The discharge of a stream will vary greatly over time depending on the weather within its watershed (the area drained by the stream). If it rains within the watershed, the discharge will increase. During a dry spell, the discharge will decrease. As a rainstorm begins, the discharge of a river will not instantaneously increase. It takes time for the rain to reach the river. Envision a rural scene with rain falling in the woods. Much of this water will infiltrate directly into soil and never reach the stream. Any water that does accumulate on the surface will flow slowly downhill. On its journey, it must negotiate its way through a twisted, difficult path through rocks, leaf litter, the blades and stems of plants, and other obstacles. Finally, this runoff may pour into a small rill. From here, it flows to a gully, to a small creek, to a larger creek, and then finally it may reach the river. In this rural area, it may take hours or days for rainfall to actually reach the river and produce an increase in discharge. The delay between when rainfall occurs and when the discharge of the river actually increases is called the "lag time." The lag time is measured as the time between the middle of the rainstorm and the time when discharge reaches its peak.
Lag times will vary depending on characteristics of the watershed. As just described for a rural area, it takes a long time for rainwater to reach the river, so lag times are long. Urban areas are very different. Most urban surfaces, like roof tops and parking lots, are impermeable. Water doesn't soak in, it immediately flows away, usually at a very high velocity. As a result, rainfall very quickly enters streams, so lag times are relatively short. Streams with short lag times are said to be "flashy." Because all of the water generated by a rainstorm moves through a flashy stream in a relatively short period of time, the resulting discharge is corresponding high. Such high discharges often produce "flash" flooding. For this reason, urban areas are generally more prone to flooding than are rural areas.
Although urban areas tend to flood more often because runoff moves through the area all at once, these impermeable surfaces also mean that floodwaters will subside relatively quickly. This is called a "rapid return to baselevel." Baselevel is the volume of discharge that was in the stream before the storm occurred and to which the steam returns after the stormflow has moved through. In urban areas, floodwater rises quickly to very high flood peaks, and then rapidly subsides. In rural areas, where lagtimes are much longer, waters rise very slowly, floodpeaks are much lower (the river may never actually flood) and the return to baselevel is very slow.
Rural and urban areas will also have different baselevels. Since rural areas very slowly release water into streams, baslevels tend to be relatively high. The impermeable surfaces of urban areas release their water all at once, so between rainstorms, baselevels are very low and often zero.
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