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Surface energy fluxes of Phragmites australis in a prairie wetland
Agricultural and Forest Meteorology 94 (1999) 31±51
Surface energy ¯uxes of Phragmites australis in a prairie wetland G.G. Burbaa, S.B. Vermaa,*, J. Kima,b a
School of Natural Resource Sciences, Center for Laser-Analytical Studies of Trace Gas Dynamics, University of Nebraska, Lincoln, NE, USA b Global Environment Laboratory Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea Received 19 June 1998; accepted 23 December 1998
Abstract Components of the surface energy balance were measured in three communities (Phragmites australis, Scirpus acutus, and open water) in a wetland located in northcentral Nebraska, USA, during the growing season of 1994. This paper includes results from the area covered by Phragmites australis (reedgrass). The Bowen ratio ± energy balance method was used to calculate sensible and latent heat ¯uxes. During daytime, with a water depth of about 0.5 m, the heat storage term (G) consumed 20±30% of the net radiation (Rn). At night, G was a signi®cant source of energy (on average, about ± 40 W mÿ2). The magnitude of the daily (24 h) averaged G was small. Evapotranspiration (ET) was a major consumer of the incoming solar energy. During early and peak growth, the daily ET ranged between 2.5 and 6.5 mm per day. During senescence, evapotranspiration was between 0.5 and 3.1 mm per day. ET was partitioned into transpiration (Ev) and evaporation (Es) using a dual-source modi®cation of the Penman±Monteith equation. Results indicated that transpiration contributed 40±45% of the total ET in the beginning of the early growth stage. During the second half of the early growth stage and the entire peak growth stage, it contributed 53±62% of ET. The contribution decreased to 50% in the beginning of senescence, and to near zero in late senescence. The daytime variation of Es did not follow Rn, and seemed to be controlled by thermal stability and air dryness. Before senescence, the ratio of the actual to equilibrium evapotranspiration (ET/ETeq) averaged 1.3. It decreased to about 0.5 during senescence. The McNaughton and Spriggs (1989, IAHS Publ. 177, 86±101) model, developed primarily for dryland vegetation, signi®cantly overestimated the ET/ETeq ratio in Phragmites when the canopy stomatal resistance was larger than 150 s mÿ1. The model prediction improved signi®cantly when the contribution of evaporation was eliminated by substituting the ET/ETeq ratio by Ev/Eveq (transpiration/equilibrium transpiration). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Wetlands; Evaporation; Transpiration; Heat storage; Radiation
1. Introduction Energy exchange is among the most important processes in wetland ecosystems, because it affects variables such as temperature, water transport, plant growth and productivity (Dennison and Berry, 1989). *Corresponding author. Tel.: +1-402-472-3679; fax: +1-402472-6614; e-mail: [email protected]
The main components of the surface energy balance are the net radiation, the heat stored in water and soil, the sensible heat ¯ux, and the latent heat ¯ux (or evapotranspiration). Evapotranspiration (ET) in vegetated wetlands is frequently the largest consumer of the incoming energy (Priban and Ondok, 1985) and has a great in¯uence not only on the energy distribution, but on water conditions (e.g. temperature, depth, salinity).
0168-1923/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0168-1923(99)00007-6
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Previous studies of wetland energy ¯uxes have reported the in¯uence of meteorological factors such as solar radiation (Rijks, 1969; Hammer, 1989; Kadlec, 1989; La¯eur, 1990), wind speed (Rijks, 1969; Snyder and Boyd, 1987; Rao, 1988; Hammer, 1989), vapor pressure de®cit (Rijks, 1969; Van der Weert and Kamerling, 1974; Jones and Muthury, 1984) on ET. Limited information, however, is available on the in¯uence of plant related variables, such as stomatal resistance and leaf area index (LAI) (Munro, 1987; Koch and Rawlik, 1993; Kim and Verma, 1996). Very little is known about the partitioning of evapotranspiration into canopy transpiration and evaporation from the water surface. The present study was designed to investigate the energy ¯uxes in a mid-latitude prairie wetland dominated with Phragmites australis. The main objective of the study was to quantify the components of the surface energy balance in this important community. To better understand the energy partitioning, we attempted to separate evaporation from transpiration using a dual-source modi®cation of the Penman± Monteith equation (Massman, 1992), and examine their diurnal and seasonal variations. 2. Materials and methods 2.1. Site This study was conducted at Ballards Marsh (428300 N, 1008250 W), located in the Sandhills region of northcentral Nebraska, USA, during the growing season of 1994. In general, the Sandhills area is characterized as a semiarid prairie consisting of grasscovered sand dunes and interdunal valleys occupied by marshes, lakes and wet meadows. These wetlands are fed primarily by precipitation and, to a smaller extent, by ground water. Dunes, located 1±2 km away from the measurement station, are up to 100 m 150 m wide and rise up to 20 m above the interdunal valley. The valley is ¯at, and measures 1.2 km 5.0 km in NE±SW direction. Ballards Marsh is approximately 1.0 km 2.5 km (NE±SW) in size. Most of the marsh is dominated by two densely vegetated monospeci®c communities (Phragmites australis and Scirpus acutus), and by open water area. Phragmites and Scirpus were 3.0 and 1.5 m tall and
occupied about 40% and 30% of the total area of the marsh, respectively. Average water depth was 0.5 m. Open water was about 30% of the marsh, with average water depth of 1 m. The 1994 growing season was warmer by about 1.48C (on average) and slightly drier (by 40 mm), as compared to the long term values measured at Valentine Wildlife Refuge (15 km from the study site). The Bowen ratio ¯ux station (described below) was placed in the northcentral part of the marsh. At this location, the upwind fetch of the exclusively Phragmites dominated vegetation was about 250±330 m in the E, W and SSE directions, about 100±120 m in the SE and SW directions, and about 90±100 m in the NW direction. 2.2. Field measurements Net radiation was measured 1 m above canopy with a net radiometer (Radiation Energy Balance Systems, Beaverton, OR, model Q7). Incoming and outgoing shortwave radiation were measured with pyranometers (LI-COR, Lincoln, NE, model LI-200SA) located 1 m above the canopy. Air temperature and humidity gradients were measured with two shielded aspirated psychrometers equipped with Vaisala temperature/humidity sensors (Campbell Scienti®c, Logan, UT, model HMP35C), that were exchanged (between the heights of 1 and 2 m above the canopy) every 5 min. The mean water temperature (Tw) was measured with a 0.5 m long RTD (resistance temperature device) bar, with the upper end attached to a ¯oating panel and the lower end attached to a load. The bar was shaded with a 0.5 m circular styrofoam panel. In addition, six thermistors were installed at different depths with 10 cm intervals. The average temperature of the top 0.1 m of the submerged soil was measured with two 0.1 m long RTD bars. Temperature of the water surface (Tws) was measured with a thermopile, which ¯oated at the surface and was shaded by 0.15 m circular styrofoam panel. Water depth was measured with a sensor described by Roulet (1991). Heat stored in water (G) was calculated from the change in mean water temperature: G cw d
Tw t
(1)
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
where (Tw/t) is the time rate of change in mean water temperature measured with the RTD bar, cw is the water heat capacity, d is the water depth. The heat storage in the submerged soil was calculated from the values measured with soil heat ¯ux plates installed 0.1 m below the surface of the submerged soil, and from the rate of change of the average temperature of the top 0.1 m of the submerged soil. The heat storage in the submerged soil was negligible (less than 5% of G), and was ignored in further computations. Mean horizontal wind speed (U) was measured at 1.0, 1.5 and 2.0 m above the vegetation with cup anemometers (Cayuga Development, Ithaca, NY, model WP-1). Wind direction was measured with a vane at the height of 3 m above vegetation. Atmospheric pressure (P) was measured with a barometer (Vaisala Barometric Pressure Sensor, model PTA427, Cambell Scienti®c). These variables were measured every 5 s and averaged for 30 min. The 30 min averages were used in further computations. Vegetative characteristics in Phragmites australis (LAI and height of the vegetation) were measured in a concurrent study by Vanyarkho (1996). Leaf area index was measured on weekly basis with an plant canopy analyzer (LI-COR, Lincoln, NE, model LAI2000) and also from destructive sampling using a leaf area meter (LI-COR, Lincoln, NE, model LI-3100). 2.3. Theoretical considerations 2.3.1. Bowen ratio ± energy balance method The energy balance can be approximated as: Rn ÿ G ÿ E ÿ H 0
(2)
where Rn is the net radiation, G is the heat stored in the water, E is the latent heat ¯ux, H is the sensible heat ¯ux. Using the ¯ux-gradient approach, latent and sensible heat ¯uxes can be expressed as: Mw =Ma @e a Kw P @z @T H ÿa Cp Kh @z
E ÿ
(3) (4)
where is the latent heat of vaporization; Mw and Ma are the molecular weights of water and dry air, respectively; P is the air pressure, a is the density of moist air, Kw and Kh are the turbulent exchange coef®cients for water vapor and sensible heat, respectively; @e/@z
33
and @T/@z are the gradients of vapor pressure and potential air temperature, respectively; and Cp is the speci®c heat of air at constant pressure. From Eqs. (3) and (4), the Bowen ratio ( H/E) can be expressed as:
Kh @T=@z Kw @e=@z
(5)
where CpP/(Mw/Ma). Assuming that Kh is equal to Kw, and that (@T/@z)/(@e/@z) T/e, Eq. (5) can be simpli®ed to:
T e
(6)
where T and e are the vertical differences of temperature and vapor pressure, respectively. The above equations lead to: E
Rn ÿ G 1
(7)
H
Rn ÿ G 1
(8)
and
2.3.2. Partitioning of evapotranspiration into evaporation and transpiration The total E (or evapotranspiration) was separated into evaporation (Es) and transpiration (Ev) using a dual-source modi®cation of the Penman±Monteith equation (Massman, 1992). Fig. 1 schematically shows the resistance formulation used in this approach. The term ra is the aerodynamic resistance; rc is the canopy stomatal resistance; rs is the surface resistance to water vapor transfer; rbc is the canopy boundary layer resistance; ru is the subcanopy resistance; eo is the vapor pressure within the canopy; ea is the above canopy vapor pressure; e*(Tws) is the saturated vapor pressure at the temperature of water surface (Tws); To is the air temperature within the canopy; Ta is the above canopy temperature; and Tv is the temperature of the canopy. The energy budget for the water surface and the canopy (Fig. 1) can be approximated as: Rns ÿ G ÿ Es ÿ Hs 0
(9)
Rnv ÿ Ev ÿ Hv 0
(10)
and
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
ratio ( s) can be calculated using the dual-source approach described in Massman (1992): rs
Rns ÿ G a Cp Tws ÿ Ta ÿ Hra s 1 s a e Tws ÿ ea ÿ Era P
(18)
Eq. (18) can be considerably simpli®ed by assuming that the surface resistance to water vapor transfer, rs 0: s Fig. 1. Schematic representation of the dual-source combination model (Massman, 1992). See the text for definition of symbols.
