INTRODUCTION - Hydrology: An Introduction - page 4

Table 1.1 Estimates of world water balance (m y -1 )

Land Oceans

(1.49 × 10 8 km 2 ) (3.61 × 10 8 km 2 )

Global

Reference P R E P E P = E

Budyko (1970, 1974) 0.73 0.31 0.42 1.14 1.26 1.02

Lvovitch (1970) 0.73 0.26 0.47 1.14 1.24 1.02

Lvovitch (1973) 0.83 0.29 0.54 − − −

Baumgartner and
Reichel (1975)

0.75 0.27 0.48 1.07 1.18 0.97

Korzun et al . (1978) 0.80 0.315 0.485 1.27 1.40 1.13

some of the calculated values and, within certain limits, they provide a useful idea of the
long-term average balance in different climatic regions of the world.
As shown in Table 1.1, the average annual precipitation and also evaporation are of
the order of 1 m for the entire Earth. Over the landsurfaces the average precipitation
intensity, P , is about 0.80 m y − 1 , whereas the corresponding average evaporation, E ,
is around 0.50 m y − 1 or about 60% to 65% of the precipitation. Under steady condi-
tions, that is for long time periods, the remainder can be considered to be runoff from
the landsurfaces into the oceans, R (expressed as height of water column per unit of
time), or

R = P − E (1.1)

Averaged over all continents and over long time periods, the annual runoff R is therefore
around 35% to 40% of the precipitation. Except for South America and Antarctica (see
Table 1.2), the values for the individual continents are not very different from the global
values. Precipitation and streamflow runoff measurements have been and are being made
routinely in many places on Earth. In contrast, evaporation has not received as much
attention and no systematic measurements are available.
Estimates of the average distribution of water in different forms expressed as depth of
water covering the globe, assumed to be a perfect sphere, are given in Table 1.3. These
indicate that the1mofaverage annual precipitation is relatively large as compared to
the active fresh water on Earth, that is the water which is not stored in permanent ice
and deep groundwater. This means that the turnover of the active part of the hydrologic
cycle is rather fast, and that the residence times in some of the major compartments of
the water cycle are relatively short; the mean residence time can be taken as the ratio
of the storage and the flux in or out of storage. For example, a continental runoff rate of
0.30 m y − 1 (Table 1.1) and a storage in the rivers of (0.003/0.29) m of water on the 29%
of the world occupied by land, gives a mean residence time of the order of 13 days for
the rivers of the world. Similarly, a global evaporation rate of1my − 1 , with 0.025 m of
storage in the atmosphere, leads to a mean residence time of the order of 9 days for the