Nulidad parcial

Earlier in the discussion cost-benefit analysis was introduced as a method to identify, quantify and value information about costs and benefits in order to determine the net worth of an enterprise. Following this definition cost and benefits were evaluated for a number of enterprises from the farmer's perspective. The evaluation, however, included only direct costs and benefits and disregarded any indirect costs and benefits associated with land degradation or resource conservation. Agricultural production on sloping lands is frequently cited as the main cause o f land degradation o f tropical and subtropical upland environments. Soil erosion has been identified as the most widespread cause of land degradation. As Brown and Brown (1984) have argued, soil erosion is a physical process but its more immediate impacts are economic and its ultimate effects are social. The economic impacts of soil erosion are felt at two levels. The immediate on-site effect is believed to be declining land productivity which results in declining yields over time. When the soil leaves the boundary of the field it can cause negative (positive) externalities

with associated off-site costs (benefits). The externalities of soil erosion are addressed in Chapter Six. This section is concerned with the on-site effects of erosion and of soil and w ater conservation which an individual may consider in evaluating alternative investments.

Section 5.8.2 included a tentative approach to evaluating the long-term effects of soil erosion on farm households' ability to generate surplus income. Three scenarios were drawn up assuming that first crop yields do not change over time, second that yields decline annually by 2 per cent and third that they decline annually by 5 per cent. While the exercise is useful in assessing the economic impacts over time it is only of a speculative nature because the three scenarios were not based on scientific evidence.

The following discussion responds to the lack of evidence provided. It focuses predominantly on the initial steps of cost-benefit analysis, the identification of costs and benefits by comparing available information on the effects of traditional agricultural practices and SWC technologies on soils and crop yields. As a starting point it reviews briefly some basic information on the extent and impact of soil erosion in the Northern Thailand and concludes with the farmers' perspective and responses to erosion and declining yields.

Until the early 1980s, quantitative determinations o f the extent and impact of soil erosion by water in the tropics have been rather sketchy. The difficulty in formulating soil loss tolerance has caused an intensive debate among soil scientists and data required to quantify the causative parameters of erosional processes are rare (El-Swaify and Dangler, 1982). For the last 10 years, knowledge gaps have been slowly filled and today soil erosion rates are m easured or m odelled under literally thousands of different circumstances, scales, and levels of accuracy (Harper, 1986).

In Thailand, it is widely believed that the most serious side effect of shifting cultivation is soil erosion (Wongsprasert, 1974). Komkris (1978, p. 66) writes that shifting cultivation "coupled with annual forest fires has made the foothills into the largest potential source in the country of sedimentation and flash floods". Soil erosion is viewed as a serious national problem (Putjaroon and Pongboon, 1987) which has reached alarm ing dimensions (Henderson and Rouysungnem, 1984). Forest encroachm ent by shifting cultivators is described as "a never-ending problem of the nation" difficult to solve (Sangcoowong and Rouysungnem , 1985). Pictures have been published of exposed maize roots which are said to indicate a soil loss of more than 10 cm (more than 1200 t/ha/year) in one growing season (Messerli, 1980). There are others claiming that the problem is not serious at all (Van der Meer, 1981) and that proof that shifting cultivation

is the cause of soil deterioration cannot be provided (Wongsprasert, 1974). For van der Meer (1981) the land degradation problem was invented by foreign experts and the Royal Forest Department was only too keen to pick up the idea and to sell it as a national problem. As can be seen, the debate on soil erosion and its effects has been dominated, particularly in the past, by hear-say rather than based on scientific evidence.

Erosion is a two phase process consisting of the detachment of soil particles and their transport by erosive agents such as water or wind from a particular site (Osuji, 1989). It can be categorized as natural (or geologic) which occurs independent from human activity and accelerated (or anthropogenic) erosion which is caused by human disturbances. The distinction between the two types of erosion is important because natural erosion rates may serve to establish benchmark soil loss tolerance rates.

The concept of tolerable soil loss is concerned with limiting erosion to levels at which no irreversible degradation or productivity losses occur (Phillips, 1989). But natural erosion as a conservation goal-setting framework has received criticism for its lack o f realism because as Phillips (1989, p. 221) argued, "accelerated erosion is virtually inevitable whenever vegetation is periodically removed and soil surfaces disturbed." The criticism aside, Hudson (1971, cited in Blaikie and Brookfield, 1987) proposed a tolerable soil loss of 13.5 t/ha/year for tropical areas. Hoey et al. (1987) argued for 15 t/ha/year as an interim value for permanent agricultural system for the highlands of Northern Thailand while the Department of Land Development selected 12.5 t/ha/year (Tangtham, 1991). As shown below, these limits are not exceeded under natural conditions and by applying erosion reducing conservation strategies.

