Dynamics of Nocturnal, Daytime, and Sum-of-Hourly Evapotranspiration and Other Surface Energy Fluxes over Nonstressed Maize Canopy

Irmak, Suat

doi:10.1061/(ASCE)IR.1943-4774.0000360

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TECHNICAL PAPERS

Feb 19, 2011

Dynamics of Nocturnal, Daytime, and Sum-of-Hourly Evapotranspiration and Other Surface Energy Fluxes over Nonstressed Maize Canopy

Author: Suat Irmak, M.ASCE [email protected]Author Affiliations

Publication: Journal of Irrigation and Drainage Engineering

Volume 137, Issue 8

https://doi.org/10.1061/(ASCE)IR.1943-4774.0000360

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Abstract

The magnitude and driving forces of nocturnal evaporative losses,

E T_{c_{night}}

, and the interactions of other surface energy fluxes and microclimatic variables under various climatic, soil, and management conditions are not well understood. Such relationships are important for ecophysiological studies. This research attempts to investigate such relationships. Furthermore,

E T_{c_{night}}

can be a sizable portion of the daily total evaporative losses. Most empirical equations, especially ones that use solar or net radiation to estimate daily evapotranspiration (ET), either ignore or poorly treat the contribution of

E T_{c_{night}}

to the daily total ET. Neglecting

E T_{c_{night}}

can lead to errors in determining the daily or the sum-of-hourly

E T_{c}

(i.e.,

E T_{c_{SOH}}

) and can also cause cumulative errors when making long-term water balance analyses. In this paper, the magnitudes, trends, and contribution to the nocturnal surface energy balance of various microclimatic variables (air temperature,

T_{a}

; vapor pressure deficit,

V P D

; relative humidity,

R H

; and wind speed at 3 m,

u_{3}

) and surface energy fluxes (

E T_{c_{night}}

; soil heat flux,

G

; sensible heat flux,

H

; and net radiation,

R_{n}

); were quantified and interpreted for a nonstressed and subsurface-drip-irrigated maize canopy. The effect of microclimatic variables and surface energy flux components on the Bowen ratio energy balance system (BREBS)-measured

E T_{c_{night}}

and daytime evaporative loss,

E T_{c_{day}}

, were investigated in the growing season of 2005 (i.e., April 22–September 30) and 2006 (May 12–September 27). The nighttime evaporative losses were high early in the season during partial canopy closure because of increased surface soil evaporation and were also high later in the season during and after leaf aging, physiological maturity, and leaf senescence. The seasonal average nighttime evaporative losses for 2005 and 2006 were 0.19 and

0.11 mm / night

, respectively. Losses of 0.50 mm or more occurred in 2005 and 2006 on eight and seven nights, respectively. The seasonal total

E T_{c_{night}}

,

E T_{c_{day}}

, and

E T_{c_{SOH}}

in 2005 were 31, 612, and 642 mm, respectively. The

E T_{c}

values in 2006 were 16, 533, and 547 mm, respectively. In both years, the percent ratio of

E T_{c_{day}}

to

E T_{c_{SOH}}

usually was more than 80–85%.

E T_{c_{night}}

was affected primarily by

u_{3}

,

V P D

, and

T_{a}

. A strong relationship between

E T_{c_{night}}

and nighttime sensible heat was observed. Some of the largest ratios of

E T_{c_{night}}

to

E T_{c_{SOH}}

occurred on rainy nights with strong winds. Because of strong winds, the

E T_{c_{night}}

was high owing to the clear coupling among all energy exchanges within and above the canopy as a result of the mixing of the lower boundary layer of the microclimate. The results of this study showed that the

E T_{c_{night}}

can be up to 5% of the

E T_{c_{SOH}}

, even for a subsurface-drip-irrigated maize canopy in which the soil surface is usually dry, thus, less evaporative losses potential compared with the surface or sprinkler-irrigated surfaces in which

E T_{c_{night}}

would be expected to be considerably higher because of wetter surface conditions.

E T_{c_{night}}

needs to be quantified for different vegetation surfaces and management practices, surface wetting, and climatic conditions to better account for nighttime water losses and better understand nighttime energy balance mechanisms.

Introduction and Background

Actual crop evapotranspiration,

E T_{c}

, is often the dominant energy flux of the water balance on a field, watershed, or a regional scale. Over the last few decades, a great deal of attention has been given to develop methodologies and techniques to quantify daytime ET,

E T_{c_{day}}

, or daily average,

E T_{c_{24 h}}

, evaporative losses from different surfaces. However, most empirical equations, especially the ones that use solar or net radiation to estimate

E T_{c_{24 h}}

, either ignore or poorly treat the contribution of nocturnal (i.e., nighttime) ET,

E T_{c_{night}}

, to daily total (i.e., sum-of-hourly,

E T_{c_{SOH}}

), which can be a sizeable portion of the

E T_{c_{SOH}}

. The surface energy balance at night has generally received less attention than during the day, primarily because the absolute values of nighttime fluxes are assumed to be small. However, because energy balance components and air temperature and humidity near the soil surface are in a delicate balance at night, small changes in these variables might have important consequences to

E T_{c_{night}}

(Kobayashi et al. 2007). Thus, it is necessary to study the interaction and balance of various microclimatic and surface energy flux terms during the nocturnal period to determine the most influential factors on

E T_{c_{night}}

and the magnitude of

E T_{c_{night}}

for different vegetation surfaces and management practices.

Neglecting

E T_{c_{night}}

can lead to a considerable source of error in determining

E T_{c_{24 h}}

(Iritz and Lindroth 1994), depending on the surface. The quantification of the portion of

E T_{c_{night}}

relative to daytime evaporative losses can be used to develop strategies and management practices that conserve water through the reduction of nighttime evaporative losses. The quantification of

E T_{c_{night}}

could also have important implications in plant genetics and breeding as reduction of nighttime water loss, through more appropriate soil and plant management, and selection and breeding of short-rotation plant species and clones, which could be beneficial for increasing the efficiency of plant production in conditions with a limited water supply. Furthermore, a more accurate quantification of

E T_{c_{24 h}}

, by including

E T_{c_{night}}

for various vegetation surfaces, can aid water resources regulatory agencies, planners, and managers to better assess the availability and total losses of water resources. Also, understanding the nighttime energy exchange mechanisms are important for ecophysiological studies because nighttime evaporative loss, which is an indication of stomatal opening at night, will also subject plants to a longer exposure to pollutants, such as

O_{3}

, reducing plant health and productivity (Matyssek et al. 1995).

Nighttime evaporative losses can be attributable to plant transpiration and respiration, soil evaporation, and the evaporation of moisture that condensates on the plant canopy, and/or a combination of these functions. In many cases, nighttime evaporative losses are considered negligible because for many plant species, it is assumed that stomata are closed at night (Jarvis and Mansfield 1981). However, for some dicotyledons, stomata can remain open in the dark (Meidner and Mansfield 1965; Muchow et al. 1980). Open stomata in the dark have also been observed for cotton (Sharpe 1973), soybeans (Turner et al. 1978), kiwifruit and apple trees (Judd and McAneney 1986; Green et al. 1989), kenaf (Muchow et al. 1980), and maize (Mutiibwa and Irmak 2010). Snyder et al. (2003) observed significant stomatal conductance and transpiration for 11 of 17 plant species with a range of life histories (i.e., herbaceous annual, perennial grass, shrub, and trees), photosynthetic pathways, and habitats. Partial stomatal opening for a diverse

C_{3}

and

C_{4}

plant species (i.e., plants that produce 3-phosphoglycerate, fix three carbon dioxides are

C_{3}

plants. They are common in cool, wet, and cloudy climates, where light levels may be low, because their metabolic pathway is more energy efficient. Plants that fix four carbon dioxides instead of the normal three during glycolysis are

C_{4}

plants. They are common in hot, dry environments and have high water-use efficiency, allowing up to twice as much photosynthesis per gram of water than

C_{3}

plants. However,

C_{4}

plant metabolism is inefficient in shady or cool environments.) have also been reported by Caird et al. (2007). Nighttime transpiration rates are typically 5–15% of

E T_{c_{day}}

, although sometimes as high as 30% have been reported (Caird et al. 2007) on the basis of the gas exchange measurements of individual leaves, whole plant sap flow, and weighing or drainage lysimetry (Benyon 1999; Snyder et al. 2003; Bucci et al. 2004, 2005; Daley and Phillips 2006; Scholz et al. 2007).

