Data-driven estimates of evapotranspiration and its controls in the Congo Basin

Abstract

Abstract. Evapotranspiration (ET) from tropical forests serves as a critical moisture source for regional and global climate cycles. However, the magnitude, seasonality, and interannual variability of ET in the Congo Basin remain poorly constrained due to a scarcity of direct observations, despite the Congo being the second-largest river basin in the world and containing a vast region of tropical forest. In this study, we applied a water balance model to an array of remotely sensed and in situ datasets to produce monthly, basin-wide ET estimates spanning April 2002 to November 2016. Data sources include water storage changes estimated from the Gravity Recovery and Climate Experiment (GRACE) satellites, in situ measurements of river discharge, and precipitation from several remotely sensed and gauge-based sources. An optimal precipitation dataset was determined as a weighted average of interpolated data by Nicholson et al. (2018), Climate Hazards InfraRed Precipitation with Station data version 2 (CHIRPS2) , and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Climate Data Record product (PERSIANN-CDR), with the relative weights based on the error magnitudes of each dataset as determined by triple collocation. The resulting water-balance-derived ET (ETwb) features a long-term average that is consistent with previous studies (117.2±3.5 cm yr−1) but displays greater seasonal and interannual variability than seven global ET products. The seasonal cycle of ETwb generally tracks that of precipitation over the basin, with the exception that ETwb is greater in March–April–May (MAM) than in the relatively wetter September–October–November (SON) periods. This pattern appears to be driven by seasonal variations in the diffuse photosynthetically active radiation (PAR) fraction, net radiation (Rn), and soil water availability. From 2002 to 2016, Rn, PAR, and vapor-pressure deficit (VPD) all increased significantly within the Congo Basin; however, no corresponding trend occurred in ETwb. We hypothesize that the stability of ETwb over the study period despite sunnier and less humid conditions may be due to increasing atmospheric CO2 concentrations that offset the impacts of rising VPD and irradiance on stomatal water use efficiency (WUE).

A harmonized global land evaporation dataset from model-based products covering 1980–2017

“…The decreasing trend in the Congo Basin is detected in ERA5 and MERRA-2, while an opposite one is observed in GLEAM3.2a. Burnett et al (2020) found the Congo Basin had become sunnier and less humid in recent years through the analysis of environmental data. In general, GLEAM ET is fairly close to land ET in tropical Africa (Schuttemeyer et al, 2007;Opoku-Duah et al, 2008;Andam-Akorful et al, 2015;Liu et al, 2016), while MERRA-2 ET has the maximum temporal variability over the Congo Basin (Burnett et al, 2020;Crowhurst et al, 2020).…”

“…Burnett et al (2020) found the Congo Basin had become sunnier and less humid in recent years through the analysis of environmental data. In general, GLEAM ET is fairly close to land ET in tropical Africa (Schuttemeyer et al, 2007;Opoku-Duah et al, 2008;Andam-Akorful et al, 2015;Liu et al, 2016), while MERRA-2 ET has the maximum temporal variability over the Congo Basin (Burnett et al, 2020;Crowhurst et al, 2020). The increase in the east of North America, west of Europe and south of Asia is detectable in all datasets.…”

Where Does Moisture Come From Over the Congo Basin?

“…Previous studies have found a higher ET contribution to the moisture budget during the spring rainy season compared to that of the fall rainy season, which could also help explain why ET contributes more moisture for the spring rainy season. On the other hand, Burnett et al (2020) calculated basin-scale ET in the Congo using water-balance methods, positing that increased radiation, as well as the availability of soil moisture, can explain increased ET in the spring: Higher direct photosynthetically active radiation (PAR) fractions combined with higher net solar radiation increase water use efficiency, while increased terrestrial water storage increases the amount of water available for transpiration. Crowhurst et al (2020), using global climate models, also calculated ET over a similar domain and found that it was higher in the spring rainy seasons compared to the fall rainy season.…”

“…10.1029/2020JG006024 7 of 14 Figure 4b shows the contributions of ET and MFC to precipitation within our Congo domain, as indicated by ET/P and MFC/P. ET/P shows consistently high values throughout the year and MFC/P is always lower than that of ET/P (e.g., Burnett et al, 2020;Crowhurst et al, 2020). These data support our conclusion that there is a larger relative contribution of ET to the moisture for the spring versus fall rainy seasons.…”

CAMELE: Collocation-Analyzed Multi-source Ensembled Land Evapotranspiration Data

“…Finally, comparing different methods, the results demonstrated that combination performance based on EIVD method generally outperformed other methods through synthetic experiments. As for the simplest averaging method, though previous studies recommended the average value as the proxy of reference (Pan et al, 2020;Burnett et al, 2020;Baker et al, 2021), our synthetic results demonstrated that the averaging method presented the lowest improvement. For gridded data, the equal weight for each grid assigned by averaging method ignored the spatial variability of different products, which could result in large uncertainty.…”