The finished graph will help students focus on the nature of the precipitation-streamflow relationship, and will eventually lead them on a search for additional data. In addition to seasonal variations, the nature of the streamflow-precipitation relationship can vary dramatically from one geographic area to another. The graphs created during this activity are based on data from a watershed in eastern Massachusetts. This particular watershed area is gently sloped and heavily vegetated. Other watersheds that feature less rainfall, larger tracts of impervious surfaces, a warmer climate or other geographic and climatic differences, will have a much different relationship between precipitation and streamflow. Graphs produced from the data gathered from these areas could show significant differences.
This chapter is most appropriate for students in grades 7-12.
After completing this chapter, students will be able to:
- download data from USGS and NOAA websites;
- format the data and import it into Microsoft Excel;
- graph the data and investigate the patterns and relationships; and
- launch inquiries that can provide them with greater insights about the water cycle and its interactions with the geosphere and the biosphere.
Note: It is not necessary for students to locate, download, and format the web-based data themselves in order to engage in inquiry and deepen their understanding of the water and energy cycles. Some other options include:
- Teachers of younger students can elect to download the data and format it per the instructions provided, so that students can just import the data into Microsoft Excel, build the graphs, and analyze the data.
- Teachers can elect to download the data but let students have the experience of formatting it to import into Microsoft Excel, import it and build the graphs, and analyze the data.
Students who work through the entire Chapter will be better prepared to launch inquiries of their own, using the data they themselves choose.
You can customize the way you introduce this investigation to your students in order to support a topic you are teaching. Some possibilities:
- Highlight the Water Cycle: Students often develop the "big picture" of the water cycle without delving into the details. This activity can be used to deepen a general study of the water cycle by focusing student attention on the details of one specific aspect of the cycle. This focus helps students move beyond the big picture to dig into the complexity of the process.
- Highlight the Earth System: As students explore the factors that effect runoff, you have an opportunity to highlight the interactions of the water cycle with the geosphere (basin size, slope of the land, soil types, and exposed ledge); the biosphere (the seasonal changes that vegetation has on the precipitation-streamflow relationship, the effect of biomass on the ground - such as matted leaves and pine needles); and the atmosphere (air temperature, humidity, and wind).
- Highlight Earth's Energy Flow: It is solar energy that drives the recycling of water on our planet. Heat energy is absorbed by water as it changes state from liquid to gas (water vapor). This energy, sometimes referred to as latent heat, is transported through the atmosphere and released as water vapor condenses, changing from a gas back to a liquid.
What Influences the Precipitation-Streamflow Relationship?
This information about factors that influence the precipitation-streamflow relationship is not complete, but it will give you enough information to help you stimulate student thinking and lead a classroom discussion. Your students may be motivated to generate additional ideas. You can also use this information to help assess the development of student thinking as it relates to Learning Goal 4: (Students will) launch inquiries that can provide them with greater insights about the water cycle and its interactions with the geosphere and the biosphere.
1. Long-standing or semi-permanent factors have an effect.
The area of the basin, the slope of the ground, the permeability of the soil, and the area of impervious surface within the basin (for example, rock ledge, rooftops, paved surfaces) have an influence. Steeper slopes and larger amounts of impervious surface will increase the amount of runoff in a watershed and decrease the amount of precipitation that seeps into the ground to become groundwater. A thick mat of dead leaves or pine needles on the ground will slow down runoff and tend to increase the amount of precipitation that evaporates or is absorbed by the soil to feed the river more gradually across time.
2. Seasonal factors may apply.
Warmer air temperature will increase evaporation and reduce the amount of precipitation that reaches a stream while frozen ground will increase the amount of runoff.
Although vegetation within a basin may be considered "long-standing", its influence can vary seasonally, depending on location. When trees and shrubs are actively growing, they pull water out of the soil, allowing more precipitation to seep in. Leaves can hold moisture on their surfaces until it evaporates, never reaching the ground. When vegetation is dormant and without leaves, it is less effective in reducing runoff.
The presence of a snow cover on the ground is another possible seasonal factor. In some situations, precipitation will cause snow to melt, resulting in a disproportionate amount of water entering the river. Under different conditions, snow can capture and hold the precipitation, so that very little of it reaches the river in the short-term, but a warm day with no precipitation can melt the soggy snow pack and greatly increase streamflow.
3. Factors related to the nature of the precipitation event.
The intensity of the precipitation will influence the runoff rate. An inch of rain in an hour will produce more runoff that an inch of rain that falls across a 24-hour period, and three days of even a light rain is likely to result in increased runoff as the ground becomes saturated across time.
The following National Science Education Standards are supported by this chapter:
- 12ASI1.4 Formulate and revise scientific explanations and models using logic and evidence.
- 12DESS1.3 Energy in Earth's system.
Three or four 45-minute classroom sessions to complete the chapter.
If you locate, download, format, and graph the data ahead of time, and students just study the graph and research the factors that influence the precipitation-streamflow relationship, then one or two 45-minute classroom sessions will be sufficient.