Using Mulch to Prevent Heat Stroke in Desert Landscapes

Oct. 24, 2020

Thanks to the progressive efforts of local non-profits like Watershed Management Group and Tucson Water, our community is well versed in rainwater harvesting for the benefit of sustainable landscapes. In our urban setting, this is typically accomplished through basins-and-berms, curb cuts, and mulch-- all critical to helping rainwater infiltrate while reducing evaporation.

Recently, I’ve taken a deeper interest in mulch. Although its benefits are universally recognized, measurements of the benefits are not available in the common literature. In response, I dedicated a small part of my yard in the Tucson Mountains to host an experiment to measure exactly what happens when mulch is applied to the bare soil. Baseline conditions were established by monitoring

Baseline data demonstrated that shallow soil temperature could exceed air temperature by 15-20 degrees Fahrenheit in this south-west facing basin in the absence of rain. On days when air temperature peaked over 110 degrees, this meant uncovered soil temperatures could exceed 130 degrees. This makes sense given that bare ground is a more efficient store of energy than air for capturing incident solar energy confirming that yes- you can likely fry an egg on a sidewalk.

On July 23, my uncovered basin witnessed a heavy two-inch rainfall event. After the rain, the relative-temperature dynamic flipped with shallow-soil temperatures trailing air temperature by about ten degrees during the hottest part of the day. That effect lasted for close to 80 hours before soil temperatures started approaching air temperatures once again.

Given these observations, I suspect heating from solar radiation was being buffered by evaporation of captured rainwater until the store of shallow moisture (where my sensor was located) was exhausted. The net effect made me realize that soil is very much like a living organism that cools itself by sweating, and when that moisture runs out, temperatures rise like heat stroke in a dehydrated person.

No rainfall was witnessed over the following two weeks, so on August 3, I chipped some yard clippings and applied approximately a one-inch layer of mulch in the basin.

In the absence of rain, I immediately witnessed the relative temperature profile flip back to what was measured after the July 23rdstorm, with soil temperature dropping below air temperature by about ten degrees during afternoon peaks. At the same time, the trend in humidity loss in the shallow soil profile briefly flipped.

I learned that by adding a thin layer mulch, not only had I significantly cooled shallow soil, I also restored a temperature dynamic briefly witnessed after the July 23rd rain event in the absence of rain!  What was even more interesting is that this dynamic was maintained for over a week before soil temperature started approaching air temperatures during the hottest part of the day—much longer than the 80 hours recorded after the initial rainfall event in an uncovered basin (See figure). This confirmed that even a relatively thin layer of mulch can have a dramatic cooling effect on soil temperature while buffering moisture loss from deeper soil, and can play a critical role in preventing “heat stroke” in our desert landscapes!

I’ll soon be upgrading the experiment with capacitive moisture sensors better suited for studying water movement through the soil column. The system is offline as of October 2, but a full summary of my setup and results are available for review and download here: . For those who want to replicate this experiment, please see the description of the video for further details and code licensed under Creative Commons.