a Cp Tws ÿ Ta ÿ Hra a =P e Tws ÿ ea ÿ Era
The aerodynamic resistance (ra) was estimated as: ra ram rb
where Rn Rns Rnv
(11)
E Es Ev
(12)
H Hv Hs
(13)
and The subscript `s' refers to the water surface ¯uxes and the subscript `v' refers to the plant canopy. Net radiation reaching the water surface (Rns) was approximated using the following equation (Ross, 1970): Rns Rn exp ÿKext LAI
(14)
where Kext is an extinction coef®cient, which was calculated as (Ross, 1970) Kext G sin b
(15)
where b is the solar elevation angle, and is the leaf angle distribution. Measurements by Vanyarkho (1996) in a concurrent study indicated that the leaf angle distribution in Phragmites was approximately spherical and therefore, G() was assumed to be 0.5. Using s Hs/Es, Es and Hs can be expressed as: Es
Rns ÿ G 1 s
Hs Rns ÿ G ÿ Es
(16) (17)
Information on s is needed to calculate the individual ¯ux terms: Es, Hs, Ev, and Hv. The surface Bowen
(19)
(20)
where ram is aerodynamic resistance to momentum transfer and rb is excess resistance term (e.g., Verma et al., 1992): ram
U u2
2 Dh 2=3 rb ku Dv
(21) (22)
where u* is friction velocity, k is von Karman's constant, Dh is the thermal diffusivity, and Dv is the molecular diffusivity of water vapor. The value of u* was estimated with following equation (e.g., Rosenberg et al., 1983): u
kU ln z ÿ d0 =z0
(23)
where U was measured at 2 m (above the vegetation). Zero plane displacement (d0) and roughness parameter (z0) were estimated from the canopy height (Stanhill, 1969; Szeicz et al., 1969). The variables Tws, Ta, ea, P were measured, e*(Tws) was calculated from the slope of saturated vapor pressure±temperature curve at Tws; and G, H, E and ra were calculated as discussed above. Using this information in Eq. (19), s was calculated. Then Es, Ev, Hs and Hv were calculated using Eqs. (11),(12) (16) and (17). During certain periods (under cloudy and cool conditions during early hours of some mornings), (Tws ÿ Ta) and (e*(Tws) ÿ ea) were very small, leading to potentially large errors in s and
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
consequently in Es. Data during such periods were excluded and the missing s values were ®lled in by interpolation (using a third-order polynomial regression developed with six half-hourly values of s preceding the missing period and six half-hourly values of s following the missing period). 2.3.3. Potential and equilibrium evapotranspiration Daily potential evapotranspiration (ETp) was calculated as follows (Penman, 1948): ETp
s Rn ÿ G a Cp Df U s
(24)
In Eq. (24), f(U) was estimated by the empirical equation of Doorenbos and Pruitt (1984):
35
U f U 0:27 1 100
(25)
where U is in km per day. Daily equilibrium evapotranspiration (ETeq) was calculated (Slatyer and McIlroy, 1961) as: ETeq s
Rn ÿ G s
(26)
where s is the slope of saturation vapor pressure± temperature curve. 3. Results and discussion The green LAI was 1.2 in mid-June, reached a maximum value of 2.6 in mid-August and then
Fig. 2. Seasonal distributions of: (A) The green LAI of Phragmites australis (adapted from Vanyarkho, 1996). (B) water depth (d).
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
declined to 0.2 in late October (Fig. 2A, adapted from Vanyarkho, 1996). Based on plant development and color of leaves and seedlings, three growth stages were approximated: early growth (mid-June to mid-July), peak growth (mid-July to early/mid-September), and senescence (early/mid-September to the end of October). The Phragmites canopy was 2.2±3.1 m tall during most of the measurement period. The water depth (d) was largest (0.53 m) in the mid-June, and decreased gradually through the season (Fig. 2B) to 0.34 m in the middle of October. 3.1. Net radiation Daytime (averaged between sunrise and sunset) incident solar radiation (Rs) and net radiation (Rn)
for the entire measurement period are shown in Fig. 3A. On clear days, daytime averaged Rs peaked around 500±600 W mÿ2 during June±July. It gradually decreased to 300 W mÿ2 by mid-October. Daytime Rn ranged about 300±370 W mÿ2 during June± July and then gradually decreased to 170±200 W mÿ2 in mid-October (Fig. 3A). Linear regression between Rs and Rn yielded: Rn ÿ4:1 0:62Rs ;
r 2 0:90
(27)
Daytime albedo ranged between 0.13 and 0.14 in midJune; it increased to 0.14±0.16 in August, and then decreased to 0.12±0.14 in mid-October (Fig. 3B). These values are close to those reported for other vegetated wetlands (0.12±0.16 for sphagnum-sedge bogs, Berglund and Mace, 1972; 0.12 for swamp
Fig. 3. Seasonal distributions of the daytime averaged values of (A) incoming shortwave (Rs) and net (Rn) radiation, and (B) albedo.
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
forest, Campbell, 1986; and 0.11±0.17 for open sphagnum fen, Kim and Verma, 1996). 3.2. Heat storage term 3.2.1. Diurnal variation The storage term (G) varied considerably from one half hour to the next most likely due to the turbulent structures of varying temperature traveling past the temperature sensors, and due to nearby animal activities. To minimize this local variability, we calculated 2 h running means of the energy storage term (G), and used them in further computations. Fig. 4A shows values of G on a clear day (18 July, 1994) during mid-season. The diurnal pattern of the magnitude of G generally followed that of Rn. The storage term became positive about 1±2 h after sunrise, implying
37
that the water body became an energy sink at that time. The peak magnitude of G (170±200 W mÿ2) occurred at about the same time (13:00±15:00 CDT) as the maximum in Rn. The storage term became negative about 1±2 h before sunset. Nighttime G varied between ÿ70 and ÿ20 W mÿ2. The pattern and magnitudes of G obtained here are similar to those measured by Smid (1975) in a 0.3 m deep reed swamp in Czechoslovakia (daytime peak of about 200 W mÿ2, and nighttime values of ÿ50 to ÿ100 W mÿ2). Data (estimated directly from changes in mean water temperature, and indirectly from Rn, H and E ¯uxes) from various water bodies (without vegetation) indicate large magnitudes of G (up to 700 W mÿ2) during daytime (Rose and Chapman, 1968; Imberer, 1985; Stannard and Rosenberry, 1991; Assouline and Mahrer, 1993; Burba et al.,
Fig. 4. (A) Storage term (G) and net radiation (Rn) on 18 July, 1994. (B) Daytime magnitude of G/Rn on 18 July, 1994.
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
1998). Large magnitudes of G in non-vegetated shallow water bodies are expected due to a more effective penetration of radiation. The storage term acts as practically the only source of energy at night and a considerable sink of energy during daytime. On 18 July, the magnitude of G/Rn gradually increased in the morning from 0.10 to 0.17 (08:30±09:00 hours) to a midday maximum of 0.30± 0.33, and then gradually decreased to 0.07±0.13 by 17:00±18:00 hours (Fig. 4B). The fraction of Rn consumed by G was highest at midday, because of a better direct penetration of solar radiation through the vegetation and into the water body. A linear regression between the half-hourly values of G and Rn on 15 days from different parts of the season yielded: G ÿ51 0:41Rn ;
r 2 0:86
(28)
3.2.2. Daytime, nighttime and 24 h averages Heat stored in water during daytime can be expected to depend primarily on the net radiation reaching the water surface (Rns). Water depth (d) should also play a role in determining the heat storage potential. Daytime averaged G as a function of daytime averaged Rns and d are plotted in Fig. 5A and B. Excluding rainy days, the individual linear regressions yield: G ÿ3 0:5Rns ;
r 2 0:70
(29)
G ÿ138 0:5d;
r 2 0:57
(30)
and
To examine the combined effect of both variables, the heat stored per mm of water depth (G/d) is shown as a function of Rns in Fig. 5C. Linear regression (excluding rainy days) yields: G 0:0242 0:0009Rns ; d
r 2 0:73
(31)
No signi®cant effects of other variables (including the difference between air and water temperatures (Ta ÿ Tws) and wind speed) on G were found. Nighttime averaged G was found to be related to Ta ÿ Tws (Fig. 5D): G ÿ37 8:5 Ta ÿ Tws ; r 2 0:60 for non-rainy days
(32)
Smaller magnitudes of nighttime G, ranging from ÿ50 to 0 W mÿ2, were typical when Ta ÿ Tws > ÿ18C. When the temperature difference was ÿ68C, G increased up to ÿ140 W mÿ2. Dependence of nighttime G on water depth was less signi®cant (r2 0.44). This can be attributed to the fact that the loss of longwave energy at night occurs mainly from the surface of the water and the role of the entire water body may be relatively insigni®cant. The loss of heat from the water body at night (Fig. 4A) was similar in magnitude to the gain during daytime and therefore, the values of G when averaged over 24 h were small in comparison with Rn. Daily averaged values of G ranged between ÿ30 and 20 W mÿ2 on non-rainy days. In some previous studies in shallow lakes (e.g., Sturrok et al., 1992; Assouline and Mahrer, 1993), short-term (hourly±daily) calculations of H and E have been based on bi-weekly measurements of G. As indicated in Fig. 4, the diel variability of G is large and needs to be considered in hourly calculations of H and E. However, G averaged over a few days is relatively small (Table 1) in comparison with other components of the energy budget, and could be considered close to zero in long-term (a few days) averages of H and E. Stewart and Rouse (1976) obtained similar results for unvegetated water bodies. 3.3. Sensible heat flux Sensible heat ¯ux on a clear day in mid-season is shown in Fig. 6A. The magnitudes of H increased from 0 to 40 W mÿ2 between 07:00 and 10:30 hours (central daylight saving time). Then it slightly decreased to 20 W mÿ2 at 13:00 and stayed near this value until 17:00 hours. Nighttime values of H ranged from ÿ40 to 30 W mÿ2. In a reed swamp in Table 1 Range of G averaged for the different periods during the season (including periods with rain) Number of days used for running mean averaging
Range of averaged G (W mÿ2)
1 3 5 9 30
ÿ36 to 60 ÿ14 to 51 ÿ4 to 22 0 to 14 0 to 12
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
39
Fig. 5. (A) Daytime averaged storage term (G) as a function of net radiation below the canopy (Rns). (B) Daytime averaged storage term (G) as a function of water depth (d). (C) Energy stored per mm of water depth (G/d) as a function of Rns. (D) Nighttime averaged storage term (G) as a function of the difference between air and water surface temperatures (Ta ÿ Tws).
