Impacts of Climate Change on Water Quality

In this chapter we present the result of two model exercises aiming at simulating the impact of climate change onto two classes of surface aquifers: lakes and rivers. Section 10.1 focuses on the impact of global warming on the thermal structure of two Italian South alpine lakes: Lake Como and Pusiano. Long term hydrodynamic simulations (1953-2050) were performed using the hydrodynamic model DYRESM (Dynamic Reservoir Simulation Model). DYRESM simulations were forced with downscaled regional climate scenarios undertaken within CIRCE. Our model simulations projected a yearly average temperature increase of 0.04°C year -1 for the period 1970-2000 and 0.03°C year -1 for the period 2001-2050 (A1b IPCC scenario). These results are in line with those detected in long term research studies carried out world-wide. This temperature increase is fi rst responsible for a general increase of the water column stability and for a reduction of the mass transfer between deep and surface waters with direct implications on the oxygen and nutrient cycles. The magnitude of the temperature increase is also suf fi cient to impact on the growth of phytoplankton populations and it is likely one of the concurrent causes promoting the massive cyanobacteria blooms, recently detected in the two Italian case studies and in different lake environments in Europe. Section 10.2 approaches the problem of establishing a methodology to estimate the average yearly nutrient (phosphorus and nitrogen) river loads under present climate conditions and under the forcing of climate change. The case study is the Po River the largest hydrological basin in Italy and the third tributary of the Mediterranean semienclosed basin. The methodology developed in this study is based on a hierarchy of different numerical models which allowed to feed the MONERIS model (MOdeling Nutrient Emissions into River System) with the necessary meteorological and hydrological forcing. MONERIS was previously calibrated (1990-1995) and validated (1996-2000) under past conditions and then run under current conditions to de fi ne a control experiment (CE). Current nutrient loads have been estimated in 170,000 and 8,000 t year -1 respectively for nitrogen and phosphorus. Approximately 70% of the nitrogen load is from diffuse sources while 65% of the phosphorus load originates from point sources. Nutrient loads projections at 2100 (under different IPCC scenarios) allowed to estimate that both nitrogen and phosphorus loads are strictly dependent on the resident population which is responsible of a 61 and 41% increase respectively for nitrogen and phosphorus. Projected nutrient load variations were found to be negligible when holding the resident population constant. Finally the phosphorus load is markedly in fl uenced by the ef fi ciency of the waste water treatment plants (WWTPs).