Water balance study of Bontanga irrigation reservoir and some environmental issues

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Irrigation has been contributing immensely in poverty alleviation, food security and general improvement in the quality of lives of those who are engaged in it. However, the sustainability of these irrigated projects is being questioned both environmentally and economically. There is very little effort put into managing the water in the reservoirs of the various irrigation projects in Ghana. There is this feeling that water is limitless, purely public good without any consequences of economic efficiency and environmental sustainability. The supply of water to irrigated farms is not done on the calculation of water demand. The overall research objective is to build a water management data processing base for the Bontanga Irrigation Project. This was achieved by: • the determination of rate of annual sediment deposition into the reservoir. • Calibrating the flumes on the right and the left bank canals. • Installation of graduated staff gauges in the reservoir and the canals. • Developing a computer based programme to determine available water in the reservoir by the use of the calibrated inflows and outflows. A three (3)-step methodology has been adopted. This was done to build a systematic sequence of information to assist in the analysis. The three steps followed for the research are as follows: • Preliminary study • Exploratory study • Main research study Relationship between the level of water in the canal H. and the discharge through the canal Q was established from the calibration of the flumes on the left and right bank canals, thus any amount of water Rowing through each of the canals can he determined. The relationship between H and Q were Q 4.4257 H1.672 for the right hand flume and Q 4.0658H1.775 for the left hand flume. The reservoir area -capacity curves were plotted to establish the relationship between the gauge levels (h) and the reservoir flooded areas (A) as A(0.0763h2 + 0.654h + 1 .3035) x I 06m2 and the gauge levels (h) and reservoir capacities (V) as V (0.4029h2 + l.6493h + 0.9301) x l06m2. The runoff (y)-rainfall (x) relationships for the reservoir catchment was as y=O. 1 598x implying that the runoff coefficient for the catchment was 0.1598 or 15.98%. Predicted reservoir capacity agreed with observed data, but showing an over - prediction of 5.46% on the average. These established relationships form part of the input into the computer program for the water balance study. Using a locally made coring device, sediments deposited at both the right and left bank valves were sampled. The annual sediment deposition at the right hand valve was determined as 0.066m (6.60cm) and that at the left hand valve as 0.0635m (6.35cm). This brings the sediment deposition within the past twenty years of operation to 1.32m at right hand valve and 1.27m at the left hand valve. There is eminent risk facing the reservoir since it cannot last beyond 23 years of operation if the sediment deposition rate continues at the rate determined because the dead storage level for both intakes is I .52m. For the effective management of the Bontanga irrigation reservoir (and by extension to other projects in Ghana) the following recommendation should he considered: • Irrigation water withdrawals from the reservoir should be based on daily water requirements calculations. This will improve water management of the irrigation system. • The leakages a long he canals and laterals should he checked to avoid losing water through these avoidable leakages. • The study results suggest that if the irrigation system at Bontanga is run as usual, the sediment deposits could raise to such levels as could block the left and right intakes in less than four (4) years. • It is recommended that serious attention be given to the minimization of sediment inflow by putting in place pragmatic measures to manage the reservoir catchment. Though expensive, it is also recommended that desilting of the reservoir be considered as a matter of urgency if irrigation must continue at Bontanga. • It is highly recommended that management of these irrigation projects be taken seriously so that timer corrective measures are taken, bearing in mind the need for adequate resource allocations for doing this.
A thesis submitted to the Department of Materials Engineering, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the award of Master of Science degree in Environmental Resources Management, 2004