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27.7:

מחזור המים

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Biology
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JoVE Core Biology
The Water Cycle

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ההידרוספירה הוא המרחב השלם שבו מתקיימים מים על כוכב לכת. מרבית אספקת המים של כדור הארץ נמצאת באוקיינוסים בעלי מים מלוחים, בהם מולקולות מים בודדות עשויות להישאר למשך כ-3000 שנה לכל היותר. מדידה המכונה זמן שהייה.מולקולות מים במקורות מים מתוקים חיוניים, כגון אגמים ונחלים, עוברות מחזור מהר יותר. המחזור מתחיל כאשר אנרגיית השמש מחממת ומאדה מים מפני שטח אוקיינוסים, נחלים ואגמים אל תוך האטמוספירה, שם הם מתעבים ויוצרים עננים. לאחר מכן, המים חוזרים כמשקע לפני שטח כדור הארץ.כעת הם יכולים לזרום חזרה אל האוקיינוס כמי נגר דרך נחלים ואגמים, לחלחל לתוך הקרקע כדי להיאגר באקוויפרים, לזרום מתחת לפני שטח כמי תהום, או להתאדות שוב באמצעות דיות בצמחי יבשה.

27.7:

מחזור המים

The Earth’s hydrosphere includes all of the areas where the storage and movement of water occurs. Since water is the basis of all living processes, the cycling of water is extremely important to ecosystem dynamics.

The water cycle begins as the sun warms surface water on the land and in oceans, causing it to evaporate and enter the atmosphere as vapor. The water vapor condenses into clouds and eventually falls as precipitation in the form of rain, snow or hail.

After falling back to the Earth, water may enter large bodies of water, evaporate again, remain on the surface as runoff, or seep into the soil, where it may be absorbed by plants and transpired (released from pores in the leaves and evaporated into the atmosphere) or become groundwater. Deep groundwater may form reservoirs, or aquifers, and shallow groundwater eventually reaches a body of water, where it can be evaporated as surface water to continue the cycle.

Long-Term Storage

Human cells are over 70% water, and almost all organisms on land require fresh water to survive. However, 97.5% of the water on Earth is saltwater, and less than 1% of freshwater is accessible through rivers and lakes. Most water on Earth exists as ice, groundwater, or saltwater in the oceans and seas, and is inaccessible to many plants, animals, and fungi, and unavailable for short-term cycling. In these forms, water is stored for extended periods of time—called residence times—before entering the water cycle.

Short-Term Water Cycle

Energy from the sun warms surface water on land and in the ocean, causing it to enter the atmosphere through evaporation, sublimation (ice vaporizing from a solid form), and transpiration (water evaporation from plants). The water vapor then condenses to form clouds and eventually falls to the Earth as precipitation (e.g., rain or snow). The water that returns to the Earth’s surface may: fill bodies of water, evaporate from the ground again, permeate the soil to be absorbed and transpired by plants, flow beneath the surface as groundwater, or be stored for prolonged periods in aquifers.

Suggested Reading

Hamlington, B. D., J. T. Reager, M.-H. Lo, K. B. Karnauskas, and R. R. Leben. “Separating Decadal Global Water Cycle Variability from Sea Level Rise.” Scientific Reports 7 (April 20, 2017). [Source]

Haddeland, Ingjerd, Jens Heinke, Hester Biemans, Stephanie Eisner, Martina Flörke, Naota Hanasaki, Markus Konzmann, et al. “Global Water Resources Affected by Human Interventions and Climate Change.” Proceedings of the National Academy of Sciences of the United States of America 111, no. 9 (March 4, 2014): 3251–56. [Source]