Czechoslovakia, Smid (1975) reported similar pattern of H. The magnitude of H in his study was larger (ranging from 0 to 95 W mÿ2), however. Larger H may be an artifact of the larger values of Rn, calculated using an empirical formula in his study.
Excluding rainy days, the daily H (Fig. 6B) ranged from ÿ6 to 20 W mÿ2 before plant senescence. Daily H derived from the data reported by Munro (1979; wooded swamp) and Smid (1975; reed swamp), for non-senescent periods, ranged from 0
40
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Fig. 6. (A) Sensible heat flux (H) and net radiation (Rn) on 18 July, 1994. (B) Seasonal distributions of the daily averaged sensible heat flux (H) and net radiation (Rn). (C) Seasonal distribution of the daily averaged ratio, H/Rn.
to 25 W mÿ2. With the onset of senescence (early midSeptember), the daily averaged magnitude of H increased to 30 W mÿ2. During early and peak growth stages, H consumed about 10% of Rn (Fig. 6C). As plants began to senesce (mid-September), stomatal
resistance increased (Vanyarkho, 1996) and transpiration decreased (as will be discussed later), more energy (20±30% of Rn) was partitioned into H. In the last few days of the study (mid-October), H consumed about 70% of Rn.
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
41
Fig. 7. (A) Latent heat flux (E) and net radiation (Rn) on 18 July, 1994. (B) Air temperature (Ta) and vapor pressure deficit (D) on 18 July, 1994. (C) The ratio E/Rn on 18 July, 1994.
3.4. Latent heat flux 3.4.1. Diurnal variation Latent heat ¯ux on a clear day in mid-season is shown in Fig. 7A. Midday values of E ranged from 300 to 380 W mÿ2. Peak magnitude of E occurred
about 1±2 h after the peak in Rn. This lag is likely due to enhanced evapotranspiration in the afternoon resulting from higher air temperature and vapor pressure de®cit (Fig. 7B). Nocturnal values of E were small (from ÿ30 to 30 W mÿ2). Small negative E ¯ux may be related to heavy fog often observed at night.
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Pattern and magnitudes of E mentioned above are similar to those observed by La¯eur (1990) for sedgedominated wetlands in Canada. La¯eur (1990) measured midday E ranging from 210 to 400 W mÿ2. Souch et al. (1996) and Spieksma et al. (1997) observed similar E over a lakeshore wetland in Indiana and in a peatland in The Netherlands, respectively. In a reed swamp in Czechoslovakia, Smid (1975) reported larger magnitudes of midday E (varying from 400 to 500 W mÿ2). However, Rn calculated by an empirical formula in his study was 100± 170 W mÿ2 higher than the net radiation measured in our study. For a sphagnum-dominated wetland in Minnesota, Kim and Verma (1996) reported an average daytime E of 230±250 W mÿ2 in mid-season. Latent heat ¯ux was the prime sink of Rn in this ecosystem (Fig. 7C). During midday, E/Rn was 0.65±0.70. Smid's (Smid, 1975) data from a reed swamp would suggest E/Rn of 0.74 during midday in the peak growth season. Unvegetated water surfaces consume considerably smaller amounts (20±55%) of Rn in evaporation during daytime (Friehe et al., 1991; Stannard and Rosenberry, 1991; Burba et al., 1998). 3.4.2. Daily evapotranspiration rates Seasonal pattern of daily integrated ET followed that of the net radiation (Fig. 8A). Sharp decreases in the magnitude of ET occurred primarily on cloudy and rainy days (DOY 169, 178, 195, 247, 277). Excluding rainy days, the ET rates ranged from 3.5 to 6.5 mm per day during the early and peak growth stages. The evapotranspiration decreased to about 2 mm per day during senescence. The average ET rate for the entire measurement period (June±October) was 3.75 mm per day. Evapotranspiration data reported from different wetlands include: 1.3±6.0 mm per day (with an average of 3.1 mm per day) for a sedge-dominated wetland in central Canada (La¯eur, 1990); 2.0±4.6 mm per day for wooded swamp in Ontario (Munro, 1979); 0.5± 4.8 mm per day for an open sphagnum fen in northcentral Minnesota (Kim and Verma, 1996); 1.4± 6.9 mm per day for a reed swamp in Czechoslovakia (Smid, 1975; Priban and Ondok, 1985); and 5.1 mm per day for freshwater marsh in Florida (Dolan et al., 1984). As discussed above, on a daily basis G and H were small. Latent heat ¯ux, therefore, was the primary consumer of Rn (Fig. 8B). On non-rainy days, E
generally consumed 80±100% of Rn during the ®rst two stages (early and peak growth: mid-June to early September) of the canopy development. During senescence, E/Rn decreased to 0.6±0.8 (Fig. 8B). By midOctober, when most plants had senesced, E/Rn was about 0.3. 3.5. Partitioning evapotranspiration into transpiration and evaporation As discussed in Section 2.3, ET was partitioned into the evaporation (Es) and transpiration (Ev) components using the dual-source modi®cation of the Penman±Monteith equation (Massman, 1992). Three clear days were selected to represent typical behavior of Ev and Es (Fig. 9) in each growth stage: 19 June (early growth stage), 18 July (peak growth stage) and 10 October (senescence). 3.5.1. Transpiration In the early growth stage, on 19 June (green LAI 1.4), the pattern of Ev followed that of Rn (Fig. 9A). Transpiration increased from 0±10 W mÿ2 during 08:00±08:30 hours to a peak magnitude of 230 W mÿ2 during 13:30±14:30 hours and then decreased to 10 W mÿ2 during 19:30±20:00 hours. Daytime averaged magnitude of Ev was 97 W mÿ2. In the peak growth, on 18 July (Fig. 9B) transpiration had a similar pattern, but substantially larger magnitudes (peak 350 W mÿ2, daytime average 177 W mÿ2). Larger Ev magnitudes are because of a larger green LAI 2.0 (midday air temperature, vapor pressure de®cit and wind speed on this day were not much different from those on 19 June, Table 2). The canopy transpiration utilized practically all (>90%) the net radiation intercepted by vegetation (Rn ÿ Rns). Substantially smaller transpiraTable 2 Air temperature, wind speed and vapor pressure (at 2 m above vegetation) averaged between 11:30 and 16:30 hours, on 19 June, 18 July and 10 October, 1994
19 June 18 July 10 October
Temperature (8C)
Vapor pressure deficit (kPa)
Wind speed (m sÿ1)
30 30 22
2.5 2.4 2.0
4.5 4.0 5.0
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
43
Fig. 8. (A) Seasonal distributions of the daily integrated evapotranspiration (ET) and daily averaged net radiation (Rn). (B) Seasonal distribution of the ratio E/Rn.
tion rates were observed during senescence on 10 October (green LAI 0.8). The midday air temperature and vapor pressure de®cit were lower than those on the other days, but wind speed was higher (Table 2). On 10 October (Fig. 9C), the peak magnitude of Ev was about 40 W mÿ2, and the daytime average was 12 W mÿ2. It is also worthwhile to examine the results on a cloudy day (5 October) during late senescence (Fig. 9D). Theoretically, due to plant senescence and low Rn, the transpiration on this day should be very low. As seen in Fig. 9D, the calculated values of transpiration were indeed close to zero throughout the day (the daytime average magnitude of Ev was 3 W mÿ2).
3.5.2. Evaporation The magnitude of evaporation on 19 June increased from 70 to 130 W mÿ2 between 08:00 and 10:30 hours (Fig. 9A), and then decreased to small values (0± 20 W mÿ2) during 13:00±15:00 hours. Later in the afternoon Es increased to 170 W mÿ2 (18:00±19:00 hours). A similar pattern of Es, with slightly smaller magnitudes, was seen on 18 July (Fig. 9B). Toward the end of the season (10 October) the evaporation rate increased to 130±140 W mÿ2 between 12:00 and 16:00 hours, with no clear evidence of a daytime depression (Fig. 9D). Friedrich (1972) suggested that the evaporation rates above water surface could be signi®cantly (up to 80%) suppressed because of stable
44
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Fig. 9. Partitioning of latent heat flux (E) into transpiration (Ev) and evaporation (Es) on: (A) 19 June (B) 18 July (C) 10 October (D) 5 October.
thermal strati®cation caused by temperature inversion over the cool water surface. Depression in the daytime Es was also observed by others (e.g., Assouline and Mahrer (1993) over a large lake; Burba et al. (1998) over an open water area of Ballards Marsh) when water temperature was cooler than the air.
3.5.3. Daily integrations The daily transpiration and evaporation are shown in Fig. 10A. Transpiration ranged between 1.3 and 4.0 mm per day through the early and peak growth stages (with an average of 2.9 mm per day). During senescence Ev ranged between 0 and 1.8 mm per day
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
45
Fig. 10. Seasonal distribution of (A) the daily evaporation (Es) and transpiration (Ev), and (B) Es/ET and Ev/ET.
(with an average of 1.1 mm per day). Evaporation was between 1.0 and 3.2 mm per day during most of the early and peak growth stages. In Fall, Es did not decrease as rapidly as Ev did, and had an average of 1.8 mm per day. During the beginning of the early growth stage, transpiration contributed about 40±45% of the total ET (Fig. 10B). Then, with an increase in plant growth and activity, the transpiration contribution increased and during the second half of early growth stage and throughout peak growth stage, it contributed 53±62% of ET. The transpiration contribution decreased from about 50% in the beginning of the senescence to
almost negligible levels at the end of the measurement period (Fig. 10B). 3.6. Actual, potential and equilibrium evaporation rates 3.6.1. Potential evapotranspiration Measured ET and ETp for the whole measurement period are shown in Fig. 11A. During the early and peak growth stages ET was 75±100% of ETp (Fig. 11B). During senescence, ET was considerably smaller (10±75%) than ETp (except on cloudy days when both were small). For sedge-dominated wetland
46
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Fig. 11. Seasonal distribution of (A) actual (ET) and potential (ETp) evapotranspiration, and (B) ET vs. ETp.