The contribution of

E T_{c_{night}}

can be a substantial portion of the

E T_{c_{SOH}}

, and the magnitude of this portion shows significant variations with microclimatic conditions, vegetation dynamics, surface soil water status related to irrigation method and frequency, rainfall patterns, and interactions of these factors.

E T_{c_{night}}

has accounted for 8% of the

E T_{c_{24 h}}

for natural tall grass prairie vegetation (Sugita and Brutsaert 1991) measured by using an eddy covariance method in northeastern Kansas; 4.1% of the

E T_{c_{24 h}}

for a willow short-rotation forest stand in clay soil near Uppsala, Sweden, where

E T_{c_{night}}

was primarily attributable to soil evaporation (Iritz and Lindroth 1994); and from 1.7% to 14% of

E T_{c_{24 h}}

for irrigated alfalfa measured by using a Bowen ratio energy balance system (Malek 1992). Tolk et al. (2006) reported measured ratios of

E T_{c_{night}}

to

E T_{c_{24 h}}

ranging from an average of 3% for rainfed cotton to 7.2% for sprinkler-irrigated alfalfa on a seasonal basis measured by using precision weighing lysimeters. In the largest event, they found that

E T_{c_{night}}

was as much as 12% of

E T_{c_{24 h}}

, with single nighttime losses approaching 2 mm. Their analyses showed that virtually all

E T_{c_{night}}

was the result of imposed atmospheric conditions, primarily vapor pressure deficit,

V P D

, and that some

E T_{c_{night}}

was associated with sensible heat transfer to the canopy. The

E T_{c_{night}}

portion of

E T_{c_{24 h}}

measured by using weighing lysimeters for rainfed alfalfa was 8% in North Carolina (England 1963).

E T_{c_{night}}

was shown to vary with the season from 7 to 21% in the spring and from 0 to 15% in the summer in eastern Nebraska, as measured with precision weighing lysimeters by Rosenberg (1969). Benyon (1999) observed that in an 8-month period from late winter to midautumn, water use rates of

0.10 mm / h

or greater occurred on 24 nights for an irrigated Eucalyptus plantation in South Australia. On average, nighttime transpiration was 5% of the daily total transpiration. Nighttime transpiration rates as high as

0.5 mm / night

was reported by Rawson and Clarke (1988) for a nonstressed wheat canopy. They also showed that this transpiration rate varied substantially among genotypes of the same plant species and that the rate would be reduced considerably by the selection of genotypes with both low cuticular and low stomatal transpiration.

The author acknowledges potential limitations and/or challenges in measuring nighttime evaporative losses by using the eddy covariance system, BREBS, scintillometers, soil-water balance, or any other type of technique; however, nighttime losses are usually detectable with these techniques. Researchers commonly used a variety of methods to quantify

E T_{c_{night}}

, including the energy balance equation or BREBS (Severini et al. 1984; Malek 1992; Iritz and Lindroth 1994; Jara et al. 1998; Baille et al. 2006; Kobayashi et al. 2007) or the eddy covariance system (Sugita and Brutsaert 1991; Fisher et al. 2007; Novick et al. 2009). None of these techniques has a significant advantage over the other in measuring

E T_{c_{night}}

. For example, one may think the eddy covariance system advantageous over the BREBS; research findings prove that the flux measurements recorded with the eddy covariance system can be in considerable error at night when turbulence is low and intermittent (Goulden et al. 1996; Cienciala and Lindroth 1999; Fisher et al. 2007). With eddy covariance, a comprehensive evaluation of energy balance closure at 22 FLUXNET sites (Baldocchi et al. 2001) showed a mean imbalance of 20% that was greatest at night (Wilson et al. 2002). Furthermore, Falge et al. (2001) reported that weak turbulence, which occurs primarily at night, creates frequent gaps that may affect well over 50% of the eddy covariance data, making it extremely challenging to use the eddy covariance system to quantify

E T_{c_{night}}

. Thus, the eddy covariance system is not more robust or accurate in quantifying

E T_{c_{night}}

than other methods, and using surface energy balance principles such as BREBS may offer a better alternative.

A literature review revealed that the dynamics between

E T_{c_{night}}

and the microclimatic conditions for maize canopy under subsurface-drip-irrigated field environments has not been studied. The aforementioned research results indicated that the magnitude of

E T_{c_{night}}

relative to

E T_{c_{day}}

and

E T_{c_{SOH}}

changes with surface wetting (irrigated versus rainfed), season, species, and management practices for the same species. Specifically, a gap of knowledge exists in the quantification of

E T_{c_{night}}

and its relationship to the microclimate variables for maize grown on a subsurface-drip-irrigated surface, which has a drier soil surface and different environmental characteristics than rainfed and surface or sprinkler-irrigated fields. In this paper, the measurements of

E T_{c_{day}}

,

E T_{c_{night}}

, and

E T_{c_{SOH}}

for a nonstressed and subsurface-drip-irrigated maize canopy are presented, analyzed, and discussed. The contribution of

E T_{c_{day}}

and

E T_{c_{night}}

to the

E T_{c_{SOH}}

was quantified separately on a daily basis throughout two growing seasons. The relationships among

E T_{c_{night}}

and other surface energy balance and microclimatic variables, including sensible heat flux,

H

; soil heat flux,

G

; net radiation,

R_{n}

; vapor pressure deficit,

V P D

; air temperature,

T_{a}

; relative humidity,

R H

; and wind speed at 3 m,

u_{3}

for the entire growing seasons in 2005 and 2006 were investigated.

Materials and Methods

General Experimental Procedures and Site Description

Soil, plant, micrometeorological, and surface energy flux measurements, including evapotranspiration, were recorded in 2005 and 2006 at the University of Nebraska-Lincoln, South Central Agricultural Laboratory (SCAL), near Clay Center, Nebraska, (latitude 40°34′N; longitude 98°08′W; elevation 552 m above mean sea level). Clay Center is in a transition zone between subhumid and semiarid zones with strong winds. The soil at the experimental site is a Hastings silt loam (fine, montmorillonitic, mesic Udic Argiustoll), with a field capacity of

0.34 m^{3} / m^{3}

and a permanent wilting point of

0.15 m^{3} / m^{3}

. The particle size distribution consists of 15% sand, 62.5% silt, 20% clay with 2.5% organic matter content. A maize (Zea mays L.) hybrid (Pioneer 33B51) with a comparative relative maturity of 113–114 days was planted at a 0.76 m row spacing and a seeding rate of approximately

73, 000 seeds / ha

at a planting depth of 0.05 m with an east–west planting direction. In 2005, the maize was planted on April 22, emerged on May 12, reached full canopy closure on July 4, reached silking stage on July 12, matured on September 7, and was harvested on October 17. In 2006, it was planted on May 12, emerged on May 20, reached complete canopy closure on July 8, reached silking stage on July 15, matured on September 13, and was harvested on October 5.

The experimental field (13 ha) was irrigated with a subsurface-drip irrigation (SDI) system. The SDI system has an advantage over other irrigation methods in that surface soil evaporation is minimized because no surface soil wetting occurs by irrigation. The drip laterals were spaced every 1.52 m (i.e., every other plant row) in the middle of the furrow and at a depth of approximately 0.40 m from the soil surface. The effective rooting depth for maize in the experimental site was 1.20 m. Irrigations were applied twice or three times a week to replenish the soil-water content in the plant root zone to approximately 90% of the field capacity. The available soil-water in the top 1.20 m soil profile was kept between field capacity and the maximum allowable depletion (approximately 40% of the water holding capacity) to avoid plant water stress (Irmak and Mutiibwa 2009; Irmak et al. 2008; Irmak and Mutiibwa 2008).

