Passage IV

A river's discharge is the volume of river water flowing past a location in the river in a given amount of time. Discharge is affected by several factors, including the area of the river's drainage basin (the land surface that delivers the rainwater that falls on it into the river) and the spatial relationship of the river to other rivers.

Two studies examined the discharge in 3 rivers—the North River, the Jones River, and the Calm River—following a rainfall event. As shown in Figure 1, the drainage basin of the North River and the drainage basin of the Jones River are within the drainage basin of the Calm River. The area of each river's drainage basin is given in kilometers².

Study 1

The rainfall event produced 25 mm of rain. As rainwater drained into the 3 rivers, the discharges of the rivers increased. The maximum discharge measured immediately after a rainfall event is called a flood.

Over the 12 days following the rainfall event, as the flood caused by the rainfall moved downstream, the discharge was continuously measured, in m³/sec, at the 3 sites shown in Figure 1. The results are shown in Figure 2. (Note: No rainfall event occurred in the Calm River drainage basin during those 12 days.)

Study 2

The normalized discharge over the 12 days following the rainfall event was calculated for each of the 3 rivers. The normalized discharge values allowed the flood to be studied as if the 3 rivers had identical drainage basin areas of 1 km². Each normalized discharge value was calculated using the following equation:

The value of the numerator, the river discharge, was obtained from Figure 2. The results are shown in Figure 3.

Figures 2 and 3 adapted from George Hornberger et al. , Elements of Physical Hydrology. ©1998 by The Johns Hopkins University Press.