Environmental flow allocations in monsoonal Hong Kong

Abstract

1. Hong Kong streams are subject to aggressive water extractions but the downstream water needs of ecosystems – i.e. environmental flow (e-flow) requirements – have not yet been addressed. This study investigated hydro-ecological relationships that could be used to establish e-flow allocations for streams in monsoonal Hong Kong. 2. Data were collected during the wet and dry seasons from 10 unpolluted streams experiencing a gradient of flow reductions (c. 0–98%). Relationships between flow conditions (percentage discharge reduction and absolute discharge volume) and responses of macroinvertebrate composition and periphyton condition were established for each season. 3. Declines in richness of Ephemeroptera and abundance of hydropsychid caddisflies, as well as increases in the proportion of predators, were linearly related to percentage discharge reduction during both seasons. Relationships were also recorded for eight other macroinvertebrate richness or compositional metrics during the dry season only. Relationships between macroinvertebrate assemblage attributes and absolute discharge volume across downstream reaches were also evident. Periphyton was relatively insensitive to flow reductions and did not provide useful hydro-ecological relationships, although declines in autotrophic index were related to percentage discharge reduction during the dry season. 4. Using hydro-ecological relationships established for macroinvertebrates, two levels of e-flow were proposed: a ‘threshold’ intended to maintain near-natural conditions and a ‘degradation limit’ that allowed no more than 25% of the maximum indicator response to flow reduction. Calculated threshold e-flows required downstream allocation of ≥74% of natural flows; degradation limit e-flows were ≥12% (wet) and ≥27% (dry). The discharge needed to maintain threshold conditions was 30–105 L s−1 (wet) and 5–14 L s−1 (dry), with degradation limit e-flows of 19–57 L s−1 (wet) and 3–6 L s−1 (dry), relative to natural mean discharges of 77–303 L s−1 (wet) and 3–18 L s−1 (dry). 5. The proposed e-flow allocations are indicative only, and significant obstacles to implementation have yet to be surmounted. Any such implementation requires monitoring of outcomes in order to refine the allocations and inform adaptive flow management for Hong Kong streams.