New PhD Thesis
Author: Prince Martin Gyekye
Title: Impacts of Floods on Soil Erosion, Urban Land Use and Soil Quality in the Greater Accra Metropolitan Area
Date: January 2023
The Ph.d thesis has been submitted for defense
Uncontrolled and unregulated land use activities have reduced arable lands and increased flood incidence in the Greater Accra Metropolitan Area (GAMA) of Ghana, especially in the periurban areas. Soil quality variability and its relationship to runoff, erosion, floods and decline in arable lands across time and space have had little or no attention in the coastal savannah agroecological systems of the Savannah zone of Ghana, especially in urban settings. Therefore, the study’s overall objective was to explore how runoff, flood, and land use impact soil quality and their implications for livelihoods. Five major sub-studies were conducted to achieve this objective. They include; (1) assessment of the amount of runoff and soil infiltration rate and its implications for/on soil quality and risks to livelihood; (2) assessment of the rates of erosion across the landscape and related effects on livelihoods, using qualitative and quantitative data triangulation; (3) assessment of the land-use suitability dynamics for selected crops; (4) evaluation of the impact of urban expansion on the dynamics of arable lands and soil quality in Accra, using Santa Maria as a test case, and; (5) analysis of land-use and soil quality-related policy considerations for the GAMA. In study (1), using a combination of soil characterisation, the double-ring infiltration method and the Gerlach runoff model, decline in soil qualities were observed to be caused by land use changes. Such soil quality decline led to reduced infiltration rates, contributing significantly to runoff and flooding and, subsequently, risks to livelihoods within the study area. It can be inferred from the study that improving the soil structure of bare lands through planting grasses is likely to decrease soil surface runoff by 52% while increasing soil infiltration by approximately 700%. However, further conversion of the limited forests/woodlands in the GAMA to grasslands or bare lands would likely reduce soil infiltration rate by 63% and 95%, respectively. This action would also likely increase soil surface runoff by about 140% in respect of grasslands and about 400% for bare lands. From study (2), it was concluded that erosion rates were generally about four times on the high grounds, i.e. 95 t ha−1 yr−1 compared to 25 t ha−1 yr−1 on the lowlands. In the case of land use systems, erosion rates followed the order; bare lands (169.8) > grasslands (4.6) > forests/woodlands (0.003 t ha−1 yr−1). The RUSLE erosion mapping model showed that 69% of the GAMA is above the international threshold or soil tolerance level of 5.0 t ha-1 yr-1. From study (3), it was concluded that the soils of Accra are suitable for the cultivation of crops such as maise, cassava, okro, rice, pepper and tomato. This result validates indigenous people’s belief that the soils are more suitable for a wider variety of crops than previously reported by researchers. The results of study (4) show that high population and migration have resulted in pressure on lands for development, leading to loss of farmlands of the indigenes within the Santa Maria area. Residents more affected include farmers in the peri-urban areas. The result of study (5) shows that out of 11 selected policies, only the National Medium-Term Development Policy Framework 2022-2025 touched on almost all the themes. None of the policy instruments focuses on the protection of urban arable lands. Soil quality is relegated to rural development strategies leaving out periurban issues. Generally, the studies conclude that urbanisation has caused the GAMA to lose arable lands of good soil quality. The metropolitan area now has to contend with reduced soil infiltration, rising soil surface runoff and erosion with varying livelihood risks. It is recommended that measures to reduce floods, urban soil erosion, and livelihood risks should include forest/woodland reserve protection, provisions of drainage facilities, and grassing of the open areas of the GAMA to enhance infiltration rate and reduce runoff amount.