Earth's hydrosphere and rational use of water resources

As previously noted, a water cycle occurs within the Earth's hydrosphere. The movement of water takes place in all directions. The distribution of water in the hydrosphere, including its various states of matter, is presented in Table 7.1.

The water surface area is 360.8⋅106 km2, and the average depth of the world's oceans is 3.8 km.

Dissolved substances determine salinity – the mass of dissolved matter in grams per 1000 g of seawater. For example, this is how the chlorinity of seawater is determined. According to the Knudsen formula:

S = 0.03 + 1.805Cl, where S is the salinity of the water and Cl is the chlorinity of the water.

The salinity of ocean waters is determined by several chemical elements – ions. These are cations 2+, Ca 2+, and anions 24−, HCO 3−. The remaining elements account for 4.2 %. The salinity S of ocean water varies within the range of 34 – 36 %. Glaciers are an important link in the water cycle. They are formed in polar and high-mountain regions and are characterized by low mineralization. Continental waters – rivers, lakes, and swamps – are formed in accordance with the local climate and are also diverse in terms of their composition of dissolved salts. At the same time, the concentration ratios of ions are quite constant and are the inverse of those in seawater:

in seawater 2 2 24 3−, in continental waters: Mg 2 2 24 3−.

Groundwater is variably mineralized and characterized by a lack of oxygen. Its upper layers – soil moisture – participate in the water cycle. Groundwater also includes thermal waters, such as geysers. Since Earth's waters are mineralized and constantly migrating, the hydrosphere participates in the cycling of substances (trace elements) in the biosphere.

Form of occurrence Volume, 103 km3 Percentage including active water exchange 4000 0.3

River water 1.2 ∼0.0001

Fresh water makes up 3 % of the mass of all water on Earth. In reality, only 0.001 of the total fresh water is accessible or suitable for human use.

Fresh water consumers

Fresh water is used to meet the domestic needs of the population, as well as by industry and agriculture. A distinction is made between returnable consumption – where withdrawn water is returned to the source (municipal services, industry, reservoirs) – and non-returnable water consumption – where it is used for filtration, evaporation, etc. (mainly in agriculture). Although river water reserves are small (1200 km3 or 0.0001 % of the total hydrosphere volume), it is river water that provides the main volume of water consumed for domestic and industrial purposes, as river waters possess a significant capacity for renewal and self-purification.

The process of urbanization and the intensive development of industry and agriculture lead to a continuous increase in water consumption (it increased more than 7-fold during the 20th century). Fig. 7.1 shows the dynamics of global water consumption by type of economic activity. By the year 2000, 63 % of total global water consumption (or 86 % of non-returnable consumption) was attributed to agriculture. Thus, during the 20th century, the area of irrigated land increased from 47 million hectares to 347 million hectares. A similar situation existed in the former USSR by 1991-92: water consumption for industrial needs accounted for about 30 %, agriculture for 57 %, and municipal services for 8 %. The situation is significantly different in the Russian Federation: in 1991, out of 117 km3 consumed, 53% went to industrial needs, more than 20 % to agriculture, and 15 % to household and drinking water needs.

On many rivers in Russia, water withdrawal has exceeded all permissible norms, amounting to more than 50 % of the average long-term flow (e.g., the Kuban, Ural, and Miass rivers, etc.). On the Don River, water withdrawal reached 64 %. This indicates that the observed degradation of water bodies, especially in the European part of Russia, the Urals, and the Kuzbass, is linked to excessive water withdrawal.

Losses of fresh water. Environmental consequences

As noted above (Section 7.2), the volume of river water accounts for a negligible fraction (0.0001 %) of the hydrosphere's volume. Meanwhile, to this day, human consumption of fresh water is carried out mainly through river runoff. This is understandable: the river network is the most accessible source for fresh water withdrawal, both due to its distribution across the Earth's surface (compared to, for example, lakes, which contain two orders of magnitude more fresh water, but are distributed extremely unevenly) and in terms of energy and other costs associated with extraction (compared to, for example, withdrawing water from underground sources). Furthermore, the waters of the river network are the most dynamic and capable of renewal and self-purification. The annual runoff of the river network into the world's oceans is estimated, on average, at (15-16)⋅103 km3/year. If we compare the global water consumption reached by the year 2000 (5190 km3/year, Fig. 7.1) to the average long-term runoff of the world's river network ((15-16) ⋅103 km3/year), it turns out that the average global water withdrawal is approaching 30% of the world's average river network runoff. Of course, this assessment does not account for water withdrawal from underground sources or lakes (this withdrawal is accounted for in the projected water consumption figure of 5190 km3/year). Therefore, the real ratio of annual water withdrawal to annual runoff will be slightly less than 30%, but it is heading in that direction.

