Atmospheric water and solar energy provide the main inputs for the generation of precipitation, which falls over the land and oceans. Rainfall can either infiltrate into the soil system, percolate to deeper ground water, evaporate or transpire through vegetation back to the atmosphere, or run into the nearest streams or rivers. Infiltrating water is the main source of recharge to the root zone and groundwater aquifers below. Rivers can also recharge aquifers or can act as discharge points for aquifer outflows. The ocean is the ultimate receptor of surface and groundwater contributions from surrounding land masses and provides the main source of water for evaporation back to the atmosphere Table 1. 1 shows the distribution of water throughout the earth. Clearly, the oceans and icecaps dominate as sources of water Table 1.1 Distribution of worlds estimated water resources Location Surface w ater Percentage Equivalent Residence area (km) Volume of total (% depth(m) ti x10° (km)x 10 Surface water Lakes and1.55 0.13 0.01 0.25 reservoirs Swamps <0.1 <0.01 007 years River channels <0.1 01 <0.01 0.003 2 weeks Subsurface water Soil moisture 130 0.07 0.01 Groundwater 130 10,000 years Icecaps and 17.8 10-1000 glaciers year Atmosphere 504 0.01 0.025 10 days Oceans and 361 1370 2500 4000 years seas Total 1460 100 2680 According to these figures, the present amount of free water on earth equals 1460 million km corresponding to a layer 2680 m thick over the whole earth surface. It seems that the supply of water on earth is nearly inexhaustible. However, a completely different picture obtained when water qual ity is taken into account Of the total amount of free water in earth, more than 94% is salt water. which cannot be used for agriculture or for domestic and industri consumption. The water present as snow or ice(2%), and as water vapour in the atmosphere (.001%), cannot be used for these purposes either, and so, for the use of our society only theAtmospheric water and solar energy provide the main inputs for the generation of precipitation, which falls over the land and oceans. Rainfall can either infiltrate into the soil system, percolate to deeper ground water, evaporate or transpire through vegetation back to the atmosphere, or run into the nearest streams or rivers. Infiltrating water is the main source of recharge to the root zone and groundwater aquifers below. Rivers can also recharge aquifers or can act as discharge points for aquifer outflows. The ocean is the ultimate receptor of surface and groundwater contributions from surrounding land masses and provides the main source of water for evaporation back to the atmosphere. Table 1.1 shows the distribution of water throughout the earth. Clearly, the oceans and icecaps dominate as sources of water. Table 1.1 Distribution of World's estimated water resources Location Surface area (km2 ) x 106 Water Volume (km3 ) x 106 Percentage of total (%) Equivalent depth (m) Residence time Surface water Lakes and reservoirs 1.55 0.13 <0.01 0.25 -10 years Swamps <0.1 <0.01 <0.01 0.007 1-10 years River channels <0.1 <0.01 <0.01 0.003 -2 weeks Subsurface water Soil moisture 130 0.07 < 0.01 0.13 2 weeks – 1 year Groundwater 130 60 4 120 2 weeks – 10,000 years Icecaps and glaciers 17.8 30 2 60 10 – 1000 years Atmosphere 504 0.01 <0.01 0.025 -10 days Oceans and seas 361 1370 94 2500 -4000 years Total 1460 100 2680 Source: Nace, 1971 According to these figures, the present amount of free water on earth equals 1460 million km3 corresponding to a layer 2680 m thick over the whole earth surface. It seems that the supply of water on earth is nearly inexhaustible. However, a completely different picture obtained when water quality is taken into account. Of the total amount of free water in earth, more than 94% is salt water, which cannot be used for agriculture or for domestic and industrial consumption. The water present as snow or ice (2%), and as water vapour in the atmosphere (.001%), cannot be used for these purposes either, and so, for the use of our society only the 2