Influence of scale in water resources management for heterogeneous African semiarid rangelands.

Semiarid rangelands (grasslands with scattered trees and shrubs) are one of Africa’s most complex and variable biomes. They are a mosaic of land uses, where extensive livestock is the main economic activity, and agriculture or conservational uses are also crucial. They are highly controlled by the availability of water, e.g., pasture and rainfed crop production. Although the vegetation is adapted to variable climatic conditions and dry periods, the increase in drought intensity, duration, and frequency, the changes in agricultural practices, and other socioeconomic and environmental factors precipitate their degradation. The combined differential functioning and characteristics of the vegetation components and communities affect water dynamics, resulting in high spatiotemporal variability that creates distinct patches. Therefore, the precision, resolution, and accuracy of the information required for water management differ according to the scales of these patches: from the local to the basin. We want to assess the optimal spatiotemporal scale when monitoring semiarid mosaic vegetation cover and its water consumption.

 

To answer this question, we evaluated the water use patterns of the typical vegetation patches (tree+grass savanna, grassland, crop area, and creek shore) estimated by different modeling approaches (FAO56 and TSEB) with spatial resolutions of 30 m, 250 m, and 1 km. From a farm/agricultural management viewpoint, we demonstrated the need for sufficient spatial and temporal resolution when evaluating water consumption and the difficulties when significant temporal gaps are present. Higher spatial-temporal scales were crucial to determining the pasture drying cycle and crop water use. In humid or denser areas that provide essential ecosystem services (e.g., wildlife habitat), transpiration rates were higher throughout the year and often underestimated when using coarse data. Over savanna patches, products with coarse resolution (1 km) reflected well the water use pattern. These metrics reflected the severe drought experienced during the 2015-2016 seasons due to an intense El Niño event, other dry events (e.g., 2002, 2007), and the recovery time of each vegetation patch.