The hydrological cycle is a closed system with stores, fluxes and processes. It is driven by solar energy and gravitational potential energy. There is a fixed, constant, finite volume of water with inputs, outputs, stores and flows.

During the ice age, water was help in the cryosphere. Climate change has changed this so now nearly 97% of water is held i the oceans, 1.9% are in ice caps and 1.1% is stored as groundwater. Blue water is on land, green water is in vegetation. Out of 97% in the ocean, 2.5% of that is freshwater, 69% of that freshwater is locked in ice and glaciers, 30% of that is deep fossil water so 1% of freshwater is left accessible to us.

Cumbria is the wettest region of England. It gets 2000mm of rain a year. Limestone becomes impermeable when joints or bedding planes fill with water which leads to floods as rivers are saturated. Precipitation acts as the input, evaporation is the output and the rest depends on Lithology and water quantity.

The global water budget is the balance of fluxes and the size of stores. Fossil water can be extracted from aquifers but it won’t recharge quick enough in some places. Residence time is how long the water spends in a store or reservoir such as ice cores and groundwater. The residence time in soil and rivers is less as it is exposed to humans and the sun. Atmospheric pressure is a short store too because it condenses and falls as precipitation.

Stores with longer residence times are more easily polluted because they sit there for a longer time.

The tropics evaporate much of the sea because it is hot so lots of rainfall falls there. They have high rates of evaporation. Trade winds transfer water to the intertropical convergence zone where convection currents cool and condense it so it falls as heavy rainstorms.

 In the tropical rainforest, there are less seasonal differences, vegetation is a store all year round, ½ precipitation returns back to the atmosphere with evapotranspiration, it recycles its own rain and they get 2000mm of rainfall a year.

 Polar regions have freeze thaw seasons which release gas and carbon, solar radiation is reflected off the snow, permafrost is impermeable, there is little stores in vegetation, rapid surface run off in summer, a cryosphere store and less than 200mm of annual precipitation. Oceans evaporate more than they precipitate and land precipitates more than it evaporates.

Much of the cryosphere is melting due to global warming. Thermohaline circulation is the way in which cooler polar water is more saline so it sinks below the hot tropical water which draws it from the surface above. In turn, this draws water from the tropics and pulls cold water up to be warmed.

In rainy seasons there are more transfers and stores than summer where there are monsoons and evapotranspiration but fewer stores. Winter has snow but less evaporation so we tend to have more groundwater.


No comments have yet been made