Measurement of Microclimate Variables, Evapotranspiration, and Other Surface Energy Balance Components

The BREBS used in this study is part of the Nebraska Water and Energy Flux Measurement, Modeling and Research Network (NEBFLUX) (Irmak 2010) that operates 10 BREBSs and one eddy covariance system over various vegetation surfaces, ranging from irrigated and rainfed grasslands, irrigated and rainfed croplands with different tillage and management practices, to Phragmites (Phragmites australis)-dominated cottonwood (Populus deltiodes var. occidentalis) and willow stand (Willow salix) plant communities. The details about instrumentation, calibration, instrumentation type, sensitivity, resolution, and other characteristics provided in Irmak (2010) will not be repeated in this paper, and only primary instrumentation and measurement details will be briefly described. Actual evapotranspiration and other surface energy balance fluxes were measured with a deluxe version of a Bowen ratio energy balance system (Radiation and Energy Balance Systems, REBS, Inc., Bellevue, WA) during extensive field campaigns. The BREBS was installed in the middle of the experimental field with a fetch distance of 520 m north–south and approximately 280 m east–west. The prevailing wind direction at the site is south–southwest. The BREBS used an automatic exchange mechanism that physically exchanged the temperature and humidity sensors every 15 min at two heights above the canopy to minimize the sensor biases affect on energy balance calculations. The lower exchanger tube was kept approximately 1 m above the canopy throughout the growing seasons. The distance between the lower and upper exchanger tubes was kept at a constant distance of 0.90 m throughout the season. Variables that were measured with the BREBS included incoming and outgoing shortwave and longwave radiation and surface albedo (REBS Model THRDS7.1 double sided total hemispherical solar radiometer sensitive to wavelengths from 0.25 to 60 μm); net radiation,

R_{n}

(REBS Model Q*7.1 net radiometer sensitive to wavelengths from 0.25 to 60 μm); sensible heat flux,

H

; soil heat flux,

G

; wind speed and direction at 3 m,

u_{3}

(Model 034B cup anemometer with a wind speed range of

0 - 44.7 m / s

and a starting threshold of

0.28 m / s

, Met One Instruments, Grant Pass, OR); air temperature,

T_{a}

(REBS Model THP04015); relative humidity,

R H

(REBS Model THP04016); rainfall (Model TR-525 rainfall sensor, Texas Electronics, Inc., Dallas, TX); and soil temperature (REBS STP-1). Soil heat flux was measured with three REBS HFT-3.1 heat flux plates and three soil thermocouples. Soil heat flux plates were placed at a depth of 0.06 m below the soil surface. Three REBS STP-1 soil thermocouple probes were installed in close proximity to each soil heat flux plate. Measured soil heat flux values were adjusted to soil temperature and soil-water content as measured by using three REBS SMP1R soil moisture probes at a depth of 0.05–0.06 m.

Both the THRDS7.1 and Q*7.1 sensors were supplied with a constant breeze blown with a fan through a desiccant tube to keep air space inside the dome dry. All variables were executed with 30 s intervals and averaged and recorded every hour (Central Standard Time, CST) for energy balance calculations with a datalogger (Model CR10X, Campbell Scientific, Inc., Logan, UT) and a relay multiplexer (Campbell Scientific Model AM416). The BREBS was closely supervised with data downloading and vigorous maintenance procedures executed on a weekly basis. Maintenance included cleaning the thermocouples and housing units (i.e., exchanger tubes), servicing the radiometers by cleaning/replacing the domes when needed, checking/replacing the desiccant tubes, and making sure that the radiometers were properly leveled.

Data Analyses

Daily energy balance can be separated into day and night components, and the nocturnal surface energy balance at the soil/canopy surface can be written as

R_{n_{night}} - G_{_{night}} - H_{_{night}} - E T_{c_{night}} = 0

(1)

The actual evapotranspiration data analyzed were (1)

E T_{c_{day}}

, which represented the evaporative losses that occurred only during daytime; (2)

E T_{c_{night}}

, which represents the total evaporative losses at nighttime; and (3)

E T_{c_{24 h}}

or

E T_{c_{SOH}}

, which is the sum of the hourly evaporative losses for a 24 h period. A positive sign of the fluxes indicate energy flux away from the surface to the surrounding microclimate, and a negative sign indicates flux toward the soil or canopy surface from the air. Daytime was defined as the period during which the incoming shortwave radiation,

R_{s}

, was positive (i.e.,

R_{s} > 5 W / m^{2}

), and nighttime was defined as the period during which

R_{s} < 5 W / m^{2}

. Whereas it changed slightly during the season as a function of day of year, the daytime period usually covered the period from approximately 7:00 a.m. to 8:00 p.m. for most of the growing seasons. The measured micrometeorological variables (

T_{a}

,

R H

,

u_{3}

, and

V P D

) were graphed for both years and were separated into three time periods [i.e., daytime, nighttime, and 24 h (daily average)], and then their seasonal distributions were analyzed. The same procedures were followed for the measured surface energy flux components (

E T_{c}

,

H

,

G

, and

R_{n}

). The percentage of

E T_{c_{night}}

and

E T_{c_{day}}

relative to

E T_{c_{SOH}}

were quantified for both years. The relationship between

E T_{c_{night}}

and

E T_{c_{day}}

versus

E T_{c_{SOH}}

were also quantified. Finally, the relationships among

E T_{c_{night}}

and the primary micrometeorological and energy flux variables (

u_{3}

,

T_{a}

,

V P D

,

R H

,

H

,

G

, and

R_{n}

) were investigated.

Results and Discussion

Weather Conditions, Seasonal Distribution of Measured Evapotranspiration, Seasonal Cumulative Evapotranspiration, and Rainfall in 2005 and 2006

A summary of the measured primary meteorological data for the 2005 and 2006 growing seasons and the long-term (26 year; 1983–2008) average values are presented in Table 1. The year 2005 was drier than a normal year with a total rainfall 72% of normal. In 2006, rainfall from April through July (245 mm) was less than the long-term average (374 mm). From March to August 2006,

T_{a_{\min}}

was, on average, 1.3°C higher than the long-term average. Overall, 2006 was warmer and drier than an average year.

Table 1. Daily Average Meteorological Parameters Measured for March–October at Clay Center, Nebraska

Period	Meteorological variable	March	April	May	June	July	August	September	October
2005	$u_{3} (m / s)$	4.7	5.1	5.0	3.7	2.4	1.7	2.4	3.1
	$T_{a_{\max}} (° C)$	11.6	17.3	23.3	28.4	30.4	27.8	27.5	19.4
	$T_{a_{\min}} (° C)$	−1.9	4.5	9.6	16.5	17.8	16.7	13.4	5.7
	$R H_{_{24 h}}$ (%)	66.8	68.5	63.6	71.2	70.7	78.3	68.2	67.2
	$R_{s} ({W / m}^{2})$	150.4	206.5	259.0	279.3	283.0	223.1	207.8	145.6
	$R_{n} ({W / m}^{2})$	71.0	108.5	138.7	172.1	174.4	132.8	111.2	59.2
	Rainfall (mm)	52.4	64.4	41.7	77.1	69.6	60.4	42.6	32.2
2006	$u_{3} (m / s)$	4.6	4.9	4.7	3.2	1.7	1.5	1.9	3.4
	$T_{a_{\max}} (° C)$	9.1	20.5	24.3	29.6	30.3	27.8	22.9	16.1
	$T_{a_{\min}} (° C)$	$- 2.7$	5.2	10.8	15.9	18.3	17.0	9.7	3.4
	$R H_{_{24 h}}$ (%)	72.7	64.6	61.0	65.2	73.4	79.8	71.3	70.3
	$R_{s} ({W / m}^{2})$	157.6	214.0	256.3	288.4	278.4	224.8	181.8	120.4
	$R_{n} (W / m^{2})$	72.0	111.4	141.8	169.5	174.0	142.1	108.5	53.8
	Rainfall (mm)	4.3	46.5	60.1	54.2	83.8	118.9	75.9	21.8
Long-term (1983–2008) average	$u_{3} (m / s)$	4.1	4.4	4.0	3.5	2.9	2.6	3.1	3.3
	$T_{a_{\max}} (° C)$	10.5	17.0	22.5	28.1	30.3	29.2	25.3	18.3
	$T_{a_{\min}} (° C)$	$- 3.2$	2.4	9.3	14.6	17.3	16.3	10.7	3.6
	$R H_{_{24 h}}$ (%)	69.8	66.3	71.3	70.2	73.2	74.5	68.8	67.2
	$R_{s} ({W / m}^{2})$	156.6	196.0	225.0	259.8	259.8	228.5	184.4	131.1
	Rainfall (mm)	40.0	59.0	112.0	110.0	93.0	83.0	63.0	45.0

Note:

u_{3}

= wind speed at 3 m;

T_{a_{\max}}

and

T_{a_{\min}}

= maximum and minimum air temperature, respectively;

R H_{_{24 h}}

= daily average relative humidity;

R_{s}

= incoming shortwave radiation; and

R_{n}

= net radiation.

