Assessment methods of watershed ecosystem health

Watershed ecosystem health is the prerequisite for sustainable development of the watershed. The aim of watershed ecosystem health assessment is to recover and manage watershed ecosystem and to realize sustainable development of the watershed. Healthy watershed ecosystems possess the following characteristics: they are resilient to normal perturbations encountered in their evolutionary history; they are free of Ecosystem Distress Syndrome; they are self-sustaining; management practices and ecosystem processes do not impair adjacent ecosystem; they are economically viable; they sustain healthy human communities. Watershed ecosystem is a social-economic-natural complex ecosystem; and watershed natural ecosystem consists of terrestrial ecosystem, land/water ecotone ecosystem and freshwater ecosystem. The assessment of watershed ecosystem health should be carried out from broad temporal-spatial scales. River continuum and temporal-spatial scales of different biological features in a watershed should be taken into account. A rational aim for ecological improvement of unhealthy watershed ecosystems could be towards a reference state of maximum naturalness attainable under the given conditions in a watershed. For anthropogenically modified watersheds, the reference state of watershed ecosystem health assessment might be ‘relative ecological integrity’. Indicator species assessment and indices assessment are the two main methods of watershed ecosystem health assessment. However, it should be careful in selecting indicator species. The sensitivity and reliability of species need to be considered simultaneously. Terrestrial ecosystem indicator species, land/water ecotone ecosystem indicator species and freshwater ecosystem indicator species should be simultaneously selected to assess watershed ecosystem health. The indices of watershed ecosystem health assessment should include four categories: ecological category, physical-chemical category, social-economic category and human health category. Every category has some relative indices. Ecological indices are those reflecting the characters and states of a watershed ecosystem and are classified into terrestrial ecosystem indices, land/water ecotone ecosystem indices, freshwater ecosystem indices and integrated watershed indices. The physical-chemical indicators relate to the abiotic environment of a watershed ecosystem. The social-economic and human health indicators are focussed on the supporting role of watershed ecosystems in human survival and watershed development, with economic parameters and environmental pressure indices of social development being adopting to measure the quality and sustainability of watershed ecological service. Because of the complexity of watershed ecosystems, the measures of watershed ecosystem health indices, especially some ecological indices, are difficult. The measurement methods should be further developed. The assessment of watershed ecosystem health has a history of only ten years. There are some problems that should be further discussed in this realm. This paper puts forward these problems and research trends in future.