Now that the monsoon is winding down, the common questions are, “How much did you get? And where does it go? How can an arroyo be four feet deep and running at 500 cubic feet per second one day and be dry two days later?” The following may help to explain some of the factors that affect our region.
Elevation
Rainfall variation in our area is studied by the US Department of Agriculture at several locations, including the Walnut Gulch Experimental Station near Tombstone. A network of rain gauges deployed over about 3,000 square acres provide data that demonstrates the relationship between rainfall amount and elevation. Higher elevations get more rain, and how much falls on a given place and at a given time varies greatly, as you see when you drive out to dry pavement on the edge of a summer storm.
Evaporation
Hydrology studies in southern Arizona indicate that only a small percentage of rainfall is retained in perennial streams or groundwater. About 95 percent of our rainfall evaporates back into the atmosphere, through a process referred to as envirotranspiration. By placing sensors at various heights above ground on “flux” towers, hydrologists can measure the rate of envirotranspiration within a given area. The time it takes for water to return to the atmosphere can be a few months or many years. In very dry places, like Walnut Gulch, more water evaporates than falls each year, and the soil often ends up drier than it was the year before. This goes on and on until the deep soils eventually dry out, and the area becomes a desert. Plants that cannot hold water within their roots for many years (until a wet one) can’t survive.
Groundwater recharge
Wells that tap groundwater are often taking water that is 10,000 years old and is not being replaced as fast as it is being pumped. Places like the central valley of California, or the areas around Phoenix, have pumped so much groundwater that the soil has fallen many feet as a result of subsidence. Even the very large Sierra Nevada mountains have risen measurable levels as the weight of the groundwater has been removed and the bedrock springs back up.
Most groundwater recharge occurs during the winter, when rainfall events are widespread and last for days at a time. Generally, winter rains falling high in the mountains infiltrate to deep bedrock, and then the water moves down the watershed, eventually emerging as surface water in streams. Or it may go deeper and become groundwater. This may take days, weeks, or months. However, estimates of this winter rain recharge indicate that it is not substantial.
Runoff
To measure runoff one needs to have a place where all the water that runs off a watershed can be measured. You might have noticed the cable bridges over some water courses—these are places where we can measure large flows. For a study of total runoff in relation to total rainfall, one needs to have similar, but smaller gauges on all the little creeks and gullies. Walnut Gulch is one such study site. Runoff is typically present only very soon after heavy monsoon rainfalls, and can be rapidly absorbed by the runoff channels. Walnut Creek reveals that, overall, the percentage of annual rainfall that goes to runoff is only about one percent.
Together with soil texture, latitude, wind patterns, air temperatures, and other conditions, these are factors that influence the long-term outcome of rainfall in our corner of Arizona.
Mark R. Stromberg, Ph.D, is Adjunct Professor with the University of Arizona.