Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

and can accumulate as ice caps and glaciers. The latter can store frozen water for thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. A portion of runoff flows into rivers in valleys, with streamflow moving water towards the oceans (Fig. 2.4). Runoff and water emerging from the ground (groundwater) may be stored as freshwater in lakes. Not all runoff flows into rivers; much of it infiltrates into the ground. Some water percolates deep into the ground and replenishes aquifers, which can store freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. In river valleys and flood plains, there is often continuous water exchange between surface water and groundwater in the hyporheic zone. Over time, the water returns to the ocean to continue the water cycle. Since the hydrological cycle involves the exchange of energy, it leads to temperature changes (https://en.wikipedia.org/wiki/water_cycle). For instance, when water evaporates, it absorbs energy from its surroundings which subsequently cools the environment. As water condenses, energy is released which warms the environment. These heat exchanges influence climate. Furthermore, the evaporative phase of the cycle purifies water which in turn replenishes freshwater resources. The flow of liquid water and ice also transports minerals across the Earth. These movements may also involve reshaping the geological features of the Earth, through processes such as erosion and sedimentation. Considering the foregoing, it is clear that the hydrological cycle is essential for the maintenance of most life and ecosystems. 2.3 THE WATER BALANCE IN VINEYARDS The water balance in a vineyard depends on a number of water gains and losses. The gains are water coming into the root zone. They are primarily rainfall and irrigation water infiltrating and percolating into the root zone (Fig. 2.5). Water moving upwards from deeper layers or a groundwater table via the capillary fringe into the root via capillary rise can also be considered as a gain. Depending on the extent to which capillary rise occurs, it can be an important source of water in the case of dryland vineyards. In the case of vineyards on slopes, sub-surface lateral flow from higher ground into the root zone is also a gain. However, if excessive lateral water flow occurs, it might be necessary to intercept it by means of cutoff drains to prevent waterlogging in the root zone. Losses in the water balance comprise evaporation from the soil surface, as well as water extracted via the roots and lost to the atmosphere (Fig. 2.5). Deep percolation or drainage beyond the lower limit of the root zone are also losses that commonly occur in vineyards, particularly after heavy rainfall or in the case of over-irrigation. The drainage water may eventually recharge deep groundwater bodies. Surface runoff usually occurs when the rate of rainfall and/or irrigation application exceeds the infiltration capacity

IRRIGATION OF WINE GRAPES 33