Spatiotemporal variability of ecosystem water use efficiency in northern Ethiopia during 1982–2014

Water use efficiency (WUE) measures the trade-off between carbon gain and water loss and is an important link between the carbon and water cycles. A better understanding of spatiotemporal variation in WUE and its controlling factors will help to improve ecosystem management for adapting to and mitigating the impacts of climatic change. We examined the spatiotemporal variations of WUE and its controlling factors for five bioclimatic zones and four land covers for 1982–2014 in the Tekeze River basin in northern Ethiopia. We quantified mean annual and seasonal net primary productivity (NPP), actual evapotranspiration (ETa), and WUE (NPP/ETa) during this period and found that each increased significantly. NPP, ETa, and therefore WUE varied spatially amongst the bioclimatic zones and land covers, but with different patterns. WUE was higher in semi-arid than humid zones. Spatiotemporal changes in temperature and precipitation could be important controls on NPP and ETa variations and therefore WUE. Mild and severe droughts in the southern regions of the basin decreased annual WUE, and moderate and extreme droughts slightly increased it, indicating that the measure of environmental rehabilitation implemented since the 1980s have increased NPP in these regions, despite the water stress caused by frequent droughts. The variability of WUE was primarily controlled by climatic change and human activity, the latter dominant since the 2000s. Ecological restoration and measures for conserving soil and water could substantially increase the vegetation productivity of this region, and be a viable management strategy to both adapt to and mitigate the impacts of climate change.