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Factors affecting dewfall, its measurement with lysimeters, and its estimation with micrometeorological equations I Huijie Xiao I Factors affecting dewfall, its measurement with lysimeters, and its estimation
... Following convention, all equations are written such that fluxes towards the surface are positive. For a detailed derivation the reader is referred to [36] or the Appendix A. ...
... Only the agreement between the EB and TVT equation is not satisfactory (Figure 9). In the BREB equation the heat and vapour conductance cancel out in the β term [36]. Hence, calculations with this equation are not affected by any difficulties in obtaining a correct value for g. ...
The energy balance (EB), turbulent vapour transport (TVT), Penman-Monteith (PM) and Bowen ratio energy balance (BREB) equation were used to estimate dewfall based on meteorological data. Initially there were big disagreements between the estimates from these four equations. However, after multiplying the heat and vapour conductance terms by 0.33 the agreement was much better. This implies that the disagreements derived from improper conductance values. Initially we did not consider the effect of atmospheric stability on the conductances. With stability correction the conductances were on average 0.5 times the values without stability correction. To arrive at the aforementioned 0.33, the conductances with stability correction still need to be lower by a factor of 0.66. The value of the von Karman constant and the relationships for the zero plane displacement and the roughness length we used in our conductance computations are widely used, but not the only possible ones. With different values and relationships also suggested in the literature one can reach this factor. However, it is also possible that our wind speed data contributed to the fact that the conductances we computed were too high. Their computation for a given canopy-atmosphere system requires wind speeds from a wind profile in equilibrium with the vegetation. This in turn requires an adequate fetch around the investigated surface. The highly varied vegetation in and around the site where the study was conducted makes adequate fetch rather doubtful. To obtain valid conductance values the atmospheric stability conditions must be considered, the appropriate values for the von Karman constant, the zero plane displacement and roughness length must be used, and there must be adequate fetch. The BREB equation does not contain a conductance term and therefore does not suffer from the problems just stated. The other three equations do. However, the BREB, like the EB and TVT equations, need the surface temperature which is not routinely measured. This then leaves the PM equation from which this temperature has been eliminated as the only option. Hence, in a future study dewfall estimates from the PM equation should be compared with direct measurements with a high precision weighing lysimeter.
Lysimeters are devices for measuring percolation of water through soils and sampling soil water for chemical analyses. Lysimeters have been used for over 300 years to determine water use by vegetation. Precision lysimetry for measuring evapotranspiration (ET) has developed mainly within the past 50 years. Weighing lysimeter designs are quite varied to suite individual research requirements. Surface areas from 1.0 m2 to over 29 m2 have been used. ET accuracy depends directly on the lysimeter area, mass, and the type of scale, but many lysimeters have accuracies better than 0.05 mm. Few weighing lysimeters exceed 2.5-m profile depth. Mechanical, floating, hydraulic, and electronic scales have been used in weighing lysimeters with varying types of data recording methods. Lysimeter wall construction can affect heat transfer to the lysimeter and water flow along the walls. ET accuracy of weighing lysimeters can be affected by many additional factors (personnel traffic, cultural operations, crop height, etc.).
Dewfall and evapotranspiration have been measured with a weighable lysimeter and computed through the energy balance equation, during day and nighttime; the Kazanskiy-Monin nomograph has been used for parameterizing the air sensible heat flux. The experimental data have been taken on a flat lawn, starting after an irrigation.The results show that the values of the computed mean net water flux are always significant, that is, their signs are always unambiguous. There is a general good agreement between derived and measured water fluxes, and a good coincidence between derived and computed dew onset, dew ending and dew duration.