The seasonal distribution of BREBS-measured daily

E T_{c}

values for the 2005 and 2006 seasons are presented in Figs. 1(a, b), respectively. The growing season in 2005 was 23 days longer than the season in 2006 (162 days in 2005 versus 139 days in 2006). In both years, the hourly

E T_{c}

showed an increasing trend from early toward the midseason and then decreasing toward the end of the season (late September) as a result of physiological maturity and plant senescence. In both years, the seasonal maximum

E T_{c}

was measured in July. In 2005, the maximum hourly

E T_{c}

(

1.16 mm / h

) occurred on July 23 at 1:00 p.m. (CST). In 2006, the maximum hourly

E T_{c}

(

1.28 mm / h

) was measured on July 19 at 3:00 p.m. (CST). The energy fluxes and micrometeorological variables were plotted for these two days, as examples of diurnal courses for 2005 and 2006, in Figs. 2(a, b), respectively. On July 23, 2005,

E T_{c}

started to increase at 7:00 a.m. and peaked at 1:00 p.m. as

1.16 mm / h

. After 8:00 p.m. until approximately 7:00 a.m.,

E T_{c}

remained very small (

0.002 - 0.003 mm / h

) with some negative values, especially during early morning hours, attributable to condensation. Total

E T_{c}

for the day was

9.7 mm / day

. The day was extremely warm and dry with only 36.1% humidity and high

R_{s}

,

R_{n}

, and

T_{a}

, and a high

u_{3}

of

4.4 m / s

. The climatic conditions on July 19, 2006, were very similar to those measured on July 23, 2005, with a smaller

R_{s}

,

R_{n}

,

u_{3}

and similar

G

, but with a 1.4°C higher

T_{a}

and an approximately 6% smaller

R H

.

Fig. 1. Seasonal distribution of BREBS-measured daily actual evapotranspiration, $E T_{c}$ , for: (a) 2005 and (b) 2006

Fig. 2. Diurnal distribution of BREBS-measured actual hourly evapotranspiration and other energy fluxes on: (a) July 23, 2005; and on (b) July 19, 2006; measured microclimatic variables are included for the peak $E T_{c}$ hour

Compared to the diurnal trend of

E T_{c}

on July 23, the

E T_{c}

on July 19 showed a steeper increase and decrease over the course of the day, likely attributable to more rapid changes in

T_{a}

from morning toward late afternoon. Both

R_{n}

and

G

followed the same trends on both days, with smaller

R_{n}

and

G

values on July 19.

E T_{c}

exceeded the available energy (i.e.,

R_{n} - G

) on both days, indicating advective conditions. The ratio of

E T_{c} / (R_{n} - G)

(i.e., the magnitude of advection) exceeded 1.0. The ratio of

E T_{c} / (R_{n} - G)

was 1.30 and 1.52 on July 23 and July 19, respectively; indicating additional source of heat energy to the research field environment on both days was attributable to regional advection rather than local advection because the research field was surrounded by hundreds of hectares of irrigated maize in all directions. During these conditions, the additional energy was used (extracted from the air) to warm the surface (positive

H

). The extraction of heat from the air, therefore, could be attributed to the regional rather than local (in the near vicinity of the experimental field) advection. As Fig. 2 indicates, during nighttime and early morning, the flux of water vapor was occasionally downward (negative

E T_{c}

). During these conditions, the downward flux of water vapor could result in dew formation on the soil and plant canopy, especially in early morning, resulting in negative

E T_{c}

values. The contribution of the fraction of

R_{n}

to

G

was very small on both days. The fractions of the

R_{n}

that were used for

G

and

H

on July 23 were 5.4 and 28.7%, respectively and on July 19 were 6.4 and 48.2%, respectively.

The cumulative sum-of-hourly

E T_{c}

and cumulative daily rainfall in the 2005 and 2006 seasons, along with the long-term average cumulative rainfall, are shown in Fig. 3. In 2005,

E T_{c}

(641 mm) exceeded rainfall (307 mm) by a factor of 2.0. The

E T_{c}

/rainfall ratio in 2006 was 1.50. The rainfall was 70 and 84% of the normal (433 mm) in 2005 and 2006, respectively.

E T_{c}

in 2005 was higher than

E T_{c}

in 2006 (541 mm) because of the longer growing season and drier conditions.

Fig. 3. Measured cumulative seasonal actual evapotranspiration, $E T_{c}$ , long-term cumulative average rainfall, and cumulative rainfall for individual seasons in 2005 and 2006

Seasonal Distribution of Daytime, Nighttime, and 24-Hour-Daily Average Data for the Primary Micrometeorological Variables

Daily fluctuations in the primary microclimatic variables, including

T_{a}

,

R H

,

u_{3}

, and

V P D

, for the 2005 and 2006 growing seasons are presented in Figs. 4 and 5. The seasonal data summary for the same variables is presented in Table 2. The daytime air temperature values,

T_{a_{day}}

, were greater than daily or 24 h temperature,

T_{a_{24 h}}

, and nighttime temperatures,

T_{a_{night}}

, at all times in both years [Figs. 4(a) and 5(a)]. For most of the period, the

R H_{_{night}}

remained more than 60% in both years, whereas the

R H_{_{day}}

fluctuated between 25 and approximately 95%. The

R H_{_{24 h}}

fluctuated between

R H_{_{night}}

and

R H_{_{day}}

. On a seasonal average basis,

R H_{_{night}}

was approximately 21% and was 11% greater than

R H_{_{day}}

in both years. The wind speeds in all three periods showed a decreasing trend from April to toward midsummer in July and August and an increasing again toward September and October [Figs. 4(c) and 5(c)]. The highest wind speed in the area usually occurred in March–April, and the lowest

u_{3}

usually is observed in July– August. Six nights in 2005

u_{3_{night}}

exceeded

6 m / s

; four of those nights

u_{3_{night}}

exceeded

8 m / s

. In 2006, one night exceeded

6 m / s

and one night exceeded

8 m / s

. The ratio of

u_{3_{day}}

to

u_{3_{night}}

in 2005 and 2006 was 1.92 and 1.98, respectively; and the ratio of daily average wind speed,

u_{3_{24 h}}

, to

u_{3_{night}}

in 2005 and 2006 was 1.49 and 0.82, respectively.

