Rain on Adobe: What 20 Test Walls Taught the Park Service About Storms and Old Missions

The adobe test walls at the Desert Research Learning Center. Image from NPS's page.

In 2018 the Mission Church of San Jose de Tumacácori lost parts of two exterior walls after the park got 1.5 inches of rain in 24 hours. Is that climate change? There isn't enough long-term monitoring to say for sure, but National Park Service (NPS) staff decided to find out how adobe stands up to heavier rain, by building walls and raining on them. Their results come with a clear threshold: between a 1-year and a 25-year storm, the damage jumps.

Why Tumacácori

Climate change is usually discussed in terms of natural resources, but it also threatens cultural ones. At Tumacácori National Historical Park, the church of San Jose de Tumacácori is more than 200 years old. After an intense early-winter rain in 2010, a hole opened around the west sanctuary window, and nearly a ton of adobe and two lintels were lost. After that year's summer monsoon, a large part of the sacristy's north wall collapsed. In 2015 a fireplace fell after unusually timed, intense autumn rain. In 2018 the church lost 46 and 4 cubic feet from two exterior walls.

The fireplace in the convento at Tumacácori in March 2015 (left) and in October 2015, after the park received 3.62 inches of rain in four days (right). Images from NPS's page.

There is not enough long-term monitoring of cultural resources to blame climate change for the damage; building condition, materials and past treatments also matter. But from 2003 to 2012, the park's mean annual temperatures exceeded more than 90 percent of those of the previous century. Southwestern projections call for higher temperatures and less precipitation overall, but more intense downpours and more variability, including heavy rain at times of year that used to be dry.

The experiment

At the Desert Research Learning Center, NPS staff and volunteers built adobe test walls under the guidance of archeologists, masons and preservationists, with hydrologists, soil scientists, geographers and computers helping to design the tests. Adobe, from the Arabic for "the bricks," is one of humanity's oldest building materials.

Experts from NPS, the Forest Service and the Bureau of Land Management taught participants to mix adobe and pour it into forms to cure for at least four weeks, then split a few bricks to check they were dry inside. The bricks, of sand, clay and silt, came from a local materials center, and Tumacácori staff tested that they resembled the park's materials. The 800 bricks made 20 walls, each 31 inches wide, 40 inches tall and 16 inches deep, with mortar of the same material. Each wall had two special bricks, one with a soil-moisture sensor and one with a soil water potential sensor, to see how easily the bricks would dry. Then each wall was laser-scanned (LiDAR) to sub-centimeter accuracy, giving a 3-D model before the rain.

Agricultural Research Service staff built a portable rain tent, with a nozzle on an oscillating arm like a lawn sprinkler, controlled by a computer to produce three rain intensities based on the park's climate record. The 20 walls were split into four sets of five: three got a different intensity each and one was an untreated control. The tent moved from wall to wall, giving each a 30-minute shower.

A wall under a simulated rainstorm. Image from NPS's page.

What the scans showed

Within a week of the rain, each wall was scanned again, and comparing the before and after models gave the material lost, the depth of the largest cavity and the surface area affected. Loss rose with rain intensity, and statistical tests showed the results were strongly significant.

Material loss rises exponentially with rainfall in a 30-minute shower. Image from NPS's page.

The moisture sensors were less useful, probably because most of the rain ran off the walls instead of soaking in, and their readings moved with the day's temperature and humidity, making it hard to say what caused any rise. The soil-moisture sensors, placed higher in the wall, responded more than the water-potential sensors.

Lessons

• Bare adobe is vulnerable. In every 30-minute test the loss was high, which matters when several monsoon storms add up.
• The big jump comes early. There was a large difference in intensity and loss between the 1-year and 25-year treatments. Tests between those would show the rate of loss better.
• LiDAR works but costs time. One week of field work needs three weeks of processing, so finding the point where erosion in 30 minutes becomes worth the scanning would make monitoring more efficient. White 150 mm reference spheres on galvanized pipes kept each scan aligned to about 4 millimeters.
• Weather is a pest. Bricks left in low spots melted in natural rain before the experiment, so tarps were among the most important equipment. Making bricks and walls in winter and testing in spring would reduce the vigilance needed.

Adobe bricks melted in natural rain. Image from NPS's page.

The team valued the cooperation among hydrology, climatology, archeology, geography and preservation, and the "mini-Stonehenge" of walls draws visitors' questions, which gives rangers a way to talk about archeology and climate change.

What comes next

Planned work on the existing walls includes testing the cumulative effects of several storms, testing plaster additives and other protective treatments, and finding the rainfall at which management concern and the cost of scanning meet. Other building methods across the Southwest need tests too. The results apply mainly to Tumacácori and Fort Bowie National Historic Site, and walls simulating the puddled adobe at Casa Grande Ruins National Monument would need to be built differently, with no mortar. The team would also like to test how water infiltration affects buried artifacts.

The project was funded by NPS's Albright-Wirth Grant program and the National Center for Preservation Technology and Training, with the Forest Service, the Agricultural Research Service, the Bureau of Land Management and Archaeology Southwest.

Sources

• National Park Service, Sonoran Desert Network, "Measuring the Effects of Rainstorm Intensity on Adobe Walls in the National Parks" (https://www.nps.gov/articles/sodn_adobe_test_walls.htm); the source's unpublished-report references are not reproduced; rewritten in hubnx's own words.