Has the Three-Gorges Dam made the Poyang Lake wetlands wetter and drier?

The Three-Georges Dam holds many records in the history of engineering. While the dam has produced benefits in terms of flood control, hydropower generation and increased navigation capacity of the Yangtze River, serious questions have been raised concerning its impact on both upstream and downstream ecosystems. It has been suggested that the dam operation intensifies the extremes of wet and dry conditions in the downstream Poyang Lake, and affects adversely important local wetlands. A floodgate has been proposed to maintain the lake water level by controlling the flow between the Poyang Lake and Yangtze River. Using extensive hydrological data and generalized linear statistical models, we demonstrated that the dam operation induces major changes in the downstream river discharge near the dam, including an average "water loss". The analysis also revealed considerable effects on the Poyang Lake water level, particularly a reduced level over the dry period from late summer to autumn. However, the dam impact needs to be further assessed based on long-term monitoring of the lake ecosystem, covering a wide range of parameters related to hydrological and hydraulic characteristics of the lake, water quality, geomorphological characteristics, aquatic biota and their habitat, wetland vegetation and associated fauna.

Hydrological Importance and Water Quality Treatment Potential of a Small Freshwater Wetland in the Humid Tropics of Costa Rica

Manon Bachelin

Rapid increases in population and growing food demand are causing widespread deterioration of tropical wetlands globally, and an increased focus on the role and function of these imperiled ecosystems is required. Objectives of this study were to investigate the hydrological dynamics and water quality treatment potential of a small freshwater wetland in the humid tropics of Costa Rica. High-resolution, spatially distributed surface water and meteorological data were combined with a detailed topographical survey to quantify the wetland water balance, hydroperiod, and seasonal variability of wetland area, volume, and residence time. The water balance was dominated by precipitation and outflow, with little contribution from runoff, except during the largest storms. Over 80% of the wetland was flooded continuously; hydroperiods in remaining areas were bi-modal. Small seasonal variations in wetland area, volume, and residence times yielded high and sustained water quality treatment potential. Potential pollutant removal efficiencies were 63.6-99.8% for biological oxygen demand; 60.0-99.8% for total suspended solids; 51.1-98.5% for total nitrogen; and 34.2-99.7% for total phosphorous. The study provides insights into the hydrological functions of this and similar small Central American wetlands and provides a template for extending in-depth hydrological monitoring to other tropical wetland sites.

2011

Evolution of stream and lake water temperature under climate change

Damien Bouffard, Wolf Hendrik Huwald, Adrien Michel, Carl Love Mikael Råman Vinnå, Bettina Schaefli, Martin Schmid, Alfred Johny Wüest

This report presents past observations and projects the future development of water temperature in Swiss lakes and rivers. Projections are made until the end of the 21st century using the CH2018 climate scenarios. Besides climate change effects on temperature, we also discuss effects on discharge for rivers, and effects on the thermal structure, and specifically the seasonal mixing regime and ice cover of lakes. Observations over the past 40 years show a clear increase in river temperatures, with a mean trend of 0.33 ± 0.03 °C per decade, corresponding to ~80% of the observed air temperature trend. This warming has been continuous over the last four decades and impacts the health of stream ecosystems (e.g. by favouring the spread of fish diseases) and their services (e.g., the water usage for industrial cooling). The temperature rise is more pronounced in the Swiss Plateau than in the Alps, where snow and glacier melt partially mitigates (for now) the effects of increasing air temperature. Conversely, annual average discharge shows no significant trend. Similar trends have also been reported for Swiss lakes with mean summer lake surface temperature increasing by 0.40 ± 0.08 °C per decade since the 1950s. This warming trend affects lake stratification. Warm periods may for instance increase the occurrence of deep-water anoxic conditions, as observed during the 2003 heat wave in Lake Zurich. In mild winters, ice cover duration is reduced in alpine lakes, and winter deep mixing is less intense in large-peri-alpine lakes. The mild winter 2006/7 limited, for instance, the seasonal mixing of Lake Constance to about 60 m depths. Effects of warming on lake thermal structure vary within and between regions, due to both lake and watershed characteristics as well as regional climate change patterns. We simulated the future evolution of stream temperature for 10 catchments in Switzerland for a historical reference period (1990–2000) and two future periods: 2055–2065 (mid-century) and 2080–2090 (end of the century). Results show that the temperature will stabilize by the end of the century for the RCP2.6 scenario (strong CO2 emission reduction), whereas the warming will accelerate with time for the RCP8.5 scenario (business as usual scenario). This expected warming will have significant impacts on the stream ecosystems. Alpine and lowland catchments will experience a similar annual mean temperature increase but display different seasonal effects. While Swiss Plateau rivers will become warmer both in winter and summer (but more in summer), alpine rivers will experience almost no warming in winter but a strong warming exceeding that of air temperature in summer. This is explained by an abrupt decrease in discharge, and by the soil warming resulting from the absence of snow and thus a lower albedo. Lake temperature projections are based on one-dimensional, vertically resolved, hydrodynamic simulations for 29 lakes. The simulated lakes cover a wide range of sizes, depths and water quality, and an altitude range from 200 to 1800 m a.s.l. Simulations indicate substantial changes in lake thermal structure for RCP8.5 with surface temperatures increasing on average by 3.3 °C at the end of the 21st century. This increase is limited to 0.9 °C in the mitigation scenario RCP2.6. We identified an altitude-dependent evolution of the durations of summer and winter stratification as well the ice-covered period. Larger changes in stratification duration are expected to occur at higher altitude lakes. Yet, these lakes will still maintain winter stratification and a shortened icecovered period while lower altitude lakes (below ~1500 m a.s.l.) risk drastic changes in the mixing regime. e.g., a complete loss of the ice cover and winter stratification under the RCP8.5 scenarios. Such changes in the mixing regime may strongly impact lake ecosystems. These low to mid altitude lakes may therefore be considered as the most vulnerable to climate change.

2021

Seasonality of density currents induced by differential cooling

Damien Bouffard, Tomy Doda, Hugo Nicolás Ulloa Sánchez, Alfred Johny Wüest

When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as a thermal siphon (TS). TSs flush the littoral region and increase water exchange between nearshore and pelagic zones; thus, they may potentially impact the lake ecosystem. Past observations of TSs in lakes are limited to specific cooling events. Here, we focus on the seasonality of TS-induced lateral transport and investigate how seasonally varying forcing conditions control the occurrence and intensity of TSs. This research interprets 1-year-long TS observations from Rotsee (Switzerland), a small wind-sheltered temperate lake with an elongated shallow region. We demonstrate that TSs occur for more than 50 % of the days from late summer to winter and efficiently flush the littoral region within ∼10 h. We further quantify the occurrence, intensity, and timing of TSs over seasonal timescales. The conditions for TS formation become optimal in autumn when the duration of the cooling phase is longer than the time necessary to initiate a TS. The decrease in surface cooling by 1 order of magnitude from summer to winter reduces the lateral transport by a factor of 2. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge, and flushing timescale to the surface buoyancy flux, mixed-layer depth, and lake bathymetry. The timing and duration of diurnal flushing by TSs relate to daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing relative to the summer diurnal cycle. Given their scalability, the results reported here can be used to assess the relevance of TSs in other lakes and reservoirs.