Flood risk reduction and flow buffering as ecosystem services: a flow persistence indicator for watershed health
M. van Noordwijk1,2, L. Tanika1, and B. Lusiana11World Agroforestry Centre (ICRAF), SE Asia program, Bogor, Indonesia 2Wageningen University, Plant Production Systems, Wageningen, the Netherlands
Received: 15 Dec 2015 – Accepted for review: 12 Jan 2016 – Discussion started: 19 Jan 2016
Abstract. Flood damage depends on location and adaptation of human presence and activity to inherent variability of river flow. Reduced predictability of river flow is a common sign of degrading watersheds associated with increased flooding risk and reduced dry-season flows. The dimensionless FlowPer parameter (Fp), representing predictability, is key to a parsimonious recursive model of river flow, Qt = FpQt−1 + (1−Fp)(Pt−Etx), with Q, P and E expressed in mm d−1. Fp varies between 0 and 1, and can be derived from a time-series of measured (or modeled) river flow data. The spatially averaged precipitation term Pt and preceding cumulative evapotranspiration since previous rain Etx are treated as constrained but unknown, stochastic variables. A decrease in Fp from 0.9 to 0.8 means peak flow doubling from 10 to 20% of peak rainfall (minus its accompanying Etx) and, in a numerical example, an increase in expected flood duration by 3 days. We compared Fp estimates from four meso-scale watersheds in Indonesia and Thailand, with varying climate, geology and land cover history, at a decadal time scale. Wet-season (3-monthly) Fp values are lower than dry-season values in climates with pronounced seasonality. A wet-season Fp value above 0.7 was achievable in forest-agroforestry mosaic case studies. Interannual variability in Fp is large relative to effects of land cover change; multiple years of paired-plot data are needed to reject no-change null-hypotheses. While empirical evidence at scale is understandably scarce, Fp trends over time serve as a holistic scale-dependent performance indicator of degrading/recovering watershed health.
van Noordwijk, M., Tanika, L., and Lusiana, B.: Flood risk reduction and flow buffering as ecosystem services: a flow persistence indicator for watershed health, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2015-538, in review, 2016.