A framework for Hydrological characterisation to support Functional Flows (HyFFlow): Application to a tropical river

Study region: Sarawak, Malaysia. Study focus: This paper presents a framework (‘HyFFlow’) designed to provide a systematic characterisation of river flow regimes. HyFFlow consists of four packages, together yielding information on rainfall and flow patterns. They include analyses to identify and characterise different types of high flow event, based on their magnitude, shape and duration, and assessment of the timing and duration of periods of high and low flow. HyFFlow also includes assessment of temporal patterns (seasonality and long-term trends) in rainfall and flow. The characterisation provides a comprehensive baseline against which future changes can be assessed. New hydrologic insights for the region: HyFFlow analyses indicated subtle and complex changes to long-term hydro-climatological conditions in the Baleh catchment, Malaysia: (i) analyses of the hydrograph indicated that there have been reductions in flow in the wet season, but not at other times of the year, and (ii) while there is no evidence of long-term trends in precipitation across the catchment (no change in monthly rainfall values over a 51- year period), there has been an increase in the number of days each month with no rainfall in some sub-catchments. HyFFlow analyses also identified four main types of high flow event in the Baleh whose functional roles need to be assessed as part of future geomorphic and ecological studies.