Resources and environment, Year 10: climate, soils and adaptations (Hot deserts option)

250

提示A quarter of a metre.

为什么It is dryness, not heat, that makes a desert. In a hot desert the little rain that falls often comes in a few sudden storms, with months or years of nothing between them.

Diurnal temperature range

提示The first word means "daily".

为什么In a hot desert it is very large: often more than 20 °C, and sometimes much more. Plants, animals and people all have to cope with scorching days and cold nights in the same 24 hours.

There are no clouds to trap the heat, so it escapes into the sky

提示Compare a night with a blanket and a night without.

为什么By day the clear sky lets the Sun heat the ground strongly. After sunset the same clear, dry air lets that warmth radiate straight out to space.

Few plants grow, so there is little dead material to rot into the soil

提示Humus has to come from somewhere.

为什么This is interdependence. Little rain means few plants. Few plants mean little humus, so the soil is thin, sandy or stony and holds water badly. Poor soil in turn makes it harder for plants to grow.

Water is drawn up and evaporates, leaving its dissolved salts behind

提示Think what remains in a pan when brine boils dry.

为什么Evaporation is far greater than rainfall, so water in the soil moves upwards, not downwards. Each time it evaporates it leaves a little more salt. Irrigating crops can make this worse, and most crops cannot grow in salty soil.

A succulent

提示The same word describes juicy food.

为什么After a rare storm the plant takes in as much water as it can and swells. It then lives on that store through the dry months. A thick waxy skin stops the water escaping.

It reaches water held deep underground

提示Rain is rare, yet moisture lies far below.

为什么Other desert plants do the opposite: they spread a wide net of shallow roots to catch rain from a large area before it evaporates. Both are answers to the same problem, a shortage of water.

A smaller surface area means less water is lost by transpiration

提示Compare drying a flat sheet with drying a needle.

为什么Broad leaves have thousands of pores that let water vapour out. Spines have almost none. The green stem does the photosynthesis instead. Spines also put off animals looking for a drink.

fat

提示Not a drink. It is an energy reserve.

为什么Keeping the store in one place on its back, and not all over the body, lets the camel lose heat more easily from the rest of its skin. The store lets it go for a long time without food.

rain

提示A rare event that briefly turns the ground green.

为什么These plants then grow, flower and make new seed within a few weeks, before the ground dries out again. The plant avoids the drought by spending it as a seed.

Around water: oases, seasonal rivers and the wetter desert edges

提示Follow the green patches on a satellite picture.

为什么This is the biodiversity issue for deserts. The few places with water are also where people farm, build and take water. So a small development can affect a large share of the wildlife. Desert species are also living close to their limits, so a small rise in temperature can be too much.