2.3 Operational Definition of Sustainable Development
Way back in 1946 Hicks (Hicks 1946; cited in Solow 1986; Maler 1990, Daly 1989, Daly 1992) defined income as the maximum amount that a community can consume over some time period and still be as well off at the end of the period as at the beginning. Under this situation the community should be able to produce the same income next year without depleting the capital stock. The criterion of sustainability is, therefore, implicit in the Hicksian concept of the maximum flow of income that could be generated while at least maintaining the stock of assets (or capital). While conventional economy applied the precondition of maintaining capital intact only to manmade capital, sustainable development requires, as a bare minimum, maintenance of manmade as well as natural capital (Daly 1994). Thus any economic growth at the cost of depletion of environmental resources (source and sink resources) actually amounts to living off capital rather than income, which is unsustainable (Victor 1991).

Pearce and Turner (1990) proposed a working definition of sustainable development from economic viewpoint; - 'it involves maximising the net benefits of economic development subject to maintaining the services and quality of natural resources over time'. They suggested that the traditional approach to define economic development as real gross national product (GNP) per-capita (or real consumption per capita) be modified and extended to include a more comprehensive set of welfare indicators, - education, health, quality of life, etc. The services and quality of natural resources can be maintained over time only through relating the scale of economic activity to the natural capacities of ecosystems to regenerate resource inputs for the economy and to assimilate (absorb and or store without long-term damage) the waste flows from the economy (Turner 1990).

Sustainability can thus be interpreted as the ability of an ecosystem to maintain long-term economic productivity under stress or shock. Stress can be viewed as a regular, relatively small and often predictable perturbation while a shock, on the other hand, is an irregular, infrequent, relatively large and unpredictable disturbance.

Goodland et al., (1991) broadened the scope of sustainable development as a pattern of social and structural economic transformation (i.e. development) which optimises the economic and other societal benefits available in the present without jeopardising the likely potential for similar benefits in the future. Opinions differ regarding the possible substitutability of natural resource by manmade capital and the extent up to which the same may be accomplished. Solow (1986) felt that to operationalise sustainable development the overall stock of capital assets should remain constant over time. Turner (1990) termed this concept as 'Solow Sustainability' or weak sustainability. Daly (1994) criticised this approach to be untenable as this is built on the false premise of perfect substitutability between manmade and natural capital. Daly (1991) stressed that in order to be sustainable both manmade capital and natural capital must be maintained intact separately as manmade and natural capital are fundamentally complements and only marginally substitutes. The complements must each be maintained intact because productivity of one depends on the availability of the other. This may be termed as 'Daly sustainability' or strong sustainability.

2.4 Approaches to Sustainable Development
It may be observed that the approaches described above while addressing the issue of intergenerational equity have ignored the issue of 'intragenerational equity'. Niu et al. (1993) attempted to address the spatial aspect by adding to the WCED (1987) definition the requirement 'to meet the need of a specific region without curtailing the ability of other regions to meet their own needs.' A conceptual framework proposed by Niu et al. (1993) is schematically presented in Figure 2.1.

Pearce et al. (1989) emphasised the necessity to provide for the needs of the least advantaged in the society, i.e. intragenerational equity. These ideas operate at all levels: within local communities, at a national level and between countries in the global community (Soussan 1992).

Munasinghe (1993) identified three inter-connected concepts of sustainable development which vary primarily on their focused objectives. The concepts are (i) economic sustainability, (ii) social sustainability and (iii) the environmental sustainability. The trade-off among the three main objectives is schematically depicted in Figure 2.2.

Goodland (1994) pointed out that the three types of sustainability, - economic, social and environmental - are clearset when kept separate.