Effect of Land Surface-Atmospheric Boundary Layer Inter-Actions on Rice Production In West Africa

Abstract

Land cover change can influence the LCL height, which in turn affects cloud formation and consequently convective precipitation. This study compares the LCL height over three land cover types (Urban, vegetation, and crop) in West Africa and assesses their influence on convective cloud and precipitation using a new land surface model eCLM, a variant of CLM5 which has the same modelling capabilities as CLM5, but it uses much “leaner” scripts for building and running a model. The forcing provided is the Global Soil Wetness Project (GSWP) version 3. Land surface fluxes (SH, sensible heat flux; LH, latent heat flux) data were collected from eddy covariance stations at 11 sites distributed across West Africa. The simulation results from eCLM for the three land covers show a good agreement compared to both measurement and ERA5 with a correlation coefficient of above 0.75 and root mean square error lower than 30 W/m2. LCL is higher for cropland compared to natural vegetation and urban areas. Monthly LCL trends were estimated from the three land cover types over West Africa. LCL increases more over crop area with a high reaching 4.5 km with an average change of 1.7 km compared to the vegetation and urban land cover where LCL height reaches a high of 4.2 km. The height of the LCL shows a strong relation with precipitation. The LCL decreases when the monsoon progresses in June-July-August (JJA) and increases during the dry season when the monsoon retreats in December-January-February (DJF). We also observed a strong correlation between LCL and surface fluxes over the study area. Over the three land cover types (urban, vegetation, crop), LCL and LH are negatively correlated, while LCL and SH are positively correlated. Thus, changing from one land cover to another affects land surface fluxes (sensible and latent), and then significantly modifies moisture conversion at the lifting condensation level (LCL) and hence at the cloud base height, thereby increasing or decreasing precipitation over West Africa. Rice (Oryza sativa L.) is the main staple food for millions of people in sub-Saharan Africa (SSA) and its production in the region is threatened by climate variability and climate change. The objectives of this study were to evaluate the performance of APSIM-ORYZA for simulating rice yield under current climate conditions in irrigated systems in the derived savannah agro-ecological zone of West Africa. Data were collected on soil properties, weather, management practices, and rice yield during two years from experimental studies considering two irrigated schemes namely Mbe and Lokakpli. These data were used to calibrate and validate the APSIM-ORYZA model to represent with acceptable accuracy rice growth and yield under the conditions of production of the sites of studyOnce validated, the future bias-corrected climate data from Representation Concentration Pathways (RCPs) climate scenarios RCP 4.5 and RCP 8.5 were compiled and formatted to inform scenarios simulations with the model. Impact of climate change on rice yield was then assessed for six alternative management practices that are mostly preferred by farmers. The results showed that APSIM-ORYZA simulated satisfactorily irrigated rice with R2 values of 0.8 to 1, a root-mean-square error (RMSE) of 0.08 t/ha, and a Nash–Sutcliffe Efficiency of 0.99. Rice yield was projected to decrease of 17 and further 18% by 2070 under RCP 4.5 and 8.5, respectively compared to the reference rice yield of 2019. Rice yield was projected to be maintained with a relative increase from its baseline value in 2019 if alternative management practices such as rice straw mulching, alternate wetting, and drying, transplanting of 35 days old seedlings, organic fertiliser application at 120 kg N ha-1 and rice sowing date after 1 April were used. This study filled a gap in the literature concerning the potential of management practices to increase rice yield under future climate change conditions and highlight the importance of adoption of improved practices for the sustainability of rice production in the region of SSA.