based on ET estimations would allow limited groundwa- ter supplies to be used more efficiently for wheat pro- Winter wheat (Triticum aestivum L.) is one of most important duction. crops in the North China Plain. However, soil water deficit (SWD) often occurs due to lack of precipitation in its growing season. In this To calculate crop ET over seasonal time, it is essential study, we introduce two semiempirical approaches, a recharge model to simplify ET-estimated methods in the NCP due to and the crop coefficient (Kc)-reference evapotranspiration (ET0) ap- lack of enough weather, soil, and crop physiological proach, to estimate wheat actual evapotranspiration (ETa) under no data. Thus, it is convenient and suitable to use empirical SWD and slight and severe SWD conditions. The recharge model approaches to estimate crop ETa. These methods are allocated ET0 to reference evaporation and reference transpiration mainly based on the Kc-ET0 approach and on soil water as a function of leaf area index. In the model, ETa is limited by soil balance (SWB) calculation (Rana and Katerji, 2000). water content, and crop water extraction for ETa is distributed through When limited by soil water content, water uptake for the soil profile as exponential functions of soil and root depth. The crop transpiration from a point in a soil profile is an ex- Kc-ET0 approach regarded ETa under the SWD condition as a logarith-
A surface energy balance model with a multilayer canopy representation has been developed to simulate the vertical temperature distribution of the foliage elements and the soil underneath. This model allows one to simulate the radiative temperature of the complex vegetation-covered surface as a function of the angle of observation. The model was implemented for a two-layer canopy and has been tested by comparing simulated radiative surface temperatures (observed under a look angle of 45°), latent heat, sensible heat, momentum, and ground heat fluxes, and soil temperatures at different depths, with observed data, gathered in a maize field. The fluxes above the canopy were measured by eddy correlation methods. This paper describes the model structure and simulation results as compared with the field measurements. Except for the ground heat fluxes, all the simulated and measured parameters match reasonably well. Further, the model was used to study the sensitivity of the radiative temperature of the complex surface to variations in the roughness length z0, the leaf area index LAI, and the drag coefficient Cd. The radiative surface temperature turned out to be very insensitive to variations in Cd and LAI, and it was found that resulting variations in the radiative surface temperature fall well within the detection error of the remotely sensed surface temperature.
Duration and amount of dew under a variety of topographic and vegetative-cover conditions were studied on the Priest River Experimental Forest in northern Idaho during the summer of 1958. Dew was measured by a continuous-recording device that registered the change in weight of an expanded polystyrene (Styrofoam) block. Dew deposits ranged from zero on cloudy, windy nights to a maximum of 0.014 inch during a clear night following a day with rain. Monthly amounts of dew ranged from 7 to 29 per cent of the monthly rainfall. Dew was deposited only on the flat of the Priest River Valley and along the Benton Creek valley bottom. Dew was not deposited under the forest canopy or on the slopes.
1. We report laboratory studies that aim to characterize the microphyte community composition and metabolic features of crusts covering dunes in the Negev. 2. The crusts are formed by at least one moss, four blue-green (Cyanobacteria) and two green (Chlorophyta) algal species with Microcoleus sociatus being dominant. The sheaths of the latter procaryotic alga, together with a contribution by moss rhizoides and protonemata, are responsible for stability of the topsoil crusts. 3. Following moistening of the dry crust, CO2 release took place, even in the light, until positive net photosynthesis was achieved. This delay was mainly due to the high CO2 buffer capacity of the soil solution. In moist crusts, growth of the microphytes soon took place, and CO2 assimilation increased continuously. Net photosynthesis seemed to be adapted to relatively low light and temperature conditions. 4. Maximal photosynthesis was still possible with a crust water content equivalent to precipitation of 0.2-0.3 mm, but was suppressed at less than 0.1 mm. Dew and fog inhibition would be expected to allow photosynthetic activity and growth. Chlorophyll-related maximal rates of carbon gain were of similar magnitude to leaves of arido-active phanerogamous Negev plants. Area-related maximal rates reach more than 20% of desert shrubs. Thus, in the short term, carbon input into the ecosystem through the soil crusts can become substantial.
The evaporation and condensation coefficients of water are extensively analyzed considering also data hitherto not taken into account. From the performed evaluation, a decline of both coefficients with increasing temperature and pressure is derived. For water, the condensation coefficients is generally higher than the evaporation coefficient. Evaporation and condensation coefficients exceed 0.1 for dynamically renewing water surfaces, while the analysis reveals coefficients below 0.1 for stagnant surfaces. The influence of impurities and surface active substances, as well as the effect of the dynamic surface tension is discussed.
Computer models of the evaporation of dew and the drying of water drops on plant leaves were combined to simulate leaf wetness duration at a depth within a corn canopy that is critical for the onset of disease. These models, for use in the field, were adapted to run on hourly climate observations over a 5 month period (May–September) at different locations in Canada. The models were used to compute the probability of particular episodes occurring in any month, the return period of extreme episodes and the average temperature associated with different lengths of leaf wetness duration. The formation of dew was inconsequential in all but a few of the episodes of leaf wetness.The simulation did not exhibit any observable pattern between average temperature and the length of surface wetness. This is not surprising since evaporation in these models is driven by radiation and the vapour pressure deficit. However, analyses of episodes of leaf wetness duration exceeding 1 day provided evidence for a geographical and seasonal variation in leaf wetness. This variation was explained with reference to monthly normals at each location and by an examination of the synoptic patterns associated with extreme episodes. The analysis of monthly normals revealed the occurrence of high wind speeds at Saint John that act to prolong leaf wetness through the advection of sea fog.
Mineral soils of different textures, containing the same amount of water, exhibit very different thermal conductivities. However, when they are compared at the same moisture tensions, their thermal conductivities are similar. Moisture variations have a much greater effect on thermal conductivity of soils than bulk density and grain size.
