From the moment water falls to theground as rain it starts making its way back to the oceans, regardless of itfalling over flatlands close to the river’s mouth, or high in the mountainsnear the drainage divide. A drainage divide, also called a watershed, is a linethat separates two drainage basins so that any water that falls on one sidegoes to one river and any water that falls on the other side even just a metreaway will make its way to a different river system. A drainage basin is an area of land where precipitation collectsand drains off into a common outlet, such as into a river or otherbody of water.
The drainage basins water system not only includes all of the surfacewater, such as from rain and snowmelt but also the groundwater underneaththe earth’s surface as well.As soon as the rainwater falls itbegins its journey to the oceans, either by percolating into the soil or porousrocks if possible, or if it lands on a surface it can’t absorb into, forexample concrete in urban and developed land, then it will run-off the surfacedue to gravity. Either way gravity moves the water downhill towards the lowestpossible point, most often the oceans; however around 18% of the Earths surfacedrainage basins are endorheic meaning that they do not drain into the ocean, anexample of this would be inner Asia that drains into the Caspian Sea or theAral Sea. Drainage basins always drain toward the lowest point meaning thatdrainage divides usually follow ridges or mountain ranges because as the namesuggests they are basins, so large lowland areas may only have a few very largedrainage basins, while mountainous areas will usually have lots of smallerbasins.In terms of flooding risks there are 5 mainfactors around the catchment zone that will affect the likelihood or amount offlooding. Topography, the shape and features of the Earth’s surface, affectswater runoff speed, for example a rocky steep environment will allow the waterto reach the river faster increasing the risk and severity of flooding. The basinshape also affects how long it will take water to drain into the river forexample a circular basin will take less time to drain than a long thin one.Size is also a factor as larger drainage basins will generally have more watermoving through them increasing the potential for flooding.
Land use contributesto the volume of water reaching the river, water that falls onto urban areaswill runoff into the river with almost no absorption into the groundwater.Possibly the single most significant factor to flooding and water drainagewould be the type of soil in the basin. Sandy soilsare very permeable, and so rainfall on sandy soil is likely to be absorbed bythe ground. However, clay soils are impermeable and therefore rainfall onclay soils will run off rather than being absorbed. Given enough rainfall evenfree-draining soils can become saturated, meaning that any additionalrainfall will run off rather than being absorbed into the ground.The amount of time it takesfor water to reach the river from when it rains down is the main factor forflooding because if all of the water gets into the river very quickly, forexample in an urban area with very high run off, then all of the water would bedischarged from the river at the same time increasing the likelihood offlooding. However if water is absorbed into the ground it will enter the riverat a slower more steady rate so the rivers peak discharge would besignificantly less.
The difference in time between the peak rainfall and thepeak discharge is called the lag time of the river, and in general the longerthe lag time the less severe any potential floods will be. A rivers runoff/discharge is measures in cumecs which stands for cubic metres per second.The absorptionof water into the soil is called infiltration, and the downward movement of water within the soil is called percolation. Thepore spaces within a soil are the conduit that allows waterto infiltrate and percolate through it.