Anthropogenic Impacts on Summer Precipitation in Central Arizona, U.S.A

Abstract

This article explores the possibility of urbanization- and irrigation-induced increases in summer precipitation totals in central Arizona. Maximum precipitation impacts are hypothesized to occur downwind of the Phoenix area in the Lower Verde basin. Results from statistical tests indicate that summer precipitation totals in the Lower Verde basin are greater than totals in nearby basins. Precipitation totals in the basin also appear to be equivalent to totals at more monsoon-impacted stations in eastern Arizona. While this research is preliminary, the results do provide encouraging evidence of the existence of anthropogenically enhanced summer precipitation in central Arizona.

Key Words:

• Arizona
• irrigation
• Phoenix
• precipitation
• urbanization

Notes

Note: N = the number of years from 1950 to 2000 with valid precipitation totals for the summer season.

^aComrie and Glenn (1997).

Note: Only totals from those years when both stations had valid data were used in the calculations. Positive values and negative values indicate higher and lower totals, respectively, at the Lower Verde basin stations. Bold, italicized values represent significant differences at α=0.05. Numbers in parentheses after these values represent the number of summers involved in the comparisons. Numbers in parentheses after the stations' names are the stations' elevations in meters above sea level. ^aComrie and Glenn (1997).

¹ Irrigated areas include turf and cropland. Brown (2001) presents actual evapotranspiration values for turf. Erie and colleagues (1982) present consumptive use estimates. Actual evapotranspiration was assumed to be 75 percent of consumptive use. Therefore, actual evapotranspiration for cotton fields and turf was 561 mm and 471 mm, respectively. Actual evapotranspiration for desert areas, which was 64 mm, was assumed to equal precipitation, since soil-moisture storage should be negligible after the extremely dry months of May and June. The precipitation value was the average of totals from 1989 to 2002 at six Arizona Meteorological Network (AZMET) stations (Coolidge, Litchfield, Maricopa, Phoenix Encanto, Phoenix Greenway, and Waddell) located throughout the Phoenix area.

² This difference was estimated using present day land-use and land-cover (LULC) data for the urbanized portion of the Central Arizona-Phoenix Long-Term Ecological Research (CAP-LTER) study region (Stefanov, Ramsey, and Christensen 2001), while also assuming that only a desert land-scape was present in 1850. The following lists the LULC classes and their respective area proportions and estimated actual evapotranspiration totals (mm) per monsoon season:

Xeric Residential (0.37, 64)

Commercial/Industrial (0.29, 64)

Mesic Residential (0.12, 476)

Canals (0.11, 64)

Asphalt and Concrete (0.08, 64)

Cultivated Grass (0.02, 476)

Water (0.01, 664)

An actual evapotranspiration total (476 mm) was available only for the turf-based classes (i.e., mesic residential and cultivated grass). This total was obtained from Brown (2001). The potential evapotranspiration total (664 mm) was used as the actual evapotranspiration total for the water class, while the precipitation total (64 mm) was used as the actual evapotranspiration total for the nonirrigated classes (i.e., xeric residential, commercial/industrial, canals, and asphalt and concrete).

³ The hypothetical water-surface coverage was based on the assumption that the proportions of actual evapotranspiration to potential evapotranspiration for irrigated crops and urbanized Phoenix were 0.85 and 0.30, respectively. Thus, the total area of the lake was 85 percent of the irrigated crop area plus 30 percent of the urbanized area. The actual evapotranspiration value for irrigated crops was 186 mm month⁻¹. Endnote 2 presents the proportions of the various LULC classes in urbanized Phoenix. The composite actual evapotranspiration estimate for urbanized Phoenix was 46 mm month⁻¹. The potential evapotranspiration value was 221 mm month⁻¹.

⁴ Saffell and Ellis (2002) determined that the Coolidge, Maricopa, Phoenix Encanto, and Phoenix Greenway AZMET stations were located in or proximate to irrigated areas, and that the Litchfield and Waddell AZMET stations were located in rural, quasidesert areas. Relative humidity and dry-bulb temperature data from 1989 to 2002 for all stations were used to calculate absolute humidity. Coolidge and Waddell had the highest (14.8 g m⁻³) and lowest (11.0 g m⁻³) average absolute-humidity values, respectively.

⁵ Since results from recent research (Rosenfeld 2000) have shown that the increase of a cloud's precipitation potential by anthropogenic CCN is a debatable impact, this article downplays possible precipitation-enhancing effects of Phoenix's CCN emissions.