Opportunities for comparing the water status of crops when the presence or absence of dew is the main environmental variable are rare. This paper reports the results of such a comparison made on successive days on a Paspalum dilatatum (Poir) pasture. Leaf water potentials (Ψ1) were measured with a pressure chamber, and the energy balance method was used to determine evapotranspiration. Crop resistances (rc) were calculated from the micrometeorological data.
The presence of dew halved the early morning crop resistances (1.7 compared to 0.9 sec cm⁻¹). The paspalum pasture did not appear to exercise stomatal control over evapotranspiration until most of the dew had evaporated. At ca 1000 hours rc was 1.6 sec cm⁻¹ on both days.
Leaf water potentials measured before sunrise were −1.3 bars and at noon were ca −13.0 bars on both days, but the pattern of change in Ψ1 between these tunes varied. In the absence of dew Ψ1 decreased rapidly to reach −12.0 bars at 0900 hours; whereas with dew present, relatively high Ψ1 were maintained until most of the dew had evaporated.
Dew, which is a significant feature of many agricultural regions, may thus have an important effect on the water balance of pastures and crops and thereby enhance production.
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Dew has broad relevance to physical geography and many human activities. This paper reviews the observation and simulation of rural and urban dew, and the implications of dew as a climatic phenomenon. There is no universal protocol for measuring dew (i.e., condensation on cooled surfaces), its component fluxes (dewfall and distillation) or guttation, which is 'dewdrops' exuded from leaves. The many methods that exist to measure dew and surface moisture range from simple visual assessment, to electronic wetness sensors, lysimetry and remote sensing. Most studies of dew are rural; urban dew data are rare and studies seldom address dew in patchy landscapes, e.g., a forest clearing. Hardware dew models are rare. Numerous numerical models exist to simulate or forecast dew on rural crops but few address dew for surfaces other than leaves, e.g., a cocoa pod or road. There is a general consensus in the literature that dew is reduced or absent in cities because of the urban heat island effect and reduced vapour supply. However, little data exist to test this. Given the broad relevance of dew to many topics, future studies of dew in complex landscapes, including urban areas, would prove valuable.
This paper describes an energy budget approach which allows the estimation of dew duration (DD) from standard weather station measurements of air temperature, dew point temperature, wind speed and cloud cover. Differences between observed and estimated values of DD were, on average, ⩽1 h for an exposed leaf and ⩽1.5 h for a shaded leaf.
Dew observations made with Duvdevani recorders (Duvdevani, 1947) at about 60 stations in India for a period of three years (1968–1971) are analysed to study seasonal frequency and total accumulation. An attempt is made to assess the extent to which dew accumulation could mitigate water deficiency in winter and augment soil moisture for possible agronomic exploitation.
This study investigates the role of dew in the moisture balance of a summer-dry climate. Dew was measured by means of the optical technique employing Duvdevani dew blocks during three summer months and one fall month in 1970. Results were compared with rainfall and potential evapotranspiration. Dew amounted to 12–14% of normal monthly rainfall in mid-summer. In the unusually dry year of 1970 it reached 154% of the rainfall in August. Dew was seen to increase with the number of clear nights and, thus, increased as clouds and precipitation decreased. Although dew was an available source of evaporative moisture until approximately 09h30 on the morning after a clear night, its total was only about 2% of monthly potential evapotranspiration. The amount of dew increased outward from a vertical forest edge as longwave cooling and wind effects increased.
The spatial distribution of leaves, stems and ears within a crop of winter wheat was determined and related to air movement and radiation. Wet and dry bulb air temperatures and leaf temperatures were measured at four heights within the crop. The quantity of dew deposited was determined by absorption on blotting paper and weighing. Although greater quantities of dew condensed in the upper regions of the crop most plant surfaces experienced dew periods of similar duration. On the ten occasions measured these lasted from 4 to 14 h with quantities of dew deposited ranging from 0.02 to 0.33 mm/night. Vapour pressure gradients indicated that the condensation above 60 cm occurred as dewfall and that below 60 cm as distillation, the former being normally about twice the latter. A surface-wetness recorder at a nearby agrometeorological station gave closely similar readings to the duration of surface wetness in the crop.
