Equilibrium Evaporation Rate Research Articles

Evapotranspiration and surface conductance in a high elevation, grass-covered forest clearcut

Pinegrass is the dominant species in clearcuts in the dry southern interior of British Columbia and is the major competitor with coniferous seedlings. This paper examines water use in such a clearcut during two successive growing seasons and the response of surface conductance to environmental variables. Evapotranspiration was derived from eddy correlation measurements of sensible heat flux density and measurements of net irradiance and soil heat flux density. The Penman-Monteith equation was used to calculate hourly surface conductances of the clearcut. Leaf area index measurements were used to calculate mean stomatal conductances from surface conductances. Mean stomatal conductance was modelled using boundary line and non-linear least squares optimization techniques. The most successful model, developed from data from the first growing season and tested on measurements from the second growing season ( R 2=0.70), was obtained using nonlinear optimization with mean stomatal conductance as a function of saturation deficit at the leaf surface ( D 0) and solar irradiance. Mean stomatal conductance was also modelled as a function of saturation deficit at 1.3 m ( D) and solar irradiance. This model was less successful ( R 2=0.62) than that based on D 0. Saturation deficit and temperature were highly correlated at 1.3 m and at the leaf surface. No relationship between mean stomatal conductance and air temperature or volumetric soil water content was found. Hourly evapotranspiration rates during the second growing season, calculated using stomatal conductance models (developed from data from the first growing season) based on either D or D 0 agreed well with measured rates ( R 2 ≈ 0.8). Hourly evapotranspiration rates were highly correlated with hourly equilibrium evaporation rates ( R 2=0.84). The mean hourly value of the Priestley-Taylor coefficient α was found to be 0.81.

Trace gas exchange above the floor of a deciduous forest: 1. Evaporation and CO2 efflux

The eddy correlation method has great potential for directly measuring trace gas fluxes at the floor of a forest canopy, but a thorough validation study has not been yet conducted. Another appeal of the eddy correlation method is its ability to study processes that regulate and modulate gas exchange between the soil/litter complex and the atmosphere that cannot be probed with chambers. In this paper we report on eddy correlation measurements of water vapor, sensible heat, and carbon dioxide exchange that were made at the floor of a deciduous forest. The validity of the eddy correlation method to measure the emission of water vapor and CO2 from a deciduous forest floor is demonstrated by our ability to close the surface energy budget during periods that meet the requirements of the technique. Water vapor fluxes from a dry forest floor are strongly influenced by large‐scale turbulent events that penetrate deep into the canopy. The frequency of these turbulent events prevents equilibrium evaporation rates from being achieved because the dynamic time constant for water vapor exchange is longer. Consequently, maximal evaporation rates are capped to rates defined by the product of the driving potential of the atmosphere and the surface conductance. On the other hand, evaporation from a wet forest floor proceeds at rates reaching or exceeding equilibrium evaporation and are highly correlated with static pressure fluctuations. CO2 efflux rates are governed by litter and soil temperature, as expected. But we also find a significant correlation between static pressure fluctuations and soil/litter CO2 exchange rates.

Tree/pasture interactions at a range of tree densities in an agroforestry experiment. III. Water uptake in relation to soil hydraulic conductivity and rooting patterns

Eucalyptus grandis was planted in a Nelder fan design in November 1983 into a previously established pasture dominated by Setaria sphacelata cv. Kazungula, at the CSIRO Sanford Pasture Research Station, Queensland, Australia. Nine concentric rings of 18 trees were planted at radii of 4.4-61.6 m, giving a range of tree densities which decreased from 3580 to 42 stems/ha. Tree transpiration was studied at three tree densities (2150, 304 and 82 stems/ha, representing high, medium and low densities) over a 'drought' period of approximately 1 yr (Nov. 1985-Sep. 1986) and related to rooting patterns and soil hydraulic properties. Over the range of soil water contents studied, the ratio of tree transpiration rate to equilibrium evaporation rate (T/Esub(eq)) decreased linearly with decreasing mean soil water content at each tree density. To investigate the effects of soil hydraulic conductivity and root length density on the total transpirational flux, overall soil conductances (Ksub(s)) were calculated, with soil conductance in each horizon weighted according to the length of root in that horizon. At each tree density, decreases in the ratio T/Esub(eq) were related to decreases in ln Ksub(s) measured at 1.2 m from the stem. A more rapid decrease in T/Esub(eq) with decrease in water content observed at the low tree density was attributed to a greater decrease in Ksub(s) as mean water contents decreased. The greater decrease in Ksub(s) at low tree densities was associated with a larger proportion of water extracted and a higher proportion of total root length in surface soil horizons, which showed a greater decrease in hydraulic conductivity than subsoil horizons for the same decrease in water content.

