Climate and Water at Tumacácori National Historical Park, Water Year 2018

Monitoring the Santa Cruz River at Tumacácori National Historical Park. Image from the National Park Service's page.

Tumacácori National Historical Park preserves the first missions founded in what is now Arizona, established by Padre Eusebio Kino in 1691. The mission stood near the Santa Cruz River, whose water let it grow grapes, grain, vegetables and medicinal plants and keep livestock.

Over the centuries, environmental change, land use and groundwater pumping took away much of the river's natural flow. Since 1972, though, the river has carried a steady flow of treated effluent from the Nogales International Wastewater Treatment Plant, 10 miles upstream. As a result, the park's stretch supports a rare southwestern cottonwood–willow riparian forest, one of the most endangered ecosystems in the country — although recent changes at the plant have reduced that flow.

The National Park Service's Sonoran Desert Network monitors streams at Tumacácori and three other park units — water quality, streamflow, the shape and makeup of the channel, streamside vegetation and aquatic invertebrates — to catch broad changes and warn managers early. It also tracks groundwater and climate, which are closely linked to the streams. Results are reported by water year, October through September.

Climate

Water year 2018 brought 12.18 inches of precipitation, 74% of normal (16.56 inches). February was wetter than normal and June–August close to normal; every other month was much drier. Temperatures ran warmer than normal except in summer, which was slightly cooler. A drought index showed the drought that began in 2000 continuing, though less severe over the last five years.

Groundwater

Groundwater is one of the Southwest's most vital resources, supplying drinking water and irrigation and sustaining springs, rivers and streams. At the park it is measured automatically and by hand in wells of different depths. In both monitored wells, 2018's average level was higher than the long-term average for that well, by 0.21 to 1.35 feet. Levels fell during the dry spring and rose quickly with monsoon rain and flows.

Groundwater monitoring wells at the park. Image from the National Park Service's page.

Streamflow

Measured at U.S. Geological Survey gauge 09481740, five kilometers downstream of the park, flow in the Santa Cruz was slightly below average. It was typical from October to May, fell during the hot, dry weeks of late May and June, and peaked on August 23, with smaller events on August 9, August 14 and September 19.

Retrieving sampling equipment from the Santa Cruz River. Image from the National Park Service's page.

Water quality

At the Santa Gertrudis index site, about 150 meters below the park's southern boundary, almost 98% of samples met state standards. There were two exceedances: one for E. coli and one for cyanide.

Aquatic insects and fish

On the Santa Gertrudis index reach, a 460-meter stretch just below the main unit, the Arizona Index of Biological Integrity has rated the park's segment at or below impaired since 2012, though Arizona has no threshold for effluent-dominated streams. Targeted riffle samples showed insect numbers declining from 2012 to 2018, a pattern not seen in reach-wide samples. The insect community also shifted: from 2012 to 2014 the most abundant insects were midges of the subfamily Orthocladiinae, and after 2015 those of the subfamily Chironominae. The reasons are unknown.

Western mosquitofish. Photo by Ryan Hagerty, U.S. Fish and Wildlife Service.

Collaborative sampling had found three fish there: longfin dace, Gila topminnow and western mosquitofish. In 2018 the measure for longfin dace fell while that for Gila topminnow and mosquitofish rose. The continued presence of these fish is a good sign that they are responding to better water quality and habitat on this reach.

Sources

• National Park Service, Sonoran Desert Inventory and Monitoring Network: "Climate and Water Monitoring at Tumacácori National Historical Park, Water Year 2018," summarizing E. L. Gwilliam, L. Palacios and K. Raymond, Status of climate and water resources at Tumacácori National Historical Park: Water year 2018 (2019).