In this study results of dew recordings by means of the Kessler dew recorder above various soil surfaces under semi-arid climatic conditions in the interior of South Africa are reported. Dew formation on lawn was heaviest in autumn but lowest in late winter and spring. Because dew deposition totalled approximately 12 mm over the year—or only 2% of the average annual rainfall—on 167 days, it was concluded that dew under the local conditions could not have a significant bearing on the water economy of the soil. However, in late summer and autumn dew occurred on 20 days per month and dew formation continued for 12–15 h at night. It was assumed that these conditions may reduce transpiration stress and favour the germination and spread of pathogenic spores on the plant surfaces.Compared with the amount of dew on lawn (100%), dewfall in growing wheat approached 200% when the sampling plate was placed slightly below the surface of the crop. The ratio between dew above uncropped soil and on lawn was approximately 55% throughout the year.It is emphasized that continuous dew recordings in crops are useful for field studies concerned with the effect of weather conditions on plant growth and plant diseases. In addition, standard recordings of dew occurrence on lawn are also necessary for reference purposes and for evaluations of the relationship between weather conditions and dew formation.
Agricultural meteorology is related to the use of pesticides in three broad areas: (1) weather affects the transportation of pesticides, (2) weather affects the transformation of pesticides, and (3) weather affects the pest and thus the need for a pesticide.In most control programs, drift is an important variable in achieving safe and efficient pesticide placement. Wind, air stability, the type of application equipment, and the formulation of the chemical influence drift. We found a great deal of useful drift research in the literature. It appears that a further need is for more widespread distribution of this information accompanied by a nation-wide system of timely drift advisories for operational use of applicators.The physical processes involving the influence of weather on chemical transformation, degradation and removal are not as well understood as the processes involved in drift. Research is the big need in this area.Modification of the meteorological conditions of the pest microenvironment offers a real challenge. Agricultural meteorologists have been included on the team of agricultural research specialists and have made notable contributions to this effort for some crop pests. Further joint research along this line can give significant increases to agricultural efficiency and public safety by reducing the need for chemical pesticides.
Liquid water on the leaves of many crop plants plays an important role in the outbreaks of foliar diseases. While rain and irrigation contribute, the majority of wetting events in many areas results from cooling of leaves below the dewpoint temperature of surrounding air. Thus, most models of crop foliar diseases include factors related to both pathogen biology and environmental regulation of the presence of water on leaves. We measured and simulated leaf wetness formation to test the ability of a detailed, mechanistic soil-canopy-atmosphere model to predict wetness formation as a function of vertical position in the canopy, and to study the role of the near-surface wetness of soil beneath. Position of water formation is important because older, lower leaves tend to be more susceptible to some diseases, and the role of soil wetness has implications for the accuracy of mesoscale modeling of leaf wetness in lieu of in-field measurements. When random dispersion of leaves was assumed our simulation model generally underestimated water formation on leaves in the lower half of the canopy. Clumping of leaves within each horizontal layer of the simulation decreased water accumulation in the upper portion of the canopy and increased accumulation on lower leaves. Local near-surface soil wetness increased the duration of leaf wetness by about 2h in our simulations. Thus mesoscale simulations that do not explicitly include local soil moisture, including irrigation, may frequently underestimate leaf wetness duration.
Observations were made of dew and guttation droplets on the grass Holcus lanatus L. (Yorkshire fog) at a site in rural Norfolk, UK, in 1985. The mean overnight dewfall was 0.14 ± 0.02 mm (N = 40 nights). Guttation droplets found on grass blade tips had an average diameter of 1.49 ± 0.16 mm (N = 1200 measurements), compared with 0.20 = 0.02 mm for true dew droplets (N = 550 measurements). The average total volume of guttation water exuded per grass blade per night was 1.0 ± 0.3 × 10−7 dm3, which represents about 0.1 mm of precipitation; guttation supplied about the same amount of water as dewfall to a short grass surface. About 8% of the mean daily June–August net radiation in southern England would be needed to evaporate the average dew and guttation-derived leaf wetness, which totalled 0.25 ± 0.04 mm.Guttation amount was significantly correlated with soil temperature and moisture (P < 0.001, r2 = 0.874). The diameter of dew droplets was proportional to gravimetrically measured dewfall amount. The average total surface area of dewdrops on short grass was about 11 m2 m−2 ground area. The corresponding average for guttation was about 5 m2 m−2 ground area.The modal pH of a mixture of dewfall and guttation was 5.0–5.2. Chemical analyses of mixed samples of dew and guttation showed a ten-fold super-saturation with respect to CO2. The equilibrium partial pressure of SO2 in these bulked samples was very low (10−4−10−3 Pa), an order of magnitude lower than atmospheric concentrations. The half-life of SO2 with respect to oxidation in the dew and guttation mixtures was a few hours.
