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A serious lack of uniform data on a global scale regarding in-lake conditions of the TWAP transboundary lakes required that their potential risks be estimated on the basis of the characteristics of their drainage basins, rather than direct analysis of their in-lake conditions. The relative lake threat ranks were determined with a scenario analysis program that allowed consideration of specific preconditions and assumptions about the nature and magnitude of their basin-derived stresses, and the possible impacts on the sustainability of their ecosystem services, as defined by the users of the ranking results. Because the transboundary lake threat ranks are based on specific lake and basin assumptions, therefore, the calculated rankings represent only one possible set of lake rankings.
Using basin characteristics to rank the transboundary lake threats does not allow consideration of the unique features that can buffer in-lake responses to basin-derived disturbances. These features include an integrating nature for all inputs from their basins, long water retention times, and complex, non-linear response dynamics. A global overview of river basin threats based on 23 basin-scale drivers considered within four thematic areas (catchment disturbance; pollution; water resource development; biotic factors) was modified for the transboundary lakes assessment. The driver weights were initially based on the collective opinions of experts exhibiting a range of disciplinary expertise, and were subsequently refined with inputs from lake scientists and managers participating in ILEC’s 15th World Lake Conference.
An interactive spreadsheet-based scenario analysis program was used to rank the relative threats to the transboundary lakes. The characteristics of the transboundary lakes were determined by superimposing the lake basin grids over the river basin grids, and scaling the driver data to lake basin scale. Selected basin drivers, weights and preconditions also were used in the scenario analysis program to calculate the relative lake threat ranks, expressed in terms of the Incident (HWS) and Adjusted (Adj-HWS) Human Water Security and Incident Biodiversity (BD) threats.
The calculated transboundary lake threats clearly indicated their lake threat rankings must be considered within the context of the specific basin conditions and assumptions used to derive them. Thus, they represent only one possible set of lake threat rankings. Other factors such as lake and basin area, basin population and density, regional location, per capita Gross National Income (GNI), and Human Development Index (HDI) can produce markedly different ranking results. Therefore, defining the appropriate context and preconditions for interpreting the lake ranking results, a task that was beyond the scope of this analysis, is an essential responsibility of those using the results, including lake managers and decision-makers.