La¯eur (1990) reported ET being 75±100% of ETp. Coef®cients of linear regression between ET and ETp from our study are: intercept, a 0.65 mm; slope, b 0.75, with r2 0.85. La¯eur (1990) obtained a 0.06±0.44 mm andb 0.61±0.83, with r2 0.81±0.89. Examination of the Penman±Monteith equation (Monteith, 1965) and Eq. (24) indicates that ET/ ETp may be linked to the canopy stomatal resistance (rc). In the following, we examine the dependence of the ET/ETp ratio on midday rc (Fig. 12A). The values
of rc used here are from a concurrent study of Vanyarkho (1996), in which leaf stomatal resistance was measured. The measurements of rc were scaled up to the canopy level using the light response relationships in an approach described in Jarvis and McNaughton (1986), and in Norman et al. (1993). As seen in Fig. 12A, the Phragmites evapotranspired at a potential rate when midday rc was less than about 100 s mÿ1. As rc increased, ET/ETp declined and approached 0.1 when rc was about 10 000 s mÿ1. The
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
47
Fig. 12. (A) Dependence of the ratio of actual to potential evapotranspiration (ET/ETp) on midday canopy stomatal resistance (rc). (B) Dependence of transpiration (Ev) on midday rc. (C) Dependence of the ratio ET/ETp on green LAI. (D) Dependence of the ratio ET/ETp on the difference between air and water surface temperatures (Ta ÿ Tws).
canopy stomatal resistance controlled the transpiration rate, Ev (Fig. 12B), and therefore, affected the ET/ETp ratio. Ev was less than 1 mm per day when rc < 250 s mÿ1. A decrease in rc from 250 to 75 s mÿ1 corresponded to an increase in Ev from 1.2 to an
average of 3 mm per day. Measured ET was close to potential rates for green LAI greater than 1.8 (Fig. 12C). Also, as mentioned earlier, thermal stability conditions affect Es (evaporation from the water surface), and, therefore, should also be considered. Although
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
there is scatter in the data (Fig. 12D), ET seems to be near ETp (ET/ETp 0.6±1.2) when Ta ÿ Tws < 28C and generally less than ETp under stronger inversion conditions (Ta ÿ Tws > 28C).
3.6.2. Equilibrium evapotranspiration It is also worthwhile to evaluate the ET/ETeq ratio, traditionally known as the Priestley±Taylor coef®cient (Priestley and Taylor, 1972). During most of the
Fig. 13. (A) Seasonal distribution of ET/ETeq. (B) ET/ETeq as a function of the canopy stomatal resistance (rc): open circles are data from this study; the curve is the ET/ETeq±rc relationship from McNaughton and Spriggs (1989). Average values from 9 days included in their Figure 3 are shown here. (C) Ev/Eveq as a function of rc. Solid squares are data from this study. The curve is the same as in B. (D) Same as (C) with results on ET/ETeq from tallgrass prairie (open triangles): Verma et al. (1992).
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
early and peak growth stages ET/ETeq ranged from 1.2 to 1.5 (with an average of 1.3), and decreased from about 1.3 to 0.5 during senescence (Fig. 13A). In Fig. 13B, ET/ETeq is examined as a function of the canopy stomatal resistance in the framework of the McNaughton and Spriggs (1989) model (developed primarily for dryland vegetation). For rc < 100 s mÿ1, ET/ETeq is close to 1.26. However, for rc > 150 s mÿ1 ET/ETeq deviated signi®cantly from the values predicted by the model. To eliminate the effect of evaporation, the ratio of the actual to equilibrium transpiration (Ev/Eveq) was computed, where Eveq s
Rnv s
(33)
As shown in Fig. 13C, the dependence of Ev/Eveq on rc is much closer to that predicted by the McNaughton and Spriggs model for ET/Eeq. The Ev/Eveq ratio was about 1.26 when rc < 100 s mÿ1 and rapidly decreased with increasing rc. The relationship between Ev/Eveq and rc is quite similar to that (ET/ETp and rc) from a tallgrass prairie in Kansas (Verma et al., 1992), where the evaporation contribution was small (Fig. 13D). 4. Summary A micrometeorological study was conducted to measure energy ¯uxes over Phragmites australis in the Sandhills of Nebraska during the growing season of 1994. During the early growth stage, the daytime albedo ranged between 0.13 and 0.14. It increased to 0.14±0.16 during the peak growth stage and decreased to 0.12±0.14 during senescence. The averaged daytime net radiation was 300±370 W mÿ2 during June±July and decreased to 170±200 W mÿ2 by mid-October. The heat storage term was the main source of energy at night and a considerable sink of energy during daytime. Averaged over a week or more, heat storage term becomes quite small. It suggests that during Bowen ratio calculations of long-term H and E over wetland heat storage component in some cases could be neglected. It also suggests that if hourly or daily H and E are of interest, heat storage is quite important, and should be measured as frequently as temperature and humidity gradients.
49
Sensible heat ¯ux was a minor component of the energy budget during most of the growing season. During early and peak growth stages, the magnitude of the daytime sensible heat ¯ux averaged about 25 W mÿ2, consuming 5±10% of Rn. It increased to about 50 W mÿ2 at the end of senescence. Daily evapotranspiration utilized about 80±90% of Rn in the early and peak growth stages. During senescence, the percentage decreased to 30±80%. During the early and peak growth stages, the averaged daily ET was between 75% and 100% of the potential rates. During senescence this percentage was 10±75%. The total evapotranspiration was successfully separated into evaporation and transpiration using a dualsource modi®cation of the Penman±Monteith equation (Massman, 1992). During the early and peak growth stages, Ev (transpiration) ranged from 1.3 to 4.0 mm per day and Es (evaporation) ranged from 1.0 to 3.2 mm per day. Transpiration decreased from 1.8 mm per day at the beginning of senescence to negligible values near the end. The McNaughton and Spriggs (1989) model, developed primarily for dryland vegetation (therefore, disregarding water surface evaporation), signi®cantly overestimated the ET/ETeq ratio in Phragmites (ETeq is the equilibrium evapotranspiration) when the canopy stomatal resistance was larger than 150 s mÿ1. The model prediction improved signi®cantly when the contribution of evaporation was eliminated by substituting the ET/ETeq ratio by Ev/Eveq (transpiration/equilibrium transpiration). Acknowledgements This study was supported by a grant from the Great Plains Regional Center of the National Institute for Global Environmental Change and by the UNL Center for Laser Analytical Studies of Trace Gas Dynamics and the Agricultural Research Division. We would like to thank Dave Earl, Sheldon Sharp and Jim Hines for technical support; Narasinha Shurpali and Rob Clement for help in data collection; and Olga Vanyarkho for the data on stomatal resistance, LAI and canopy height. We thank Drs. Tim Arkebauer and Bill Massman for their valuable comments on the manuscript. This paper has been assigned Journal Series No. 12252, Agricultural Research Division, University of Nebraska ± Lincoln.
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References Assouline, S., Mahrer, Y., 1993. Evaporation from Lake Kinneret 1. Eddy correlation system measurements and energy budget estimates. Water Resour. Res. 29(4), 901±910. Berglund, E.R., Mace Jr., A.C., 1972. Seasonal albedo variation of black spruce and Sphagnum-sedge bog cover types. J. Appl. Meteorol. 11, 806±812. Burba, G.G., Verma, S.B., Kim, J., 1998. Surface Energy Fluxes of an Open Water Area in a Mid-Latitude Prairie Wetland. Boundary-Layer Meteorol., submitted for publication. Campbell, G.S., 1986. An Introduction to Environmental Biophysics. Springer, New York, 159 pp. Dennison, M.S., Berry, J.F., 1989. Wetlands. Guide to science, law, and technology. Noyes publications, Park Ridge, NJ, USA, 439 pp. Dolan, T.J., Hermann, A.J., Bayley, S.E., Zoltek Jr., J., 1984. Evapotranspiration of a Florida U.S.A., freshwater wetland. J. Hydrol. 1974, 355±371. Doorenbos, J., Pruitt, W.O., 1984. Crop Water Requirement, Irrigation and Drainage. Paper No. 24. FAO, Rome, 179 pp. Friedrich, K., 1972. On the evaporation from lake in warm and dry environment. Tellus XXIV 2, 116±121. Friehe, C.A., Shaw, W.J., Rogers, D.P., et al. 1991. Air±sea fluxes and surface layer turbulence around a sea surface temperature front. J. Geophys. Res. 96(C5), 8593±8609. Hammer, D.A., 1989. Creating freshwater wetlands. In: Hammer, D.A. (Ed.), Constructed Wetlands for Wastewater Treatment, Municipal, Industrial and Agricultural. Lewis Publishers, Boca Raton, FL, pp. 151±153. Imberer, J., 1985. Thermal characteristics of standing waters, an illustration of dynamic processes. In: Davis, B.R., Wamsley, R.D. (Eds.), Perspectives in Southern Hemisphere Limnology. Dr W. Junk Publishers, Dordrecht, pp. 7±30. Jarvis, P.G., McNaughton, K.G., 1986. Stomatal control of transpiration: scaling up from leaf to region. Adv. Ecol. Res. 15, 1±49. Jones, M.B., Muthury, F.M., 1984. The diurnal course of plant water potential, stomatal conductance and transpiration in a papyrus canopy. Oecologia 63(2), 252±255. Kadlec, R.H., 1989. Hydrologic factors in wetland water treatment systems. In: Hammer, D.A. (Ed.), Constructed Wetlands for Wastewater Treatment, Municipal, Industrial, and Agricultural. Lewis Publisher, Boca Raton, FL, pp. 21±40. Kim, J., Verma, S.B., 1996. Surface exchange of water vapor between an open sphagnum fen and the atmosphere. BoundaryLayer Meteorol. 79, 243±264. Koch, M.S., Rawlik, P.S., 1993. Transpiration and stomatal conductance of two wetland macrophytes (Cladium jamaicense and Typha domingensis) in the subtropical everglades. Am. J. Bot. 80(10), 1146±1154. Lafleur, P.M., 1990. Evapotranspiration from sedge-dominated wetland surfaces. Aq. Bot. 37, 341±353. Massman, W.J., 1992. A surface energy balance method for partitioning evapotranspiration data into plant and soil components for a surface with partial canopy cover. Water Resour. Res. 28(6), 1723±1732.
