etude de la qualite des eaux d'un hydrosysteme fluvial ... - LTHE

NH 4 at three different temperatures (20, 25 and 30 [°C]). It thus proves that the cause of lowDO in the downstream Nhue river in summer 2003 is due to increase of temperature.Figure 3.1.17: Evolution of DO at differenttemperatures in average discharge conditionFigure 3.1.18: Evolution of NH 4 at differenttemperatures in average discharge conditionThis simulation also shows that the temperature causes large change on the DO evolution.Precisely, at point NT1, the simulated DO is 5.2, 4.3 and 3.0 (mg O 2 /l) at 20, 25 and 30 (°C),respectively. Another simulation performed at 25°C (normal condition) assuming total closureof the To Lich yields to an increase of DO level from 4.3 to 5.4 (mg O 2 /l) only. It means thatbetween the first and the second scenarios; (the first: the To Lich effluence is close andtemperature of 30°C; the second: the To Lich effluence is open and temperature of 25°C), theDO level in the first one is about 0.2 mg (O 2 /l) lower than the DO at the second one. That isgenerally coincident with our experimental results represented in the figure 3.1.16.Different from the high sensibility of DO to temperature, the sensibility of NH 4 to temperatureis much feebler than its sensibility to the effluence from the To Lich river. It indicates thatwhen the To Lich effluence is close, the NH 4 drops significantly. This sensibility observationis proved by the experimental results (figure 3.1.15).184