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We investigated processes contributing to periodic acidification events in headwater streams of Nee Soon Forest Catchment (NSFC) in Singapore by monitoring hydrochemical changes in response to rainfall inputs. Stream chemistry response to most rainfall events was characterized by decreases in pH from means ranging from 5.1 to 5.3 to below 4.8–5.0 and corresponding increases in specific electrical conductivity from baseflow values of 15–30 to 50–80 μS cm−1, indicative of low-total dissolved solids in stormflow. The decreases in pH in the streams, which are typically acidic year-round, were related to occasional highly acidic rainfall inputs (pH ~4.05) and likely the flushing of organic acids into the stream by shallow subsurface flow interacting with surface litter and/or organic-rich soil horizons. The interaction of rainwater runoff with organic matter in the soil matrix possibly alters the chemical composition of stormflow, influencing pH. Decomposition of instream organic matter also reduces stream water pH. Leaching experiments revealed that the overland flow passing through organic matter and A horizon material has the potential to lower stream water pH by approximately a half unit or more, in part, by flushing nitrates that were produced by microbial decomposition of organic matter and/or precipitated sulphur that enterred the forest by wet or dry atmospheric deposition. The observed periodic acid events are a natural phenomenon in the stream system in this urban environment because of naturally acidic rainfall and granitic soils with low buffering potential. However, acid events are likely amplified in frequency and magnitude by anthropogenic pollution emissions of sulphur and nitrogen species (e.g., SO2 and NOx) from local and regional sources that lower rainfall pH. Although, acid runoff events are typically short-lived (

Many coastal cities are experiencing growing risk to hydrological hazards through the combination of uncontrolled urban development and exposure to natural phenomena linked to climate change, including rising sea levels, intensified storms, and amplified storm surges. This growing risk is particularly acute in Asian coastal mega-cities, many of which have yet to develop adequate adaptation policies to address plausible impacts of climate change. In this analysis, we review how Hong Kong and Singapore, two of the most affluent coastal cities in East Asia, have initiated many flood mitigation strategies policies that have allowed them to reduce the impacts flooding. These strategies, once relying largely on building flood control structures, have now evolved to include holistic flood risk management approaches that include socio-economic factors. Arguably these two success stories provide inspiration for other coastal Asian cities. However, as climate change and uncontrolled development are likely to increase urban flooding in the future, general improvements could be made to improve knowledge transfer: e.g., develop means to work across policy silos and strike compromises between conflicting sectoral responsibilities, and develop long-term integrated strategies using planning tools and practices to address growing risk. While knowledge transfer cannot be direct because of different geographical settings, socio-economic situations, and political situations, we encourage governments to look beyond engineering-based flood control structures as to develop flood governance programs.