Data Series 284
U.S. GEOLOGICAL SURVEY
Data Series 284
Complete micrometeorological data sets are available in appendixes A–F of this report. Appendix A lists daily mean micrometeorological data and daily total precipitation for 2001–05. Hourly mean micrometeorological and hourly precipitation data for 2001–05 are listed in appendixes B–F.
Appendix A lists daily mean values of micrometeorological data computed from instantaneous readings sampled every 10 seconds during the day (8,640 samples). These daily mean values are equivalent to daily mean values computed by averaging hourly mean values (appendixes B-F) computed from (the same) instantaneous readings sampled every 10 seconds during the hour (360 samples). In this report, daily mean values are included only in appendix A. Any reference to daily mean values computed from hourly mean values can be looked up in appendix A. However, daily maximum and minimum values of micrometeorological data computed from instantaneous readings sampled every 10 seconds throughout a day are not equivalent to daily maximum and minimum values computed from hourly mean values of the same data. In this report, discussions of micrometeorological data will focus on the daily mean values (appendix A) with reference to daily maximum and minimum values from hourly mean values (appendixes B–F).
Daily maximum and mean solar radiation computed from hourly averaged values at ADRS are shown in figure 4. Complete daily and hourly solar-radiation data are in appendixes A–F. Solar radiation is the amount of incident radiation from the sun that reaches the surface of the earth at the point of detection. Both daily and hourly mean and maximum solar-radiation data in this report and solar-radiation data presented in the previous report (Johnson and others, 2002) are based on instantaneous readings sampled every 10 seconds. Earlier ADRS publications (Wood and Fischer, 1991, 1992; Wood and others, 1992; Wood and Andraski, 1992, 1995; Wood, 1996) also reported daily mean and maximum solar radiation derived from hourly mean values based on instantaneous readings sampled every 10 seconds.
The daily maximum and mean solar-radiation curves (fig. 4) show the annual solar-energy cycle and the difference between the daily mean and maximum for any given day of the year. The maximum radiation for the year occurs during the summer solstice in late June during clear sky days with maximum daily values generally averaging about 1,020 W/m2 during the 5 years of record. Conversely, during the winter solstice period in late December during clear sky days maximum daily values generally average about 580 W/m2 during the 5 years of record. The downward spikes in figure 4 show days where average daily solar radiation is reduced during extended periods of cloud cover. As indicated in figure 4, the difference between the daily maximum and daily mean time-series values varies from more than 720 W/m2 during the summer to an average of about 410 W/m2 during the winter.
Figure 4 shows a shift in the data set, which occurred on May 5, 2004, with the replacement of one pyranometer with another. The shift is about 5 percent of the maximum expected absolute error in natural daylight for this kind of sensor. During any given 1-year period an individual sensor can range ±2 percent.
Daily maximum and mean net radiation computed from hourly averaged values are shown in figure 5. Complete daily and hourly net-radiation data are in appendixes A–F. Net radiation is the difference between all downward and upward radiation fluxes of both short- and long-wave radiation. Net radiation is a measure of the energy available at the surface of the Earth that can be partitioned into such energy-consuming processes as heating the soil and air, ET, and plant growth. By convention, net radiation is defined as positive when downward components (incoming radiation) exceed upward components (outgoing radiation), and this typically occurs after sunrise and before sunset. Allocation of this net radiative energy varies with seasonal surface conditions. For example, in late spring after a rainstorm, more energy is used to evaporate water from wet soils and for plant transpiration than is used to heat the air and soil. Later in the summer under dry conditions almost all the net radiative energy goes to heat the air and soil.
During 2001–05, the daily mean net radiation generally ranged from about 20 W/m2 in early winter to about 160 W/m2 in early summer (fig. 5; appendix A). The daily maximum generally ranged from a low of about 290 W/m2 in early winter to a high of about 570 W/m2 in early summer. The mean net radiation that occurs from 8 a.m. to 4 p.m. PST also is shown in figure 5 to illustrate that a large part of daily net radiation available to the site occurs during the 4 hours before and after noon. This daily 8 a.m. to 4 p.m. mean radiation often is within 100 W/m2 of the daily maximum radiation.
