Evapotranspiración y Balance de Energía en El Cultivo de Alfalfa

8/11/2019 Evapotranspiracin y Balance de Energa en El Cultivo de Alfalfa

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EVAPOTRANSPIRATION AND ENERGY BALANCE ON AN ALFALFA CROP

Eduardo Moguel-Ordez, Leonardo Tijerina-Chvez, Abel Quevedo-Nolasco,

Guillermo Crespo-Pichardo2 y Gabriel Haro-Aguilar2

Divisin Acadmica de Ciencias Biolgicas. Universidad Jurez Autnoma de Tabasco. Km. 0.5

Carretera Villahermosa-Crdenas. Villahermosa, Tabasco. ([email protected]). 2Especialidad

de Postgrado en Agrometeorologa. IRENAT. Colegio de Postgraduados. 56230, Montecillo,Estado de Mxico. ([email protected]).

ABSTRACT

Bowens micrometeorological method of energy balance was usedto learn about the accuracy

of evapotranspiration estimation during short periods of time and to study energy balance on

a crop of alfalfa (Medicago sativa L.) established at the Colegio de Postgraduados in

Montecillo, State of Mexico. Lysimeter readings and Bowens estimates of latent heat fluxes

(LE) were compared at hourly intervals for four days. Soil heat fluxes (G) and sensible heat

(H) were also calculated. Due to the presence of obstacles around the study area, atmospheric

stability conditions required to assume equal turbulent exchange rates for sensible heat (Kh)and vapor water (Kw) were not met; therefore, it was necessary to calculate these rates to

correct Bowens values (). When values (assuming Kh=Kw) were compared with corrected

values it was found that fluctuated in the winter from 129.3 to 63.4 when Kh=Kw and

from0.879 to 2.48 when was corrected for calculated Kh and Kw. During the summer,

fluctuated from 1.37 to 1.40 when Kh=Kw, and from -0.59 to 0.10 when was corrected for

calculated Kh and Kw. In general, diurnal flux of LE estimated from Bowens method

underestimated lysimeter flux readings due to the advection of sensible heat; which reached

values of 92.8 Kw in the summer. Diurnal balance of energy indicated that advection of

sensible heat represented slightly more than 40% of the net radiation.

Key words:Medicago sativa L., agrometeorology, flux heat, Bowen ratio method, microclimate.

INTRODUCTION

Knowledge about crop energy balance helps to

understand and manage mass and energy fluxes

and their effects on plant yield. Measurements

of energy balance components (including

evapotranspiration as latent heat flux) are used

in agriculture and meteorology to study

different processes for water management

which, among others, include: calibration andvalidation of water balance models in cultivated

areas, evaluation of remote sensors on crops,

protection of crops against freezing, planning

and design of irrigation systems, irrigation

schedules, yield prediction, and modeling of

physiological processes.

The study of evapotranspiration processes helps

to model, predict, and increase crop yields. As a

consequence, there has been an increase in the

use of methodologies that involve exchange of

energy and mass and how this exchange is

influenced by environmental and plant

physiological variables. Micrometeorological

methods, supported by phisiologicalinformation, are one of the best means to

examinate the interaction between a crop and

the environment (Baldocchi et al., 1983).

Among micrometeorological methods of energy

and mass balance, Bowens Energy Balance

Ratio has been used successfully to estimate
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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crop evapotraspiration in real time; moreover,

the equipment required to use this method is

more accessible than that required by other

methods (Ashktorab et al, 1989).

Waggoner (1975) and Heilman et al. (1989)

claimed that in order to use Bowens method, as

for other methods, it is necessary to have stable

atmospheric conditions or wide open areas

where one can assume that vertical transfer

factors for water vapor (Kw) and sensible heat

(Kh) are the same. Otherwise, if these

conditions are not met due to the effect of

obstacles (such as wind break barriers) on the

layer of air in which measurements are to be

taken; then one should calculate turbulent flux

coefficientsKh andKw (Brown and Rosenberg,1971).

