Popular and political thinking in Thailand is dominated by the belief that deforestation leads to drastic changes in water yields in the affected catchments (Thongtham in Bangkok Post 1978 and 1979; cited in Gibson, 1983). In this context shifting cultivators are viewed as the culprits, because the potential downstream effects of logging activities in upland or highland areas do not appear to have been a deterrent to granting logging licences before the logging ban (Anon, 1989e). In general, it is frequently contended that the clearing o f forests results in lower water yields during the dry season and higher yields in the wet season causing frequent floods in the lowlands reaching as far as the central plains and Bangkok. How far the myth and misunderstanding about the role of
forests has penetrated the people's minds becomes even more evident by referring to a high ranking politician who according to a Bangkok Post article (Anon, 199 lh, p.3) had said that the Green Isan Project had "achieved its objectives by helping bring more rain and increase water levels in ground-level reservoirs".
Before turning to water yields, I summarize briefly the effects of land use change on runoff generation. The research results in Thailand are very site specific but ample evidence has been documented which indicates that, generally, runoff from undisturbed natural forests and plots which have been cultivated applying soil and water conserving practices, is lower than from disturbed forests, plots which were cultivated in the traditional way and bare soil plots (Table 6.1).
Table 6.1 Runoff under different land use practices (data from Tangtham, 1991).
Land use Runoff in mm Runoff in per
cent of rainfall
Slope steepness
Unbumt mixed decidious forest 77 5.5 15
Unbumt dry dipterocarp forest 51 3.6 15
SWC practices 102-113 7.5 - 8.4 3 0 - 4 0
2 9 -8 7 2.1 - 3.2 2 5 - 3 0 38 - 105 2.3 - 6.4 2 0 - 5 0
2 1 -6 1 1.8 - 5.1 1 8 -4 0
2 0 -2 5 1.7 - 2.1 n.a.
Burnt mixed decidious forest 153 10.9 15
Burnt dry dipterocarp forest 85 6.1 15
Traditional upland rice 204 12 54
135 10 3 0 - 4 0 117 8.5 2 5 - 3 0 150 9.1 2 0 - 5 0 35 2.9 1 8 -4 0 Bare soil 111.8 9.4 40 149 11 3 0 - 4 0 210 12.7 2 0 - 5 0 150 12.5 1 8 - 4 0 21.5 1.8 n.a.
Most available data are not accompanied by synoptic considerations. The data from some experimental plots lack information on soil, rainfall, slope steepness, the history of the plots or the year in which the studies were conducted. Therefore, a comparative analysis is difficult and at best tentative. The evidence suggests that cultivation practices, in particular traditional practices, result in soil compaction, the collapse of macropores and the decline in soil organic matter (Turkelboom et al., 1991). All of these factors lead to
decreased surface infiltration rates thereby increasing runoff. However, this does not mean that increases in infiltration rates accompanying, for instance, reforestation significantly reduce the incidence of downstream flooding (Gilmour et al, 1987).
Turkelboom et al. (1991) also found that subsoils (20-100 cm) are generally drier during the dry season and the first half of the rainy season under a forest cover. This result is of particular importance in the examination of the overall effect of land use or management changes on water yields. It indicates that though soils under forests have generally higher infiltration rates and storage capacities than soils with less organic matter (Pritchett, 1979; cited in Bruijnzeel, 1990) much of this water is consumed again by the forest and does not sustain streamflow. Clear-felling of forests on the other hand, results in a drastic reduction of transpirational demand, especially during the dry season, and is primarily responsible for the large increases in low flow of streams (Oyebande, 1988).
After reviewing almost a hundred paired catchment basin experiments Bosch and Hewlett (1982, p. 3) concluded that "no experiments, with the exception of perhaps one, have resulted in reductions in water yield with reductions in cover, or increases in yield, with increases in cover". Qian (1983) did not detect systematic changes in streamflow patterns on the island of Hainan in South China despite a 30 per cent loss of tall forest between 1950 and 1980. Catchment studies in Northeast Thailand also failed to demonstrate any significant relation between deforestation and water yield (Dyhr-Nielsen, 1986).