McNaughton, K.G., Spriggs, T.W., 1989. An evaluation of the Priestley and Taylor equation and the complementary relationship using results from a mixed layer model of the convective boundary layer, in estimation of aerial evapotranspiration. IAHS Publ. 177, 86±101. Monteith, J.L., 1965. Evaporation and Environment, Proc. XIX Symp. Soc. Exp. Biol., Cambridge University Press, London, pp. 205±403. Munro, D.S., 1979. Daytime energy exchange and evaporation from a wooded swamp. Water Resour. Res. 15(5), 1259±1264. Munro, D.S., 1987. Surface conductance to evaporation from a wooded swamp. Agric. For. Meteorol. 41, 249±258. Norman, R., Figer, L., Titus, D., Gearheart, R., 1993. Review of wetland evapotranspiration literature. Environmental Resour. Eng. Dept., Humboldt State University, Arcata, CA, 115 pp. Penman, H.L., 1948. Natural evapotranspiration from open water, bare soil and grass. Proc. R. Soc. London Ser. A 193, 120±145. Priban, K., Ondok, J.P., 1985. Heat balance components and evapotranspiration from a sedge-grass marsh. Folia. Geobot. Phyttotaxon. 20, 41±56. Priestley, C.H.B., Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev. 100, 81±92. Rao, A.S., 1988. Evapotranspiration rates of Eichhornia crassipes (Mart.) Solms, Salvinia molesta D.S. Mitchess in a humid tropical climate. Aq. Bot. 30(3), 215±222. Rijks, D.A., 1969. Evaporation from a papyrus swamp. Quart. J. Roy. Meteorol. Soc. 95, 643±649. Rose, C.W., Chapman, A.L., 1968. A physical analysis of diurnal temperature regimes in clear and turbid water layers, a problem in rice culture. Agric. Meteorol. 5, 391±409. Rosenberg, N.J., Blad, B.L., Verma, S.B., 1983. Microclimate. The Biological Environment. Wiley, New York, 495 pp. Ross, J., 1970. Mathematical models of photosynthesis in a plant stand. In: Ross, J. (Ed.), Predictions and Measurement of Photosynthetic Productivity. PUDOC, Wageningen, pp. 29±45. Roulet, N., 1991. Methane flux from drained northern peatlands. Effect of a persistent water table lowering on flux. Global Biogeochem. Cycles 7, 749±769. Slatyer, R.O., McIlroy, I.C., 1961. Practical Microclimatology. CSIRO, Australia and UNESCO, 160 pp. Smid, P., 1975. Evaporation from a reedswamp. J. Ecol. 63, 299± 309. Snyder, R.L., Boyd, C.E., 1987. Evapotranspiration by Eichhornia crassipes (Mart.) Solms and Typha latifolia L. Aq. Bot. 27, 217±227. Souch, C., Wolfe, C.P., Grimmond, C.S.B., 1996. Wetland evaporation and energy partitioning: Indiana Dunes National Lakeshore. J. Hydrol. 184, 189±208. Spieksma, J.F.M., Moors, E.J., Dolman, A.J., Schouwenaars, J.M., 1997. Modelling evaporation from a drained and rewetted peatland. J. Hydrol. 199, 252±271. Stanhill, G., 1969. A simple instrument for field measurement of turbulent diffusion flux. J. Appl. Meteorol. 8, 509±513. Stannard, D.1., Rosenberry, D.O., 1991. A comparison of shortterm measurements of lake evaporation using eddy correlation and energy budget methods. J. Hydrol. 122, 15±22.
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51 Stewart, R.B., Rouse, W.R., 1976. A simple method for determining the evaporation from shallow lakes and ponds. Water Resour. Res. 12(4), 623±628. Sturrok, A.M., Winter, T.C., Rosenberry, D.O., 1992. Energy budget evaporation from Williams Lake, a closed lake in northcentral Minnesota. Water Resour. Res. 28(6), 1605±1617. Szeicz, G., Endrodi, G., Taichman, S., 1969. Aerodynamic and surface factors in evaporation. Water Resour. Res. 5, 380± 394.
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Van der Weert, R., Kamerling, G.E., 1974. Evapotranspiration of water hyacinth (Eichhornia crassipes). J. Hydrol. 22, 201±202. Vanyarkho, O., 1996. Seasonal changes in vegetative characteristics and gas exchange of Phragmites australis and Scirpus acutus in mid-latitude prairie wetland ecosystem. M.S. Thesis. University of Nebraska ± Lincoln, NE, 85 pp. Verma, S.B., Kim, J., Clement, R.J., 1992. Momentum, water vapor and carbon dioxide exchange at a centrally located prairie site during FIFE. J. Geophys. Res. 97(17), 18629±18693.
Surface energy ¯uxes of Phragmites australis in a prairie wetland G.G. Burbaa, S.B. Vermaa,*, J. Kima,b a
School of Natural Resource Sciences, Center for Laser-Analytical Studies of Trace Gas Dynamics, University of Nebraska, Lincoln, NE, USA b Global Environment Laboratory Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea Received 19 June 1998; accepted 23 December 1998
Abstract Components of the surface energy balance were measured in three communities (Phragmites australis, Scirpus acutus, and open water) in a wetland located in northcentral Nebraska, USA, during the growing season of 1994. This paper includes results from the area covered by Phragmites australis (reedgrass). The Bowen ratio ± energy balance method was used to calculate sensible and latent heat ¯uxes. During daytime, with a water depth of about 0.5 m, the heat storage term (G) consumed 20±30% of the net radiation (Rn). At night, G was a signi®cant source of energy (on average, about ± 40 W mÿ2). The magnitude of the daily (24 h) averaged G was small. Evapotranspiration (ET) was a major consumer of the incoming solar energy. During early and peak growth, the daily ET ranged between 2.5 and 6.5 mm per day. During senescence, evapotranspiration was between 0.5 and 3.1 mm per day. ET was partitioned into transpiration (Ev) and evaporation (Es) using a dual-source modi®cation of the Penman±Monteith equation. Results indicated that transpiration contributed 40±45% of the total ET in the beginning of the early growth stage. During the second half of the early growth stage and the entire peak growth stage, it contributed 53±62% of ET. The contribution decreased to 50% in the beginning of senescence, and to near zero in late senescence. The daytime variation of Es did not follow Rn, and seemed to be controlled by thermal stability and air dryness. Before senescence, the ratio of the actual to equilibrium evapotranspiration (ET/ETeq) averaged 1.3. It decreased to about 0.5 during senescence. The McNaughton and Spriggs (1989, IAHS Publ. 177, 86±101) model, developed primarily for dryland vegetation, signi®cantly overestimated the ET/ETeq ratio in Phragmites when the canopy stomatal resistance was larger than 150 s mÿ1. The model prediction improved signi®cantly when the contribution of evaporation was eliminated by substituting the ET/ETeq ratio by Ev/Eveq (transpiration/equilibrium transpiration). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Wetlands; Evaporation; Transpiration; Heat storage; Radiation
1. Introduction Energy exchange is among the most important processes in wetland ecosystems, because it affects variables such as temperature, water transport, plant growth and productivity (Dennison and Berry, 1989). *Corresponding author. Tel.: +1-402-472-3679; fax: +1-402472-6614; e-mail: [email protected]
The main components of the surface energy balance are the net radiation, the heat stored in water and soil, the sensible heat ¯ux, and the latent heat ¯ux (or evapotranspiration). Evapotranspiration (ET) in vegetated wetlands is frequently the largest consumer of the incoming energy (Priban and Ondok, 1985) and has a great in¯uence not only on the energy distribution, but on water conditions (e.g. temperature, depth, salinity).
0168-1923/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0168-1923(99)00007-6
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Previous studies of wetland energy ¯uxes have reported the in¯uence of meteorological factors such as solar radiation (Rijks, 1969; Hammer, 1989; Kadlec, 1989; La¯eur, 1990), wind speed (Rijks, 1969; Snyder and Boyd, 1987; Rao, 1988; Hammer, 1989), vapor pressure de®cit (Rijks, 1969; Van der Weert and Kamerling, 1974; Jones and Muthury, 1984) on ET. Limited information, however, is available on the in¯uence of plant related variables, such as stomatal resistance and leaf area index (LAI) (Munro, 1987; Koch and Rawlik, 1993; Kim and Verma, 1996). Very little is known about the partitioning of evapotranspiration into canopy transpiration and evaporation from the water surface. The present study was designed to investigate the energy ¯uxes in a mid-latitude prairie wetland dominated with Phragmites australis. The main objective of the study was to quantify the components of the surface energy balance in this important community. To better understand the energy partitioning, we attempted to separate evaporation from transpiration using a dual-source modi®cation of the Penman± Monteith equation (Massman, 1992), and examine their diurnal and seasonal variations. 2. Materials and methods 2.1. Site This study was conducted at Ballards Marsh (428300 N, 1008250 W), located in the Sandhills region of northcentral Nebraska, USA, during the growing season of 1994. In general, the Sandhills area is characterized as a semiarid prairie consisting of grasscovered sand dunes and interdunal valleys occupied by marshes, lakes and wet meadows. These wetlands are fed primarily by precipitation and, to a smaller extent, by ground water. Dunes, located 1±2 km away from the measurement station, are up to 100 m 150 m wide and rise up to 20 m above the interdunal valley. The valley is ¯at, and measures 1.2 km 5.0 km in NE±SW direction. Ballards Marsh is approximately 1.0 km 2.5 km (NE±SW) in size. Most of the marsh is dominated by two densely vegetated monospeci®c communities (Phragmites australis and Scirpus acutus), and by open water area. Phragmites and Scirpus were 3.0 and 1.5 m tall and
occupied about 40% and 30% of the total area of the marsh, respectively. Average water depth was 0.5 m. Open water was about 30% of the marsh, with average water depth of 1 m. The 1994 growing season was warmer by about 1.48C (on average) and slightly drier (by 40 mm), as compared to the long term values measured at Valentine Wildlife Refuge (15 km from the study site). The Bowen ratio ¯ux station (described below) was placed in the northcentral part of the marsh. At this location, the upwind fetch of the exclusively Phragmites dominated vegetation was about 250±330 m in the E, W and SSE directions, about 100±120 m in the SE and SW directions, and about 90±100 m in the NW direction. 2.2. Field measurements Net radiation was measured 1 m above canopy with a net radiometer (Radiation Energy Balance Systems, Beaverton, OR, model Q7). Incoming and outgoing shortwave radiation were measured with pyranometers (LI-COR, Lincoln, NE, model LI-200SA) located 1 m above the canopy. Air temperature and humidity gradients were measured with two shielded aspirated psychrometers equipped with Vaisala temperature/humidity sensors (Campbell Scienti®c, Logan, UT, model HMP35C), that were exchanged (between the heights of 1 and 2 m above the canopy) every 5 min. The mean water temperature (Tw) was measured with a 0.5 m long RTD (resistance temperature device) bar, with the upper end attached to a ¯oating panel and the lower end attached to a load. The bar was shaded with a 0.5 m circular styrofoam panel. In addition, six thermistors were installed at different depths with 10 cm intervals. The average temperature of the top 0.1 m of the submerged soil was measured with two 0.1 m long RTD bars. Temperature of the water surface (Tws) was measured with a thermopile, which ¯oated at the surface and was shaded by 0.15 m circular styrofoam panel. Water depth was measured with a sensor described by Roulet (1991). Heat stored in water (G) was calculated from the change in mean water temperature: G cw d
