Introduction to Physical Hydrology

Richard J. Chorley

• Publisher: Taylor & Francis
• Year: 2019
• eBook ISBN: 9781000007077
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

Originally published in this form in 1971, the content of this book was originally part of a larger composite volume 'Water, Earth and Man' (1969) which provided a synthesis of hydrology, geomorphology and socio-economic geography. This volume brings together the systematic theme of physical hydrology while maintaining a link with the original book which emphasised the benefit of the study of water being considered in the widest sense within the physical and social environments.

1.I. The World Hydrological Cycle

R. G. BARRY

Institute of Arctic and Alpine Research, University of Colorado

1. Global water and the components of the hydrological cycle

We begin our consideration of water in the global context with some figures to illustrate the storage capacity of the earth-atmosphere system. The oceans, with a mean depth of 3.8 km and covering 71% of the earth's surface, hold 97% of all the earth’s water (1.31 × 10²⁴ cm³). 75% of the total fresh water is locked up in glaciers and ice sheets, while almost all of the remainder is ground water. It is an astonishing fact that at any instant rivers and lakes hold only 0.33% of all fresh water and the atmosphere a mere 0.035% (about 12 × 10¹⁸ cm³).

Fig. 1.I.1 The global hydrological cycle and water storage.

The exchanges in the cycle are referred to roo units which equal the mean annual global precipitation of 85.7 em (33.8 in.). The storage figures for atmospheric and continental water are percentages of all fresh water. The saline oceans make up 97% of all water (From More, 1967).

Fig. 1.I.2 Annual evaporation in em (After Budyko et al., 1962).

In hydrological studies the primary focus of interest is the transfer of water between these stores (fig. 1.I.1). The exchanges of water involved in the various stages of the hydrological cycle are evaporation, moisture transport, condensation, precipitation, and run-off. The global characteristics of these components will now be examined to provide a framework for the discussion in subsequent chapters.

2. Evaporation

Evaporation (including transpiration) provides the moisture input into the atmospheric part of the hydrological cycle and may be taken as our starting-point. The oceans provide 84% of the annual total and the continents 16%. Figure 1.I.2 shows the general pattern, although the magnitudes are only to be regarded as approximate in view of our present limited knowledge concerning evaporation. The highest annual losses, exceeding 200 cm, occur in the sub-tropics of the western North Atlantic and North Pacific, where evaporation over the respective Gulf Stream and Kuro Shio Currents is very pronounced in winter, and in the trade-wind zones of the southern oceans. The land maximum occurs primarily in equatorial regions in response to high solar radiation receipts and the growth of luxuriant vegetation. It is noticeable that amounts over land are two-three times less than over the oceans in equivalent latitudes. The factors which determine evaporation rates are discussed fully in Chapter 4.1.

3. Atmospheric moisture

The atmospheric moisture content, comprising water vapour and water droplets and ice crystals in clouds, is determined by local evaporation, air temperature, and the horizontal atmospheric transport of moisture. The cloud water may be ignored on a global scale, since it amounts to only 4% of atmospheric moisture.

Air temperature sets an upper limit to water-vapour pressure – the saturation value (i.e. 100% relative humidity) – consequently we may expect the distribution of mean vapour content to reflect this control (fig. 1.I.3). In January minimum values of 0.1–0.2 cm (equivalent depth of water) occur in continental interiors and high latitudes, with secondary minima of 0.5–1 cm in tropical desert areas. Maximum vapour contents of 5–6 cm are over southern Asia during the summer monsoon and over equatorial latitudes of Africa and South America.

The average water content of the atmosphere is about 2.5 cm (1 in.), which is sufficient only for some ten days’ supply of rainfall over the earth as a whole. Clearly, a frequent and intensive turnover of moisture through evaporation, condensation, and precipitation must occur. While atmospheric moisture is essential for precipitation, the relationship between these two items is determined by the efficiency of rain-producing weather systems (Chapter 3.1.2) in any particular climatic region. For example, observations show that on average only 5% of the water vapour crossing Illinois is precipitated there, and in the case of the Mississippi basin only about 20%.

4. Precipitation

The major types of precipitation are drizzle, rain, snow, and hail, although dew, fog drip, hoar frost, and rime may also make significant contributions to the total (see Chapter 3.1). The distribution of summer and winter precipitation is shown in fig. 1.I.4. The least reliable parts of the maps are the oceans, especially in the southern hemisphere. Analysis of precipitation frequency at North Atlantic weather ships suggests that previous estimates of annual totals in the north-western sector are 20–50 cm too low, while around 45^° N, 15^° W they are 40–50 cm too high.

The patterns reflect many complex weather factors and geographical influences, such as topography and the land-sea distribution, but the most significant features are:

1. The ‘equatorial’ maximum, which is displaced into the northern hemisphere. This is related primarily to the converging trade-wind systems and monsoon regimes of the summer hemisphere, particularly i...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Original Title Page
• Original Copyright Page
• Contents
• Preface
• Acknowledgements
• Introduction
• 1.I. The World Hydrological Cycle
• 2.I. The Basin Hydrological Cycle
• 2.II. The Drainage Basin As The Fundamental Geomorphic Unit
• 3.I(i). Precipitation
• 3.I(ii). The Assessment of Precipitation
• 4.I. Evaporation And Transpiration
• 4.II. Soil Moisture
• 5.I. Infiltration, Throughflow, And Overland Flow
• 6.I. Ground Water
• 7.I. Open Channel Flow
• 8.I. The Hydrology Of Snow And Ice
• 9.I. The Flood Hydro Graph
• 10.I. River Regimes
• 11.I. Long-Term Precipitation Trends
• Index