Landscape and (sub) catchment scale modeling of effects of forest conversion on watershed functions and biodiversity in Southeast Asia
Natural forests are, rightly or wrongly, the global benchmark for both ‘watershed functions’ and ‘biodiversity conservation’. While both these functions can be affected by forest conversion and further intensification of agriculture, the trajectories of bothfunctions are essentially different. ‘Watershed functions’ can be defined as the way landscapes determine quantity, timing and quality of river flow, by the way they 1) transmit, 2) buffer and 3) gradually release the rainfall that is received, 4) modify waterquality and 5) maintain the integrity of the soil capital in the catchment area. For these 5 ‘criteria’ we developed quantitative indicators, applicable in assessments at different scales. There is only a very partial direct overlap between watershed functions in this sense and the ability to conserve, provide habitat and connectivity for biological diversity in landscapes. The relationships between land use change, watershed functions andbiodiversity conservation are captured in a series of 10 hypotheses and 5 major questions studied in this report. We tested the hypotheses for internal consistency through theconstruction and use of quantitative simulation models that can be compared with actual data sets. We concentrated on the first three criteria and indicators for this report. Two ASB benchmark areas in Southeast Asia were the focus of this study, MaeChaem in northern Thailand and Sumber Jaya (Way Besai) in Lampung in the southern part of Sumatra (Indonesia) have an annual rainfall of about 1.5 and 2.5 m year-1, respectively. Total water yield (after subtraction of an estimated evapotranspiration of 1.3 m year-1) is about 0.2 and 1.2 m year-1, or 15 and 50% of rainfall. These values may broadly represent the hydrology in subhumid and humid tropics. In Mae Chaem the difference between actual and potential evapotranspiration dominates the water balance via total water yield. In Sumber Jaya (Way Besai) changes in soil structure that partition total water yield over quick and slow flows are the main feature that needs to be betterunderstood.
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Publication year
2022
