The hydrologic cycle has received considerable attention in recent years with particular interest in the dynamics of land–atmosphere exchanges as it relates to global climate change and the need for more accurate numbers in global circulation models (GCMs). Recent advance in remote sensing and operational weather forecasts have significantly improved the ability to monitor the hydrologic cycle over broad regions (Vörösmarty and Peterson, 2000). The application of hydrologic models in understanding interactions between the watersheds and estuaries is critical when examining seasonal changes in the biogeochemical cycles of estuaries. Water is the most abundant substance on the Earth’s surface with liquid water covering approximately 70% of the Earth. Most of the water (96%) in the reservoir on the Earth’s surface is in the global ocean. The remaining water, predominantly stored in the form of ice in polar regions, is distributed throughout the continents and atmosphere—estuaries represent a very small fraction of this total reservoir as a subcomponent of rivers. Water is moving continuously through these reservoirs. For example, there is a greater amount of evaporation than precipitation over the oceans; this imbalance is compensated by inputs from continental runoff. The most prolific surface runoff to the oceans is from rivers which discharge approximately 37,500 km3 y−1 (Shiklomanov and Sokolov, 1983). The 10 most significant rivers, in rank of water discharge, account for approximately 30% of the total discharge to the oceans (Milliman and Meade, 1983; Meade, 1996). The most significant source of evaporation to the global hydrologic cycle occurs over the oceans; this occurs nonuniformly and is well correlated with latitudinal gradients of incident radiation and temperature. The flow of water from the atmosphere to the ocean and continents occurs in the form of rain, snow, and ice. Average turnover times of water in these reservoirs can range from 2640 y in the oceans to 8.2 d (days) in the atmosphere (Henshaw et al., 2000; table 3.1). The aqueous constituents of organic materials, such as overall biomass, have an even shorter turnover time (5.3 d). These differences in turnover rate are critical in controlling rates of biogeochemical processes in aquatic systems.
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