Passive dew harvesting and rainwater collection requires a very small financial investment but can exploit a free, clean (outside urban/industrial zones) and inexhaustible source of water. This study investigates the relative contributions of dew and rain water in the Mediterranean Dalmatian coast and islands of Croatia, with emphasis on the dry summer season. In addition, we evaluate the utility of transforming abandoned roof rain collectors (“impluviums”) to collect dew water too. Two sites were chosen, an exposed open site on the coast favourable to dew formation (Zadar) and a less favourable site in a cirque of mountains in Komiža (Vis Island). Between July 1, 2003 and October 31, 2006, dew was collected two or three times per day on a 1 m2 inclined (30°) test dew condenser, together with standard meteorological data (air temperature and relative humidity, cloud cover, windspeed and direction). Maximum yields were 0.41 mm in Zadar and 0.6 mm in Komiža. The mean yearly cumulative dew yields were found to be 20 mm (Zadar) and 9.3 mm (Komiža). Because of its physical setting, Komiža represents a poor location for dew collection. However, during the dry season (May to October), monthly cumulative dew water yield can represent up to 38% of water collected by rainfall. In both July 2003 and 2006, dew water represented about 120% of the monthly cumulative rain water. Refurbishing the abandoned impluviums to permit dew collection could then provide useful supplementary water, especially during the dry season. As an example, the 1300 m2 impluvium at Podšpilje near Komiža could provide, in addition to rain water, 14,000 L dew water per year.
The aim of this study was to investigate dew amounts in different urban landscapes and to examine the correlations between dew amounts and meteorological factors in Guangzhou, China. Results indicated that the dew amounts at fine night were different from landscapes to landscapes. A significant difference was found in average dew amounts between forest landscape and residential landscape (forest landscape and commercial landscape, industrial landscape and residential landscape). The highest mean dew amounts in urban area were observed in forest landscape (0.034 mm night−1), followed by industrial landscape (0.022 mm night−1), commercial landscape (0.013 mm night−1) and residential landscape (0.009 mm night−1), respectively. For maximum dew amounts, a similar relationship like the mean dew mounts was found in urban measured landscapes, whose values in turn were 0.104 mm, 0.08 mm, 0.03 mm and 0.109 mm, respectively. Both the mean dew amounts night−1 and maximum dew amounts in urban landscapes were significantly less than those (0.077 mm in mean value and 0.224 mm in maximum value) in their countryside. The mean dew amounts correlated positively with mean relative humidity; meanwhile it correlated negatively with daily evaporation and urban heat island, respectively. It was therefore concluded that urban forest landscape was an important site for dew deposit, urban environment was not favorable for dew condensation, and relative humidity, daily evaporation and urban heat island might be the most important three meteorological factors responsible for the dew amounts in urban area.
Microlysimeters were used to measure dewfall over seven nights on a grazed grass and clover pasture in the Collie River basin in southwestern Australia. The Penman-Monteith combination equation was used over the same period to estimate potential condensation. Measured dewfall ranged between 0.002 and 0.100 mm night−1; the highest recorded dewfall rate was 0.032 mm h−1. Dewfall was shown to increase with pasture height and biomass. Potential condensation was between 0.20 and 0.36 mm night−1. There was good agreement between measured and potential vapour flux trends. Low overnight wind speeds resulted in a low ratios of dewfall to distillation. The literature suggests that annual dewfall could be up to 12 mm and may occasionally contribute significantly to daily evaporation. However, these amounts are not significant when compared with the 694 mm average annual rainfall of the area. Hence dewfall is not considered to be a significant component of the water balance at this site.
During nighttime, latent heat fluxes to or from the soil surface are usually very small and the absolute amounts of dew deposition are accordingly very small. The detection of such small fluxes poses serious measurement difficulties. Various methods for measuring dew have been described in the literature and most of them rely on the use of artificial condensing plates with physical properties that are very different from those of soil surfaces. A system that detects the actual dew deposition on the soil surface under natural conditions would be advantageous and microlysimeters (MLs) appear to be the obvious answer. The objectives of this work were to test the adequacy of microlysimeters to estimate condensation amounts, and to compare these amounts with those measured by a Hiltner dew balance in order to validate the long term data collected using the latter. The research was carried out at the Wadi Mashash Experimental Farm in the Northern Negev, Israel, during two measurement periods. A micro-meteorological station was installed in the field next to a modified Hiltner balance. A microlysimeter with an undisturbed soil sample was placed nearby. During the first period, the depth of the microlysimeter was 15 cm while at the second period it was 55 cm. The results show that for measuring dew, the minimum depth of a microlysimeter should exceed the depth at which the diurnal temperature is constant, which for a dry loess soil in the Negev Desert is 50 cm.
Equations are presented which relate saturation vapor pressure to temperature for moist air. The equations are designed to be easily implemented on a calculator or computer and can be used to convert in either direction. They are more accurate than the commonly used Goff-Gratch equations for the meteorologically interesting region of 80 to +50°C. Equations also are given for the enhancement factor.
