Water and drought
How warming changes water through thirstier air, shrinking snowpack and overdrawn groundwater, and how to tell the climate's share of a water crisis from the part people control.
THIRSTY AIR
Warmer air pulls more water
You met this in the last lesson: each 1 °C lets air hold about 7% more water vapour. That cuts both ways. Warm air pulls more water out of soils, plants, rivers and reservoirs. And when that moisture does fall, it can fall as heavier downpours. So warming can bring both drier ground and more intense rain, sometimes in the same region.
A reservoir under a hotter summer sun loses more water to evaporation even if the same amount of rain fell upstream last winter.
Two kinds of dry
Less rain
The classic drought: supply falls. Less rain and snow reach the ground than usual.
Hotter drought
Demand rises: even with normal rain, hotter air evaporates more, so soils and crops dry out faster. Warming makes this kind more common, and it can stack on top of the first.
Check yourself
Reza farms wheat. His rain gauge shows about the same yearly rainfall as his father recorded decades ago, yet his soil dries out sooner every spring. What's the most likely reason?
- His gauge must be broken, because dry soil always means less rain
- Hotter air evaporates more, so the same rain leaves drier soil
- The soil has simply got older and can no longer hold water
- Heavier rain is washing all the water straight into the sea
Show the answer
Hotter air evaporates more, so the same rain leaves drier soil
Yes. With warmer temperatures, evaporation from soil and plants rises, so the same rainfall leaves less water in the ground.
MOUNTAIN WATER TOWERS
Snow is a reservoir nobody built
In many dry regions, including Iran's Alborz and Zagros, winter snowpack stores water on the mountains and releases it slowly through spring and early summer, just when farms and cities need it. Warming means more rain and less snow, and snow that melts earlier. Rivers peak sooner and run lower in late summer.
Same total precipitation, but half of it arrives as rain in January: much of it runs off in winter, and there's less left when the heat comes in July.
A mountain water year, then and now
- Winter
Then: most precipitation falls as snow and stays put. Now: more falls as rain and runs off straight away.
- Spring
Then: a slow melt feeds rivers for months. Now: the melt starts earlier and finishes sooner.
- Summer
Then: rivers still carry meltwater into the hot months. Now: flows drop earlier, just when irrigation and cities need the most.
Check yourself
Maryam's town had a winter with normal total precipitation, but it fell mostly as rain instead of snow. For summer water supply, that's just as good as a snowy winter.
Show the answer
False
False. Snow stores water on the mountains and releases it slowly into the warm months. Rain runs off in winter, often faster than dams can hold it, so less is left for summer.
UNDERGROUND
Groundwater: savings, not salary
Aquifers are layers of rock, sand and gravel holding water underground. Many refill very slowly, over decades or far longer. When wells pump out more than rain and rivers put back, the water table falls: wells must go deeper, pumping costs more, and qanats and springs dry up. In several plains in Iran, including around Tehran, satellites show the ground itself sinking as aquifers are drained.
A drought year makes people pump harder, just when the aquifer is getting the least new water. The deficit builds up year after year.
A savings account you can overdraw
Rain and river flow are your yearly income; the aquifer is savings built up over centuries. You can live off savings in a bad year, but spending more than you earn every year empties the account. Where the analogy breaks: when fine clay layers dry out and compact, the ground sinks and they can't hold water again. Some of the account is gone for good, not just empty.
Check yourself
Is each pressure on water mainly from the climate, or mainly from how people use water?
- More winter precipitation falling as rain, not snow
- Pumping wells faster than rain refills the aquifer
- Hotter summers evaporating more from reservoirs
- Planting thirsty crops across a dry plain
- City pipes leaking a big share of treated water
- Mountain snow melting weeks earlier
Show the answer
Mainly climate: More winter precipitation falling as rain, not snow, Hotter summers evaporating more from reservoirs, Mountain snow melting weeks earlier
Mainly water use: Pumping wells faster than rain refills the aquifer, Planting thirsty crops across a dry plain, City pipes leaking a big share of treated water
What actually helps
- Worldwide, farming uses around 70% of the fresh water people withdraw, and in dry countries often more, so the biggest savings are in which crops are grown and how.
- Better irrigation only saves water if total pumping is capped; otherwise the savings get spent on more fields.
- Fixing leaks, reusing treated wastewater and letting aquifers recharge in wet years all add to supply.
- Household saving matters most at peak times, when city systems are stretched in summer.
Check yourself
Farmers in Ali's village switch from flood irrigation to drip lines and use about a third less water per field. With the water they save, they plant new fields. What happens to the aquifer?
- It recovers, because drip irrigation always saves water
- Probably no relief: the saved water is pumped for new fields
- It fills up faster, because drip lines push water underground
- Nothing changes either way, because irrigation doesn't affect groundwater
Show the answer
Probably no relief: the saved water is pumped for new fields
Right. Efficiency cuts water per field, but if the savings go into more fields, total pumping stays the same or even rises. Pairing efficiency with a cap is what protects the aquifer.
Lesson recap
- Warmer air pulls more water from soils and reservoirs, and can also dump it in heavier downpours.
- Drought can come from less rain, from more evaporation, or both.
- Warming shrinks mountain snowpack and makes it melt earlier, leaving less water for late summer.
- Groundwater refills slowly; overpumping lowers water tables and can make land sink for good.
- Climate adds stress, but how water is used, especially in farming, decides much of the outcome.