Variations in measured net radiation values can occur over time as the transparent polyethylene shields on the net radiometer age and become less translucent from the UV radiation component of solar radiation or from the scouring from wind driven particles. Cracks in the shield will cause abrupt changes in net radiation values as shown in figure 5 from August 14, 2004, to October 25, 2004, when the shield was replaced. UV radiation can dry the polyethylene shield to the point where the shield is shattered from heavy rains during summer thunderstorms or from wind driven particles such as sand. Radiation values can increase or decrease when the radiometer is not level, such as when birds damage the shields or tilt the sensor head. An example of an abnormal increase in radiation can be seen from July 3, 2001, to September 5, 2001 (fig. 5). Most sensor problems can be easily fixed by replacing shields and leveling the sensors. The extended periods of poor-quality data reflect the timing of the occurrence of sensor problems with the scheduled field visits limited to quarterly visits.
Monthly maximum, minimum, and mean air temperatures computed from hourly averaged values are listed in table 1. Daily maximum, minimum, and mean air temperatures are shown in figure 6. Complete data for daily and hourly air temperatures are in appendixes A–F. During 2001–05, air temperatures averaged 18.3°C with the warmest month averaging 31.7°C during July and the coldest month averaging 6.4°C during December. From 2001 through 2005, the maximum hourly average temperature recorded was 44.7°C in July 2005 and the minimum temperature recorded was -8.5°C in January 2002.
Daily and seasonal temperature fluctuations are large at the study site. Differences between daily maximum and minimum temperatures commonly exceed 20°C as shown in figure 6. Differences between winter minimum and summer maximum temperatures exceed more than 40°C, and sometimes more than 50°C. The desert environment, with its high percentage of clear skies, has a large heating capacity from incoming solar (short-wave) radiation during the day, and a proportionally large heat discharge by terrestrial (long-wave) radiation during the night. The large gains and losses in energy on a daily basis and their relative variation on a seasonal basis are what regulate the temperatures that occur in this environment.
Daily maximum, minimum, and mean relative-humidity computed from hourly averaged values is shown in figure 7. Complete daily and hourly relative-humidity data are in appendixes A–F. Relative humidity is the ratio of the amount of water vapor in the air at a specific temperature (vapor pressure) to the maximum amount of water vapor that the air can hold at that temperature (saturated vapor pressure), expressed as a percent. Daily mean values ranged from 5 to 95 percent. In contrast, hourly mean values ranged from 1 percent during the drier summer months to 100 percent during winter storms. During mid-day hours in the summer, relative-humidity values of less than 10 percent are common in the Amargosa Desert where prevailing high summer temperatures lower the relative-humidity measurements of available vapor content of the air relative to what saturated levels would be if maximum water vapor were available in the area.
Daily mean vapor pressures computed from hourly averaged values are shown in figure 8. Complete daily and hourly vapor pressure data are in appendixes A–F. Ambient vapor pressure is the partial pressure exerted by water vapor present in the air and indicates the water-vapor content of the air under prevailing atmospheric conditions. The ambient vapor pressure is the product of the saturated vapor pressure and the relative humidity. Saturated vapor pressure is the highest concentration of water vapor that can exist in equilibrium over a free-water surface at that temperature. The data logger calculates the saturated vapor pressure in kilopascals from measured air temperature using an algorithm from Lowe (1977).
