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

250

HintA quarter of a metre.

The whyIt 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

HintThe first word means "daily".

The whyIn 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

HintCompare a night with a blanket and a night without.

The whyBy 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

HintHumus has to come from somewhere.

The whyThis 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

HintThink what remains in a pan when brine boils dry.

The whyEvaporation 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

HintThe same word describes juicy food.

The whyAfter 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

HintRain is rare, yet moisture lies far below.

The whyOther 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

HintCompare drying a flat sheet with drying a needle.

The whyBroad 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

HintNot a drink. It is an energy reserve.

The whyKeeping 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

HintA rare event that briefly turns the ground green.

The whyThese 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

HintFollow the green patches on a satellite picture.

The whyThis 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.