Deserts: water balance, energy and adaptation
Compare definitions of a desert, convert rainfall and evaporation into the same units, explain huge day–night temperature swings with an energy budget, and measure how surface area controls water loss.
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There is no single definition of a desert. In Meigs' classification, extremely arid land has at least 12 months in a row without rain, arid land less than 250 mm a year and semiarid land 250–500 mm (usually grassland steppe). Using different definitions, the USGS says about one-third of Earth's land is desert and National Geographic more than one-fifth: the same evidence can give different numbers when the categories differ.[1][3]
Many deserts lie near 30° north and south, where air that rose near the equator has already lost much of its moisture and high pressure keeps skies clear. In such places at least 90% of the Sun's rays reach the surface, while humid lands receive only about 40% of the possible solar radiation. Rain-shadow deserts form downwind of mountains that remove moisture from rising air.[4]
Desert water budgets are strongly negative: evaporation in American deserts can be 70–160 inches a year (about 1,800–4,100 mm), far more than the rain that falls. Rainfall depth converts directly to volume: 1 mm on 1 m² is 1 litre, so Cairo's 28 mm a year is about 28 litres per square metre, and the driest Atacama gets less than 2 litres.[4][2][3]
The day–night energy budget explains the cold nights. Dry, clear air lets about 90% of the day's heat radiate back to space at night, against about 50% in moister climates, so daily ranges of 50 °F (about 28 °C) or more are common. NASA gives averages of about 38 °C by day and −3.9 °C at night for hot deserts.[4][2]
Plants survive drought by storing water (cacti), cutting water loss (few leaves), reaching more water (large root systems) or escaping the dry season altogether (ephemerals that finish their life cycle within weeks of rain; perennials that go dormant). Cacti are found wild only in the Americas, yet cactus-like Euphorbia species in Africa have a similar fleshy form in a similar climate.[2][6][3]
Animals combine behaviour and physiology: the kangaroo rat is active at night, rests in a burrow by day, does not sweat or pant, and gets its water from seeds. Desertification of semiarid land is usually driven by deforestation and human demands on the land, not by drought alone.[5][3]
Try it: Surface area and the rate of water loss
You need
- Three identical paper towels
- 10 ml measuring cylinder or syringe
- Electronic balance (0.1 g)
- Three Petri dishes or saucers
- Cling film and sticky tape
- Ruler
- Stopwatch
- Spreadsheet or graph paper
Safety
Use only water. Keep water away from the balance's plug and socket and wipe up spills. No heaters or lamps are needed.
What to do
- Add exactly 10.0 ml of water to each towel on its own dish.
- A: lay flat. B: roll into a tight cylinder and tape it. C: roll into a cylinder and wrap the curved side in cling film, leaving only the two ends open.
- Estimate the exposed surface area of each (A: both faces of the sheet; B: curved surface plus ends; C: the two ends only) with a ruler.
- Record the mass of each dish and towel at t = 0, then every 15 minutes for 90 minutes. Keep all three in the same place, away from heaters.
- Plot mass against time and find the rate of water loss (g/h) from the gradient.
- Calculate the rate per 100 cm² of exposed surface and estimate the uncertainty from the balance and your area measurement.
What happens, and why?
Water evaporates only from surfaces open to the air, so the rate of loss rises with exposed area. Rolling and wrapping reduce the exposed area for the same amount of water, just as a thick water-storing stem and few leaves reduce water loss in cacti and desert bushes. The rate per unit area should be roughly similar for all three, showing that area, not shape itself, is the main factor.[2]
Play: Avoid or endure?
Does this living thing avoid the heat and drought, or endure it by storing or saving water? Sort each strategy.
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Other ages: Ages 6–7 · Ages 10–11