Journal of Cave and Karst Studies - ISSN 1090-6924
Volume 61 Number 2: 108-120 - August 1999

A publication of the National Speleological Society

Microclimate Study of Kartchner Caverns, Arizona
Robert H. Buecher


A detailed two-year study of the microclimate in Kartchner Caverns determined that the most significant problem in maintaining the microclimate of the cave is the potential for drying out due to increased airflow. Two factors—a small, hypothesized upper second entrance and a slight geothermal warming of the cave—control natural airflow and increase the amount and intensity of winter air exchange.

The average amount of water reaching the cave is 7.9 mm/yr, only twice the amount lost by evaporation from cave surfaces. Kartchner Caverns has an average relative humidity (RH) of 99.4%. Useful measurement of RH required a dewpoint soil psychrometer rather than a sling psychrometer. Moisture loss from cave surfaces is proportional to relative humidity, and small changes in RH have a dramatic effect on evaporation from cave surfaces. A lowering of RH to 98.7% would double the evaporation rate and start to dry out the cave.

The volume of air exchange in the cave was estimated from direct measurement, changes in CO2 concentration, and temperature profile models. All of these methods are consistent with a volume of 4,000 m³/day entering the cave during the winter. During the summer, the direction of airflow reverses and the volume of air leaving the cave is much smaller than during the winter months. Surface air is almost always drier than cave air—only during the summer months when rain occurs does outside air contain more moisture. However, the rate of air exchange is greatly reduced during the summer, which minimizes any potential effect of increased outside moisture.

Radon concentrations in the cave are high enough to be of concern for long-time employees but not for the general public. Radon222 concentrations average 90 pCi/L and radon daughters average 0.77 Working Levels (WL) in the main part of the cave. During the winter, radon levels in the Echo Passage are up to six times higher than the rest of the cave due to the passage’s stable microclimate and limited air movement, which greatly reduces radon removal by plateout. Natural removal by ventilation is only a minor factor in determining radon levels in the rest of the cave.

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