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LessonGrades 10 - 12

Watershed Balance

A landscape drawing shows a river winding through a valley into the sea. A dotted line along a surrounding ridge of mountain peaks ("the divide") marks the area from which rain and any other water flows to the river in the valley below. In this watershed, any water that falls inside of the dotted lines, theoretically drains to the river.Figure 1. The boundaries of a watershed mark the location where all the water outlets into the same water source.

Students learn about the water cycle and its key components. First, they learn about the concept of a watershed and why it is important in the context of engineering hydrology. Then they learn how we can use the theory of conservation of mass to estimate the amount of water that enters a watershed (precipitation, groundwater flowing in) and exits a watershed (evaporation, runoff, groundwater out). Finally, students learn about runoff and how we visualize runoff in the form of hydrographs.

Engineers must be able to understand, calculate and graph water runoff for many reasons, including flood control and management, water pumping, designing and building dams, recreational planning, and probability and statistics concerning future flood scenarios. These skills are particularly useful for civil engineers involved in modeling hydrological processes and studying water resource management for the benefit of human communities and natural ecosystems.

After this lesson, students should be able to:

  • Describe the main components of the water cycle.
  • Explain the types of data displayed in a runoff hydrograph.
  • Calculate the regional water balance equation using conservation of mass.
  • Estimate the amount of runoff leaving a watershed using the regional water balance equation.

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