Mark L. Johnson
Upper Wild Burro Canyon Watershed (7/31/22)
Hiked the Wild Burro Trail and Alamo Springs Loop Trail early this morning after nice monsoonal rains early this morning and yesterday. Got me thinking that a watershed and stormwater 101 would be beneficial.
A watershed is an area of land where all the precipitation that falls on it goes to a common outlet via washes, streams and rivers.
The 10 major watersheds within the Tortolita Fan are shown on the map (left). Precipitation that falls within these watersheds flows from the Tortolita Mountains to the I-10/Santa Cruz River.
The Wild Burro Canyon Watershed is highlighted in red. It has a total area of 7.88 square miles and extends from elevation at 4,524' in the Tortolita Mountains to elevation 2,040' at the I-10.
When it rains on the Wild Burro Canyon Watershed after a long dry period, the rain is first absorbed by the dry soil and will not make it to the washes. That is what I witnessed today, as there was no visible signs of water flow in the Wild Burro Canyon Wash after receiving 0.8" of rain over the last 24 hours. It will take a considerable amount of precipitation over a period of days to see flow in the Wild Burro Canyon Wash like we had last summer.
Most communities, including Marana, utilize the 100-year return frequency standard to protect homes and business from stormwater flows.
The 100-year return frequency means there is a 1/100 or 1% chance that a storm with a certain amount of rainfall and duration will occur.
The National Oceanic and Atmospheric Administration (NOAA) actually has a chart (right) that shows the rainfall amount and duration for various return frequencies for the Dove Mountain Area. For example, a 100-year return frequency would be equivalent to 2.35" of rain in 1 hour (see red arrows and box). If we got 3.22" of rain in 1 hour it would be a 1,000-year return frequency!
Most monsoonal rains are short duration and medium to high intensity. For example, yesterday we got 0.75" of rain in 1 hour which is a 1-year return frequency or 100% chance of occurrence.
Hydrologists and hydraulic engineers calculate the elevation of the 100-year return frequency flood and all homes and businesses must be located above that elevation. Some communities, like Marana, also include an erosion setback of 25-50' to give a little more protection. Stormwater facilities (cross gutters, culverts, plunge pools, etc) are designed to handle the 100-year return frequency flows.
Climate change and the aridifciation of the Southwest are changing the dynamics of preciptation events. We are now experiencing long dry periods that are briefly interrupted by short intense rainfall events. For example; back-to-back 300-year storm events in the Coachella Valley, CA and recent flooding in Las Vegas. Then there is the recent 1,000-year storm events in St. Louis and Kentucky.
Cities, towns, HOAs, etc. need to make sure stormwater facilities are free of vegetation, accumulated sand/silt and properly armored to make sure they are ready for the big one. Let's hope we do not experience any stormwater events greater than the 100-year return frequency.
Other Interesting Stuff Seen On Today's Hike
Below is a collage of photos taken this morning which include: (1) Wild Burro Trail Sign, (2) damp rocks with moss and other surrounding greenery, (3) caliche (which prevents water from seeping into the ground), (4) Wild Burro Canyon, (5) rare rattlesnake siting on Alamo Springs Loop Trail on a cool, damp morning, (6) staghorn cholla, (7) damp, green trail, (8) barrel cactus with blooms, (9) grass turning green again and (10) black-tailed jackrabbit.