Following the Water: Mexican Middle Schoolers Model Their Local Watershed

TOM EKMAN ( teaches 7th grade biology at the Sierra School, a Mexican Civic Association (AC) and a non-profit organization that is an accredited, bilingual school for Mexican and international youth in Todos Santos, BCS, Mexico. The Sierra School ( offers and English and Spanish course of study in the humanities, literature, maths, art, ecology, science, and sport.

As a 7th grade biology teacher at the bilingual Sierra School in Mexico, I try to get my students outside as often as possible. The pristine desert around our school presents a paradox: while the entire landscape is dramatically carved by arroyos (dry washes) and broad fluvial plains, we rarely actually see the rain that so palpably shapes our environment. Apart from a huge hurricane-related storm every other year, rain is very rare.

I explain to students that a "watershed" is a useful model for scientists, who use it to calculate the amount of water being "shed" out of a given geographic basin. This calculation is useful for studies of agriculture, drinking supply, and power generation. It also allows scientists to trace water pollutants back to their source. My 7th grade students are just on the cusp of grasping abstract concepts in science. "Watershed" offers the dual challenge of not only being partly abstract, but also a somewhat dormant element in our environment for the 360+ days of the year without rain!

Real-life problems make for great science labs. The dirt road near our school regularly floods during hurricane events, effectively cutting off the north part of our village. In an introductory lab, I have students create a scale model of an arroyo and experiment with simulated river flows. (They enjoy building it up with houses and cars and bridges, and then, with great scientific interest, unceremoniously unleashing huge "storms" on it).

In a subsequent lab, I ask students to imagine that they are the town engineers and have been asked to design a culvert to mitigate these flood events. We develop the hypothesis that we could come up with a rough estimate of the necessary diameter of the culvert by estimating maximum flood levels.

The first task is to create a map of the watershed. To incorporate technology, I cobbled together secondhand smartphones from the U.S. and introduced them as our "computer lab in a box." (Real computers are far beyond our lean budget as a social-mission school.) The phones have their SIM cards removed and are used for research and photodocumentation of student work during discrete periods of time.

I ask students to use Google Images to examine different graphical representations of watersheds. Seeing multiple artistic renderings of spatial scientific models always proves more powerful than using the one diagram provided in the textbook. This approach helps students better understand how abstract models are communicated and how scientists use them.

Next, students look at Google Earth imagery draped on top of a digital elevation model ("3D view") to find the ridgetops. Fortunately, social trails on the ridges make them easy to identify. I provide the students with a printout of the Google Earth image and a sheet of graph paper and have them copy the Google Earth image of the watershed onto the graph paper.

The second task is to field-check our maps. The Sierra School is surrounded by an extraordinary "natural laboratory" that includes pristine desert, a lagoon, and a high-energy Pacific Ocean beach. We cover up to protect ourselves from the tropical sun and hike the social trails around the watershed as we spot-check our hand-drawn maps for accuracy. We also examine the junction of the arroyo with the road where the flooding occurs. Students can see the banks of 100-year flood events high up on the arroyo wall. We resolve that no culvert could reasonably mitigate these megafloods, but still plan to see if a culvert could contain waters from less extreme events.

The third task involves calculating the watershed runoff area. Students add scale to their maps and count the number of graph paper cells up-watershed from the road crossing. Comparing totals, we are able to identify anomalous results, and then we average all our totals to determine the area of the watershed.

In a fourth task, I have students calculate hurricane rainfall. The students all agreed that Hurricane Odile in 2015 was the biggest flood they had seen in the last ten years, with three days of constant, heavy rain. Students use the smartphones to estimate that 30 cm (12 inches) of rain fell during that Category 4 hurricane. We multiply the area of the watershed by this number and divide it by time (3 days x 24 hours x 60 minutes) to determine a rough estimate of the volume of water that might clear the watershed in a minute. (We also acknowledge that flood levels might vary significantly in intensity with microbursts.)

Finally, students estimate the diameter of a culvert that could contain such water flow. They use the estimated flow per minute to propose a realistic size. We also develop ideas for future study, including fluctuations in water flow and the type of debris that might need to clear the culvert.

The overall goal of this lab is to provide students with firsthand experience with a watershed, which they gain by creating the two maps and hiking the ridgelines around the arroyo basin. This activity also calls for new skills, such as field-checking a map, calculating precipitation from a storm, and using aerial imagery from Google. The activity was designed to be relevant and fun.

After they completed this lab, I asked students to reflect on what they liked or did not like about it. Student E. wrote: "This experiment was the best one we did in the whole year. In this experiment we solve[d] one of our community problems by having fun and learning. The bests[sic] part of it was doing some map and hiking because we examinate[sic] the terrain."

To conclude our study, we named our watershed "The Sierra School Arroyo."