Reservoir phosphorous sources evaluation and water quality modeling in a transboundary watershed

The Alqueva dam, constructed in the Portuguese section of the Guadiana River, is the centre of a multipurpose project under development in Alentejo, Portugal, which includes strategic water storage, hydropower production, irrigation and urban water supply. Aiming to adequately evaluate nutrient control strategies along the Alqueva watershed in Portuguese territory, total phosphorous (P) quantification methodologies were implemented for urban, industrial and diffuse (nonpoint) sources and used as input to the two-dimensional hydrodynamic and water quality model CE-QUAL-W2. As a first approach two scenarios of pollution reduction in Portuguese territory were simulated: 90% reduction of urban waste water pollution sources (scenario 2) and 100% reduction of all pollution sources (scenario 3). According to the results of the ongoing study, even a theoretical 100% reduction of the pollution loads originated in Portuguese territory would not insure an overall satisfying water quality in the Alqueva reservoir. Nevertheless, simulation results show not only that urban waste water discharges are yet a significant contribution for unsatisfactory water quality, but also that the overall contribution from the Portuguese area may be an important factor on biomass control. Also, water quality deterioration has been observed along reservoir branches, which are probably only affected by loads originated in Portuguese territory.

The presented study aimed to evaluate the influence of nutrient loads originated in Portuguese territory on the water quality of the Alqueva reservoir, a major water source for most of the southern area of Portugal (4150 hm3 storage capacity). The Alqueva dam is located in the Guadiana river, a transboundary river with 83% of its basin located in Spanish territory. The construction of the Alqueva dam (started in 1998) and the recent flooding of the Alqueva reservoir (2002) have necessarily enhanced the need for a deeper understanding of all water quality issues related, towards the achievement of better water quality conditions. It is a known fact that meteorological conditions observed in the region of the Alqueva reservoir — Alentejo — are most unfavorable regarding water quality: high temperatures, irregular precipitation and long reservoir retention times. Even though severe water quality problems were expected to occur in the Alqueva reservoir, based on the knowledge of other reservoirs in the Alentejo region, much was left to be done Fig. 7. Simulated and observed values of PO4 at the Alqueva–Mourão monitoring station.