Initial Publication Date: February 8, 2021

The Freshwater pcMEL

The Freshwater pcMEL asks students to weigh the connections between evidence and alternative explanations about the quantity and quality of freshwater resources. The availability of usable freshwater as the world's population grows is a critical environmental issue facing Earth and its inhabitants.

Below are links to resources that will help students use the Freshwater pcMEL and develop an understanding of the spatial complexity of access to freshwater resources.


This article provides an introduction to the Freshwater build-a-MEL (baMEL). It is useful even if you are using the Freshwater pcMEL with your students. The primary difference between the pcMEL and the baMEL is that with the pcMEL students are given two models and four lines of evidence in a preconstructed diagram where as with the baMEL students choose two models from three and select four lines of evidence from eight to create their own MEL diagram.

Freshwater Resources Article Cover
Freshwater Resources: The Challenges of Quantity and Quality (Acrobat (PDF) 529kB Feb8 21)

Next Generation Science Standards Performance Expectations

MS-ESS2-4: Earth's Systems

  • Develop a model to describe the cycling of water through Earth's systems driven by energy from the sun and the force of gravity.

MS-ESS3-1: Earth and Human Activity

  • Construct a scientific explanation based on evidence for how the uneven distributions of Earth's mineral, energy, and groundwater resources are the result of past and current geoscience processes.

MS-ESS3-3: Earth and Human Activity

  • Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.

HS-ESS3-2: Earth's Systems

  • Analyze geoscience data to make the claim that one change to Earth's surface can create feedbacks that cause changes to other Earth systems.

HS-ESS3-1: Earth and Human Activity

  • Construct an explanation based on evidence for how the availability of natural resources, occurrence of natural hazards, and changes in climate have influenced human activity.

The Models

Model A: Earth's freshwater challenges will be solved by engineering solutions.
Model B: Earth has a shortage of freshwater, which will worsen as our world's population increases.

Student Handouts

Lines of Evidence

Evidence #1: Land use changes have generated large pressures on freshwater resources. These changes are affecting both water quality and availability.
Evidence #2: The world's population is increasing. This stresses the supply of freshwater.
Evidence #3: Water reclamation and desalination costs are expensive. These costs vary depending on location.
Evidence #4: Advances in engineering have led to better access to quality drinking water. At the same time life expectancy and quality of life have improved.

Student Handouts

Other Resources

This Plausibility Ranking Task (PRT), which may be completed prior to using any MELs, helps students to understand the role of evidence in supporting or refuting models.

Virtual MEL

The Google Drive folder below contains MEL resource documents in the Google formats. In order to maximize the flexibility of our activities, the MEL Project Team wanted to make more resources compatible with virtual settings. We have made our MEL scaffolds available in Google formats, so that they can be copied, modified, and uploaded more easily on a variety of platforms and devices. Please note that to access all the materials for each MEL scaffold, you will need a Google account. View the README file first.

Digital Resources

Extreme Weather NCA
This site on Freshwater Availability from the National Climate Assessment provides an introduction to extreme weather events, including heat waves, droughts, heavy downpours, floods, hurricanes, and other storms.