The mean saturated vapor pressures are higher during the summer months and lower during winter months (fig. 8) because warm air can potentially hold considerably more water vapor than cold air. For this reason, the summer rain events cause generally somewhat higher ambient vapor pressures than the larger winter precipitation. Figure 8 generally shows these higher ambient vapor pressures predominantly during the summer months. However, the mean ambient vapor pressure during the months of June through August average only about 0.15 kPa more than the mean ambient vapor pressure during the months of December through February, or about 0.60 kPa and 0.45 kPa, respectfully. Daily variations in ambient maximum and minimum vapor pressure from the mean vapor pressure typically are less than 0.15 kPa. During 2001–05, ambient-vapor pressures averaged about 0.5 kPa with daily means ranging from 0.08 to 2.1 kPa, while saturated-vapor pressures averaged 2.6 kPa with daily means ranging from 0.6 to 6.5 kPa. This large difference between saturated and ambient vapor pressure is an indication of the large water deficit that exists in this desert environment.
Daily mean wind speeds are shown in figure 9. Complete daily and hourly wind speed data are in appendixes A–F. Annual mean wind speed computed from daily mean wind speeds for the 5 years of record (2001–05) was 2.8, 2.9, 2.8, 3.0, and 2.7 m/s, respectively. During these 5 years, the mean wind speed was 2.8 m/s, and the maximum daily mean wind speed was 8.8 m/s on February 2, 2003 (fig. 9). The maximum hourly mean wind speed was 13.0 m/s and occurred on February 2, 2003. The distribution of hourly mean wind speed for the 5-year monitoring period is shown in figure 10. For example, 4 percent of the time the wind speed was below 1.0 m/s, and about 80 percent of the time the wind speed was less than 4.0 m/s. Winds of less than the anemometer threshold of 0.28 m/s are set to zero by the field recorder to indicate they are not measurable. Hourly mean wind speeds of less than 0.28 m/s occurred less than 0.6 percent of the time during the 5 years of record, as shown in figure 10. However, most days had at least one 10-second sampling interval with wind speeds below the instrument threshold giving most days a daily minimum based on a 10-second sampling interval of 0.0 m/s.
Daily and hourly mean wind-vector directions in degrees Azimuth of true north with standard deviation are listed in appendixes A–F. Mean horizontal wind-vector direction was calculated by vectorially summing the individual wind vectors consisting of wind magnitude and direction using available data logger commands. The daily mean wind-vector directions are shown in figure 11. Daily wind directions indicate seasonal variability and annual recurrent patterns for 2001–05. Wind at the ADRS predominantly was from the northwest from September through February, and generally associated with regional frontal systems moving in from the west coast during the autumn and winter seasons. Winds from March to September are more evenly distributed from the northwest, southwest, and southeast. The ill-defined summer to autumn winds have at times been observed (D.I. Stannard, U.S. Geological Survey, oral commun., 2006) to be strongly katabatic, blowing up the valley during the day and down the valley at night. Southwest and southeast winds typically are associated with the counter-clockwise rotation of subtropical lows moving inland from the west coast of Mexico or southern California.
Daily mean barometric pressure and hourly mean barometric pressure data are listed in appendixes A–F. Barometric pressures at the ADRS facility are corrected to sea level using an altitude of 847.2 m. Daily mean barometric-pressure values from hourly averaged values are shown in figure 12. The mean barometric pressure for 2001–05 was about 101.5 kPa. Higher pressures generally occurred during clear winter days and lower pressures occurred during storm periods and summer. An hourly reading of 103.77 kPa occurred on February 10, 2002, and an hourly reading of 99.48 kPa occurred on April 15, 2002.
Daily total precipitation is summarized in table 2 (from appendix A), and monthly total precipitation is shown in figure 13. Hourly precipitation is listed in appendixes B–F. During the times that the tipping-bucket precipitation gage malfunctioned, precipitation volumes from the waste-facility precipitation gage adjacent to ADRS were used. These substituted volumes for individual storm events were accurately measured and only the time distribution was estimated for one to several days. The procedure for estimating daily precipitation followed technical guidelines established by the U.S. Geological Survey.