In this study we evaluated the accuracy of

Bowens micrometeorological method for

evapotranspiration estimation in short periods

of time; in addition, we studied energy

partitioning in alfalfa (Medicago sativa L.) for

two days in the winter and two days in the

summer.

MATERIALS AND METHODS

The study was conducted in an area cultivated

with the variety of alfalfa Castilla, within the

lysimeter and agrometeorological station area

of the Colegio de Postgraduados in Montecillo,

State of Mexico (19o 29 N and 98o 54 W), at

an altitude of 2250 m. The soil was classified

within the Montecillo series as a deep sand

loam with low texture variability and without

salinity problems. Wind break barriers were

present around the area cultivated with alfalfa;barriers consisted of 20+ m trees at the East and

greenhouses, not higher than 5 m, at the West.

Based on information available from the

agrometeorological station, data from two

winter (13 and 15 January 1990) and two

summer (8 and 9 August 1991) days were used;

where readings were taken every hour from

7:00 to 18:00. Evapotranspiration was measured

using a lysimeter, outfitted with a combined

mechanical-electronic recorder system, with an

undisturbed structure soil monolith measuring

1.80 m in length and width, with a depth of 1.5m. The system allows the detection of weight

changes corresponding to a water depth of 0.06

mm.

An automatic meteorological station model CR-

10 (Campbell Scientific, Logan, U.S.A.) was

set alongside the lysimeter, and it was

programmed to register averages for the

analyzed variables every 30 min. The reference

heights for both temperature and relative

humidity sensors (Young trademark), and windspeed sensor (two cup anemometers: Models

03001, Young trademark and H2996, Met One

trademark) were 50 and 194 cm in summer and

75 and 225 cm in winter. Net radiometer

(Model Q7, Radiation and Energy Balance

Systems, Inc.) height was 195 cm in the

summer and 100 cm in the winter. Soil

thermometers (Model 107 B, Campbell

Scientific) were set at depths of 2 and 12 cm in

the summer and at 0, 10, and 20 cm in the

winter.

Bowens microclimatic method of energy

balance

As stated by Rosenberg et at, (1983) and

Ashktorab et al, (1989):

=H /LE = (Cp P /L) (T / e) (Kh /Kw) (1)

WhereH is the sensible heat flux (Wm-2):

H = (Rn + G +LE) (1.1)

G is the soil heat flux (Wm-2), LE is the latent

heat flux (Wm2). L is the evaporation latent

heat (cal g-1) obtained by:

L = 595.9 0.55T, con T enoC (1.2)


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Where Cp is the airs specific heat at constant

pressure (0.24 cal g-1 oC); is the ratio of

waters molecular weight to that of airs

(18.016/ 28.960=0.622); P is the atmospheric

pressure (mb), T is the air temperature (oC)

difference (T2T1) at two different heightsfrom the soil surface (oC), e is the water vapor

pressure (mb) difference (e2e1) at two

different heights from the soil surface (mb), Kh

is the turbulent exchange coefficient for

sensible heat (cm2s-1), and Kw is the turbulent

exchange coefficient for water vapor (cm2s-1).

From equation (1):

H = LE (2)

And, when substituting this equation on the

energy balance equation

Rn + G +LE +H = 0 (3)

LE = (Rn + G)/(1 + ) (4)

Which is known as Bowens Energy Balance

Ratio.

This method assumes that, when fetch

conditions are adequate, meteorologicalinstruments within the sublayer are in complete

equilibrium () under stable atmospheric

conditions, or Kh=Kw. Nevertheless, in this

study Kh and Kw values were calculated, using

lysimeter data, in order to verify adequate fetch

conditions. With this purpose, the following

procedures were used (Motha et al., 1979):

Kh= (H Z) /{aCp (T2T1)} (5)

And

Kw = (ETlisZ)/(Ha2Ha1) (6)

where Z = height difference between two

reference points (cm); a = air density (0.00125

g cm-3); T2 and T1 = air temperatures (oC) at

reference heights 2 and 1; ETlis = lysimeter

evapotranspiration (g cm-1

s-1

); andHa2andHa1

= air absolute moisture (g cm-3

) at reference

heights 2 and 1.