In this situation, it is extremely important to minimize non-recoverable water consumption to the greatest possible extent. That is, such water consumption where a certain share of water withdrawn by a water user, for example, from a river network, is not returned to the river network. Unfortunately, today's technologies in industry, agriculture, and municipal services predetermine significant losses of freshwater. Table 7.2 presents data on total water consumption and non-recoverable losses (in the denominator, with the percentage of losses also shown) for the specified types of economic activity in the territory of the former USSR in 1900, 1980, and 2000.

As follows from Table 7.2, in 2000, in the territory of the former USSR, out of 440 km3 of freshwater consumed, 270 km3 (or 61.4%) was non-recoverably withdrawn, primarily from river runoff. If we assume that in the territory of the former USSR, as on average in the world, water intake constitutes ∼30% of the runoff, then taking into account that non-recoverable losses of freshwater in 2000 were estimated at 61.4%, it turns out that the river network of this territory currently loses 0.614·30% ≈ 18.5% of its runoff non-recoverably. This same figure can, as a first approximation, be applied to the world as a whole.

Total water consumption and non-recoverable losses, in km3/year by type of economic activity in the territory of the former USSR

Agriculture 41.6 130.5 250.5 26.4(63.5%) 60.2(46.1%) 205.2(81.9%)

0.1 (10%) 5(7.1%) 34 (26.15%)

Municipal services 1.6 9.7 34 0.6 (37.5%) 2 (20.6%) 7 (20.5%)

Reservoirs 0 14.6 23.4

0 14.6 (100%) 23.4 (100%) (rounded) 28 (63.6%) 107 (47.5%) 270 (61.4%)

Ecological consequences of water resource scarcity

Of course, these losses are not absolutely non-recoverable for the biosphere. In fact, in the process of water use, part of the water from the river network is converted into atmospheric moisture through evaporation (as, for example, in cooling devices for heated water at thermal and nuclear power plants) or into soil and atmospheric moisture (as during irrigation in agriculture) and, thus, remains in the biosphere.

However, this brings no relief to water bodies, or more precisely, the biota of water bodies, primarily rivers: a decrease of almost 20% in the mass of water in rivers leads to a sharp change in the living conditions of all participants in the ecological processes within these water bodies. Intense simultaneous pollution of river water with municipal, industrial, and agricultural runoff leads to the degradation of the rivers in the river system of the territory of the former USSR.

Characteristic examples:

• the rivers of Central Asia – the Amu Darya and Syr Darya, which were practically "dismantled" for the irrigation of cotton plantations and other economic needs along their path to the Aral Lake-Sea, thereby predetermining the onset of the Aral Sea's demise;
• the Don River, the water intake of which reached 64%, which corresponds to non-recoverable losses of its waters of 40%, and in June-August, which account for more than 60% of the water intake, the legendary quiet Don is barely hanging on.

The impact of water intake on ecosystems and estuaries

With non-recoverable water diversion, the ecological consequences affect more than just the river itself. Swamps along many rivers are drying up or have already dried up, as swamps are fed to a much lesser extent by periodic floods. This leads to the disappearance of a large number of waterfowl and many species of animals and plants that previously inhabited these places.

This also applies to estuaries—bays where freshwater flowing from river mouths into the world ocean gradually mixes with seawater. Estuaries are among the most productive ecosystems in the biosphere: excellent breeding grounds for many species of fish, mollusks, and water birds.

When river runoff decreases, the salinity of the water in estuaries increases, which sharply changes the ecology of these waters. For example, the ecological situation in the northern part of the Gulf of California (Mexico) is changing noticeably due to a significant decrease in the inflow of freshwater from the Colorado River: water withdrawal for irrigation and the water supply of the city of Los Angeles is rapidly approaching 100% of its average annual runoff, and the place where the river flows into the Gulf of California now often represents a dry riverbed.