land use type; four ~ individual measurements 29 June - 5 July 1978. ro >-t Protected pasture mixed with 1.01 t ha-l June-Oct. Impat 1981 0 Middle Mountai~s: identical location to Mulder tn 1978; two 10 m bounded plots each land use forest." .... type; K-value' of surface soil given as 0.35; Overgrazed land. 9.85 t ha-l June-Oct. 0 daily measurements 11 June - 15 Oct. 1979. :J .... Middle Mountains: Tamagi, Phewa watershed, near Dense forest. 0.43 t ha-l July-Oct. Impat 1981 :J Pokhara; northeast aspect, elevation 1800 m; r1- one 10 m2 bounded plot; well drained clay loam ::J< derived from grey schist and quartzite schist; ro 11 composite measurements 01 July - 07 Oct. 1979. :J:: .... High Mountains: Namche Bazaar to Dingboche, Heavily grazed pasture. 10.5-715.4 g/trough Byers 1986 S Mar-Oct. i:U Sagarmatha National Park; 35.unbounded plots N with 0.5 m long collection troughs; elevations Utilized forest (litter/moss 0-16.2 g/trough i:U 3440-4412 m; weekly measurements Mar-Oct. 1984. layer intact). Mar-Oct. \C i:U , Soil erodibility (Wischmeier et al., 1971) "Same fenced pasture as in Mulder's study; trees were seedlings at the time of Impat's study (author) N ~ w 244 W.J.H.Ramsay immediately north of Kathmandu but is derelict (1983), and no results from it are known to have been published. All the records reported are short-term, site-specific, and under no circumstances should be generalized regionally (see Roels (1985) for a critical discussion of plot studies). For example, Mulder's results were based on only 4 individual measurements made following rain events between 29 June and 5 July 1978 (Mulder, 1978), and the bounded plot size (10 m2) is not representative of a longer slope. However, the figures in Table 1 illustrate both the high absolute rates of surface erosion under some circumstances (e.g. at Gagretal), and the relative differences between different land use types, e.g. overgrazed pasture (9.85 t ha-l, 11 June - 15 ~ct. 1979), protected pasture (1.01 t ha-l for the same period), and forest (0.43 t ha-l, 01 July - 07 Oct. 1979) at Banpale and Tamagi in the Phewa watershed near Pokhara (Impat, 1981). Of particular interest is Impat's finding that although precipitation peaked in August, soil loss in the Phewa Valley was greatest at the beginning of the measurement period in June. This can be ascribed to an increase in the vigour of the vegetation cover during the monsoon (CaIson, 1985). Byers (1986) reported relatively low surface erosion rates in the subalpine environment of the Sagarmatha Nation- al Park partially due to sediment trapping by shrub growth on ter- racettes. Laban's (1978) method for determining soil loss from silt accumu- lated behind gully check dams included the use of sediment delivery ratio and trap efficiency curves developed for use in very different environments, as well as assumptions on gully shape and sediment contribution from gully-wall slumping. Other soil loss figures reported by Laban are based on a preliminary exercise in stream sediment sampling in the Kathmandu Valley undertaken by Kandel (1978). Sampling was carried out once or twice weekly, and the results extrapolated to give annual soil loss figures. All these figures should be treated with caution. Measured data on soil loss from cultivated land in Nepal are not available. Sastry and Narayana (1984) report markedly reduced runoff and erosion from bunded areas compared to rates under forest in the Doon Valley near Dehra Dun in India, but this work has limited application away from similar environments such as the Chitawan Valley in Nepal. Pandey et al. (1983/1984) report average soil losses of only 64 kg ha-lyear-l from a 100 m2 plot of 38° slope under maize cultivation at Naini Tal, but were working in a lime- stone area with subsurface flow systems.

DISCUSSION: DEFORESTATION AND EROSION

A general discussion of forests, erosion, sediment and water yield, and floods is available in Hamilton (1986). Although it cannot be assumed that natural erosion rates are low or even moderate under dense forest cover (Pearce, 1986), there is a presumption that removal of tree and shrub cover will affect erosion to some degree. This is due to effects on both slope stability and surface erosion. Deforestation and erosion in the Himalaya 245