In their pioneering research, W ischmeier's and Smith's (1978; cited in Harper, 1986) developed a soil erosion equation, the Universal Soil Loss Equation (USLE). Following Seckler (1987) the USLE may be written:

E = f(C, S, T, L)

where E is the average annual erosion expressed in t/ha/year. C is the climatic factor (rainfall erosivity). S is the soil factor (erodibility). T is the topography factor comprised of slope gradient and slope length, and L is the land utilization factor which expresses the plant cover in relation to bare soil. While the application of the USLE in environments for which it was originally not designed is not w ithout its problem s (Harper, 1986, Stocking, 1987) an analysis of the four physical parameters shows the following:

- slope topography can be m odified by changing gradients (e.g. terrace construction) and slope length (e.g. alley cropping), thereby influencing run-off, and

- the land utilization factor can be influenced by vegetation management or mulching, effectively reducing splash erosion and increasing water infiltration rates.

The geological formation of northern Thailand and the physiography of the mountains result in comparably high erosion rates on steeper slopes even under natural conditions. They range from 0.02 t/ha/year in the hill evergreen forest to up to 8 t/ha/year in the sloping mixed deciduous forest (Tangtham, 1991). Anecksam phant et al. (1991) measured erosion rates on forest plots (no detailed description) exceeding the above rates slightly. On a 35 per cent slope, Sombatpanit (1991) obtained erosion rates for imperata grassland and bamboo forests of only 1 t/ha/year.

It is often contended that trees per se can prevent erosion. While the vegetative cover exerts a m ajor influence on run-off and soil erosion (Van der Linden, 1983), the beneficial effect of tropical forests or tree plantations on these processes only exists in connection with understory and litter layers (Wiersum, 1985). Where these protective layers have been disturbed or have not been established, erosion rates under forests can even exceed erosion under annual crops due to increased erosive power of the rainfall (Wiersum, 1984; Besler, 1987; Brandt, 1988; Thornes, 1989, El-Swaify, 1992). In this context, annually burnt forests produce erosion rates which are twice as high as those from unburnt forests (Tangtham, 1991). Hutacharoen (1987) reports erosion rates of greater than 300 t/ha/year in annually burnt dry dipterocarp forests with 10 to 30 per cent crown cover. The documented results do not only indicate the severity of soil erosion but point out potential opportunities for successfully reducing erosion rates by designing appropriate land management practices.

The effects of modified T and L on soil erosion are currently the focus of intensive research efforts in Northern Thailand. Soil erosion rates as well as related variables (e.g. run-off, soil nutritional status, crop yields) are measured in plot experiments by the Departm ent of Land Developm ent partially in cooperation with the Thai-Australian H ighland Agricultural and Social Development Project (TA-HASD), with the Thai- German Highland Development Programme (TG-HDP) and the International Board for Soil Research and Management (IBSRAM).

Before discussing erosion in more detail it should be emphasized that although techniques for its measurement have been improved over the years, available data can still produce more confusion than clarification. Often it is difficult if not im possible to draw

conclusions from the mass of haphazard data available in the literature (Lai, 1990). This is compounded by emotional connotations and a lack of objectivity, and Lai (1990, p.

132) continued:

"The literature is full of horror stories. From the level of technical information presented, however, it is often difficult to judge whether an author is 'crying w olf or the threat to natural resources and the environment is genuine."

An apposite exam ple is found in Panichapong and V ijam sorn (1985; cited in Chaiwanakupt and Changprai, 1991) who reported annual soil loss ranging from 125 to 6000 t/ha in the uplands of Thailand. As the example indicates not very much appears to have changed since the 1970s when it was observed at a UN Conference on Desertification (1977, p. 177; from Seckler, 1987) that "statistics [on soil erosion and deforestation] are seldom in the right form, are hard to come by and are even harder to believe, let alone interpret." Soil loss data for the highlands in Northern Thailand are more than plentiful, but as six measurements from different locations with varying experimental set-ups for 1989 indicate they are sometimes hard to interpret or to believe (Table 5.25).

Table 5.25 Soil loss in Northern Thailand (1989).

Location Land use Soil loss

Lao Che Guay (Chiang Rai)1 Trad, rice 917.3 t/ha

Ja Bo (Mae Hong Son)2 Bare plot 282.5 t/ha

Doi Tung (Chiang Rai)3 Agroforestry 97.9 t/ha

Chiang Dao (Chiang Mai)3 Bare plot 10.0 t/ha

Doi Tung (Chiang Rai)3 Forest 8.4 t/ha

Chiang Dao (Chiang Mai)3 Trad, com 1.9 t/ha

Note: Data supplied by 1 Hoey (pers. comm., 1991) 2 Boonchee (pers. comm., 1991) 3 Anecksamphant et al. (1991)

The experimental fields referred to are located throughout the northern provinces of Chiang Mai, Chiang Rai and Mae Hong Son representing the highly diverse environmental conditions. This may explain at least partially the enormous discrepancies between the data in table 5.25. The plots have a slope of 30 to 55 per cent and are situated in altitudes varying from 480 to 1000 m*. The main objective of the field research is to

20C

With the exception of the Ja Bo site (this site is located in the study area) soils are classified as Ultisols. Their topsoils are characterized by low pH-levels (4.8-5.8) and they are generally poor in available phosphorus with the exception o f one site in Chiang Rai. The Ja Bo site has a rather high pH (7.1) and is characterized by a very high content of exchangeable calcium (2718 ppm).

investigate the influence of soil and water conserving land management practices