Increased evaporative loss and higher transpiration from trees during dry periods have the effect of reducing stream flows. W hile it is frequently contended in Thailand that reforestation increases water yields, evidence indicates the opposite: the drying up of streams (Vajirajutipong, 1990, pers. comm.), a decrease o f well levels in dry seasons and a general decrease in total water yield (Chunkao, 1981; cited in Hamilton and King, 1983). The analysis of runoff data from northern and eastern tributaries to the Chao Phraya also did not show systematic changes in water yield and Dyhr-Nielsen (1986, p. 6) concluded that changes "have been statistically insignificant compared to the natural, climatically determined variations in the ru n o ff.
The evidence presented shows that while runoff on individual fields increases due to forest conversion and can be reduced again by practicing soil and water conservation methods, the impact on overall water yields is statistically insignificant. Furthermore, locally an increase in forest cover leads at least temporarily to reduced water yields.
Up to this point only changes in total water yield were considered. Theoretically total water yield may not be effected if wet season flow and flood peaks compensate for a
reduced dry season flow. In an examination of the seasonal distribution of water flow for the Ping River (Station PI at Nawarat Bridge in Chiang Mai) no statistically significant changes between 1927 and 1989 could be identified (Figures 6.1 and 6.2).
Figure 6.1 Dry-Season Water Yield* of the Ping River as measured at PI (Nawarat Bridge) between 1927-1990 (data from the Royal Irrigation Department).
Dry season runoff Running mean (10Y) 4 5 0 -
1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990
Figure 6.2 W et-Season W ater Yield o f the Ping River as measured at PI (Nawarat Bridge) between 1927-1990 (data from the Royal Irrigation Department).
Wet season runoff 4000 -
Running mean (lOy) 3500 - 3000 - S 2500 -
2ooo -
1000 - 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990The water diverted for irrigation purposes at the Mae Taeng and the Mae Faek irrigation facilities have been included in the analyses of lx>th, the dry-season and the wet-season water yields.
To investigate the relation between discharge and rainfall is not possible because continuous long-term rainfall data for Northern Thailand only exist for the major cities located in the valleys. A rather tentative analysis between the existing rainfall data (from Chiang Mai) and the discharge indicates that the strong year to year variability of the weather seems to "dwarf" any effects associated with land cover transformations in the watersheds. Further complications arise in interpreting the data because water use for agriculture, industrial and domestic purposes has increased significantly. This has happened particularly in the last 10 years during which the Chiang Mai Valley experienced rapid economic growth and crop cultivation intensified.
As discussed in the preceding paragraphs significant changes in total flow and flow regime for Northern Thailand's major rivers could not be verified yet. Dyhr-Nielsen (1986) detected a small increase in dry season flow for the Yom, Wang and Nan but was unable to relate it to deforestation. He failed to show the same effect for the Pasak River whose basin is more effected by deforestation than the basin of the former three rivers. No statistically significant changes were detected for the Ping River north of Chiang Mai. Much more rigorous analyses need to be conducted to assess the impacts of land cover transformations on water yields. Geological and climatic parameters which determine the initial hillslope hydrological response need to be taken into account. Catchment size and basin and channel geometry also determine the magnitude of particular events (Baron et a l., 1980; Bruijnzeel, 1986) and of annual runoff (Anyadike and Phil-Eze, 1989). With regard to storm flows, effects of locally increased flows whose magnitude may have been increased by unfavourable land use are moderated by differences in lag time between tributaries and by spatial and temporal variations in rainfall (Bruijnzeel, 1990).
The example of the rainfall events in the second half of August 1987 in the northwestern Mae Hong Son Province may illustrate this fact. The Huai Mae Ya River, a tributary to the Pai River experienced maximum streamflow (32.8, 22.4 and 21.9 m /sec) after heavy rains on August 17, 1 8 , 2 3 and 24 (Rainfall data for the particular basin do not exist but heavy rains were recorded to a maximum of 134 mm/day at neighboring stations). While runoff (expressed in liter/sec/km^) in the Huai Mae Ya basin developed quickly into a stormflow for the river (August 19, 24 and 25) the effect was far less pronounced in the Pai River (Figure 6.3). Land cover data indicate that the upper reaches of the Huai Mae Ya basin were (1976) characterized by extensive grasslands, described as Imperata savanna (Gibson, 1983). Despite the government reforestation programmes many of these grasslands still exist today. The grasslands may have intensified the stormflow in the particular basin. However, as figure 6.3 indicates with an increase in catchment area
this effect was lost further downstream, an effect that was also reported by Engineering Consultants INC. (1971) for the Ping River and its smaller tributaries.
