| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Surface water > Lake levels and salinity |
Brief Description: Lakes are dynamic systems that are sensitive to local climate and to land-use changes in the surrounding landscape. Some lakes receive their water mainly from precipitation, some are dominated by drainage runoff, and others are controlled by groundwater systems. On a time scale ranging from days to millennia, the areal extent and depth of water in lakes are indicators of changes in climatic parameters such as precipitation, radiation, temperature and wind speed. Lake level fluctuations vary with the water balance of the lake and its catchment, and may, in certain cases, reflect changes in shallow groundwater resources. Especially useful as climatic indicators are lakes without outlets (endorheic), widely distributed in North America, Africa, Central Asia, Middle East, and Australia. In arid and semi- arid areas, the levels and areas of lakes with outflows are also highly sensitive to weather. Where not directly affected by human actions, lake level fluctuations are excellent indicators of drought conditions. For example, lake levels throughout the southern prairie provinces of Canada dropped in response to the warm, dry weather of the 1980s. Ephemerally or seasonally-flooded lake basins (playas) are dynamic landforms, the physical character and chemical properties of which reflect local hydrologic changes, and which react sensitively to short-term climate changes (e.g. rate of evaporation). Fluctuations in lake water salinity (e.g. CaHCO3, MgHCO3, CO3, MgSO4, NaSO4) also provide an indication of changes in conditions at the surface (climate, inflow/outflow relations) and in shallow groundwater. Significance: The history of fluctuations in lake levels provides a detailed record of climate changes on a scale of ten to a million years. Lakes can also be valuable indicators of near- surface groundwater conditions. Human or Natural Cause: Natural, but can be influenced by human-induced climate change, and by engineering works, such as dams and channels. For example, as a result of diversion into irrigation projects of rivers that flowed into the formerly stable Aral Sea between Kazahkstan and Uzbekistan, the volume and extent of this huge inland lake has been dramatically reduced: between 1960 and 1989 its level dropped by 14 m, its volume decreased by 68%, and its salinity tripled. Environment Where Applicable: Arid and semi- arid regions, continental mid- latitudes and tropical and subtropical latitudes. Types of Monitoring Sites: Shallow and, in particular, saline and hyposaline lakes in sand- rich basins (i.e. where groundwater responds rapidly to climate), ideally located along vegetation and elevation transects that include agricultural and non- agricultural settings. Spatial Scale: Patch to mesoscale / regional Method of Measurement: Lake levels are generally measured with shoreline gauges. Areal extent is assessed primarily using successive air photos, supplemented with ground- level surveys, radar altimetry, and satellite images. Salinity is measured by standard analytical means. Past variations in levels and salinity can be recognized by studying old shorelines, lake-side archaeological sites, and the geochemistry, mineralogy, isotopic composition and fossil content of sediment cores. Remains of diatoms, chrysophytes, chironomids, ostracods and other bio-indicators in lake sediments are widely used to infer past lakewater salinity. Frequency of Measurement: Lake level and lake water composition monthly to annual. Areal extent every 5 years. Limitations of Data and Monitoring: Limited by availability of gauge data, resolution of photographic and satellite images, and by climatic records for baseline data. Applications to Past and Future: Good index of water balance and changes in precipitation and evaporation. Records of lake dynamics in historic and pre- historic periods provide baseline data on past responses to climate change. With the establishment of threshold values, lakes may provide an early warning of shallow groundwater depletion. Possible Thresholds: When evaporation exceeds precipitation, as in semi-arid environments, lake area and salinity can change markedly. The utility of lakes as sources of water for human use depends on water availability and quality: thresholds for human health can be rapidly crossed as chemical concentrations (salinity) increase with evaporation. |
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