The condition of atmospheric stability was

verified using Richardsons Number

adimensional parameter (Ri). This parameter

describes the relative importance of floating and

mechanical forces, that is, the relative

importance of free against forced convection

(Rosenberg et al., 1983).Ri is obtained by:

Ri = [(g / T) (d/ dz)] / (dU / dz)2 (7)

Where g is the acceleration due to gravity

(9.8 m s-2

); (d/dz) and (dU/ dz) are vertical

gradients for average potential temperature

(), and average horizontal wind speed,respectively; and T is the average absolute

temperature (oK). The sign for Ri is

determined by the potential temperature

gradients; Ris sign is negative in unstable

conditions and positive in stable or thermal

inversion conditions. When Ri approaches

zero, it means that conditions are close to

neutrality. The T gradient was used instead

of the potential temperature () because it is

possible to do so within the first two

meters, in height, from the soil surface.

Null alignment method

Soil heat flux (G) was obtained using the

null alignment method proposed and

described by Kimball and Jackson (1975).

In order to prevent soil moisture from

limiting alfalfas evapotranspiration,

sprinkler irrigation was applied for the

duration of the study to maintain water

potentials between 0.3 and 0.5 atmospheres.

Inner limiting layer and fetch conditions


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According to Munro and Oke, cited by

Rosenberg et al. (1983), the density

(thickness) of the inner limiting layer (CLI)

may be calculated by:

CLI =X4/5

z01/5

(8)

WhereX is the distance between the edge of the

crop field (where the wind is coming from) and

the reference point, and z0 is the rugosity

parameter of the underlying surface. The

thickness of the sublayer at complete

equilibrium () is considered a 10th of CLI

thickness, measured above the zero plane of

displacement (d).

Values for z0 and d are obtained from ratiosproposed by Stanhill, cited by Hatfield (1989):

z0= 0.13 hc (9)

where hc is crop height in meters, and

log d = 0.979 log hc0.154 (10)

RESULTS AND DISCUSSION

Micrometeorological conditions recorded in this

studywere different between winter and

summer.

1) Oscillation of air and soil temperature and

relative humidity were higher in the winter

(24.7, 12.9 oC, and 79.3%, respectively) than in

the summer (11.3, 6.1 oC, and 36.2%,

respectively).

2) 9 hours of positive Rn were recorded in the

winter with an average flux density of 226.83

Wm-2, and an

average maximum value of 425.74 Wm-2.

During these nine hours an average total flux of

122.5 Kw was received.

3) 11 hours of positive Rn were recorded in the

summer with an average flux density of 224.97

Wm-2, and an average total flux of 161.2 KW.

4) Wind speed in the summer fluctuated

between 0.20-0.96 and 0.0-0.4 m s-1 at heights

of 194 and 50 cm, respectively. The lower wind

speeds were recorded in the first hours of the

day. Direction of predominant winds was NW

and S. Calm conditions prevailed during the

first part of the day in the winter; but, even

though wind speed increased from 12:00, wind

speed remained low at less than 0.4 m s-1. This

was observed at both 225 and 75 cm heights.

During these days, predominant winds came

from the NW.

Inner limiting layer and fetch conditions

Measurements for the inner limiting layer

thickness obtained for the winter were wide

enough to cover the meteorological instruments

set in the field. Nevertheless, the sub-layer

width at absolute balance () had values

fluctuating between 0.75 and 1.51 m in the

winter and 0.82 to 1.65 m in the summer. With

these values, it was not possible to completely

cover all of the equipment set in the field. Thiscould have favored a transportation momentum

higher than 10%; which is, as stated by

Heilman et al. (1989) and Rosenberg et al.

(1983), the maximum transportation momentum

for the completely fitted sub-layer for ideal

placement of meteorological instruments.

