Assessment of Impact of Turbidity on Reservoir Storage, A Case Study of Gilgel Gibe I Reservoir, Southwest, Ethiopia

Abstract

Turbidity is one of the responsible factor for changing the quantity of water stored in reservoir by increasing surface water temperature and evaporation. Therefore, determination of the impact of turbidity on water stored behind the dam is crucial to understand several environmental phenomena and social issues. This study was centered on, assessing the impact of turbidity on reservoir storage of Gilgel Gibe I reservoir. The samples(data) were taken from the reservoir near to rainfall event ceased by stratifying the reservoir water randomly during different time, based on turbidity variation along the longitudinal axis of reservoir. To attain the goal of this study, both in laboratory and on field experiments were performed. To determine the relationship that turbidity has with surface water temperature and downward vertical alteration of water temperature along with turbidity variation, ten pools were burrowed and they were covered with white transparent plastic to preclude turbid water infiltration and entrance of soil water into the pools. Two Class A pans were used to determine surface water evaporation and storage change was evaluated from volume of water evaporated from Gilgel gibe I reservoir obtained at full supply level of the reservoir. Water temperature in pools and evaporation from pans were measured at a different time interval of a day and during different days of a month respectively. Water temperature data were recorded for 15 days at different time of interval and average value were taken to analysis the data. To determine surface water evaporation,40days of data were taken within 24hours interval and average value were exploited to analysis it. Data were analyzed using SPSS version20 software and MS excel 2016 spreadsheet to determine correlation and regression analysis respectively. Therefore, the study concluded that: turbidity had direct strong positive relation with water temperature at 9:00 and 13:00 observation hours and strong negative relation at 17:00 observation hours. The spearman`s rank correlation coefficient(r) value at 9:00 and 13:00 was +1 and at 17:00 was -1. The tests statistic values (F, 𝑅 2 and p) which obtained by simple linear regression and ANONA were given as: F (1, 8) = 126.730, 28.989 and 219.301, 0.965, 0.784 and 0.941 and P <.001, P=.001and P <.001 respectively for the three observation hours sequentially (9:00,13:00 and 17:00), water temperature decrease vertically from the top to the bottom layer of the pools which revealed by box and whisker plot, in which there is greater extinction of sunlight in most turbid than least turbid pool with the difference in water temperature of 9.78°C and 1.53°C for both water samples and F (2, 27) = 39.587 and p <.001 obtained by univariate general linear model, turbidity had direct strong positive relation with reservoir water evaporation with spearman`s rank correlation coefficient(r) value of +1. The test statistics attained from simple linear regression and ANOVA were given as: F (1, 8) =14443999.960, p<.001 and 𝑅 2 = 1 and turbidity has an impact on the water stored in a reservoir by engendering storage change. The difference in volume of the water evaporated from a reservoir for most turbid (226NTU) and least turbid (43.7NTU) water was, 65.812 cubic meters. The value of coefficient of determination was ( 𝑅 2 ) = 1, which obtained from simple linear regression. All tests were statistically significant. The results revealed that, if reservoir water turbidity increase, it immensely affects the quantity of the water stored in the reservoir by causing significant loss of the water as water vapor