Forest loss impact on river flow regimes of the Singkarak - Ombilin basin, West Sumatra, Indonesia
Creators: name: Jeanes, Kevin Wayne
Publisher: name: The Australian National University
Year: 2015

description: The impact of forest loss on catchment watershed functions has been the subject of scientific and public debate since the 1840's in the temperate world and since the 1920's in the tropical world. The thesis seeks to explore this debate and the impact of tropical native forest loss and soil degradation on catchment hydrologic response, through literature review, a wet tropical case study, and a combined bio-physical science, social survey and numerical analysis and modelling approach. The thesis identifies the impact of forest loss on aquifer recharge, low flows, flood patterns and flow seasonality, and the existence of the soil degradation-induced 'infiltration trade-off' effect (Bruijnzeel, 1988; Bruijnzeel, 1989), as the main disputed issues of the science and debate. These issues and the debate have arisen due to a rift between: 'mainstream' temperate hydrologists who have focussed on short-term and small-scale research studies on non-degraded soils; and 'soil-focused' tropical hydrologists and the tropical public, agency practitioners and policy-makers who have focussed on long-term, larger-scale studies with degraded soils. A case study of the wet tropical Singkarak-Ombilin river basin in West Sumatra, Indonesia was conducted to identify whether field evidence can address the issues or support the tropical 'soil-focused' hydrologist's, public and policy maker's views. Social survey, empirical analyses and parametric and non-parametric modelling methodologies were employed to explore the forest loss and land degradation impact on hydrologic response across five varying-scaled study catchments (150 - 2,200 sq km), a 38 year discharge record and with daily time-step models. This combination of data input, models, social and numerical analysis proved a successful hydrological analysis approach in the data-sparse tropical study conditions. The combined methodologies identified that forest loss had led to widespread basin land degradation, and that these combined influences had led to: increased erosion and sediment transport (basins 150 - 1100 sq km); reduced groundwater recharge (basins &lt; 100 sq km); increased flood peaks and reduced low flows (baseflow) (basins &lt; 320 sq km); and reduced total annual yield (basins &gt; 540 sq km). In larger basins (540 - 2,200 sq km), a combined forest loss, land degradation and rainfall fluctuation influence was indicated to drive the pattern of reduced low flows and baseflow. These patterns indicate that the land degradation-induced 'infiltration trade-off' effect does exist within basin, and that it is primarily driving hydrologic response independent of rainfall variation in small basins (&lt; 320 sq km) and secondarily driving baseflow response and total annual yield in larger basins (540 - 2200 sq km). The thesis case study social survey, numerical and modelled findings support the 'soil-focused' hydrologist's and tropical public and policy maker's views of a positive forest cover effect on catchment function. But they challenge the views of the 'mainstream' hydrologists. The findings contribute evidence that wet tropical native forest cover does sustain dry season and annual baseflow, low flows and reduce the seasonal variability of river flow patterns, and thus contributes to the broader policy debate to suggest that wet tropical forests require preservation to provide the environmental service of water supply.