Total annual precipitation averaged 130.3 mm for the 5 years of record (2001–05) with measured totals of 164.8 mm in 2001, 3.5 mm in 2002, 131.8 mm in 2003, 173.6 mm in 2004, and 177.7 mm in 2005 (table 2). Based on a combination of waste-facility and ADRS precipitation records for 25 years (1981–2005), the long-term average annual precipitation was 112 mm at ADRS. Annual precipitation for the previous 20 years of record (1981–2000) averaged 108 mm (Johnson and others, 2002). Precipitation during the 5 years of record (2001–05) was about 147, 3, 117, 115, and 158 percent, respectively, of the 25-year long-term average. The low annual precipitation in 2002 was associated with a La Niña event which occurred along the eastern Pacific Ocean during that year. Winter frontal systems typically account for 70 percent of ADRS precipitation (Andraski and Stonestrom, 1999, p. 459), whereas summer storms typically account for 30 percent of precipitation. In the Amargosa Desert, the predominant winter precipitation comes from regional winter frontal systems moving in from the west coast. However, in this desert environment, precipitation is scant and unpredictable because of the rain shadow effect of the Sierra Nevada Mountains, which similarly shadow Death Valley west of the Amargosa Desert. Summer precipitation, mainly from localized convective storms, is even more uncertain within different locations of the Amargosa Desert. Summer storms are dependent on water vapor transported into the area by southwesterly winds, which bring water vapor primarily from subtropical low-pressure systems from off the west coast of either Mexico or Southern California. Previous comparisons between the ADRS weather station and two National Oceanic and Atmospheric Administration (NOAA) sites (Wood and Andraski, 1995, p. 15) indicate monthly values also differ considerably between sites within the Amargosa Desert.
Monthly total precipitation is highly variable from year to year for a given month, and from month to month in a given year as shown in figure 13. For example, significant precipitation fell during January 2001 and 2005, but not during intervening Januarys. Other months also show monthly variability. In 2002, precipitation was minimal for the 12-month period, defining the driest year for the 25-years of record at the ADRS, although the other 4 years were wetter than normal and increased the long-term average annual precipitation.
Daily and hourly mean values of near-surface soil temperature and soil-heat flux are listed in appendixes A–F. Daily mean soil temperatures and soil-heat-flux values are shown in figures 14 and 15, respectively. The averaging soil-thermocouple probe measures the temperature in soil between the soil surface and the 8-cm depth. The soil-heat-flux plates measure thermal energy that enters or leaves the soil. As the thermal energy enters the soil and moves downward, the soil-heat flux is defined as positive, and as the thermal energy leaves the soil the soil-heat flux is defined as negative. The mean soil temperature for 5 years of record (2001–05) was 21.4°C. Daily mean soil temperatures ranged from 42.9°C on July 23, 2003, to -0.4°C on January 4, 2004 (fig. 14). The mean values of soil-heat flux for the 5 years of record were 1.3 W/m2. Daily mean soil-heat flux ranged from 16.0 W/m2 on July 10, 2001, to -19.0 W/m2 on October 21, 2004 (fig. 15).
Daily and hourly mean values of near-surface soil-water content are listed in appendixes A–F. Daily mean values of soil-water content are shown in figure 16. The mean soil-water content for 5 years of record (2001-05) was 0.05 m3/m3 (5 percent). The near-surface soil-water content ranged from a low daily mean of less than 0.016 m3/m3 (1.6 percent) during June, July, and August 2002 to a high of about 0.31 m3/m3 (31 percent) on January 4, 2005 (fig. 16). Spikes in the soil-water content shown in figure 16 are indications of rain events occurring at the research site (fig. 13).
When power availability is low, the soil-moisture probe does not operate and data loss occurs. The soil-moisture probe requires higher levels of power than the other sensors used at the weather station. Hourly averaged soil-water content values were obtained only during daytime when solar energy met power requirements (appendix B and start of appendix C). High resistance wiring and the solar panel were replaced on January 17, 2002. The new equipment enabled the batteries to meet the soil-moisture probe power requirements and continued soil-water content monitoring.
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