Land for Life, Water for Life: Managing the Extremes by World Meteorological Organization
W
ater and land are closely intertwined. When a rain drop falls, the land surface determines which way it will initially flow and, to some extent, the variety of uses it will be put to. Water can evaporate directly from the surface, infiltrate the ground, or flow into rivers, dams, or other areas. It can be used by plants and animals to give life, consumed by humans, or used for a variety of purposes before being channeled back into the system. Water can be stored naturally, but it can take many years to reenter the surface land system. But this story about water begins with the eastern tropical Pacific Ocean, where intense solar heating produces high evaporation, which turns into moisture. This moisture then moves into the lower levels of the atmosphere and plays a key role in defining the weather and distribution of water over land surfaces. This low-level atmospheric moisture is carried westward by the trade winds across the Earth and thus shapes the spread and location of the drylands around the globe. The results from this global picture are further influenced by how the moisture flow interacts with, among other factors, the different oceans. Now let’s go to South America, where this story continues to unfold. The coastal region is virtually a dryland for a reason. Along the coast, a breeze also forms and flows from the ocean over the land, but barely brings enough moisture with it to form a cloud canopy that can supply rain water
to the nearby areas. The outcome is land with an arid or semiarid climate, where plants can cope with a short rainy season and dry conditions that last two-thirds of the year. Then around Christmas, the trade winds vanish and the previously cold waters in the ocean warm up. An ocean breeze full of moisture flows and rises over the land, but the hills block its flow, and the moisture turns into clouds and falls as rain. A rainy season with dark clouds arrives to deliver hour after hour of rain on the land. Plants, animals, and humans are prepared for their most productive period of the year and a burst of activities takes place in the rural areas. Vegetation grows, insect populations explode, and landscape changes in a matter of days. The land changes color as the soil moisture is replenished by constant rainfall, and the water reservoirs, which were dry, dusty surfaces, are refilled. But the climate, like humans, is not a static artifact; it is dynamic. The next year, a phenomenon termed la Niña may appear. It happens when the ocean waters are cooler than at the start of the rainy season and the trade wind system fails to weaken enough. The moisture that should form over the South American coast travels and falls way out, causing flooding in the western Pacific islands. There is still rain for the plants, animals and farmers, but only toward the end of the season. The land may be green, but there
L and for Life . WATER
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