Modelling the Irrigation Potential of Besease Wetlands
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Date
2012-08-10
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Abstract
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.
Description
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