Tw t
(1)
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
where (Tw/t) is the time rate of change in mean water temperature measured with the RTD bar, cw is the water heat capacity, d is the water depth. The heat storage in the submerged soil was calculated from the values measured with soil heat ¯ux plates installed 0.1 m below the surface of the submerged soil, and from the rate of change of the average temperature of the top 0.1 m of the submerged soil. The heat storage in the submerged soil was negligible (less than 5% of G), and was ignored in further computations. Mean horizontal wind speed (U) was measured at 1.0, 1.5 and 2.0 m above the vegetation with cup anemometers (Cayuga Development, Ithaca, NY, model WP-1). Wind direction was measured with a vane at the height of 3 m above vegetation. Atmospheric pressure (P) was measured with a barometer (Vaisala Barometric Pressure Sensor, model PTA427, Cambell Scienti®c). These variables were measured every 5 s and averaged for 30 min. The 30 min averages were used in further computations. Vegetative characteristics in Phragmites australis (LAI and height of the vegetation) were measured in a concurrent study by Vanyarkho (1996). Leaf area index was measured on weekly basis with an plant canopy analyzer (LI-COR, Lincoln, NE, model LAI2000) and also from destructive sampling using a leaf area meter (LI-COR, Lincoln, NE, model LI-3100). 2.3. Theoretical considerations 2.3.1. Bowen ratio ± energy balance method The energy balance can be approximated as: Rn ÿ G ÿ E ÿ H 0
(2)
where Rn is the net radiation, G is the heat stored in the water, E is the latent heat ¯ux, H is the sensible heat ¯ux. Using the ¯ux-gradient approach, latent and sensible heat ¯uxes can be expressed as: Mw =Ma @e a Kw P @z @T H ÿa Cp Kh @z
E ÿ
(3) (4)
where is the latent heat of vaporization; Mw and Ma are the molecular weights of water and dry air, respectively; P is the air pressure, a is the density of moist air, Kw and Kh are the turbulent exchange coef®cients for water vapor and sensible heat, respectively; @e/@z
33
and @T/@z are the gradients of vapor pressure and potential air temperature, respectively; and Cp is the speci®c heat of air at constant pressure. From Eqs. (3) and (4), the Bowen ratio ( H/E) can be expressed as:
Kh @T=@z Kw @e=@z
(5)
where CpP/(Mw/Ma). Assuming that Kh is equal to Kw, and that (@T/@z)/(@e/@z) T/e, Eq. (5) can be simpli®ed to:
T e
(6)
where T and e are the vertical differences of temperature and vapor pressure, respectively. The above equations lead to: E
Rn ÿ G 1
(7)
H
Rn ÿ G 1
(8)
and
2.3.2. Partitioning of evapotranspiration into evaporation and transpiration The total E (or evapotranspiration) was separated into evaporation (Es) and transpiration (Ev) using a dual-source modi®cation of the Penman±Monteith equation (Massman, 1992). Fig. 1 schematically shows the resistance formulation used in this approach. The term ra is the aerodynamic resistance; rc is the canopy stomatal resistance; rs is the surface resistance to water vapor transfer; rbc is the canopy boundary layer resistance; ru is the subcanopy resistance; eo is the vapor pressure within the canopy; ea is the above canopy vapor pressure; e*(Tws) is the saturated vapor pressure at the temperature of water surface (Tws); To is the air temperature within the canopy; Ta is the above canopy temperature; and Tv is the temperature of the canopy. The energy budget for the water surface and the canopy (Fig. 1) can be approximated as: Rns ÿ G ÿ Es ÿ Hs 0
(9)
Rnv ÿ Ev ÿ Hv 0
(10)
and
34
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
ratio ( s) can be calculated using the dual-source approach described in Massman (1992): rs
Rns ÿ G a Cp Tws ÿ Ta ÿ Hra s 1 s a e Tws ÿ ea ÿ Era P
(18)
Eq. (18) can be considerably simpli®ed by assuming that the surface resistance to water vapor transfer, rs 0: s Fig. 1. Schematic representation of the dual-source combination model (Massman, 1992). See the text for definition of symbols.
a Cp Tws ÿ Ta ÿ Hra a =P e Tws ÿ ea ÿ Era
The aerodynamic resistance (ra) was estimated as: ra ram rb
where Rn Rns Rnv
(11)
E Es Ev
(12)
H Hv Hs
(13)
and The subscript `s' refers to the water surface ¯uxes and the subscript `v' refers to the plant canopy. Net radiation reaching the water surface (Rns) was approximated using the following equation (Ross, 1970): Rns Rn exp ÿKext LAI
(14)
where Kext is an extinction coef®cient, which was calculated as (Ross, 1970) Kext G sin b
(15)
where b is the solar elevation angle, and is the leaf angle distribution. Measurements by Vanyarkho (1996) in a concurrent study indicated that the leaf angle distribution in Phragmites was approximately spherical and therefore, G() was assumed to be 0.5. Using s Hs/Es, Es and Hs can be expressed as: Es
Rns ÿ G 1 s
Hs Rns ÿ G ÿ Es
(16) (17)
Information on s is needed to calculate the individual ¯ux terms: Es, Hs, Ev, and Hv. The surface Bowen
(19)
(20)
where ram is aerodynamic resistance to momentum transfer and rb is excess resistance term (e.g., Verma et al., 1992): ram
U u2
2 Dh 2=3 rb ku Dv
(21) (22)
where u* is friction velocity, k is von Karman's constant, Dh is the thermal diffusivity, and Dv is the molecular diffusivity of water vapor. The value of u* was estimated with following equation (e.g., Rosenberg et al., 1983): u
kU ln z ÿ d0 =z0
(23)
where U was measured at 2 m (above the vegetation). Zero plane displacement (d0) and roughness parameter (z0) were estimated from the canopy height (Stanhill, 1969; Szeicz et al., 1969). The variables Tws, Ta, ea, P were measured, e*(Tws) was calculated from the slope of saturated vapor pressure±temperature curve at Tws; and G, H, E and ra were calculated as discussed above. Using this information in Eq. (19), s was calculated. Then Es, Ev, Hs and Hv were calculated using Eqs. (11),(12) (16) and (17). During certain periods (under cloudy and cool conditions during early hours of some mornings), (Tws ÿ Ta) and (e*(Tws) ÿ ea) were very small, leading to potentially large errors in s and
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
consequently in Es. Data during such periods were excluded and the missing s values were ®lled in by interpolation (using a third-order polynomial regression developed with six half-hourly values of s preceding the missing period and six half-hourly values of s following the missing period). 2.3.3. Potential and equilibrium evapotranspiration Daily potential evapotranspiration (ETp) was calculated as follows (Penman, 1948): ETp
s Rn ÿ G a Cp Df U s
(24)
In Eq. (24), f(U) was estimated by the empirical equation of Doorenbos and Pruitt (1984):
35
U f U 0:27 1 100
(25)
where U is in km per day. Daily equilibrium evapotranspiration (ETeq) was calculated (Slatyer and McIlroy, 1961) as: ETeq s
Rn ÿ G s
(26)
where s is the slope of saturation vapor pressure± temperature curve. 3. Results and discussion The green LAI was 1.2 in mid-June, reached a maximum value of 2.6 in mid-August and then
Fig. 2. Seasonal distributions of: (A) The green LAI of Phragmites australis (adapted from Vanyarkho, 1996). (B) water depth (d).
36
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
declined to 0.2 in late October (Fig. 2A, adapted from Vanyarkho, 1996). Based on plant development and color of leaves and seedlings, three growth stages were approximated: early growth (mid-June to mid-July), peak growth (mid-July to early/mid-September), and senescence (early/mid-September to the end of October). The Phragmites canopy was 2.2±3.1 m tall during most of the measurement period. The water depth (d) was largest (0.53 m) in the mid-June, and decreased gradually through the season (Fig. 2B) to 0.34 m in the middle of October. 3.1. Net radiation Daytime (averaged between sunrise and sunset) incident solar radiation (Rs) and net radiation (Rn)
for the entire measurement period are shown in Fig. 3A. On clear days, daytime averaged Rs peaked around 500±600 W mÿ2 during June±July. It gradually decreased to 300 W mÿ2 by mid-October. Daytime Rn ranged about 300±370 W mÿ2 during June± July and then gradually decreased to 170±200 W mÿ2 in mid-October (Fig. 3A). Linear regression between Rs and Rn yielded: Rn ÿ4:1 0:62Rs ;
r 2 0:90
(27)
Daytime albedo ranged between 0.13 and 0.14 in midJune; it increased to 0.14±0.16 in August, and then decreased to 0.12±0.14 in mid-October (Fig. 3B). These values are close to those reported for other vegetated wetlands (0.12±0.16 for sphagnum-sedge bogs, Berglund and Mace, 1972; 0.12 for swamp
Fig. 3. Seasonal distributions of the daytime averaged values of (A) incoming shortwave (Rs) and net (Rn) radiation, and (B) albedo.
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
forest, Campbell, 1986; and 0.11±0.17 for open sphagnum fen, Kim and Verma, 1996). 3.2. Heat storage term 3.2.1. Diurnal variation The storage term (G) varied considerably from one half hour to the next most likely due to the turbulent structures of varying temperature traveling past the temperature sensors, and due to nearby animal activities. To minimize this local variability, we calculated 2 h running means of the energy storage term (G), and used them in further computations. Fig. 4A shows values of G on a clear day (18 July, 1994) during mid-season. The diurnal pattern of the magnitude of G generally followed that of Rn. The storage term became positive about 1±2 h after sunrise, implying
37
that the water body became an energy sink at that time. The peak magnitude of G (170±200 W mÿ2) occurred at about the same time (13:00±15:00 CDT) as the maximum in Rn. The storage term became negative about 1±2 h before sunset. Nighttime G varied between ÿ70 and ÿ20 W mÿ2. The pattern and magnitudes of G obtained here are similar to those measured by Smid (1975) in a 0.3 m deep reed swamp in Czechoslovakia (daytime peak of about 200 W mÿ2, and nighttime values of ÿ50 to ÿ100 W mÿ2). Data (estimated directly from changes in mean water temperature, and indirectly from Rn, H and E ¯uxes) from various water bodies (without vegetation) indicate large magnitudes of G (up to 700 W mÿ2) during daytime (Rose and Chapman, 1968; Imberer, 1985; Stannard and Rosenberry, 1991; Assouline and Mahrer, 1993; Burba et al.,
Fig. 4. (A) Storage term (G) and net radiation (Rn) on 18 July, 1994. (B) Daytime magnitude of G/Rn on 18 July, 1994.