Fig. 4. Seasonal distribution of daytime, nighttime, and daily (24 h) primary micrometeorological variables, including: (a) air temperature, $T_{a}$ ; (b) relative humidity, $R H$ ; (c) wind speed at 3 m, $u_{3}$ ; and (d) vapor pressure deficit, $V P D$ , for the 2005 season

Fig. 5. Seasonal distribution of daytime, nighttime, and daily (24 h) primary micrometeorological variables, including: (a) air temperature, $T_{a}$ ; (b) relative humidity, $R H$ ; (c) wind speed at 3 m, $u_{3}$ ; and (d) vapor pressure deficit, $V P D$ , for the 2006 season

Table 2. Measured Daytime, Sum-of-Hourly, and Nighttime Maximum, Minimum, and Average Values of Primary Micrometeorological Variables and Surface Energy Fluxes for 2005 and 2006

Variable	2005			2006			Two-year average
Variable	Maximum	Minimum	Average	Maximum	Minimum	Average	Maximum	Minimum	Average
$H_{_{SOH}}$	4.2	$- 2.76$	0.91	4.46	−3.57	1.03	4.33	−3.17	0.97
$H_{_{day}}$	4.4	$- 2.22$	1.24	4.66	−2.96	1.24	4.54	$- 2.59$	1.24
$H_{_{night}}$	$- 0.02$	$- 1.29$	$- 0.33$	0.05	$- 1.02$	$- 0.21$	0.02	$- 1.15$	$- 0.27$
$G_{_{SOH}}$	0.95	$- 0.68$	0.15	0.68	$- 0.70$	0.12	0.81	$- 0.69$	0.13
$G_{_{day}}$	1.6	$- 0.26$	0.46	1.12	$- 0.16$	0.45	1.34	$- 0.21$	0.45
$G_{_{night}}$	0.04	$- 0.89$	$- 0.31$	$- 0.04$	$- 0.74$	$- 0.33$	0.00	$- 0.81$	$- 0.32$
$u_{3_{24 h}}$	10.4	0.69	3.1	9.82	0.57	2.66	10.09	0.63	2.88
$u_{3_{day}}$	12.3	1.09	3.8	9.82	0.57	2.66	11.05	0.83	3.23
$u_{3_{night}}$	8.6	0.23	2.3	8.60	0.15	1.96	8.58	0.19	2.13
$T_{a_{24 h}}$	29.7	2.51	20.5	30.1	10.1	21.4	29.9	6.31	20.9
$T_{a_{day}}$	32.9	2.59	23.3	32.8	12.5	24.2	32.9	7.56	23.8
$T_{a_{night}}$	26.5	0.71	17.2	27.6	6.5	17.9	27.1	3.61	17.6
$R H_{_{24 h}}$	96.9	37.2	71.0	98.5	38.1	72.2	97.8	37.7	71.6
$R H_{_{day}}$	95.1	27.1	61.1	99.8	24.4	62.7	97.5	25.77	61.9
$R H_{_{night}}$	98.9	49.2	82.6	99.9	47.6	83.6	99.4	48.8	83.1
$V P D_{_{24 h}}$	2.29	0.08	0.87	2.88	0.03	0.90	2.58	0.06	0.89
$V P D_{_{day}}$	3.04	0.13	1.28	3.76	0.00	1.32	3.40	0.07	1.30
$V P D_{_{night}}$	1.24	0.03	0.39	1.84	0.00	0.40	1.54	0.01	0.39
$R_{n_{24 h}}$	0.32	0.03	0.21	0.32	$- 0.001$	0.22	0.27	0.015	0.22
$R_{n_{day}}$	0.58	0.12	0.43	0.58	0.01	0.43	0.58	0.07	0.43
$R_{n_{night}}$	$- 0.01$	$- 0.08$	$- 0.04$	$- 0.003$	$- 0.06$	$- 0.03$	$- 0.007$	$- 0.07$	$- 0.04$

Note:

H

= sensible heat flux (mm);

G

= soil heat flux (mm);

u_{3}

= wind speed at 3 m (

m / s

);

T_{a}

= air temperature (°C);

R H

= relative humidity (%);

V P D

= vapor pressure deficit (kPa);

R_{n}

= net radiation (mm);

24 h

= daily (24 h) average; and

SOH

= sum-of-hourly.

The vapor pressure deficit showed a significant fluctuation, not only during the season, but also from day to day, as a function of

T_{a}

and

u_{3}

[Figs. 4(d) and 5(d)]. In 2005, the daytime average vapor pressure deficit,

V P D_{_{day}}

, ranged from 0.13 to 3.04 kPa, with a seasonal average of 1.28 kPa. Whereas the nighttime average vapor pressure deficit,

V P D_{_{night}}

, was smaller than

V P D_{_{day}}

and

V P D_{_{24 h}}

, it had considerable magnitude, with respect to the nighttime conditions and the absence of

R_{n}

, ranging from 0.03 to 1.24 kPa, with a seasonal average of 0.39 kPa. A larger range of

V P D

values existed for all three analyses periods in 2006, with larger maximum

V P D

values and smaller minimum values, and the seasonal average

V P D

values were larger than in 2005 (Table 2). No clear increasing or decreasing trend was evident in

V P D

in the 2005 season, whereas it showed a decreasing trend from early season toward the late season in 2006.

Seasonal Distribution of Daytime, Nighttime, and Sum-of-Hourly Evapotranspiration and Surface Energy Fluxes

Seasonal distributions of measured daily

E T_{c}

,

H

,

G

, and

R_{n}

for the 2005 and 2006 seasons are presented in Figs. 6 and 7, respectively. The summary of maximum, minimum, and average

E T_{c_{SOH}}

,

E T_{c_{day}}

, and

E T_{c_{night}}

for 2005 and 2006 seasons are presented in Table 3. The magnitudes of the

E T_{c_{SOH}}

and

E T_{c_{day}}

were similar between years, with

E T_{c_{SOH}}

greater than

E T_{c_{day}}

at all times [Figs. 6(a) and 7(a)]. The

E T_{c_{night}}

ranged from 0 to 1.10 mm in 2005 and from 0 to 0.95 mm in 2006. Losses of 0.50 mm or more occurred in 2005 and 2006 on eight and seven nights, respectively. The largest single

E T_{c_{night}}

loss for nonstressed maize occurred during the night ending on June 28, 2005 when 1.10 mm of

E T_{c_{night}}

occurred, which was 14.1% of the

E T_{c_{SOH}}

of 7.8 mm and 16.4% of

E T_{c_{day}}

of 6.7 mm. In 2006, the largest

E T_{c_{night}}

occurred on May 26 as 0.95 mm, and this amount was 18.8% of the

E T_{c_{SOH}}

of 5.1 mm and 23.1% of

E T_{c_{day}}

of 4.1 mm. All the high

E T_{c_{night}}

values were measured during nights when

u_{3}

,

T_{a}

, and

V P D

were high. For example, on the night of June 28, 2005, the

u_{3}

,

T_{a}

, and

V P D

were

3.5 m / s

, 23.5°C, and 1.03 kPa, respectively. The seasonal average values for the same variables at nighttime were

2.3 m / s

, 17.2°C, and 0.39 kPa, respectively. Similar observations were obtained in 2006, that is, the high nighttime evaporative losses usually, but not always, were associated with high

u_{3}

,

T_{a}

, and

V P D

. Mean nighttime temperatures were similar in both seasons (17.2°C in 2005 and 17.9°C in 2006). In 2005, the total

E T_{c_{night}}

,

E T_{c_{day}}

, and

E T_{c_{SOH}}

were 30.6, 612, and 642 mm, respectively. In 2006, the

E T_{c}

totals were 15.8, 533, and 547 mm, respectively. The smaller proportion of

E T_{c_{SOH}}

as

E T_{c_{night}}

in 2006 might be attributable to the shorter growing season in 2006 (162 days in 2005 versus 139 days in 2006) or some combination of less favorable meteorological conditions for

E T_{c_{night}}

in 2006. Also, the nighttime average

u_{3}

values were smaller in 2006.