Fetch:height ratios obtained in this study (at

lysimeteredge distances of 48 m N, 63 m W, 40

m S, and 26 m E) for the summer, were 80:1,105:1, 67:1, and 43:1 to N, W, S, and E

respectively. In the winter, this ratio was higher

at 100:1 to the N, W, and S, and 74:1 to the E.

When associating the completely fitted layer ()

thickness to fetch, it indicated that the study

area would require a 106 m windward fetch in


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the winter and a 92.7 mfetch in the summer in

order to obtain a minimum thickness of 2.25 m

for which would totally cover meteorological

instruments, and therefore, would allow to

assume that Kh=Kw.

Ri showed that, for both winter and summer,

conditions of atmospheric stability were present

during the first hours in the morning (until

9:30). Unstable conditions started at 10:00, and

prevailed until 18:00 in the winter; however,

stable atmospheric conditions showed again at

about 16:00 in the summer.

Kh and Kw values

Because the area where the study was carried

out did not present the conditions of

atmospheric stability, required by Bowens

method, neither had enough fetch; calculation

of Kh and Kw was done at hourly intervals for

the days included in this study. Kh and Kw

changed during the day, and this was reflected

by the Kh/Kw ratio (Table 1). In general, the

Kh values were lower than Kw ones. Perhaps

this happened because of the effect of obstacles

(wind break barriers and greenhouses) existing

around the study area. These obstacles reduceKh more than Kw (Brown and Rosenberg,

1971).

Correction of value

The correction of values using the calculated

ratio Kh/Kw allowed, for both study periods, to

reduce the oscillation of , and consecuently,

these values fell within the intervals obtained

by Duning et al. (1991) in wheat (Triticum

aestivum L.), Hanks et al. (1971) in sorghum(Sorghum bicolor L.), and Fritschen and Van

Bavel (1964) in Sudangrass (Sorghum

halapense).

Table 1. Bowens b values uncorrected (1:

Kh=Kw) and corrected (2 when Kh and Kw

are calculated) for a day in winter and a day

in summer.

The occurrence of very high values during the

first hours in the morning, or at the end of the

afternoon (Table 1), is a common situation;

because at those times of the day LE is low

(Williams and Stout, 1981).

Calculus of latent heat flux (LE) using Bowens

method Lysimeter (LElis) and Bowens method

(LEBow) latent heat fluxes showed a tendencyof LEBow to underestimate LElis mainly in the

last hours in the afternoon (15:00-18:00 hours)

with values higher than 69.79 Wm-2. Bowens

method tendency to underestimate LE at the

end of the afternoon was due to the fact that

LEBows calculation was related to net

radiation flux (Ashktorab et al., 1989). In

contrast to the summer, LEBow overestimated

LElis in the winter. LElisLEBow differences

reached hourly values of up to 335 Wm-2

(12:00 on 9 August 1990) and 265.21 Wm-2

(11:00 on 13 January 1991). In the first case

LElis was underestimated by LEBow, and in

the second one LElis was overestimated. These

results coincided with those by Dunin et al.

(1991) who found out, under advection

conditions, that Bowens method has a


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tendency to underestimate lysimeter

measurements of LE. LE overestimation on

January 13 could have been due to the presence

of calm conditions during the first half of the

day (wind speed under 0.2 m s-1) that reduced

Kw for that period, and also to predominatinglow temperatures that induced stomata closure

in alfalfa.

When analyzing the relationship between

LEBows estimation of LElis measurements,

both in Wm-2 units, we fitted the following

models:

a) 13 January: LElis = 0.0956 + 0.5453

LEBow, r2 = 0.7318

b) 15 January: LElis = 0.1265 + 0.7631

LEBow, r2 = 0.7631

c) 8 August: LElis = 0.1675 + 1.5044 LEBow,

r2 = 0.9063

d) 9 August: LElis = 0.0169 + 1.3558 LEBow,

r2 = 0.7583

Balance of energy in alfalfa

Balance of energy analysis (Figure 1) showed a

sensitive heat (H) advection process which was present

in both winter and summer. This phenomena had direct

repercussions on energy partitioning in terms of ground

(G) and latent (LE) heat flow. Advection favored a

higher diurnal LE flow in the summer with values of up

to 203.1 KW; while G heat flow was 31.3 KW. In the

winter, advection contributed more to G flow (up to 93

KW) than LE flow (with a maximum value of 166.8

KW on 15 January).

