Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 7 7Map 4.2: Mean annual precipitation0 50 250 500 750 1,000 2,000 and more(in mm/year)The world pattern of precipitation shows strong disparities between large annual rainfall in the tropics (some areas get in excess of 10,000 mm), and semi-arid and aridregions (such as the Sahara Desert). Differences within the African continent are particularly significant.Source: Map prepared for the World Water Assessment Programme (WWAP) by the Centre for Environmental Research, University of Kassel, based on an Analysis by the Global Precipitation ClimatologyCentre (GPCC) (data extracted from the GPCC website in 2002 and Rudolf et al., 1994).where totals are small. The world’s deserts and semi-deserts arelocated in these areas, some vast such as the Sahara Desert, othersvery local in nature. In certain arid regions there may be no rain forseveral years, in marked contrast to locations where heavyprecipitation occurs virtually every day and annual totals areenormous, for example in Hawaii, United States (11,000 mm). Suchextremes in precipitation give rise to floods on the one hand anddroughts on the other, with few parts of the globe left immune: infact, deserts can experience flash floods, while humid areas maysuffer from prolonged droughts.Shiklomanov (1998a) estimates the total precipitation on theland surface to be 119,000 cubic kilometres (km 3 ) per year withother estimates ranging from 107,000 to 119,000 km 3 .EvaporationThe pattern of evaporation is conditioned by the availability ofwater to evaporate. Where water is readily available, such as in anopen water surface, evaporation is at the potential rate and onlyrestricted by atmospheric conditions. Where water is in limitedsupply, in an arid area for example, the actual rate of evaporationfrom the land surface is much lower than the potential. In generalterms, potential evaporation rates are highest in the arid subtropics(more than 2,000 mm per annum), decreasing poleward to about500 mm at latitude 50° and also decreasing with altitude. Actualrates are highest in the tropics and in mid-latitudes where largeprecipitation totals ensure a plentiful supply of soil moisture.Evaporation from the land surface is estimated by Shiklomanov(1998a) to be 74,200 km 3 a year, with the lowest of otherestimates being 70,000 km 3 .Soil moistureThe soil acts as a significant reservoir when it is well developed,partitioning precipitation between runoff and infiltration andreleasing water for plant growth. Soil moisture storage is dependenton a number of factors in addition to precipitation and evaporation:factors such as soil type, soil depth, vegetation cover and slope. Theconsequence is that even within a small basin, the pattern of soilmoisture can be very heterogeneous. Consequently, the best guideto the global distribution of soil moisture storage may be thebalance between precipitation and evaporation. This balance has amarked seasonal pattern over much of the world within the top partof the soil profile. This dries during the summer and returns to a