38
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
1998). Large magnitudes of G in non-vegetated shallow water bodies are expected due to a more effective penetration of radiation. The storage term acts as practically the only source of energy at night and a considerable sink of energy during daytime. On 18 July, the magnitude of G/Rn gradually increased in the morning from 0.10 to 0.17 (08:30±09:00 hours) to a midday maximum of 0.30± 0.33, and then gradually decreased to 0.07±0.13 by 17:00±18:00 hours (Fig. 4B). The fraction of Rn consumed by G was highest at midday, because of a better direct penetration of solar radiation through the vegetation and into the water body. A linear regression between the half-hourly values of G and Rn on 15 days from different parts of the season yielded: G ÿ51 0:41Rn ;
r 2 0:86
(28)
3.2.2. Daytime, nighttime and 24 h averages Heat stored in water during daytime can be expected to depend primarily on the net radiation reaching the water surface (Rns). Water depth (d) should also play a role in determining the heat storage potential. Daytime averaged G as a function of daytime averaged Rns and d are plotted in Fig. 5A and B. Excluding rainy days, the individual linear regressions yield: G ÿ3 0:5Rns ;
r 2 0:70
(29)
G ÿ138 0:5d;
r 2 0:57
(30)
and
To examine the combined effect of both variables, the heat stored per mm of water depth (G/d) is shown as a function of Rns in Fig. 5C. Linear regression (excluding rainy days) yields: G 0:0242 0:0009Rns ; d
r 2 0:73
(31)
No signi®cant effects of other variables (including the difference between air and water temperatures (Ta ÿ Tws) and wind speed) on G were found. Nighttime averaged G was found to be related to Ta ÿ Tws (Fig. 5D): G ÿ37 8:5 Ta ÿ Tws ; r 2 0:60 for non-rainy days
(32)
Smaller magnitudes of nighttime G, ranging from ÿ50 to 0 W mÿ2, were typical when Ta ÿ Tws > ÿ18C. When the temperature difference was ÿ68C, G increased up to ÿ140 W mÿ2. Dependence of nighttime G on water depth was less signi®cant (r2 0.44). This can be attributed to the fact that the loss of longwave energy at night occurs mainly from the surface of the water and the role of the entire water body may be relatively insigni®cant. The loss of heat from the water body at night (Fig. 4A) was similar in magnitude to the gain during daytime and therefore, the values of G when averaged over 24 h were small in comparison with Rn. Daily averaged values of G ranged between ÿ30 and 20 W mÿ2 on non-rainy days. In some previous studies in shallow lakes (e.g., Sturrok et al., 1992; Assouline and Mahrer, 1993), short-term (hourly±daily) calculations of H and E have been based on bi-weekly measurements of G. As indicated in Fig. 4, the diel variability of G is large and needs to be considered in hourly calculations of H and E. However, G averaged over a few days is relatively small (Table 1) in comparison with other components of the energy budget, and could be considered close to zero in long-term (a few days) averages of H and E. Stewart and Rouse (1976) obtained similar results for unvegetated water bodies. 3.3. Sensible heat flux Sensible heat ¯ux on a clear day in mid-season is shown in Fig. 6A. The magnitudes of H increased from 0 to 40 W mÿ2 between 07:00 and 10:30 hours (central daylight saving time). Then it slightly decreased to 20 W mÿ2 at 13:00 and stayed near this value until 17:00 hours. Nighttime values of H ranged from ÿ40 to 30 W mÿ2. In a reed swamp in Table 1 Range of G averaged for the different periods during the season (including periods with rain) Number of days used for running mean averaging
Range of averaged G (W mÿ2)
1 3 5 9 30
ÿ36 to 60 ÿ14 to 51 ÿ4 to 22 0 to 14 0 to 12
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
39
Fig. 5. (A) Daytime averaged storage term (G) as a function of net radiation below the canopy (Rns). (B) Daytime averaged storage term (G) as a function of water depth (d). (C) Energy stored per mm of water depth (G/d) as a function of Rns. (D) Nighttime averaged storage term (G) as a function of the difference between air and water surface temperatures (Ta ÿ Tws).
Czechoslovakia, Smid (1975) reported similar pattern of H. The magnitude of H in his study was larger (ranging from 0 to 95 W mÿ2), however. Larger H may be an artifact of the larger values of Rn, calculated using an empirical formula in his study.
Excluding rainy days, the daily H (Fig. 6B) ranged from ÿ6 to 20 W mÿ2 before plant senescence. Daily H derived from the data reported by Munro (1979; wooded swamp) and Smid (1975; reed swamp), for non-senescent periods, ranged from 0
40
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Fig. 6. (A) Sensible heat flux (H) and net radiation (Rn) on 18 July, 1994. (B) Seasonal distributions of the daily averaged sensible heat flux (H) and net radiation (Rn). (C) Seasonal distribution of the daily averaged ratio, H/Rn.
to 25 W mÿ2. With the onset of senescence (early midSeptember), the daily averaged magnitude of H increased to 30 W mÿ2. During early and peak growth stages, H consumed about 10% of Rn (Fig. 6C). As plants began to senesce (mid-September), stomatal
resistance increased (Vanyarkho, 1996) and transpiration decreased (as will be discussed later), more energy (20±30% of Rn) was partitioned into H. In the last few days of the study (mid-October), H consumed about 70% of Rn.
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41
Fig. 7. (A) Latent heat flux (E) and net radiation (Rn) on 18 July, 1994. (B) Air temperature (Ta) and vapor pressure deficit (D) on 18 July, 1994. (C) The ratio E/Rn on 18 July, 1994.
3.4. Latent heat flux 3.4.1. Diurnal variation Latent heat ¯ux on a clear day in mid-season is shown in Fig. 7A. Midday values of E ranged from 300 to 380 W mÿ2. Peak magnitude of E occurred
about 1±2 h after the peak in Rn. This lag is likely due to enhanced evapotranspiration in the afternoon resulting from higher air temperature and vapor pressure de®cit (Fig. 7B). Nocturnal values of E were small (from ÿ30 to 30 W mÿ2). Small negative E ¯ux may be related to heavy fog often observed at night.
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Pattern and magnitudes of E mentioned above are similar to those observed by La¯eur (1990) for sedgedominated wetlands in Canada. La¯eur (1990) measured midday E ranging from 210 to 400 W mÿ2. Souch et al. (1996) and Spieksma et al. (1997) observed similar E over a lakeshore wetland in Indiana and in a peatland in The Netherlands, respectively. In a reed swamp in Czechoslovakia, Smid (1975) reported larger magnitudes of midday E (varying from 400 to 500 W mÿ2). However, Rn calculated by an empirical formula in his study was 100± 170 W mÿ2 higher than the net radiation measured in our study. For a sphagnum-dominated wetland in Minnesota, Kim and Verma (1996) reported an average daytime E of 230±250 W mÿ2 in mid-season. Latent heat ¯ux was the prime sink of Rn in this ecosystem (Fig. 7C). During midday, E/Rn was 0.65±0.70. Smid's (Smid, 1975) data from a reed swamp would suggest E/Rn of 0.74 during midday in the peak growth season. Unvegetated water surfaces consume considerably smaller amounts (20±55%) of Rn in evaporation during daytime (Friehe et al., 1991; Stannard and Rosenberry, 1991; Burba et al., 1998). 3.4.2. Daily evapotranspiration rates Seasonal pattern of daily integrated ET followed that of the net radiation (Fig. 8A). Sharp decreases in the magnitude of ET occurred primarily on cloudy and rainy days (DOY 169, 178, 195, 247, 277). Excluding rainy days, the ET rates ranged from 3.5 to 6.5 mm per day during the early and peak growth stages. The evapotranspiration decreased to about 2 mm per day during senescence. The average ET rate for the entire measurement period (June±October) was 3.75 mm per day. Evapotranspiration data reported from different wetlands include: 1.3±6.0 mm per day (with an average of 3.1 mm per day) for a sedge-dominated wetland in central Canada (La¯eur, 1990); 2.0±4.6 mm per day for wooded swamp in Ontario (Munro, 1979); 0.5± 4.8 mm per day for an open sphagnum fen in northcentral Minnesota (Kim and Verma, 1996); 1.4± 6.9 mm per day for a reed swamp in Czechoslovakia (Smid, 1975; Priban and Ondok, 1985); and 5.1 mm per day for freshwater marsh in Florida (Dolan et al., 1984). As discussed above, on a daily basis G and H were small. Latent heat ¯ux, therefore, was the primary consumer of Rn (Fig. 8B). On non-rainy days, E
generally consumed 80±100% of Rn during the ®rst two stages (early and peak growth: mid-June to early September) of the canopy development. During senescence, E/Rn decreased to 0.6±0.8 (Fig. 8B). By midOctober, when most plants had senesced, E/Rn was about 0.3. 3.5. Partitioning evapotranspiration into transpiration and evaporation As discussed in Section 2.3, ET was partitioned into the evaporation (Es) and transpiration (Ev) components using the dual-source modi®cation of the Penman±Monteith equation (Massman, 1992). Three clear days were selected to represent typical behavior of Ev and Es (Fig. 9) in each growth stage: 19 June (early growth stage), 18 July (peak growth stage) and 10 October (senescence). 3.5.1. Transpiration In the early growth stage, on 19 June (green LAI 1.4), the pattern of Ev followed that of Rn (Fig. 9A). Transpiration increased from 0±10 W mÿ2 during 08:00±08:30 hours to a peak magnitude of 230 W mÿ2 during 13:30±14:30 hours and then decreased to 10 W mÿ2 during 19:30±20:00 hours. Daytime averaged magnitude of Ev was 97 W mÿ2. In the peak growth, on 18 July (Fig. 9B) transpiration had a similar pattern, but substantially larger magnitudes (peak 350 W mÿ2, daytime average 177 W mÿ2). Larger Ev magnitudes are because of a larger green LAI 2.0 (midday air temperature, vapor pressure de®cit and wind speed on this day were not much different from those on 19 June, Table 2). The canopy transpiration utilized practically all (>90%) the net radiation intercepted by vegetation (Rn ÿ Rns). Substantially smaller transpiraTable 2 Air temperature, wind speed and vapor pressure (at 2 m above vegetation) averaged between 11:30 and 16:30 hours, on 19 June, 18 July and 10 October, 1994
19 June 18 July 10 October
Temperature (8C)
Vapor pressure deficit (kPa)
Wind speed (m sÿ1)
30 30 22
2.5 2.4 2.0
4.5 4.0 5.0
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43
Fig. 8. (A) Seasonal distributions of the daily integrated evapotranspiration (ET) and daily averaged net radiation (Rn). (B) Seasonal distribution of the ratio E/Rn.
tion rates were observed during senescence on 10 October (green LAI 0.8). The midday air temperature and vapor pressure de®cit were lower than those on the other days, but wind speed was higher (Table 2). On 10 October (Fig. 9C), the peak magnitude of Ev was about 40 W mÿ2, and the daytime average was 12 W mÿ2. It is also worthwhile to examine the results on a cloudy day (5 October) during late senescence (Fig. 9D). Theoretically, due to plant senescence and low Rn, the transpiration on this day should be very low. As seen in Fig. 9D, the calculated values of transpiration were indeed close to zero throughout the day (the daytime average magnitude of Ev was 3 W mÿ2).
