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U.S. GEOLOGICAL SURVEY
Data Series 284

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Selected Evapotranspiration Data

ET data sets are available in appendixes I–M. Daily ET measurements are listed in appendix I. Appendixes J–M list the four principal energy budget components of latent-heat flux, sensible-heat flux, soil-heat flux, and net radiation during 15-minute intervals for 2002–05. Other field data, including air temperature, air-vapor pressure, soil-heat flux at individual flux plates, and soil-temperature and soil-water content above the flux plates, were collected and used to derive variable coefficients used in the calculations (appendixes J-M).

Daily Evapotranspiration and Energy Fluxes

Daily total ET is summarized in appendix I and is shown graphically in figure 20 for 2002–05. ET data collection started on February 13, 2002. Total recorded ET was 48 mm (based on 321 days) for 2002, 148 mm for 2003, 198 mm for 2004, and 233 mm for 2005 (appendix I). ET totals for each year in this desert environment are highly variable and dependent on available moisture primarily from precipitation and precipitation-derived water stored in the upper soil profile. A much smaller component of ET is supplied by water moving upward through the deep unsaturated zone to the land surface. ET totals also can include the net effect of daily recycling of atmospheric moisture and surface dew. In addition, moisture from localized convective storms can be advectively transported by katabatic winds over the ADRS. Thus, measured ET will include moisture inputs that are not limited to just precipitation measured at the site.

ET in 2002 (fig. 20) is limited throughout the year by a lack of precipitation (fig. 13). In figure 20, daily ET spikes after a rainstorm and gradually decreases until the next rain event. Elevated ET right after a storm is derived from evaporation of surface and near-surface soil moisture that supplements the increased plant transpiration. This high rate of evaporation can last for one to several days depending on the amount of precipitation. Depending on the duration and intensity of the rain event, deeper soil-moisture percolation may occur and maintain the decreasing daily ET curve for longer periods. This more sustained daily ET is attributed primarily to loss of root-zone soil-moisture by plant transpiration.

During 2002, available soil moisture sustained daily ET of less than 0.2 mm/d during summer when there is maximum energy available for ET. In late autumn 2002, with less energy available, daily ET was estimated at less than 0.05 mm/d indicating a lower base discharge, though these values are within the measurement error. In contrast to 2002 (fig. 20), with an average daily ET of 0.15 mm/d for the period of record, ET in 2003–05 (figs. 20) averaged about 0.5 mm/d. During 2003–05, ET increased substantially due to increased precipitation at the site or in the basin near the site that introduced moisture directly or through advection. Because 2002 was an extremely low precipitation year with maximum depletion of the upper several meters of soil moisture, data from 2003 can provide a comparison of measured ET to precipitation from a water budget perspective. Average daily ET for 3 years of record (2003–05) exceeded the available precipitation measured at the site by 0.086 mm/d. On an annual basis, ET during those three years averaged about 193 mm exceeding average precipitation of 161 mm by about 32 mm. ET uncertainty, (conservatively) estimated to be about 0.05 mm/d, or on an annual basis about 18 mm, would not account for all the excess water. This excess water between ET and measured precipitation needs to be attributed to some combination of long-term, upward-moving soil moisture from depth, additional ET from advection of precipitation moisture near the site but not recorded by the precipitation gage, and measurement uncertainties.

Figures 21A and B show the change in energy budget fluxes before and after a rain event of 22.1 mm, which occurred at the end of July 2003. The results of those changes on the daily ET (fig. 20) are shown for three days before (July 25–27) and the three days after (August 1–3) the rain event of July 28–31. Prior to the rain event, there was a fairly stable but low latent-heat flux with most available energy being used as sensible-heat flux (fig. 21). Latent-heat flux is the energy used to drive the ET process and the sensible-heat flux is the energy used to heat the air. Both the latent-heat flux and the sensible-heat flux partition the total available energy (net radiation at the surface of the earth less energy used to heat the soil). With little soil moisture for plant transpiration or soil evaporation, the energy used for sensible-heat flux would nearly equal the available energy. Conversely, with sufficient moisture the energy used for latent-heat flux would dominate. As shown in figure 21, under dry desert conditions (July 25-27), the latent-heat flux during mid-day is about 20 to 30 W/m², which is less than one-twelfth the sensible-heat flux, generating about 0.2 mm of daily ET (fig. 20). After the rain event, the latent-heat flux on August 1 dominates the sensible-heat flux on a cloudy day with limited available energy (fig. 21B). During the 3 days after the rain event (and for many days after), latent-heat flux levels were above pre-rain levels. During the 3 days, mid-day latent-heat flux decreased from about 130 to 80 W/m² and daily ET decreased from 1.7 to 1.0 mm as shown in figure 20 (for days 213, 214, and 215). The highest daily ET of about 2.6 mm (fig. 20) occurred during the last day of the rain event ending on calendar day 212 (July 31). During this day, rain ended at 1000 hours followed by clear skies with mid-day latent-heat flux energy exceeding 400 W/m², sensible-heat flux less than 130 W/m², and wet surfaces evaporating water and drying.

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