Evaporation from bare Sharpsburg s.c.l. was measured with precision weighing lysimeters at Mead, Nebr., during summer and fall, 1966. A number of drying cycles were included. Maximum daily evaporation was about 9 mm/day in August and about 3 mm/day in late October. The ratio E/Rn was frequently greater than unity especially when the lysimeters and a 1-ha area surrounding them was visibly wet and the surface of the adjacent fields dry. A significant advectional contribution of energy in indicated. Nocturnal evaporation accounted for 10 to 50% of the total daily evaporation, the greater proportions occurring in fall. Dew was detected almost every morning. A maximum dew deposition of about 0.5 mm was measured. The Penman method yielded lower estimates of bare soil evaporation than did van Bavel's combination method. Both types of estimates varied from lysimetrically measured evaporation by from 3 to 50% on a daily basis.
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Since 1990, agriculture in the five new federal states of Germany has experienced a fundamental structural change. As much as 10 per cent of the 6·2 million ha of previously intensively farmed agricultural land were set-aside abruptly. In the spring of 1991, a lysimeter trial (filled with soils common in the catchment area of the Elbe River), was set up to investigate the impact of set-aside on the water and solute balance. The studies proved that restricting agricultural use in areas previously farmed intensively by converting them into permanent or rotation fallow will result in measurable changes in deep percolation (ground-water recharge) and water quality in less than one year. The results of the lysimeter studies were extrapolated to calculate the effects of set-aside in a catchment area (about 2 500 ha) with similar meteorological and soil conditions. The calculations showed that increasing the area under rotation fallow from 8 to 15 per cent increases the nitrogen load of the stream draining the catchment by about 5 per cent. Copyright © 1999 John Wiley & Sons, Ltd.
The amount of dewfall and dewrise to a corn canopy has been estimated
over 7 nights by using the Bowen ratio energy balance technique and the
soil diffusivity technique, respectively.In addition, the distribution
of free liquid water within the plant community has been measured by
using Leick plates. By combining the Leick plate results with the
foliage area distribution of the plant canopy, the total amount of dew
deposition within the plant canopy can also be estimated. On average,
both techniques did not show a systematic difference and agreed to
within 10%. Moreover, it appeared that the nondimensionized dew profiles
within the stand showed a more or less similar shape for all nights.The
potential dew formation was calculated for all nights. On average, the
potential dew was 30% higher than the actual dew. This offers the
opportunity to make reliable total dew estimates, and also, due to
similarity, dew profile estimates for a corn canopy from simple
micrometeorological data.Model calculations have been executed, where
the total amount of dew as well as the mean dew profiles within the
canopy were simulated. Deviations from the total measured dew amounts
occurred from +10% until 30%. Moreover, it appeared that the simulated
dew profiles were strongly smoothed with regard to the measured ones.
The importance of dew to agriculture, together with the absence of dew
measurements at standard weather station sites, resulted in the
development of predictive models for dew formation in the Umatilla River
Basin, Oregon. Meteorological data were obtained at the Pendleton
Experiment Station in close proximity to dew-measurement devices, and
from the Pendleton National Weather Service Office. Dew measurements,
made during the months of April, May and June for the years 1971-76,
were compared with values of wind speed, relative humidity and minimum
temperature at the Experiment Station. All three variables were found to
have significantly different average values when dew occurred as opposed
to when dew did not occur. The duration of relative humidity above 90%
exhibited the strongest correlation with dew duration (r = 0.60). Using
these boundary values beyond which the occurrence of dew was unlikely,
dew occurrence was predicted with an accuracy of 81%. With the addition
of a boundary value for soil moisture, the predictive accuracy increased
to 90%. When the technique for forecasting dew occurrence was combined
with a regression model for estimating dew duration, the result was an
overall system for the prediction of dew duration (within 3 h of actual)
which was accurate 71% of the time. This predictive accuracy rose to 85%
for years when soil moisture data were available. Use of climatic data
from the Pendleton Weather Service Office led to diminished forecasting
accuracy, apparently due to differences in geography and differences in
height of the measuring instruments at the two stations. The forecasting
system developed for the Pendleton Experiment Station failed to produce
a statisfactory level of predictive accuracy when used at a nearby
Weston site. The findings of this research, together with the findings
of other similar studies, illustrate the difficulty of constructing dew
prediction models with general applicability and emphasize the need for
widespread dew measurements at standard weather stations.
Wind and temperature profiles for a wide range of stability conditions
have been analyzed in the context of Monin-Obukhov similarity theory.
