Impact of drainage operation on water quality of Daourat Reservoir
The Oum Rbia River is the longest river in Morocco with a total length of about 550 km. Studies conducted on this river have shown that agricultural activity is the main factor affecting the water quality. The current study focused on Daourat Reservoir, a part of the Oum Rbia River. The aim of this study was to investigate the water quality of the Daourat reservoir (physico-chemical parameters, nutrients and chlorophyll a) in order to estimate its eutrophication status. The influence of draining reservoir on water quality was also analysed. This survey also included a regression analysis that permitted the determination of the relationship between phosphorus and chlorophyll a, as well as an assessment of the level of eutrophication of the Daourat reservoir. A monthly survey was conducted to monitor changes in the hydrological cycle as well as possible seasonal variations during the period from 2001-2002. Water parameters (T°, pH, dissolved oxygen, suspended solids), nutrients (NH4 +, NO3-, P-PO4, Tot-P) and chlorophyll a were determined according to standard French protocols (AFNOR, 1994). Annual nutrient inputs and outputs were calculated from the monthly discharge from the river and the monthly mean nutrient concentrations. The annual nutrient output was calculated similarly. The physico-chemical parameters and nutrient concentrations varied temporally in the Daourat Reservoir. The highest concentrations of nutrients were observed during summer and winter, with the lowest concentrations occurring in the spring. This evolution seemed to be influenced by the nature of the loads. Nutrient loading from the upstream regions of the reservoir was minimal compared to the contributions from the local catchment area and from internal loading from the sediments. Significant nutrient loading to the reservoir came from sediment resuspension on December 23, 2001 and by internal loading from sediments during the summer. The upstream reservoirs (Al Massira and Imfout) had minimal influence and were in fact responsible for the reduced fluvial contributions to the Daourat Reservoir. The increase in nutrient loads in the reservoir was responsible for high phytoplankton biomass, indicated by the high chlorophyll a values (mean 35 μg/L). The trophic state of the Daourat Reservoir was classified as hypereutrophic. Due to the large intra-annual variability of water quality, a properly designed monitoring program of reservoir water is recommended.