Handbook for Irrigation of Wine Grapes in South Africa

2.2 THE HYDROLOGICAL CYCLE The hydrological cycle describes the dynamics of water on, above and below the surface of the Earth (Fig. 2.4). The mass of water remains fairly constant over time, but can be partitioned into ice, fresh water, saline water and atmospheric water changes depending on climatic variables (https://en.wikipedia.org/wiki/ water_cycle). The sun, which drives the water cycle, heats water in oceans and other large water bodies (Fig. 2.4). Water evaporates as water vapour (gas) into the air. Rain (liquid), as well as ice and snow (solids) can sublimate directly into water vapour. Evapotranspiration (ET) occurs when water simultaneously transpires from plants and evaporates from the soil.

Solar energy

Rain

Snow & ice

Rain

ET

Evaporation

ET

Evaporation

Rivers/ Lakes

O cean

FIGURE 2.4. Schematic representation of the Earth’s hydrological cycle, where ET is evapotranspiration by vegetation, e.g. forests and crops (redrawn from Hillel, 1980).

Water vapour molecules are less dense, compared to the major components of the atmosphere, i.e. nitrogen and oxygen. Due to the difference in molecular mass, water vapour rises in open air as a result of buoyancy (https://en.wikipedia.org/ wiki/water_cycle). However, as altitude increases, air pressure decreases and the temperature drops. The lower air temperature causes water vapour to condense into liquid water droplets which are heavier than the air. Consequently, these small droplets fall unless they are supported by an updraft. A huge concentration of these droplets over a large space in the atmosphere becomes visible as a cloud. On the other hand, fog is formed (i) when water vapour condenses near the ground, (ii) as a result of moist air and cool air collision or (iii) an abrupt decrease in air pressure. As air currents transport water vapour around the Earth, cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation (https:// en.wikipedia.org/wiki/water_cycle). Some precipitation falls as snow, sleet or hail,

32 CHAPTER 2 – THE DYNAMICS OF WATER IN AND AROUND VINEYARDS