Hydrologic Partitioning of Evapotranspiration, Flows and Storage Across Landscapes and Climate Regimes in the Congo Basin

We use a distributed hydrological modeling framework to simulate the spatial and temporal variability of water-balance components in the Congo River basin (CRB) in Central Africa. In particular, our goal is to develop a predictive framework suitable for describing surface-water runoff, evapotranspiration (ET), and terrestrial-water storage and the responses of these processes to changes in climate and land cover. Information on the region's climate, soil properties, land cover, and topography are used to develop the model. The CRB is divided into 1,600 sub basins. The model computes the partitioning of precipitation (P) into quick flow (Qs), and soil wetting (W = P - Qs) at the sub basin level. The soil wetting component is further partitioned into ET, base flow (Qb), and storage. We calibrate the model by minimizing an objective function defined as the sum-of-squared differences between calculated and measured monthly average total flows, base flows and water yield at 27 stream-gage locations within the CRB. Annual precipitation within the basin varies between 1,000 to over 1,800 mm. The central parts of the basin, where the tropical evergreen forests are located, receive the highest amount of precipitation, whereas the northern and southern headwater regions are dryer. The model calculations of the fraction of annual precipitation returned to the atmosphere as ET from the land surface, lakes and wetlands varies from 0.30 to 0.90 across the CRB. The Aridity Index (AI), defined as the ratio of annual precipitation to potential ET, varies from 0.50 to 1.20. Nearly ten percent of the CRB area, mostly the southeastern headwater region, falls under semi-arid to dry sub-humid category (AI <= 0.65). The mean value of the runoff fraction, defined as the ratio of annual runoff to precipitation, is 0.17 (±0.10). The highest runoff-producing areas lie mostly within the forest land cover types in the central and northwestern part of the CRB, which also receive high precipitation. The coefficient of determination between calculated and measured flows at the 27 gage locations has an average value of 0.51 (±0.22), whereas, the Nash-Sutcliffe model calibration efficiency varies from -200 to 0.85. Further calibration is being done to improve the model prediction skill. The spatial variations in ET from the terrestrial ecosystem, and in the sum of groundwater and surface water runoff arises from local-scale variability in vegetation and climate. Their understanding is important to quantify the hydrological response to climate and land cover changes, and to quantify the accessible water resources for human appropriation within the CRB in space and time.