Total diurnal Rn received in each study period was

considered as a basis for result comparison. In thesummer (with an average Rn of 161.4 KW), an average

Figure 1. Energy balance for winter and

summer days in alfalfa. Rn=net radiation;

G= heat to soil flow; LE=latent heat flow;

H=sensitive heat flow.

of 25.2 KW (15.6% of Rn) was used as G, and

202.6 KW (125.5% of Rn) as LE. In the winter(with an average Rn of 149.6 KW), 61.3 KW

(41.0% of Rn) was used as G, and 150.8 KW

(100.8% of Rn) as LE. Energy deficits were

covered by H advection, with averages of 66.3

KW in the summer (41.1% of Rn) and 64.9 KW

(43.4% of Rn) in the winter.

The advection of sensitive heat cannot always

be detected by Bowens method. This was

evident by underestimation of lysimeter

measured LE flow. Based on advection types

shown by Hanks et al.(1971) and on

micrometeorological information obtained in

this work; it can be infered that large scale

(thermal inversion) and border (temperature and

vapor pressure horizontal gradients) advection

types allowed for heat entrance into the

cultivated area.

As indicated, when employing Bowens method

in evapotranspiration studies, it is

recommended to work in areas free of obstacles

within at least a 110 m radius, avoiding wind

breakers and buildings. Even so, results from

the present study indicate that in such cases it is

feasible to use lysimeter information that allows

calculation of Kw for the analyzed area, crop,

and season of the year. It is recommended to


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deepen the understanding of advections role in

the partition of energy and, specially, in the

flow of LE.

CONCLUSIONS

In order to use the Bowens method of energy

balance it is necessary to calculate Kh and Kw

(interchangeable turbulence of heat and water

vapor coefficients, respectively) if the area to be

studied has obstacles that impede free

circulation of air. In this study correction of

Bowens value () using Kh/Kw was more

significant in the winter (January) than in the

summer (August). In Montecillo, State of

Mexico, comparison of Bowens energy

balance estimated evapotranspiration to

lysimeter measurements resulted in Bowens

method underestimating (summer) and

overestimating (winter) evapotranspiration

calculated as LE. This allowed to detect that

advection of sensitive heat is an important

source of energy that increases

evapotranspiration in alfalfa. Rn and H had

similar behavior in energy balance for both

winter and summer; even so, LE flow was

greater in the summer, in good measure becausethe ground in the summer has less heat flow (G)

than in the winter.

BIBLIOGRAPHY

Dunin, F. X., H. D. Barrs, W. S. Meyer, and A.

C. F. Trevitt. 1991.

Foliage temperature and latent heat flux of

irrigated wheat. Agric. For. Meteorol. 55: 133-

147.

Fritschen, L. J., and C. H. M. van Bavel. 1964.

Energy balance as affected by height and

maturity of Sudangrass. Agron. J. 56: 201-204.

Hanks, R. J., L. H. Ailen, and H. R. Gardner.

1971. Advection and evapotranspiration of

wide-row sorghum in the Central Great Plains.

Agron. J. 63: 520-527.

Hatfield, J. L. 1989. Aerodinamics properties of

partial canopies. Agric. For. Meteorol. 46: 15-

22.

Heilman, J. L., C. L. Brittin, and C. M. U.

Neale. 1989. Fetch requeriments for Bowen

ratio measurements of latent and sensible heat

fluxes. Agric. For. Meteorol. 44: 261-273.

Evapotranspiración y Balance de Energía en El Cultivo de Alfalfa

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