Mountains of the Moon: Reduced riverflows?

Last week we looked at how the Rwenzori Mountain's melting glaciers are having dramatic societal and agricultural impacts. This post explores the direct effects of a reduction in glacial ice on alpine riverflow in these Ugandan Mountains, focusing on Taylor et al's (2009) paper in the Journal of African Earth Sciences. What are the effects of a warming world (and thus glacier loss - covered here) on alpine water supply?

River Mubuku, the principle river
recieving meltwater discharges at
the base of the Rwenzori Mountains.
(Source)

As covered in an earlier post, these glaciers are vital to sustaining meltwater discharges, especially during the dry season, and also act as a store of seasonal precipitation (Taylor et al 2009). Thus, concerns have arisen over potential reductions in river discharge following the recent loss of Uganda's frozen reservoir. Our understanding at present is somewhat limited due to a lack of hydrological measurements in the East African Highlands (Taylor et al 2009). Besides a few spot measurements taken by Temple in 1968, no other data exists for glacial meltwater discharges in the Rwenzoris.

By taking spot measurements of alpine riverflow along numerous altitudinal cross-sections of River Mubuku (the mountain's principal river), draining alpine icefields, Taylor et al (2009) assessed the contribution of glacial ice on the Rwenzori Mountains to river flow. Using this newly collected dataset alongside historical records, the authors found that accelerated glacial retreat since the 1960s has had minimal impact on alpine riverflow. Through their study, they conclude that meltwater from glacial ice contributes to under 2% of river flow in the Mubuku during both wet and dry seasons. So although glaciers continue to rapidly recede in the Rwenzori Mountains (see my first post in this 'mini-blog' series), it seems this is having a minor impact on alpine riverflow. River Mubuku's headwaters are provided by glacial meltwater from the Rwenzori Mountains. Thus, one may presume that due to a disproportionately high specific discharge (1730mm/year), a significant amount of riverflow originates from meltwater (Taylor et al 2009). However, this high specific discharge is actually attributed to high precipitation rates in Heath-moss and Montane forest areas (below the icefields), which occupy over 50% of the river's catchment.

The authors also argue that trivial contributions of glacial meltwaters to alpine riverflow found in the Rwenzoris may apply to similar tropical alpine regions, where glaciers contribute a small percentage of the basin area, e.g. Mount Kilimanjaro and Mount Kenya. If this is true, how is a reduction in dry season riverflow on Kilimanjaro (Desanker 2002) explained, if not due to deglaciation? Taylor et al (2009) argue this is likely to be from declining rainfall and land-use changes on the catchment (e.g. deforestation), rather than a loss of alpine glaciers.

Thoughts and reflections:

I think Taylor et al (2009)'s research is a great contribution to existing literature on this topic, and reveals that glacial recession does not impact alpine riverflow as one might initially expect. It appears that climate change's impact on the hydrological cycle (intensification and altered precipitation patterns) is more of an influence on alpine riverflow in this location than changes to alpine icefields, caused by temperature rises. However, just because glacial meltwater discharges do not have a large contribution to alpine riverflow does not mean that riverflow will not be affected in other ways by climate change in the near-future. For example, increased evapotranspiration will reduce surface water availability, and less frequent, but more intense rainfall will alter patterns of seasonal discharge in River Mubuku. All these changes, regardless of whether they result from glacial loss or through changing hydrological patterns, will have significant direct impacts on those communities who rely on water from and around the Rwenzori Mountains. Climate change is manifesting itself in a number of ways, and the impacts and consequences of this are not entirely straightforward. There is clearly a great need to increase hydrological and meteorological measurements, improving the data available for meltwater and river discharge in this area, if we are to learn more about the impact of climate change and water supply in this vulnerable part of East Africa.

I highly recommend delving into the rest of Taylor et al's paper if you're interested in this topic regarding the Rwenzori Mountains, and fancy something intellectually stimulating for a bedtime read!