| Summary: | Limited water resources, climate change and food security needs in West Africa present a special set of challenges in the years to come as the population grows. An optimized irrigation scheme for agriculture can change regional climate by increasing rainfall in specific areas, possibly increasing the water availability for agricultural activities by causing changes in the background large-scale climate circulations which could lead to more precipitation overall in areas with water scarcity. Both observational and model studies have looked at irrigation impacts around the world, including West Africa. However, the intermediate mechanisms, such as specific roles of the atmospheric structures of the Planetary Boundary Layer (PBL) and Lifting Condensation Level (LCL), or how background wind patterns are affected under certain land-use changes have not been thoroughly explored. This thesis analyzes the atmospheric changes due to land-use and land-cover changes (LULCC) by analyzing the PBL, the LCL, surface wind, surface pressure and other atmospheric variables to quantify the underlying physical mechanisms which shape rainfall. We analyze this by using the MIT Regional Climate Model (MRCM) to test different LULCC scenarios. For the irrigation experiment, LCL is more sensitive and drops more than does PBL especially in the north, yet rainfall only increases south of the irrigation area. There also exists a transitional zone, north of which there is less rainfall. Desertification increases both the PBL and LCL heights, but the increase in LCL is greater. This pushes the cloud base higher than the PBL, preventing cloud formation and rainfall. However, the simulated rainfall changes do not mirror this development. At a certain latitude, there is again a transitional zone, north of which the rainfall decreases and south of which the rainfall increases intermittently. Given the patterns of the precipitation changes, we believe that different mechanisms are at work for both the desertification and irrigation experiments. This study hypothesizes a blocking mechanism that prevents the monsoon from travelling northward due to the presence of a high surface pressure anomaly being observed in the north of the irrigated zone under the irrigation scenario.
The changes of the atmospheric structure, specifically the PBL and LCL, surface pressure, and wind patterns, as analyzed in this thesis, provide us with another dimension to understand the effects of irrigation and desertification on rainfall, enabling more optimal irrigation strategies. It also provides insights on the locations where natural vegetation or croplands may benefit from the additional rainfall, which could facilitate soil carbon sequestration, a nature-based solution for combatting climate change.
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