Modelling the Irrigation Potential of Besease Wetlands

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The harsh climate, shallow and erodible soils of low fertility uplands have led to farmers extending their cultivable areas to wetlands for optimal crop production since these systems have the potential for irrigation in the dry season. Inland valleys have been cited as having a high potential for the development of rice-based, small-holder farming systems at the village level, due to their specific hydrological conditions and relatively high soil fertility. To ensure its sustainable use, the physico-chemical and the hydrological processes of the valley bottom should be ascertained. The irrigation potential of wetlands was studied at Ejisu-Besease in the Ashanti Region of Ghana by analysing the hydrodynamics of the valley bottom through a groundwater flow modelling process using MODFLOW. Groundwater recharge estimates from the water table fluctuation method was used as the recharge input into the model. The results showed that annual water level rise ranged from 1105–3115 mm in 2009 and from 397–3070 mm in 2010. A range of specific yields were extracted from the values determined from the soil textural classification triangle. The estimated recharge for the study area ranged from 133–467 mm for the fourteen (14) piezometers, representing 9–31% of 2009 annual rainfall and 47.6–427.9 mm in 2010 representing 4–34 % of the annual rainfall. Groundwater recharge was also estimated using the Kalman filter method. Using the mathematical model developed, the infiltration parameters were determined. The infiltration factor for the years 2009 and 2010 varied from 0.0–16.70 % of the incident rainfall. The results from the groundwater flow model showed that groundwater levels ranged from 259.10–259.97 m in the wet season and 258.19–258.86 m in the dry season of the simulation period. It also exhibited a form of interaction between the inland valley wetland and the Oda River which varied from period to period depending on the river stage. Sensitivity analysis was performed, and model outputs were found to be highly sensitive to the parameters such as horizontal hydraulic conductivity, specific yield and specific storage. The hydrochemical study of the area revealed that alkaline earths exceed alkalis and weak acids exceed strong acids in groundwater which presented a CaHCO3 groundwater type. Results from the groundwater chemistry of the two boreholes indicated that the groundwater is of good quality for irrigation. The Inland Valley Bottom was also classified into three hydrological regimes as a management tool for developing wetlands for crop production. The regimes are WTF Class I acute slopes segment varying from 0–30 %, WTF Class II acute slopes segment varying from 30–45% and WTF Class III acute slope segment > 45 %. The study unravelled the relationship between recurrent spatial and temporal patterns of watertable response within the inland valley bottom and their controlling factors. It is concluded that a controlled water table offers a distinguishing criterion for the development of Inland Valley Bottoms for year round crop production.
A Thesis submitted to the Department of Agricultural Engineering, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY IN SOIL AND WATER ENGINEERING College of Engineering. August,2012