Tree/pasture interactions at a range of tree densities in an agroforestry experiment. II. Water uptake in relation to rooting patterns

Patterns of water uptake throughout a drying period of approximately one year were investigated under trees and pasture at three tree densities in an agroforestry experiment, and related to tree and pasture rooting patterns and water use. A greater proportion of soil water was extracted from deep in the soil profile under the densely planted trees, owing to lower soil water contents in upper horizons and deeper and more dense rooting systems than at lower tree densities. As the drought period progressed, the ratios of tree transpiration rate and pasture evaporation rate to equilibrium evaporation rate tended to decrease at each tree density as soil water contents in upper horizons decreased, and an increasing proportion of water was extracted from deeper soil horizons. At each tree density, the rate of water uptake per unit root length was lowest in surface soil horizons and tended to increase with increasing soil depth. The rate of water uptake per unit root length tended to increase with time in deeper, wetter soil horizons and decrease with time in surface soil horizons as soil water content decreased.

Evaluation of the ventilated chamber for measuring evaporation from a forest

AbstractEvaporation from a regenerating forest was determined concurrently by atmospheric measurements of the Bowen ratio, soil water depletion, and by weighing lysimeter. The methods agreed closely over 18 days in spring and 11 days in summer. Accordingly, the Bowen ratio technique was then used as the control against which any effect on evaporation by enclosure of the lysimeter with a chamber of varied ventilation rate could be quantified hourly and daily. Accuracy of gas analysis was checked against lysimeter values.Daily evaporation by the lysimeter was generally unaffected by enclosure—on a few afternoons there was a statistically significant enhancement of hourly values by the chamber. This was accounted for by difference in turbulence between chamber and forest. The general agreement in daytime hourly values is attributed to the frequent occurrence of equilibrium evaporation (rate at which evaporation is independent of ventilation). At night, evaporation was higher during enclosure. Comparisons were not possible with rain or dew. Determination of evaporation by gas analysis agreed within about 5 per cent of lysimeter values during a dry period.We conclude that the ventilated chamber as used provides realistic estimates of evaporation by forests. We show how periods of bias can be anticipated and corrected by theory.

Water use by sheltered kiwifruit under advective conditions

Abstract The transpiration of 2 kiwifruit vines grown within sheltered orchards in the Nelson locality was estimated in late summer by measuring their water uptake for 2 days following excision. Total daily transpiration by vines with vertically projected canopy areas of 16–17 m2 varied between 80 and 100 liters of water per day (4.8–6.1 mm/day) and ranged from 1.6 to 2.4 times the equilibrium evaporation rate. Stomatal conductances on excised vines were lower than those of adjacent uncut vines after the first day. Since transpiration estimates using the Penman-Monteith formula agreed well with the measured water uptake by cut vines, this equation was used to predict the transpiration of adjacent intact vines. Advective enhancement of evaporation contributed between 21 and 49% of the daytime water use despite the presence of multiple, closely spaced shelterbelts. Transpiration continued throughout the night as a consequence of incomplete stomatal closure, so that in total, advected energy was responsible for between 0.44 and 0.59 of the total water consumption over 24 h. Because of the mountain ranges and draughted pastoral lands upwind of the orchards, together with the very windy conditions at the time of the experiments, these measurements are likely to provide a practical upper limit for the advective contribution to water use by kiwifruit grown under New Zealand conditions. A pragmatic approach to providing evaporation estimates for irrigation scheduling under advective conditions is discussed.

Evapotranspiration from Douglas fir stands exposed to soil water deficits

The rate of evapotranspiration from thinned and unthinned stands of Douglas fir was measured using energy and water balance methods. At high values of soil water storage in the root zone the evapotranspiration rate was approximately 80% of the equilibrium evaporation rate. Below a critical value of soil water storage the ratio of the evapotranspiration rate to the equilibrium evaporation rate (E/Eeq) tended to decrease linearly with decreasing soil water storage. The critical values of soil water storage in the root zone were 11.8 and 8.3 cm for the thinned and unthinned stand, respectively. Below these critical storage values, there was approximately 3.5 cm of water remaining in both root zones that was extractable by the trees. The relationship between E/Eeq and the fraction of extractable water in the root zone for both stands was very similar for sunny days. In this relationship, E/Eeq began to decrease when there was approximately 40% of the extractable water remaining in the root zones of both stands.

