Climatological Controls on Congo Basin Transpiration

The Congo Basin has two rainy seasons, with peak rainfall in March and November. On the climatological average, rainfall is higher in November than in March, yet evaporation is lower in November than in March. Previous work by the authors has suggested that lower November transpiration explains the lower November evaporation, and the aim of this paper is to quantify the processes responsible. We find that to the North of the Congo, transpiration from evergreen tropical rainforests is uniform throughout the year. Transpiration is limited by leaf area, which is maximised due to constraints on space for vegetation, and stomatal conductance, which is unchanging. To the North, soil moisture and radiation do not set limits on transpiration. On the contrary, limits to transpiration from savannah and shrubs to the South vary seasonally. The northward migration of the tropical rainbelt in boreal spring and summer generates a substantial depletion of soil moisture and leaf area, which severely limits transpiration. In addition, increasing vapour pressure deficit reduces stomatal conductance, which sets further limits on transpiration. When the tropical rainbelt migrates back southward in boreal autumn, leaf area cannot fully return to boreal spring values, because soil moisture accumulation begins relatively late. In addition, stomatal conductance is kept at low values, and these factors ensure that transpiration in the South is not able to fully recover. This provides a complete explanation as to why evaporation is lower in November than in March on the domain-wide average, despite higher November rainfall.