Having reviewed the documented research results and having examined the seasonal distribution of water yield for the Ping River and the influence of catchment size on storm flow events it is obvious that statistically significant changes in flow patterns due to land cover transformations cannot be detected, particularly for a larger basin. Investigations of small catchments down to plot size however indicate runoff and flow responses due to land cover changes or land management manipulations. As discussed above soil and water conservation practices reduce runoff and reforestation results in reduced water yields while selective logging increases it (Suksawang, 1991). Runoff also increases under grassland (Tangtham et al., 1972; cited in Gibson, 1983).
Figure 6.3 Runoff from the Pai River Catchment between August 17 and 27, 1987 (data from National Energy Administration).
■ Sop Mae Samat
S Pang Mu
H
Ban Paeng E3 Ban Mae Na □ Sop Huai Mae Y a 4 0 0 “17.08 18.08 19.08 20.08 21.08 22.08 23.08 24.08 25.08 26.08 27.08
August 1987
Note: Size of the drainage areas - Sop Mae Samat: 5,530 sqkm - Pang Mu: 3,770 sqkm - Ban Paeng 1,760 sqkm - Ban Mae Na 172 sqkm - Sop Huai Mae Ya 85 sqkm
W ith regard to assessing the economic im pacts o f the observed effects in small catchments, a crucial factor is whether they are viewed as beneficial or cause downstream problems. While it is generally believed that an increase in available water poses a problem, Fujisaka (1991, pers. comm.) for M adagascar and Gibson (1983) for Thailand
reported that especially increased runoff early in the wet season is viewed as beneficial by wet-rice planting lowlanders because it allows earlier planting dates and reduces crop failures.
A final point needs to be made with regard to the hydrological impacts before turning to the effects of land use and land management changes on soil erosion and sedimentation. The flooding o f Bangkok in 1983 is often said to be the consequence of indiscriminate resource destruction in Northern Thailand (Attaviroj, 1986 and 1990). However, major floods in the Bangkok Metropolitan and adjacent areas have been recorded for the last 200 years, with an extreme event on Dec. 2, 1785 (ONEB, undated) and another event comparable to the 1983 flooding in 1831 (Terwiel, 1989a). An official investigation described the heavy rains throughout the entire country from August to October 1983 with up to three times the average precipitation as the main cause of the flood. Furthermore, the ONEB (undated) cited an inefficient water drainage system and flood control problems as contributing effects to the magnitude of the flood. Flooding in Bangkok has also been aggravated by land subsidence which also appears to be a problem in the city of Chiang Mai (Rtiland, 1992). The root cause of both, flooding and land subsidence, has been directly related to excessive (and long-term) groundwater withdrawal, in which more water is pumped out of aquifers than recharged (Yong et al., 1991). The result is that Bangkok has sunk some 50 to 60 cm in the past 25 years (Rush, 1991). Since the altitude of Bangkok is between 0 and 1.5 m asl it is not surprising that high tides which usually occur between October and December and rise up to 1.35 m asl can set major parts of the city under water.
Examining catastrophes in their historical context also reveals that droughts and floods are not only incidents of the recent past. Terwiel (1989b) reported rice crop failures for several successive season owing to droughts and floods during the 1840s. Sittitrai (1988) described major floods in the Saraphi district in the Chiang Mai valley for 1918-20 and again for 1953. Benchaphun (1991, pers. comm.) remembered another big flood in the early 1970s.
The last point indicates that the effect of reforesting the major part of Northern Thailand will at best have a very minor effect on the flooding of the Chiang Mai valley, the Central Plains and especially Bangkok. Assum ing that the costs associated with flooding incidents in Bangkok can be reduced and make a substantial difference in the economic analysis of soil and water conservation or reforestation projects in the highlands of Northern Thailand is therefore not based on any scientific evidence, even considering that what is known is characterized by great uncertainties.