Direct measurements of heat and momentum fluxes enabled determination of
the Obukhov length L, a key independent variable in the steady-state,
horizontally homogeneous, atmospheric surface layer. The free constants
in several interpolation formulas can be adjusted to give excellent fits
to the wind and temperature gradient data. The behavior of the gradients
under neutral conditions is unusual, however, and indicates that von
Kármán's constant is 0.35, rather than 0.40 as usually
assumed, and that the ratio of eddy diffusivities for heat and momentum
at neutrality is 1.35, compared to the often-suggested value of 1.0. The
gradient Richardson number, computed from the profiles, and the Obukhov
stability parameter z/L, computed from the measured fluxes, are found to
be related approximately linearly under unstable conditions. For stable
conditions the Richard on number approaches a limit of 0.21 as stability
increases. A comparison between profile-derived and measured fluxes
shows good agreement over the entire stability range of the
observations.
Micrometeorological flux-gradient relations have been used to deduce average evaporation rates for most hours of the Wangara experiment, using net radiation and ground heat transfer values. At night, dewfall is found to have been a common occurrence, at rates that averaged about 0·015mmh−1 and sometimes approached (but never exceeded) the value of about 0·07mmh−1 that has been proposed as a natural limit. The average nocturnal dewfall was about 0·22 mm. During the 44 days of the experiment, about 20mm of rain fell at the central site, augmented by about 9mm of dewfall. Evaporation amounted to about 26mm of water.
In this paper, a new dew model is presented which is easy to apply and requires only a very limited amount of synoptic data, viz., cloud cover, relative humidity, air temperature and wind velocity. In the dew model, the contribution of the fluxes of infrared radiation, sensible and latent heat to the energy balance of a dew plate are parametarised according to detailed micrometeorological measurements of Holtslag and De Bruin. The model is tested with measurements of dew on the glass surface of an automatic sequential dew sampler. Results of model calculations and measurements of dew occurrence show a fairly close correspondence. The frequency and duration of dew deposition over a year, for 3 places in the Netherlands, have been calculated with the model, using only synoptic data. The numbers of dewy nights and dewy hours were calculated for the year 1987. This resulted in about 220 dewy nights and 1600 dewy hours. Taking the hours of rain into account, it appears that about 75% of the time a surface is wet during the nocturnal hours because of dew or rain. This fact may be important for the quantification of the dry deposition of easily soluble air pollutants and for the effect of air pollutants on vegetation.
Apparatus has been designed to record the amount of water deposited on plant shoots by rain, dew and guttation, and how long the surfaces remain wet. These factors greatly influence the extent to which plants are infected by fungi causing such diseases as potato blight.
The water on a cut potato shoot, sealed into a water-filled chamber placed on a balance, can be weighed by recording, on a rotating drum, the changes in equilibrium of the beam. Deposits from rain appear rapidly; their persistence depends on the weather. In contrast, dew is deposited slowly over a long period and dries more rapidly. The heaviest dew deposit recorded was 6.9 × 10−3 g cm−2 compared with 9.6 × 10 g cm−2 for the amount of water retained during rain.
Dew variability during the autumn dewy season within a small arid drainage basin in the Negev Highlands, Israel, is studied. Dew measurements were carried out at 18 stations on four exposures, using the Cloth‐Plate Method (CPM) and Duvdevani dew gauges. The study also included periodical wind and substrate temperature measurements.
Dew variability within the drainage basin was high. Average daily dew values obtained by the CPM were between 0.07 and 0.31 mm, whereas dew duration ranged between 1.6 and 4.1 hours per dewy morning. Dew amounts monitored by the Duvdevani gauges were lower, between 0.09 and 0.20 mm. Both methods show, however, consistent variability and correspond to a similar pattern. Whereas near‐ground dew measurements were the highest at the hilltops and at the bottom of the sun‐shaded northern and western exposures, wadi bed stations and, especially, the south facing midslope station obtained the lowest dew quantities.
The near‐surface dew patterns are not in agreement with the classical model of both Geiger and Oke, which predicts high dew quantities at the wadi beds (due to nocturnal down‐slope wind) and at the lee side of the prevailing wind, i.e. the south‐facing midslope station. The low quantities at the south‐facing midslope station is explained by the paramount role of surface temperatures, whereas variability in radiational cooling is seen as responsible for the high near‐ground dew quantities at the hilltops and the low quantities at the wadis.
This conclusion is supported by dew measurements al 40 cm above ground. Dew measurements at 40 cm above ground at the south‐facing midslope station and at both wadi beds were significantly higher ( p < 0.05) than at 0.7 cm above ground. Facilitating an efficient radiational cooling, and beyond the impact of the surface temperatures, dew measurements at this height correspond to the classical model, highlighting the important impact of surface temperatures and ventilation upon near‐ground dew condensation in an arid drainage basin.
