Runoff and rivers

The Great Salt Lake is salty because evaporation is the only way water leaves it.

Where rain goes when it lands

Stand under a gutter during a downpour and you see runoff in action. Rain that lands on the ground has three choices. It can evaporate again, it can soak into the soil, or it can flow over the surface. NOAA explains that runoff happens when rain comes faster than the ground can absorb it, or when the soil is already full. Water then follows the slope into rills, streams and rivers.

The surface matters a great deal. Forest soil with leaf litter acts like a sponge. A car park, a road or a roof acts like a tray, and nearly all the rain runs off at once. NOAA notes that water can flow over impermeable surfaces like concrete, which is why cities need drains and why heavy rain in a paved area can flood streets within minutes, while the same rain on a forest barely raises the stream.

A river is the end of a whole basin

Every river collects water from an area of land called its drainage basin or watershed. Ridges and hilltops form the edges; rain falling on one side of a ridge ends up in one river, rain on the other side in another. In the model the river starts on the mountain, where snowmelt and rain gather, and grows wider as it heads for the coast.

Rivers hold surprisingly little water at any one moment. The USGS table gives about 2,120 km³ for all the world's rivers, less than a sixth of what is in the air. Rivers matter because water passes through them quickly and because, as the USGS points out, they and lakes are where people get much of the water they use every day. Rivers are also fed from below: a USGS study found that more than half of the streamflow in the Upper Colorado River Basin starts as groundwater.

Lakes with no way out

Most river water returns to the ocean. But some basins have no outlet. NOAA gives the Great Salt Lake in Utah and the Dead Sea as examples. Rivers carry small amounts of dissolved salts into these lakes. The only way water leaves is evaporation, and evaporation takes only the water. Over thousands of years the salts build up and the lake becomes far saltier than the sea.

Over land, precipitation is larger than evaporation; over the ocean, evaporation wins. The USGS points out that rivers and groundwater flowing back to the sea make up the difference. Without them, it says, the oceans would gradually empty. Runoff is the return leg that keeps the whole cycle balanced.

Rain adds up faster than most people expect. The USGS works out that 1 inch of rain on a single acre is 27,154 US gallons. In metric units the rule is simpler: every millimetre of rain on every square metre is one litre. So 25 mm of rain on a car park 100 m by 70 m, 7,000 m², is 175,000 litres, and on a paved surface nearly all of it has to drain away within minutes. On a forest of the same size much of it would soak in and reach the stream over hours or days.