Activated release of alkalis during the vesiculation of molten basalts under high vacuum: implications for lunar volcanism

Knudsen cell-quadrupole mass spectrometry was used to study the high-temperature vaporization of Hawaiian basalts, plagioclase, tektites, and samples from the Allende meteorite. Procedures are described by which mass loss rates and vapor pressures of Na and K were measured quantitatively. Gas-rich glassy basalts were observed to vesiculate under vacuum over the 900–1000°C region and simultaneously evaporate alkalis in nonequilibrium fashion at rates (units of μg/g/hr) of approximately 200–300 Na and 75–250 K. Degassed residues of the same basalts demonstrated equilibrium evaporation rates (over the same temperature range) of 60–120 Na and 30–60 K. The gas-deficient plagioclase and tektite sample showed only equilibrium vaporization with rates of 60 Na, 10 K (plagioclase) and 10 Na, 5K (tektites) at 900–1000°C. The Allende meteorite vaporized at rates of 2400 Na and 200 K at 900–1000°C, possibly by the reaction of Na 2O and K 2O with C or S 2, or by the thermal decomposition of nepheline or sodalite. The nonequilibrium vaporization of alkalis from the gas-rich basalts is attributed to vigorous agitation of the melt during its vesiculation by a gas phase composed principally of SO 2, CO 2, H 2O, CO, and H 2S. The major gases released from the Allende meteorite at 900–1000°C are, in order of decreasing abundance, CO, S 2, CO 2, H 2O, SO 2, and H 2S. It is proposed that nonequilibrium vaporization of alkalis during the vesiculation of lunar lavas was responsible for the production of alkali-rich vapors which subsequently deposited plagioclase crystals in the vugs of lunar rocks. The vesiculative, nonequilibrium vaporization of Na and K during a lunar volcanic eruption should be expected to occur at a high rate upon initial extrusion of the lava into vacuum but then decrease by a factor of approximately three when degassing is nearing completion. Vaporization losses remain inadequate to explain the uniformly low alkali concentrations in lunar basalts.

Kinetics and Thermodynamics of the Deposition of Zinc Films on Germanium (110) and (100) Surfaces

The initial stages of the deposition of thin zinc films on the (110) and (100) surfaces of germanium single crystals have been studied. A mass-spectrometric molecular beam method was used in ultrahigh vacuum. The evaporation rate of the thin film was measured during depositions at various temperatures and beam intensities. On both substrates, the adcoverage approaches complete saturation of available adsorption sites before nucleation of bulk zinc starts. Nucleation therefore occurs on a composite substrate. On (110) substrates, the critical nucleation stage of thin film formation is determined by the coverage only. The thermodynamic parameters of the critical deposit, such as the equilibrium evaporation rate, the thermodynamic potential, and the differential binding energy, have been determined. These thermodynamic parameters determine the critical beam intensity necessary for nucleation. The deposition of zinc on (100) substrates is more complex. The critical nucleation state of the deposit is determined not solely by the coverage, but also by the deposition conditions of temperature and beam intensity. In such a case, thermodynamic parameters cannot be obtained by the present technique.

The adsorption of silver on tungsten (110)

Previous studies of the adsorption of metals on metals, by a mass spectrometric molecular beam technique, have been extended to the case of silver adsorbing on tungsten (110). The adsorption process has been studied by both the adsorption transient technique and the flash filament technique. Two adsorbed phases were observed, having heats of desorption of 66 and 41 kcal/mol. The more tightly bound phase saturated in coverage at 6 × 10 14 molecules/cm 2. The other phase showed no fixed coverage limit. The equilibrium total coverage, n a, at any value of impingement rate, İ, and substrate temperature, T, can be represented by the relation n a= I ̇ τ 1 1+ I ̇ τ 1 n 0 1+ I ̇ τ 2(T) n 0 , in which τ 1( T) and τ 2 ( T) are the mean-stay-times for adsorption in the two adsorbed phases and no is the saturation coverage in the tightly bound phase. It was also observed that at impingement rates greater than the equilibrium evaporation rate of the bulk crystal phase of silver, this phase nucleated and grew without requiring an observable critical supersaturation, with a material accommodation coefficient of unity, and epitaxially with the substrate.

Relaxation Time and Steady Evaporation Rate of Freely Falling Raindrops

Abstract Relaxation times and equilibrium temperatures and evaporation rates of stationary and falling water drops are predicted using beat and mass transfer analyses. Except possibly for very large drops the relaxation time is small and steady-state evaporation takes place over most of the lifetime of a drop. The steady-state temperature is equal to the wet-bulb temperature for both the stationary and the translating cases. The form of the ventilation coefficient measured by Kinzer and Gunn for low Reynolds number is predicted by an asymptotic theory of Acrivos and Taylor. Limiting analyses for heat transfer to spheres from fluids with high and low Prandtl numbers bracket the high Reynolds number data. Analytical expressions are suggested for predicting mass transfer rates at various Reynolds numbers.