Temperatures of springs in the vicinity of Crater Lake, Oregon, in relation to air and ground temperatures by Manuel Nathenson, 1990
The chemistry of springs in the vicinity of Crater Lake, Oregon, has been discussed by Thompson and others (1990) and by Nathenson and Thompson (1990). The purpose of this study is to extend those works by using careful measurements of spring temperatures to relate spring temperature to air and ground temperatures and by using these data to explore the notions of thermal, nonthermal, and cold springs.
The temperatures of springs involve a complex relationship between circulation path, flow rate, average annual air temperature, and local climate effects. A special class of springs known as thermal springs are ones “whose water has a temperature appreciably above the mean annual temperature of the atmosphere in the vicinity of the spring (Meinzer, 1923, p. 54.).” Meinzer does not define appreciable. Meinzer (p. 55) goes on to state that: “Nonthermal springs may be divided into (1) those whose waters have temperatures approximating the mean annual temperatures of the atmosphere in the localities in which they exist, and (2) those whose waters are appreciably colder.” The second group of nonthermal springs are cold springs. Waring (1965, p. 4) agrees with Meinzer that any spring that “is noticeably above the mean annual temperature of the air at the same locality may be classed as thermal” but uses 150F (8.30C) above mean annual temperature of the air to define thermal springs in the United States. Reed (1983, p.2) in the U.S. Geological Survey’s assessment of low-temperature geothermal resources of the United States uses a minimum temperature function that is 100C above the mean annual temperature at the surface and increases with depth by 25IC/km to define low-temperature geothermal resources. Muffler (1987) in a letter to the Director of the National Park Service concerning “Significant Thermal Features” proposes that 100C above mean annual air temperature be used to define thermal. In most situations, the question of using a numerical temperature criterion for thermal springs is not important, because measured temperatures are sufficiently anomalous. In the Cascade Range, however, large quantities of cold ground water can mix with thermal water making the magnitude of the temperature anomaly very small.
The definitions for thermal spring use mean annual air temperature, because there are abundant data available from weather stations. Conceptually, a more appropriate quantity for comparison to spring temperatures would be the average ground temperature at the surface. Although a small number of weather stations do measure ground temperatures, many more ground-temperature values can be calculated by projecting temperature versus depth data obtained in drill holes back to the surface. An advantage of this method is that it provides an integrated measure that is less sensitive to small-scale variations in soil properties. Bodell and Chapman (1982) used this method to study part of the Colorado Plateau. They found that ground temperatures had a lapse rate of -6.90C/km over an elevation interval of 1100 to 2900 m. The U.S. Standard Atmosphere (U.S. Committee on Extension to the Standard Atmosphere, 1976) assumes a lapse rate for air temperatures of -6.50CIkm from 0 to 11 km altitude, agreeing with the slope found by Bodell and Chapman (1982). Powell and others (1988) added mean annual air temperatures from weather stations to the data set of Bodell and Chapman and found that both had a lapse rate of about -7OCkm. Ground temperatures averaged approximately 30C warmer than air temperatures. Delisle (1988) gives a general rule that ground temperatures have a lapse rate of -7 ‘C/km in dry climates and -4 ‘C0km in wet climates, and values cited in Birch (1950, Table 5) generally agree. Thus ground temperatures are not necessarily the same as air temperatures.