Clement Aga Alo

Identifying Systematic Land-Cover Transitions Using Remote Sensing and GIS: The Fate of Forests inside and outside Protected Areas of Southwestern Ghana

We use remote sensing and GIS to map changes in land cover and to identify systematic land-cover transitions in Southwestern Ghana. Landsat Thematic Mapper satellite imagery of 1990 and 2000 is used to create two land-cover classifications, and the two maps are then compared to produce transition matrices both for protected and for unprotected areas. These matrices are analyzed according to their various components to identify systematic landscape transitions based on deviations between the transitions observed and the transitions expected owing to random processes of change. The results show that closed forest regions inside the protected area transition systematically to bare ground or bush fire, but closed forest outside the protected area transitions systematically to open cultivated woodland. These results are consistent with the hypothesis that logging is the main cause of the loss of closed forest inside the protected areas whereas farming is the main cause of the loss of closed forest outside the protected areas. The research highlights the need for the implementation of this methodological approach to landscape change. Identification of strong signals of forest transformation is particularly important in the light of efforts by policy makers to curb deforestation in Ghana.

Droughts, hydraulic redistribution, and their impact on vegetation composition in the Amazon forest

Hydraulic redistribution (HR), the nocturnal transport of moisture by plant roots from wetter to drier portions of the root zone, in general can buffer plants against seasonal water deficits. However, its role in longer droughts and its long-term ecological impact are not well understood. Based on numerical model experiments for the Amazon forest, this modeling study indicates that the impact of HR on plant growth differs between droughts of different time scales. While HR increases transpiration and plant growth during regular dry seasons, it reduces dry season transpiration and net primary productivity (NPP) under extreme droughts such as those during El Niño years in the Amazon forest. This occurs because, in places where soil water storage is not able to sustain the ecosystem through the dry season, the HR-induced acceleration of moisture depletion in the early stage of the dry season reduces water availability for the rest of the dry season and causes soil moisture to reach the wilting point earlier. This gets exacerbated during extreme droughts, which jeopardizes the growth of trees that are not in dry season dormancy, i.e., evergreen trees. As a result, the combination of drought and HR increases the percentage of drought deciduous trees at the expense of evergreen trees, and the fractional coverage of forest canopy is characterized by sudden drops following extreme droughts and slow recovery afterwards. The shift of the tropical forest towards more drought deciduous trees as a result of the combined effects of extreme drought and HR has important implications for how vegetation will respond to future climate changes.

Changes in Precipitation Seasonality in West Africa Predicted by RegCM3 and the Impact of Dynamic Vegetation Feedback

Using a regional climate model asynchronously coupled to a dynamic vegetation model, this study examines future climate predictions for the West Africa region and how dynamic vegetation feedback may influence such predictions. Without accounting for the impact of vegetation dynamics, the model predicts a future decrease of annual rainfall over Sahel. Dynamic vegetation feedback reverses this trend, leading to a substantial increase of annual rainfall. Regardless of how vegetation is treated, the predicted future trend of precipitation in the Sahel region follows a specific seasonal pattern, with a decrease during the pre- and early-monsoon season (May-June and early July) due to the warming-induced enhancement of spring convective barrier and an increase after the monsoon is fully established (typically in July-August-September) due to enhanced moisture import from a warmer ocean. Dynamic vegetation feedback reduces the magnitude of the predicted rainfall reduction in the early season and increases the magnitude of the predicted rainfall increase later in the rainy season. The future decrease of early-season rainfall has significant agronomic implications.

Evaluation of Groundwater Recharge Estimates in a Partially Metamorphosed Sedimentary Basin in a Tropical Environment: Application of Natural Tracers

This study tests the representativeness of groundwater recharge estimates through the chloride mass balance (CMB) method in a tropical environment. The representativeness of recharge estimates using this methodology is tested using evaporation estimates from isotope data, the general spatial distribution of the potential field, and the topographical variations in the area. This study suggests that annual groundwater recharge rates in the area ranges between 0.9% and 21% of annual precipitation. These estimates are consistent with evaporation rates computed from stable isotope data of groundwater and surface water in the Voltaian Basin. Moreover, estimates of groundwater recharge through numerical model calibration in other parts of the terrain appear to be consistent with the current data in this study. A spatial distribution of groundwater recharge in the area based on the estimated data takes a pattern akin to the spatial pattern of distribution of the hydraulic head, the local topography, and geology of the terrain. This suggests that the estimates at least qualitatively predicts the local recharge and discharge locations in the terrain.

Hydrogeological characterization of a tropical crystalline aquifer system

This research used a numerical groundwater flow model, calibrated under steady-state conditions to develop the groundwater flow system in the West Mamprusi District of Northern Ghana. It was aimed at conceptualizing the flow system to initiate a thorough hydrogeological study of the rocks in the area. Stable isotopes were used to relate groundwater recharge in the area to recent meteoric water that had been evaporated in transit down the surficial material. On this basis, direct vertical groundwater recharge from precipitation was applied in the numerical modeling. This study suggests that the prospects of commercial development of the aquifers are high as the estimated recharge ranges between 3.3% and 29% of the annual precipitation. Estimated horizontal hydraulic conductivity ranges between 3.2 and 48 m/d in the area. The variability in the horizontal hydraulic conductivity has led to the development of four prominent groundwater flowpaths in the area. However, a prominent NE–SW flow has been observed and is in consonance with the reported structural grain of the country. A groundwater flow divide noted in the southern part of the study area has been attributed to the structural heterogeneity rather than topographical complexities as the area is largely flat.