Deforestation and slope stability

The ways in which forest cover can influence slope stability and erosion have been summarized by O'Loughlin & Ziemer (1982): positive influences depend upon modification of soil moisture distribution and soil pore water pressures caused by forest evapotranspira- tion, accumulation of an organic forest floor layer, and mechanical reinforcement of the soil by tree roots; negative influences result from root wedging and windthrow, and surcharge due to weight of the tree crop. On balance the net influence of forests on slope stability is positive, with the major factor being root reinforcement which may provide an artificial soil cohesion of 1.0-20 kPa (O'Loughlin, 1984). Under conditions of soil saturation this cohesion may be critical to slope stability on some sites. However, none of the papers on mass wasting in Nepal reviewed above present data illustrating a clear relationship between forest clearance and mass movement activity, although all concur on the importance of geological factors in precipitating landslides. On the basis of visual observations alone, it is reasonable to suggest that the small, shallow, translational failures ubiquitous in the Middle Mountains may be due to loss of root reinforcement. These slides are usually located in mid or upper slope positions often recently cleared of forest, and involve mantle material within 3 m of the surface. This evidence supports the view that where creep or shear failure is a shallow phenomenon (less than 2-3 m) loss of root strength can be significant (Swanston & Swanson, 1976), and the role played by vegetation most important (Starkel, 1972). Observations near Pokhara (Ramsay, 1985; 1987) suggest that, although common, these shallow slides are only minor contributors to total sediment production. Large failures in the Middle Mountains are associated with under- cutting, unfavorable geology, and structural discontinuities. Particularly dynamic failure complexes develop where these factors combine, and may become self-reinforcing through the interaction of mass movement and mass transport processes, area exposed, and precipitation (Ramsay, 1985). Runoff generated in the larger slides often results in the rapid integration of failure scars into the drainage net, and these large failures are major contributors of sediment to stream channels. The effect of forest influences on such failures appears to be minimal. Wadia (1926) commented that "denudation in the dense forests of the hill-slopes in the Eastern Himalayas recalls that of the tropical lands." Carson (1985) gives several examples of very large landslides occurring on forested land in Nepal, and Brunsden et al. (1981) report that during a heavy storm in 1974 in an area of phyllite in eastern Nepal mass wasting was common on steep forested land but less so on more gently sloping cultivated land. These observations reinforce the need to distinguish between deep and shallow failures.

Forest clearance, surface erosion and gullying

In Nepal the processes involved in forest degradation and clearance 246 W.J.H.Ramsay include repeated lopping for fodder and fuel, the removal of the litter layer which inhibits regeneration, browsing and trampling by livestock, fire, and the felling of individual trees for construction purposes. Except in the Terai commercial sales of forest products are rare. Most of these processes are directly or in- directly related to the maintenance of livestock, an essential component of the agroecosystem over much of the Himalaya. Land cleared of forest is used for grazing, rainfed crops, or irrigated terraces. Grazing land is generally seriously overstocked and poorly managed. Forest degradation and change to other land uses affects surface erosion through reduced infiltration and exposure of soils to rain drop impact. Forest clearance in eastern Nepal has been found to decrease soil aggregate stability (Chakrabarti, 1971). In the Middle Mountains rain splash during high intensity monsoonal storms3 detaches soil particles and assists in sealing the soil surface, which is further compacted by livestock. Although very limited data show static infiltration rates to be relatively high in Nepal (40- 500 mm h-l; Gilmour, 1986), in the Middle Mountains surface runoff from sloping grazing land is often observed. This is illustrated by Impat's figures for runoff from erosion plots in the Phewa Valley: mean monthly runoff from protected pasture varied from 5.5-17% of total rainfall, but runoff from adjacent plots on overgrazed land ranged form 11-53% (Impat, 1981). This high rate of runoff causes sheetwash, rilling, and gullying, which has been noted to be severe in many parts of Nepal (Laban, 1978; Brunsden et ai., 1981; Caine & Mool, 1982), and which is usually associated with communal grazing areas and marginal agricultural land. Surface erosion is also thought to be severe from outward-sloping terraces in high rainfall areas. Level, irrigated terraces are generally kept in good repair by farmers (Johnson et ai., 1982).

CONCLUSIONS

As Pearce (1986) states, it is essential to distinguish between soil erosion on which agriculture depends, and total erosion. Total erosion in Nepal is very high, and at 1-5 mm year-l (Ramsay, 1986) approximately in balance with orogenic uplift. Locally, forest degradation and the conversion of forest to poorly managed grazing land or rainfed terraces has resulted in increased sediment mobili- zation through surface erosion, gullying, and shallow landsliding. Despite high sediment delivery ratios in the Himalaya due to effi- cient slope/channel coupling (Ramsay, 1987), such increased production represents only a small portion of the total sediment contribution to river channels (Carson, 1985). However, it is of crucial

3At Pokhara Airport 100 mm of rain in 30 min has a return period of approximately 3 years (Ramsay,1985). Deforestation and erosion in the Himalaya 247

importance to the communities affected by the loss, and for the development of effective conservation strategies. The persistence of the public association of rapid mass failures with deforestation in Nepal can be attributed to three factors; firstly, the credibility afforded to the link by institutions, the media, and "experts", e.g. Kollmansperger who regards mass wasting in the Midlands, the Mahabharat and Siwaliks "as being a man-made erosion process on an exclusive basis" (Kollmansperger, 1978/79); secondly the existence of predisposing evidence, e.g. Wilson (1973) who found that on an international scale land use outweighed both precipitation and relief as the single most important factor affect- ing sediment yield; thirdly, the undoubted connection between forest clearance and surface erosion, gullying, and small shallow slides in the Middle Mountains. These adverse effects are largely due to poor management of cleared areas rather than to removal of the forest itself.

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