3.5.2. Evaporation The magnitude of evaporation on 19 June increased from 70 to 130 W mÿ2 between 08:00 and 10:30 hours (Fig. 9A), and then decreased to small values (0± 20 W mÿ2) during 13:00±15:00 hours. Later in the afternoon Es increased to 170 W mÿ2 (18:00±19:00 hours). A similar pattern of Es, with slightly smaller magnitudes, was seen on 18 July (Fig. 9B). Toward the end of the season (10 October) the evaporation rate increased to 130±140 W mÿ2 between 12:00 and 16:00 hours, with no clear evidence of a daytime depression (Fig. 9D). Friedrich (1972) suggested that the evaporation rates above water surface could be signi®cantly (up to 80%) suppressed because of stable
44
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Fig. 9. Partitioning of latent heat flux (E) into transpiration (Ev) and evaporation (Es) on: (A) 19 June (B) 18 July (C) 10 October (D) 5 October.
thermal strati®cation caused by temperature inversion over the cool water surface. Depression in the daytime Es was also observed by others (e.g., Assouline and Mahrer (1993) over a large lake; Burba et al. (1998) over an open water area of Ballards Marsh) when water temperature was cooler than the air.
3.5.3. Daily integrations The daily transpiration and evaporation are shown in Fig. 10A. Transpiration ranged between 1.3 and 4.0 mm per day through the early and peak growth stages (with an average of 2.9 mm per day). During senescence Ev ranged between 0 and 1.8 mm per day
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
45
Fig. 10. Seasonal distribution of (A) the daily evaporation (Es) and transpiration (Ev), and (B) Es/ET and Ev/ET.
(with an average of 1.1 mm per day). Evaporation was between 1.0 and 3.2 mm per day during most of the early and peak growth stages. In Fall, Es did not decrease as rapidly as Ev did, and had an average of 1.8 mm per day. During the beginning of the early growth stage, transpiration contributed about 40±45% of the total ET (Fig. 10B). Then, with an increase in plant growth and activity, the transpiration contribution increased and during the second half of early growth stage and throughout peak growth stage, it contributed 53±62% of ET. The transpiration contribution decreased from about 50% in the beginning of the senescence to
almost negligible levels at the end of the measurement period (Fig. 10B). 3.6. Actual, potential and equilibrium evaporation rates 3.6.1. Potential evapotranspiration Measured ET and ETp for the whole measurement period are shown in Fig. 11A. During the early and peak growth stages ET was 75±100% of ETp (Fig. 11B). During senescence, ET was considerably smaller (10±75%) than ETp (except on cloudy days when both were small). For sedge-dominated wetland
46
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
Fig. 11. Seasonal distribution of (A) actual (ET) and potential (ETp) evapotranspiration, and (B) ET vs. ETp.
La¯eur (1990) reported ET being 75±100% of ETp. Coef®cients of linear regression between ET and ETp from our study are: intercept, a 0.65 mm; slope, b 0.75, with r2 0.85. La¯eur (1990) obtained a 0.06±0.44 mm andb 0.61±0.83, with r2 0.81±0.89. Examination of the Penman±Monteith equation (Monteith, 1965) and Eq. (24) indicates that ET/ ETp may be linked to the canopy stomatal resistance (rc). In the following, we examine the dependence of the ET/ETp ratio on midday rc (Fig. 12A). The values
of rc used here are from a concurrent study of Vanyarkho (1996), in which leaf stomatal resistance was measured. The measurements of rc were scaled up to the canopy level using the light response relationships in an approach described in Jarvis and McNaughton (1986), and in Norman et al. (1993). As seen in Fig. 12A, the Phragmites evapotranspired at a potential rate when midday rc was less than about 100 s mÿ1. As rc increased, ET/ETp declined and approached 0.1 when rc was about 10 000 s mÿ1. The
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
47
Fig. 12. (A) Dependence of the ratio of actual to potential evapotranspiration (ET/ETp) on midday canopy stomatal resistance (rc). (B) Dependence of transpiration (Ev) on midday rc. (C) Dependence of the ratio ET/ETp on green LAI. (D) Dependence of the ratio ET/ETp on the difference between air and water surface temperatures (Ta ÿ Tws).
canopy stomatal resistance controlled the transpiration rate, Ev (Fig. 12B), and therefore, affected the ET/ETp ratio. Ev was less than 1 mm per day when rc < 250 s mÿ1. A decrease in rc from 250 to 75 s mÿ1 corresponded to an increase in Ev from 1.2 to an
average of 3 mm per day. Measured ET was close to potential rates for green LAI greater than 1.8 (Fig. 12C). Also, as mentioned earlier, thermal stability conditions affect Es (evaporation from the water surface), and, therefore, should also be considered. Although
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G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
there is scatter in the data (Fig. 12D), ET seems to be near ETp (ET/ETp 0.6±1.2) when Ta ÿ Tws < 28C and generally less than ETp under stronger inversion conditions (Ta ÿ Tws > 28C).
3.6.2. Equilibrium evapotranspiration It is also worthwhile to evaluate the ET/ETeq ratio, traditionally known as the Priestley±Taylor coef®cient (Priestley and Taylor, 1972). During most of the
Fig. 13. (A) Seasonal distribution of ET/ETeq. (B) ET/ETeq as a function of the canopy stomatal resistance (rc): open circles are data from this study; the curve is the ET/ETeq±rc relationship from McNaughton and Spriggs (1989). Average values from 9 days included in their Figure 3 are shown here. (C) Ev/Eveq as a function of rc. Solid squares are data from this study. The curve is the same as in B. (D) Same as (C) with results on ET/ETeq from tallgrass prairie (open triangles): Verma et al. (1992).
G.G. Burba et al. / Agricultural and Forest Meteorology 94 (1999) 31±51
early and peak growth stages ET/ETeq ranged from 1.2 to 1.5 (with an average of 1.3), and decreased from about 1.3 to 0.5 during senescence (Fig. 13A). In Fig. 13B, ET/ETeq is examined as a function of the canopy stomatal resistance in the framework of the McNaughton and Spriggs (1989) model (developed primarily for dryland vegetation). For rc < 100 s mÿ1, ET/ETeq is close to 1.26. However, for rc > 150 s mÿ1 ET/ETeq deviated signi®cantly from the values predicted by the model. To eliminate the effect of evaporation, the ratio of the actual to equilibrium transpiration (Ev/Eveq) was computed, where Eveq s
Rnv s
(33)
As shown in Fig. 13C, the dependence of Ev/Eveq on rc is much closer to that predicted by the McNaughton and Spriggs model for ET/Eeq. The Ev/Eveq ratio was about 1.26 when rc < 100 s mÿ1 and rapidly decreased with increasing rc. The relationship between Ev/Eveq and rc is quite similar to that (ET/ETp and rc) from a tallgrass prairie in Kansas (Verma et al., 1992), where the evaporation contribution was small (Fig. 13D). 4. Summary A micrometeorological study was conducted to measure energy ¯uxes over Phragmites australis in the Sandhills of Nebraska during the growing season of 1994. During the early growth stage, the daytime albedo ranged between 0.13 and 0.14. It increased to 0.14±0.16 during the peak growth stage and decreased to 0.12±0.14 during senescence. The averaged daytime net radiation was 300±370 W mÿ2 during June±July and decreased to 170±200 W mÿ2 by mid-October. The heat storage term was the main source of energy at night and a considerable sink of energy during daytime. Averaged over a week or more, heat storage term becomes quite small. It suggests that during Bowen ratio calculations of long-term H and E over wetland heat storage component in some cases could be neglected. It also suggests that if hourly or daily H and E are of interest, heat storage is quite important, and should be measured as frequently as temperature and humidity gradients.
49
Sensible heat ¯ux was a minor component of the energy budget during most of the growing season. During early and peak growth stages, the magnitude of the daytime sensible heat ¯ux averaged about 25 W mÿ2, consuming 5±10% of Rn. It increased to about 50 W mÿ2 at the end of senescence. Daily evapotranspiration utilized about 80±90% of Rn in the early and peak growth stages. During senescence, the percentage decreased to 30±80%. During the early and peak growth stages, the averaged daily ET was between 75% and 100% of the potential rates. During senescence this percentage was 10±75%. The total evapotranspiration was successfully separated into evaporation and transpiration using a dualsource modi®cation of the Penman±Monteith equation (Massman, 1992). During the early and peak growth stages, Ev (transpiration) ranged from 1.3 to 4.0 mm per day and Es (evaporation) ranged from 1.0 to 3.2 mm per day. Transpiration decreased from 1.8 mm per day at the beginning of senescence to negligible values near the end. The McNaughton and Spriggs (1989) model, developed primarily for dryland vegetation (therefore, disregarding water surface evaporation), signi®cantly overestimated the ET/ETeq ratio in Phragmites (ETeq is the equilibrium evapotranspiration) when the canopy stomatal resistance was larger than 150 s mÿ1. The model prediction improved signi®cantly when the contribution of evaporation was eliminated by substituting the ET/ETeq ratio by Ev/Eveq (transpiration/equilibrium transpiration). Acknowledgements This study was supported by a grant from the Great Plains Regional Center of the National Institute for Global Environmental Change and by the UNL Center for Laser Analytical Studies of Trace Gas Dynamics and the Agricultural Research Division. We would like to thank Dave Earl, Sheldon Sharp and Jim Hines for technical support; Narasinha Shurpali and Rob Clement for help in data collection; and Olga Vanyarkho for the data on stomatal resistance, LAI and canopy height. We thank Drs. Tim Arkebauer and Bill Massman for their valuable comments on the manuscript. This paper has been assigned Journal Series No. 12252, Agricultural Research Division, University of Nebraska ± Lincoln.
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