Watersheds buffer the temporalpattern of river flow relative to the temporal pattern of rainfall. This“ecosystem service” is inherent to geology and climate, but buffering alsoresponds to human use and misuse of the landscape. Buffering can be part ofmanagement feedback loops if salient, credible and legitimate indicators areused. The flow persistence parameter Fp in a parsimonious recursive model ofriver flow (Part 1, van Noordwijk et al., 2017) couples the transmission ofextreme rainfall events (1 − Fp), to the annual base-flow fractionof a watershed (Fp). Here we Compare Fp estimates from four meso-scale watersheds in Indonesia(Cidanau, Way Besai and Bialo) and Thailand (Mae Chaem), with varying climate,geology and land cover history, at a decadal timescale. The likely response ineach of these four to variation in rainfall properties (including the maximumhourly rainfall intensity) and land cover (comparing scenarios with either moreor less forest and tree cover than the current situation) was explored througha basic daily water-balance model, GenRiver. This model was calibrated for eachsite on existing data, before being used for alternative land cover andrainfall parameter settings. In both data and model runs, the wet-season(3-monthly) Fp values were consistently lower thandry-season values for all four sites. Across the four catchments Fp values decreased with increasing annual rainfall, butspecific aspects of watersheds, such as the riparian swamp (peat soils) inCidanau reduced effects of land use change in the upper watershed. Increasingthe mean rainfall intensity (at constant monthly totals for rainfall) aroundthe values considered typical for each landscape was predicted to cause adecrease in Fp values by between 0.047 (Bialo) and0.261 (Mae Chaem). Sensitivity of Fp to changes in land use change pluschanges in rainfall intensity depends on other characteristics of the watersheds,and generalisations made on the basis of one or two case studies may not hold,even within the same climatic zone. A wet-season Fp valueabove 0.7 was achievable in forest–agroforestry mosaic case studies.Inter-annual variability in Fp is large relative to effects of landcover change. Multiple (5–10) years of paired-plot data would generally beneeded to reject no-change null hypotheses on the effects of land use change (degradationand restoration). Fp trends over time serve as a holisticscale-dependent performance indicator of degrading/recovering watershed healthand can be tested for acceptability and acceptance in a wider social-ecologicalcontext. |
