Dry years and a falling water table at Casa Grande Ruins National Monument

Casa Grande Ruins National Monument sits in the Sonoran Desert, where water is the main limit on plant and animal life. The Sonoran Desert Network monitors both climate and groundwater there, because the two are closely linked and together help managers judge the condition of the park's plants and animals and its cultural resources. The latest findings, for water years 2016 and 2017, show a monument that is getting drier above ground and below it.

The Great House at Casa Grande Ruins National Monument. Image from the National Park Service's page.

How dry is arid?

The monument's climate is classed as arid, using an aridity index. The index, used worldwide to sort climate zones, compares long-term average annual precipitation with average annual potential evapotranspiration, the amount of water that would evaporate or be used by plants if water were unlimited. On a scale that runs from hyperarid to subhumid, the monument falls at arid.

On a scale of hyperarid to subhumid, Casa Grande Ruins National Monument is classified as arid. Graphic from the National Park Service's page.

Findings are reported by water year, which runs from October to the following September.

Climate findings

Temperature. In water year 2017 air temperatures were generally above normal for most of the year, by as much as 7.0°F, except for slightly cooler-than-normal summer months (down to 1.5°F below). In water year 2016 temperatures were generally 1 to 4°F below normal from November through January, then soared in February and March relative to 1981 to 2010 normals. Minimum temperatures were generally above normal through the warm season, and extremely cold days (below 30°F) were less frequent than average in 2017, at 5 days against about 17.

Precipitation. Rainfall was below normal in both years:

Normals are the 1981 to 2010 climate data. In 2017 fall and winter precipitation was 69 percent of normal, winter rains tapered off early, and February and March rainfall was under 39 and 35 percent of normal. An extremely dry spring and September left the warm months' rain at 80 percent of normal, despite slightly above-normal July and August. In 2016 very little fell in December, February and March, normally among the wettest months, though June had about five times its normal rain (0.25 inches against 0.05).

Rainfall was below normal at the monument in water years 2016 and 2017. Image from the National Park Service's page.

The reconnaissance drought index, which measures drought severity and extent against long-term climate, reflects the extended regional drought since 2000. A five-year moving mean of annual precipitation from 1981 to 2017 suggests the monument had a brief water surplus in recent years, but the below-average rain in 2016 and 2017 pulled it below the 1981 to 2017 mean. Neither year had an extreme precipitation event (more than 1 inch).

Five-year moving mean of annual precipitation at the monument, 1981 to 2017. Blue shows periods of water surplus and red shows deficit. Graphic generated by climateanalyzer.org.

Groundwater findings

The monument lies in the north-central part of the Eloy groundwater subbasin, which has been in groundwater deficit since the early twentieth century: more water has been pumped out of the aquifer than has been naturally replenished. The results are substantial changes to natural flow, declining water levels and land subsidence, the gradual collapse, fissuring or slumping of ground where deep basins filled with sediment have been drained.

Groundwater is the drinking water source for both the monument and the neighboring city of Coolidge, so its loss could affect the water supply and the stability of structures at the monument. Groundwater levels near the monument in 2017 were 13.3 feet lower than in 2016, in line with a general downward trend since 1995. Land subsidence in the monument area from 2010 to 2016 was 0 to 1 centimeter. From March 2016 to April 2017 it was 0 to 1 centimeter across most of the monument and up to two centimeters in some areas. Only a few minor fissures are known in the Coolidge area.

Sources

• National Park Service, Sonoran Desert Network, "Climate and Groundwater Monitoring at Casa Grande Ruins National Monument, 2017" (https://www.nps.gov/articles/sodn_cagr_h20.htm), summarizing C. Filippone and K. Raymond's 2018 reports on the status of climate and water resources at the monument for water years 2016 and 2017; the Arizona Department of Water Resources subsidence map is left out; rewritten in hubnx's own words.