Topic 3.6

Water Resources

The Water Cycle

Water Budget• 70% of the Earths surface is covered by water• Only 3% of this is drinkable• 69% of this 3% is locked up in polar icecaps• The atmosphere only holds 0.3% of the total

water• We consider water to be a replenishable rather

than a renewable resource

Water Budget• Some water is more easily replenishable than others• Turnover time is the time it takes for a drop of water to leave and then

return to the same part of the systemLocation Typical Turnover TimePolar Ice caps 10 000 yearsIce in permafrost 10 000 yearsOceans 2 500 yearsGroundwater 1 500 yearsMountain glaciers 1 500 yearsLarge lakes 17 yearsBogs 5 yearsUpper soil moisture 1 yearRivers 12 daysAtmosphere 12 daysLiving things 16 days

Sustainability

• The use of global resources at a rate that allows replenishment and minimises damage to the environment

• Water use in MEDCs and LEDCs are increasing (due to population increase and greater per capita need)

• In MEDCs water is exploited for many non-essential uses, such as:– Washing cars– Watering gardens– Frequent bathing– Swimming pools

Case Study – Mexico City• Mexico City is in a closed basin and has a population of around 21.4

million people• Water availability is around 165m3/inhabitant/year• Most of this is extracted from groundwater (which is overexploited

by 120%)• Ground water availability is decreasing and there is a big problem of

subsidence, especially in Centro Historico• The government is trying to regulate water use

– Car washes and watering public green spaces use secondary-treated wastewater

– The remaining wastewater is reused by being channeled to the Tula Valley where it is used on the largest wastewater-irrigated area in the world

– Here is recharges the local aquifer and brings huge economic benefits. However there is also a risk of local health problems