Incoming short‐wave radiation S , reflected short‐wave radiation α S and net radiation R were measured over bare soil and crops from 1957 to 1959, and net long‐wave radiation ( L ) was deduced from
For grass, α increased from 0·23 at solar elevation 60° to 0·28 at 20° with daily mean 0·26. For bare soil, the corresponding increase was from 0·16 to 0·19 with mean 0·17. In mid‐June, L for bare soil decreased from – 0·1 cal cm ⁻² min ⁻¹ during the night to – 0·4 cal cm ⁻² min ⁻¹ in the early afternoon. For long grass, in August, the corresponding change was from – 0·05 to – 0·22 cal cm ⁻² min ⁻¹ . Under clear skies the incoming long‐wave component varied much less than the outgoing component, and net flux L was closely related to surface temperature.
With a heating coefficient β = – dL/dR , the observed linear dependence of R on S in the absence of cloud may be expressed as
Where, formally, R = L 0 when S = 0. For grass, sugar beet and potatoes, β lay between 0·15 and 0·22 with a variation which may depend on wind speed rather than on crop. The value for dry bare soil was higher (0·41) because there was greater surface heating.
Measurements under clear skies and over grass at Cambridge and Kew agree well with Rothamsted values (β = 0·22, L 0 = – 5·9 cal cm ⁻² hr ⁻¹ ). Over Nebraska prairie, β = 0·25, L 0 = – 4·5 cal cm ⁻² hr ⁻¹ from selected observations during Projects Great Plains and Prairie grass.
Exact information about soil water flow is needed to quantify solute transfer within the unsaturated zone. Water flux densities are often measured indirectly, e.g., with water-balance, water content–change, or tracer methods, and, therefore, often predicted with notable uncertainties. Over the last years, direct lysimetry methods have been increasingly used to study water and solute migration in soil profiles. A large weighable lysimeter is the best method to obtain reliable drainage data, but it requires relatively high investment and maintenance expenses. To reduce cost and improve comparability with undisturbed sites, a new technology to collect large monolithic soil columns with a surface area of 0.5–2 m2 and a depth of 1–3 m as well as a containerized polyethylene (PE-HD) lysimeter station were developed. In addition, the station was fitted with a new high-precision weighing technique. In this paper, the latter is demonstrated with data from a newly constructed gravitation lysimeter. Besides recording rainfall and seepage, its weighing precision makes it possible to register mass input by dew, fog, or rime. It also permits a very accurate calculation of actual evapotranspiration. Because this new type of lysimeter allows a very high temporal resolution, it is ideally suited to develop and test models for soil hydrologic processes.
Passive dew collection experiments were initiated in late 2003 in the centre of The Netherlands within a grassland area. A specially designed 1 m2 insulated planar dew collector, set at a 30° angle from horizontal, was covered with a thin (0.39 mm) polyethylene foil and subsequently replaced with 4 mm polyvinyl chloride. A second dew collector, in the shape of an inverted pyramid, was constructed to reduce the view angle to only the nighttime sky. A simple surface energy-budget model and an aerodynamic model were used to simulate the dew collected by both collectors. The planar collector collected about 90% of the dew at the grass cover while the pyramid collector collected about 1.20% of the grass cover. The aerodynamic model was able to predict the amount of collector data to within 50% for the planar collector and 60% for the inverted pyramid collector. The pyramid collector design was able to collect about 20% more dew than the inclined planar collector.
The arid Elqui valley, part of the Norte Chico of Chile between 27°S and 33°S latitude, is located south of the hyper-arid Atacama desert. The region is characterized by complex terrain and great different of the surface properties, the latter due to cultivated and irrigated areas along the valley floor and sparse vegetation in the arid surrounding. Here the energy balance of the valley has been investigated to compare the evapo-transpiration of the cultivated area with the one of the natural vegetation and to estimate the dew deposition, which proves to be an important water resource in arid areas. Analyzes are based on two Bowen ratio and two eddy covariance stations, operated in the Elqui valley from 2000 to 2002 and in November 2004, respectively.The Bowen ratio of the natural vegetation is about β≈11, but much lower in the cultivated and temporally irrigated area along the valley floor. A typical value of the Bowen ratio of a field covered by potatoes is β≈2.5 after harvesting or during dry periods and is β≈0.7 to 1 after irrigation events or precipitation.For an area with natural vegetation, the annual accumulated evapo-transpiration reaches 65 mm yr−1, which is 65% of the mean annual precipitation and 3.5% of the equivalent net radiation Rn/L. For the cultivated fields along the valley floor, the annual accumulated evapo-transpiration reaches 750 mm yr−1, which is 40% of the equivalent net radiation, Rn/L, and about 100% of the sum of estimated irrigation and mean annual precipitation (I+P). The annual nocturnal dew deposition is about 5–10 mm yr−1 and, thus, serves as an additional water source for natural vegetation, especially during dry years.