Fig. 6. Seasonal distribution of daytime total, nighttime total, and daily total (sum-of-24-h) measured: (a) evapotranspiration, $E T_{c}$ ; (b) sensible heat flux, $H$ ; (c) soil heat flux, $G$ ; and (d) net radiation, $R_{n}$ , for the 2005 season

Fig. 7. Seasonal distribution of daytime total, nighttime total, and daily total (sum-of-24-h) measured: (a) evapotranspiration, $E T_{c}$ ; (b) sensible heat flux, $H$ ; (c) soil heat flux, $G$ ; and (d) net radiation, $R_{n}$ , for the 2006 season

Table 3. Summary of Maximum, Minimum, Average, and Total (Sum-of-Hourly) Evapotranspiration

Variable	2005				2006				Two-year average
Variable	Maximum	Minimum	Average	Total	Maximum	Minimum	Average	Total	Maximum	Minimum	Total
$E T_{c_{SOH}}$ (mm)	9.48	0.63	3.98	642.4	9.89	0.37	3.97	547.1	9.68	0.50	1,190
$E T_{c_{day}}$ (mm)	9.12	0.65	3.79	611.9	9.64	0.13	3.86	532.5	9.38	0.39	1,144
$E T_{c_{night}}$ (mm)	1.10	$- 0.11$	0.19	30.6	0.95	$- 0.10$	0.11	15.8	1.03	$- 0.10$	46.4
% of ( $E T_{c_{day}} / E T_{c_{SOH}}$ )	104.5	66.6	94.2	NA	104.7	33.6	96.7	NA	104.6	50.1	NA
% of ( $E T_{c_{night}} / E T_{c_{SOH}}$ )	33.4	$- 4.5$	4.8	NA	66.4	$- 3.5$	2.9	NA	49.9	$- 4.01$	NA
% of ( $E T_{c_{night}} / E T_{c_{day}}$ )	50.1	$- 4.3$	5.0	NA	197.4	$- 3.4$	3.0	NA	123.8	$- 3.86$	NA

Note:

E T_{c_{SOH}}

, daytime

E T_{c}

(

E T_{c_{day}}

), nightime

E T_{c}

(

E T_{c_{night}}

), and NA = not applicable.

The magnitude of measured sensible heat loss is presented in Figs. 6(b) and 7(b). Early and late in the season in both years, as much as 4 mm, or more, sensible heat was used to warm the field environment during the daytime hours. During these conditions, additional heat was extracted from the soil (positive

H

) for evaporation. The daytime total sensible heat flux,

H_{_{day}}

, and daily total sensible heat flux,

H_{_{SOH}}

, gradually decreased starting from plant emergence to midseason, as the plant canopy closure increased, and the magnitude increased again toward the end of the season in both years. The increase in

H

toward the end of the season is most likely attributable to leaf aging, physiological maturity, leaf senescence, and the increase in the fraction of bare soil as the green vegetation decreased. The increase in

H

might also be attributable to less canopy evaporative cooling as a result of reduced transpiration. The nighttime total sensible heat flux,

H_{_{night}}

, fluctuated in a very narrow range. In 2005, the

H_{_{day}}

was higher than

H_{_{night}}

in the early season, from emergence until late May, and in the late season, from early September to the end of the month. In 2006, no considerable difference was observed between the two

H

values throughout the season. On a seasonal average basis,

H_{_{night}}

was

- 0.33 mm

in 2005 and

- 0.27 mm

in 2006 (Table 2). The

H_{_{SOH}}

was less than

H_{_{day}}

because of the inclusion of negative

H_{_{night}}

values in

H_{_{SOH}}

. Except for a few nights, the maize canopy was likely a sink for

H

(positive flux from soil surface toward the canopy) at nighttime throughout the season in both years.

It is clear from Figs. 6(b) and 7(b) that a consistent negative

H

was measured during midsummer in both years. It is likely that this pattern in

H

is related to the surface conditions. The fact that the surface soil layer (approximately the top 0.10–0.15 m) remained extremely dry (i.e., the soil-water content can be close to the wilting point in the top 0.10–0.15 m) in a subsurface-drip-irrigated field, especially during midsummer months, negative sensible flux was observed because the soil surface was dry but shaded and acted as a sink for heat energy (i.e., the shaded soil surface will likely absorb heat energy, resulting in negative

H

). The negative sensible heat indicates heat energy from the surface and air toward the soil. The author did not intend, in this paper, to separate the canopy transpiration and evaporation during nighttime periods because partitioning the

E T_{c}

into transpiration and evaporation is challenging, even in the daytime. However, it would be logical to assume that at least some of the

E T_{c_{night}}

values were a result of canopy transpiration because evaporation from the dry soil surface is not likely to be large. However, because the soil surface was wet during and a few days after any rain event, the soil evaporation and dew formation on the canopy were likely contributed to the

E T_{c_{night}}

.

The nighttime soil heat flux,

G_{_{night}}

, ranged from 0.04 to

- 0.89 mm

, with a seasonal nighttime average of

- 0.31 mm

. The contribution of

G

to

E T_{c_{night}}

was slightly higher in 2006, and the

G

values ranged from

- 0.04

to

- 0.74 mm

, with a seasonal nighttime average value of

- 0.33 mm

in 2006 [Figs. 6(c) and 7(c)]. The daytime net radiation,

R_{n_{day}}

, and daily net radiation,

R_{n_{24 h}}

[Figs. 6(d) and 7(d)] showed a typical pattern in both years, increasing from early season to midseason and decreasing again toward the end of the season with the physiological maturity and leaf senescence.

R_{n_{night}}

was a small portion of the

R_{n_{24 h}}

in both years, ranging from

- 0.10

to

- 1.03 mm

in 2005 and from

- 0.03

to

- 0.70 mm

in 2006. The seasonal average values were very similar (

- 0.46 mm

for 2005 and

- 0.37 mm

in 2006), and

R_{n_{night}}

was never positive during nighttime. Although it is not very obvious or considerable, a tendency for

R_{n_{night}}

to increase from early in the growing season toward the midseason was observed in both years.

Percent Ratios of Nighttime and Daytime Evapotranspiration to Sum-of-Hourly Evapotranspiration

Percent ratios of

E T_{c_{night}}

and

E T_{c_{day}}

to

E T_{c_{SOH}}

for the 2005 and 2006 seasons are presented in Fig. 8. The relationship between

E T_{c_{day}}

and

E T_{c_{SOH}}

was very strong for both seasons (

r^{2} = 0.99

). No relationship was evident between daytime and nighttime

E T

. The percent ratio of

E T_{c_{night}}

to

E T_{c_{SOH}}

ranged from 0 to 33.4% in 2005 and from 0 to 34.6% in 2006. The seasonal average ratio of

E T_{c_{night}}

to

E T_{c_{SOH}}

for 2005 and 2006 was 6.1 and 3.5%, respectively. For most of the season, in both years, the percent ratio of

E T_{c_{day}}

to

E T_{c_{SOH}}

was more than 80–85%. The percentage was lower in the early season during partial canopy from early April until mid-June because of increased bare soil evaporation. The percentage remained more than 90% until late August and decreased again during and after physiological maturity and leaf senescence. This pattern was also observed for

E T_{c_{night}}

. The largest percentage of

E T_{c_{night}}

to

E T_{c_{SOH}}

(33.4%) in 2005 occurred on May 11, one day after emergence and 19 days after planting, with the following nighttime average microclimatic conditions:

E T_{c_{night}} = 0.50 mm

;

E T_{c_{SOH}} = 1.5 mm

;

E T_{c_{day}} = 0.98 mm

;

u_{3} = 8.6 m / s

;

V P D = 0.25 kPa

;

R H = 87.4 %

;

T_{a} = 16.6 ° C

;

G = - 0.33 mm

; and

H = - 0.71 mm

. The greatest values measured for wind speed occurred during the nighttime for the entire season. A total of 35 mm of rainfall was recorded, starting from 6:00 p.m. on May 11 until 4:00 a.m. on May 12. Wetter than normal surface soil conditions resulted in higher

E T_{c_{night}}

than normal on the night of May 11 until the early morning hours of May 12, 2005. The largest percentage ratio of

E T_{c_{night}}

to

E T_{c_{SOH}}

in 2006 was measured on September 21 as 34.6%, with a relatively high

u_{3}

of

3.2 m / s

, a moderate

T_{a}

of 15.0°C, an

R H

of 83.4%, a 0.32 kPa

V P D

, a soil heat flux value of

- 0.14 mm

, and

- 0.35 mm

sensible heat. A total of 17 mm of rainfall was recorded from 8:00 a.m. until 10:00 p.m. on that day, with no rainfall even for the previous 10 days. Despite rain on the nights of May 11 and September 21, the

E T_{c_{night}}

rates were relatively high because of strong winds. The strong winds on these two days resulted in a high

E T_{c_{night}}

because of the clear coupling among all energy exchanges (fluxes) within and above the canopy attributable to the mixing of the lower boundary layer of the atmosphere.

