Abstract
Abstract. East African forested mountain regions are vital in generating and supplying water resources to adjacent arid and semi-arid lowlands. However, these ecosystems are under pressure from both climate and land use changes. This study aimed to analyze the effects of climate and land use changes on water yield using the Budyko framework as a first-order conceptual framework assuming steady-state for pristine/protected forested areas. For nine selected forested water towers in East Africa, the amount and distribution of water resources and their decadal changes were analyzed. Results show that most areas inside and outside the water towers are under pressure from human influences. Water yield was more sensitive to climate changes compared to land use changes within the selected East African water towers themselves. However, for the surrounding lowlands, the effects of land use changes had greater impacts on water yield. We conclude that the East African water towers have seen a strong shift towards wetter conditions, especially in the period of 2011–2019, while, at the same time, the potential evapotranspiration is gradually increasing. Given that most of the water towers were identified as non-resilient to these changes, future water yield is likely to also experience more extreme variations.
Highlights
Many mountainous areas act as water towers by generating and supplying runoff and streamflow to adjacent lowlands that would otherwise be much drier
Our results indicate that changes in precipitation and potential evapotranspiration are currently the major determinants of water availability from high elevated forested water towers in the East African region
Water yield are larger in the adjacent regions surrounding the water towers
Summary
Many mountainous areas act as water towers by generating and supplying runoff and streamflow to adjacent lowlands that would otherwise be much drier. Beyond the supply of water, the elevated water towers maintain high actual evapotranspiration, playing a key role in regional rainfall recycling (WWF, 2005). Dewi et al (2017) introduced the following quantification criterion for defining water towers as an area that satisfies two conditions: (a) an aridity–humidity index (i.e., a ratio of precipitation to potential evapotranspiration) of above 0.65 and (b) where the aridity–humidity index relative to the elevation is greater than 2.77. The United Nations Environment Programme (UNEP, 2010) defined water towers as elevated areas of land (generally at least 200 m above the surrounding area) that receive at least 750 mm of rainfall and 250 mm of runoff per year and are significant water sources for populations beyond their immediate delineated boundaries
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