ABSTRACT
The increasing demand for food and water due to global population growth has made it necessary to assess the quality of the available water resources for drinking and irrigation. This study aimed at assessing the water quality of selected dams/reservoirs namely; Oyan, Owiwi, Lekan Are and Owena dams/reservoirs located within Benin-Owena and Ogun-Osun River Basins in Southern Nigeria.
Population forecast for the study area was carried out by using three methods namely; arithmetic progressive method, geometric increase method and incremental increase method and the best method was selected by finding their percentage error. The water quality of the selected dams/reservoirs was determined by evaluating the physico-chemical parameters of the water samples
through laboratory test. The suitability of water for irrigation and drinking was obtained by using irrigation water quality index (IWQI) formular and the drinking water quality index (DWQI) formular. Other water quality parameters such as Sodium Absorption Ratio, Kelly Index, Magnesium Hazard, Sodium Percent and Permeability Index was calculated by using the appropriate formulae. A water quality database of the study area, and a spatial distribution map was created by transferring the results of the chemical analysis of the water samples to a GIS environment. The effectiveness of the irrigation practice in the study area was evaluated by administering questionnaires to irrigation practitioners and was analysed using ANOVA (Analysis of Variance).
Geometric increase method of population projection was selected because it has the lowest minimum percentage error of 0.0309. The population forecast of the study area for 40year period (2016-2056) was 30,023,834 which suggests that there will be a continuous increase in the demand for food and water supply. The physico-chemical parameters of the water samples showed that the mean values of pH were within the range of 6.5-8.5 which fall within the allowable range of WHO standard for drinking and irrigation water use. The obtained values of the Calcium ion (Ca2+) indicated that the water is suitable for drinking and irrigation with the observed highest value of 38.29 mg/l and the lowest value of 8.57 mg/l. The average values of the Magnesium ion (Mg2+) showed that the observed lowest value of 31.29 mg/l and the highest value of 52.29 mg/l, fall within the acceptable limit for drinking and irrigation. The Potassium ion (K+) also has an observed highest value of 4.20 mg/l and the lowest value of 2.16mg/l which when compared to water quality standards indicates that the water is suitable for irrigation. The values of IWQI for the four dams/reservoirs namely Oyan, Owiwi, Lekan Are and Owena were 82.938, 81.172, 81.453 and 86.016 respectively, these are within low restriction and no restriction level for irrigation. Likewise, the DWQI values for the four dams/reservoirs namely Oyan, Owiwi, Lekan Are and Owena, were 55.005, 52.070, 58.171 and 56.593 respectively, this shows that the water has an excellent quality rating and is safe for human health. The evaluation of the effectiveness of irrigation practice shows that the irrigation farming currently been practiced in southern Nigeria is effective and increasing the crop yield. However it is recommended that an efficient database for population records should be put in place and further water treatment to reduce excess concentrations should be carried out along with smart irrigation technologies and innovations introduced to increase food and water security in the study area.