Fig. 8. Percent ratios of nighttime and daytime evapotranspiration, $E T_{c_{night}}$ and $E T_{c_{day}}$ , to sum-of-hourly evapotranspiration, $E T_{c_{SOH}}$ , for the 2005 and 2006 seasons

Fig. 9 represents the relationship between

E T_{c_{night}}

versus

E T_{c_{SOH}}

and

E T_{c_{day}}

versus

E T_{c_{SOH}}

for the same period. On the basis of regression analyses,

E T_{c_{SOH}}

was a poor indicator of

E T_{c_{night}}

in both years with low

r^{2}

values. The relationship between

E T_{c_{day}}

and

E T_{c_{SOH}}

was strong for both seasons (

r^{2} = 0.99

).

Fig. 9. Relationships among: (a) nighttime measured evapotranspiration, $E T_{c_{night}}$ , and sum-of-hourly evapotranspiration, $E T_{c_{SOH}}$ ; and (b) daytime measured evapotranspiration, $E T_{c_{day}}$ , and sum-of-hourly evapotranspiration, $E T_{c_{SOH}}$ , for the 2005 and 2006 seasons

Relationship among Nighttime Evaporative Losses, Primary Micrometeorological Variables, and Surface Energy Fluxes

Relationships among measured

E T_{c_{night}}

, nighttime micrometeorological variables, and other surface energy balance components, including

u_{3}

,

T_{a}

,

V P D

,

R H

,

H

,

G

, and

R_{n}

, are presented in Fig. 10. A general tendency for

E T_{c_{night}}

to increase with increasing

u_{3}

was observed in both years. Although

E T_{c_{night}}

generally increased at high wind speeds, the largest

E T_{c_{night}}

was not always associated with the largest

u_{3_{night}}

. The relationship between

E T_{c_{night}}

and

u_{3}

was stronger in 2006 (

r^{2} = 0.35

) than in 2005 (

r^{2} = 0.20

) because of the larger magnitude of

u_{3_{night}}

and

V P D_{_{night}}

in 2006. The lowest

E T_{c_{night}}

values were usually associated with calm nights with small

V P D

. In conditions of small

V P D_{_{night}}

, the low boundary layer conductance of individual leaves and of the canopy would mask any cuticular or stomatal effects, particularly during calm nights, reducing

E T_{c_{night}}

(Grace 1977; Jarvis and McNaughton 1986; Rawson and Clarke 1988). In 2006,

u_{3_{night}}

alone was able to explain 35% of the variation in

E T_{c_{night}}

. The inferior relationship between

E T_{c_{night}}

and

u_{3_{night}}

in 2005 was primarily attributable to the three outlying points in Fig. 10(a) that represent the highest

u_{3}

values measured in the 2005 season as 8.4, 8.5, and

8.6 m / s

that occurred on April 22, May 17, and May 11, respectively. The

E T_{c_{night}}

for the same

u_{3_{night}}

values were 0.52, 0.50, and 0.50 mm, respectively. A close examination of the measured hourly microclimatic data on these three days indicated that a sudden change in wind direction occurred during all three nights. It is possible that some larger scale weather disturbance passed over the experimental site during those nights and that this might be responsible for mixing or overturning the lower boundary layer of the atmosphere, rapidly changing how

u_{3}

interacted with and affected the surface conditions, especially by the changing

V P D_{_{night}}

. When these three outliers (8.4, 8.5, and

8.6 m / s

) were excluded from the analyses, the

r^{2}

between the

E T_{c_{night}}

and

u_{3}

in 2005 was 0.14.

Fig. 10. Relationships among measured nighttime evapotranspiration, $E T_{c_{night}}$ , nighttime micrometeorological variables, and other surface energy balance components, including: (a) wind speed at 3 m, $u_{3}$ ; (b) air temperature, $T_{a}$ ; (c) vapor pressure deficit, $V P D$ ; (d) relative humidity, $R H$ ; (e) sensible heat flux, $H$ ; (f) soil heat flux, $G$ ; and (g) net radiation, $R_{n}$ , for the 2005 and 2006 seasons

No clear relationship between

E T_{c_{night}}

and

T_{a}

[Fig. 10(b)] was observed, although the influence of

T_{a}

on

E T_{c_{night}}

is indirect through

V P D

. However, air temperature may also influence

E T_{c_{night}}

through its effect on sensible heat flux. Data suggest that the

E T_{c_{night}}

losses seem to be greater after hot days when air and soil temperatures remain high. Tolk et al. (2006) found that with limited nighttime radiant energy, energy for

E T_{c_{night}}

was primarily provided by

H_{_{night}}

flux to the canopy when nighttime air temperatures averaged at least 2°C higher than plant canopy temperatures in irrigated alfalfa and cotton. A subsidiary correlation was observed between

E T_{c_{night}}

and

V P D_{_{night}}

in both seasons [Fig. 10(c)] with a clear trend of increase in

E T_{c_{night}}

with increasing

V P D_{_{night}}

. For a nonstressed maize canopy, on a two year average,

V P D_{_{night}}

alone was able to explain 35% of the variability in

E T_{c_{night}}

. Thus, the

u_{3_{night}}

and

V P D_{_{night}}

had a somewhat equal influence on

E T_{c_{night}}

. Tolk et al. (2006) observed that, over the entire season,

V P D_{_{night}}

explained 31, 30, 59, and 29% of the variation in

E T_{c_{night}}

for fully irrigated alfalfa, deficit-irrigated cotton, fully irrigated cotton, and rainfed cotton, respectively, as measured with the weighing lysimeters in Bushland, Texas. On an irrigated Eucalyptus grandis plantation in Australia, Benyon (1999) found that approximately 54% of the variation in mean

E T_{c_{night}}

over eight months of measurement was associated with

V P D_{_{night}}

alone, whereas wind speed accounted for 37% of the

E T_{c_{night}}

. A close examination of the data in this study suggests that high nocturnal

V P D_{_{night}}

values were usually associated with high

u_{3}

. This correlation probably reflects the common observation that nocturnal ground-based inversions, which usually form as the surface cools at night by radiation loss, are weak when winds are strong, and the

V P D_{_{night}}

near the soil surface does not fall to near zero; rather, the

u_{3}

generates turbulence in the lower boundary layer of the atmosphere, and this turbulence promotes the transport of air from aloft down to the surface (Green et al. 1989). This air usually has a larger vapor pressure deficit, reflecting conditions in the mixed layer, which most likely was formed during the preceding day (McNaughton 1988).

The relationship between

E T_{c_{night}}

and

R H_{_{night}}

was marginal with very similar slope and intercept for the 2005 and 2006 data [Fig. 10(d)]. The trend of the regression line is similar to that of the

E T_{c_{night}}

versus

V P D_{_{night}}

relationship.

R H_{_{night}}

alone explained 28% of the variability in

E T_{c_{night}}

. Most of the

R H_{_{night}}

values were within the range of 60–100% with some low values (around 50%) in 2006. The relationship between

E T_{c_{night}}

versus

H_{_{night}}

and

G_{_{night}}

[Fig. 10(e, f)] are particularly noteworthy. In both years, the

E T_{c_{night}}

increased as

H_{_{night}}

increased toward the soil. In 2005, the relationship between

E T_{c_{night}}

and

H_{_{night}}

was relatively strong (

r^{2} = 0.35

). In 2006, the relationship between the two variables was the strongest among all microclimatic and surface energy balance components with an

r^{2}

of 0.69. In 2005, the largest

E T_{c_{night}}

(1.10 mm) occurred when the

H_{_{night}}

was also highest (

- 0.95 mm

). The same results were found in 2006; the largest

E T_{c_{night}}

(0.95 mm) occurred when

H_{_{night}}

was largest (

- 0.94 mm

). The stronger relationship between

E T_{c_{night}}

and

H_{_{night}}

in 2006 might be related to the

u_{3_{night}}

pattern. During the daytime, a strong coupling usually existed between the above-canopy microclimate regime and the within-canopy state. Then, the above-canopy exchange mechanism dominated the within-canopy state (Seginer et al. 1976; Raupach et al. 1989; Goudriaan 1989; Jacobs et al. 1992, 1994, 1996). During the nighttime, however, when the above-canopy wind regime droped, only a weak coupling between both mechanisms by downdraught penetration occurred. In some cases, the coupling was not weak, but did not exist. The coupling was reduced because the above-canopy state became thermally stable by longwave radiative cooling at the top of the canopy while the within-canopy state became thermally unstable by the supply of the heat from the soil (Jacobs et al. 1996). Thus, a strong dependence of

H_{_{night}}

and

E T_{c_{night}}

is on

u_{3}

because during calm nights, when

u_{3}

is very small or close to zero, a free convection state can develop in within-canopy layers (Leclerc et al. 1991; Jacobs et al. 1992, 1996). Thus, free convection can dominate the energy exchange mechanism. Therefore, during calm nights with the absence of radiant energy,

H_{_{night}}

would be the dominant supplier of evaporative losses. The results in this paper are supportive of the aforementioned hypotheses and findings because the coupling between the above-canopy microclimate regime and the within-canopy state might have been stronger because of the larger wind speeds in 2005 than in 2006. The seasonal average nighttime wind speed was

2.31 m / s

in 2005 and

1.96 m / s

in 2006 with a significantly larger standard deviation (

1.6 m / s

) in 2005 than in 2006 (

1.22 m / s

).

E T_{c_{night}}

was primarily affected by

H

in 2006 because of a smaller

u_{3}

. The correlation between nocturnal

E T

and

V P D

and nocturnal

E T

and

H

was stronger than the correlation between nocturnal

E T

and

u_{3}

in 2005. The contribution of

G_{_{night}}

to

E T_{c_{night}}

during calm nights, therefore, would also be expected to be larger than during windy nights. Although the relationship between

E T_{c_{night}}

and

G_{_{night}}

in Fig. 10(f) is weak for both seasons, the relationship in 2006 (the calmer season) was found to be slightly stronger. During calm nights, the coupling between above- and within-canopy became weak, and the above-canopy fluxes of heat and water vapor also became very low because of the buildup of a temperature inversion above the canopy. This may also indicate that the above-canopy evaporative losses (

E T_{c_{night}}

) must be primariliy compensated by the

H_{_{night}}

and

G_{_{night}}

. Thus, one would expect to observe a stronger relationship between

E T_{c_{night}}

versus

H_{_{night}}

and

G_{_{night}}

during calm nights, which was denoted with the data in this paper.

The relationship between

E T_{c_{night}}

and

R_{n}

[Fig. 10(g)] was poor with very low

r^{2}

values in both years. The

E T_{c_{night}}

did not respond to changes in

R_{n}

in the

E T_{c_{night}}

range of

- 0.53

to approximately 0.40 mm. However, a tendency of

E T_{c_{night}}

to decrease with decreasing

R_{n}

was observed after that range in 2006. The trend of

E T_{c_{night}}

in relation to

R_{n}

in 2005 remained almost unchanged.

Summary and Conclusions

Quantifying nighttime evaporative losses,

E T_{c_{night}}

, can help better account for the total water losses during a 24 h period. The micrometeorological variables (

T_{a}

,

R H

,

u_{3}

, and

V P D

) measured over a subsurface-drip-irrigated and nonstressed maize canopy were separated into three time-steps [daytime, nighttime, and daily average (24 h)], and their seasonal distributions were analyzed and discussed. The same procedures were followed for the measured surface energy fluxes (

E T_{c}

,

H

,

G

, and

R_{n}

). The percent ratios of

E T_{c_{night}}

and

E T_{c_{day}}

relative to the sum-of-hourly

E T_{c}

(

E T_{c_{SOH}}

) were quantified for both years. The relationship between

E T_{c_{night}}

and the primary micrometeorological and energy flux variables (

u_{3}

,

T_{a}

,

V P D

,

R H

,

H

,

G

, and

R_{n}

) were investigated. The nighttime evaporative losses were high early in the season during partial canopy closure attributable to increased surface soil evaporation and were also high later in the season during and after leaf aging, physiological maturity, and leaf senescence. The seasonal

E T_{c_{night}}

averages were 0.19 and

0.11 mm / night

for the 2005 and 2006 growing seasons, respectively. The total

E T_{c_{night}}

,

E T_{c_{day}}

, and

E T_{c_{SOH}}

in 2005 were 31, 612, and 642 mm, respectively. The

E T_{c}

totals were less in 2006: 16, 533, and 547 mm, respectively. In both seasons, the primary microclimatic variables affecting

E T_{c_{night}}

were

u_{3}

,

V P D

, and

T_{a}

. A strong relationship between

E T_{c_{night}}

and

H_{_{night}}

was observed. The

E T_{c_{night}}

was a considerable portion of

E T_{c_{SOH}}

, even during a short period (growing season) of the calendar year in both years. The results of this study showed that the

E T_{c_{night}}

can be up to 5% of the

E T_{c_{SOH}}

, even for a subsurface-drip-irrigated maize canopy in which the soil surface was extremely dry, with less evaporative losses potential, compared with surface or sprinkler-irrigated fields in which

E T_{c_{night}}

would be expected to be higher because of the wetter surface soil conditions. Thus,

E T_{c_{night}}

needs to be quantified for different vegetation surfaces under different management, surface wetting, and climatic conditions to better account for nighttime water losses and to better understand nighttime energy balance mechanisms.

Acknowledgments

Disclaimer: The mention of trade names or commercial products is solely for the information of the reader and does not constitute an endorsement or a recommendation for use by the author or the University of Nebraska-Lincoln.

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Information & Authors

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Published In

Journal of Irrigation and Drainage Engineering

Volume 137 • Issue 8 • August 2011

Pages: 475 - 490

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Received: Oct 7, 2010

Accepted: Feb 17, 2011

Published online: Feb 19, 2011

Published in print: Aug 1, 2011

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Suat Irmak, M.ASCE [email protected]

Associate Professor, Dept. of Biological Systems Engineering, Univ. of Nebraska–Lincoln, 241 L.W. Chase Hall, Lincoln, NE 68583. E-mail: [email protected].

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Dynamics of Nocturnal, Daytime, and Sum-of-Hourly Evapotranspiration and Other Surface Energy Fluxes over Nonstressed Maize Canopy

Abstract

Introduction and Background

Materials and Methods

General Experimental Procedures and Site Description

Measurement of Microclimate Variables, Evapotranspiration, and Other Surface Energy Balance Components

Data Analyses

Results and Discussion

Weather Conditions, Seasonal Distribution of Measured Evapotranspiration, Seasonal Cumulative Evapotranspiration, and Rainfall in 2005 and 2006

Seasonal Distribution of Daytime, Nighttime, and 24-Hour-Daily Average Data for the Primary Micrometeorological Variables

Seasonal Distribution of Daytime, Nighttime, and Sum-of-Hourly Evapotranspiration and Surface Energy Fluxes

Percent Ratios of Nighttime and Daytime Evapotranspiration to Sum-of-Hourly Evapotranspiration

Relationship among Nighttime Evaporative Losses, Primary Micrometeorological Variables, and Surface Energy Fluxes

Summary and Conclusions

Acknowledgments

References

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Abstract

Introduction and Background

Materials and Methods

General Experimental Procedures and Site Description

Measurement of Microclimate Variables, Evapotranspiration, and Other Surface Energy Balance Components

Data Analyses

Results and Discussion

Weather Conditions, Seasonal Distribution of Measured Evapotranspiration, Seasonal Cumulative Evapotranspiration, and Rainfall in 2005 and 2006

Seasonal Distribution of Daytime, Nighttime, and 24-Hour-Daily Average Data for the Primary Micrometeorological Variables

Seasonal Distribution of Daytime, Nighttime, and Sum-of-Hourly Evapotranspiration and Surface Energy Fluxes

Percent Ratios of Nighttime and Daytime Evapotranspiration to Sum-of-Hourly Evapotranspiration

Relationship among Nighttime Evaporative Losses, Primary Micrometeorological Variables, and Surface Energy Fluxes

Summary and Conclusions

Acknowledgments

References

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