European Journal of Agronomy 13 (2000) 155–163

Measurement and modeling of evapotranspiration of olive(Olea europaea L.) orchards

F.J. Villalobos a,b,*, F. Orgaz a, L. Testi a, E. Fereres a,b

a Instituto de Agricultura Sostenible, CSIC, Apartado 4084, 14080 Cordoba, Spainb Dep. Agronomıa, Uni6ersidad Cordoba, Apartado 3048, 14080 Cordoba, Spain

Received 19 February 1999; received in revised form 6 July 1999; accepted 4 February 2000

Abstract

Efficient irrigation management requires a good quantification of evapotranspiration. In the case of olive orchards,which are the dominant crop in vast areas of southern Europe, very little information exists on evaporation.Measurements of aerodynamic conductance and evaporation above and below an olive orchard allowed thecalibration of a transpiration model of olive trees based on the Penman–Monteith equation. The model wascombined with Ritchie’s soil evaporation model and tested against an independent data set, indicating its validityunless a substantial fraction of the soil surface is wetted by irrigation emitters, which is not taken into account by themodel and deserves further research. Simulated crop coefficients of olive orchards in southern Spain changed duringthe year in response to changes in vapor pressure deficit (VPD) and evaporation from the soil surface. The averageannual crop coefficient (0.62) was rather low due to the low ground cover and to the enhanced control of canopyconductance by stomatal responses to VPD. According to our results the crop coefficient will vary among locationsand even among years, depending on rainfall and temperature. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Evaporation; Evapotranspiration; Olive; Olea europaea L.; Eddy covariance

www.elsevier.com/locate/eja

1. Introduction

Olive orchards are the main component of agri-cultural systems in many semiarid regions aroundthe Mediterranean, with more than 2 million ha inSpain and around 5 Mha in the whole EuropeanUnion (Civantos, 1997). Most olive orchards arerainfed, with yields limited mainly by water sup-

ply. Traditional olive orchards in Spain have typi-cally around 100 trees per ha with ground coverrarely exceeding 25%. Modern orchards are usu-ally drip-irrigated, with 200–300 trees per ha andground cover of 40–50%. Drip irrigation has alsoextended to numerous traditional orchards usinggroundwater of poor quality and uncertainsupply.

Good irrigation management requires an accu-rate quantification of olive evapotranspiration.The most common approach to calculate evapo-transpiration (ET) has been as the product of

* Corresponding author. Tel.: +34-957-499234; fax: +34-957-499252.

E-mail address: aglvimaf@uco.es (F.J. Villalobos).

1161-0301/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 1161 -0301 (00 )00071 -X

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163156

reference (grass) ET (ETo) by the crop coefficientKc=crop ET/grass ET, which depends on groundcover and crop characteristics (FAO method,Doorenbos and Pruitt, 1977; Allen et al., 1998). Inthe case of olive the information on Kc is scarceand obtained mainly from ET measurements us-ing the soil water balance (e.g. Mickelakis et al.,1994). Orgaz and Fereres (1997) reported cropcoefficients from 0.45 to 0.75 in different locationswhich are far below the values of annual crops,typically from 1.0 to 1.2 (Doorenbos and Pruitt,1977). The variability of Kc measured at differentlocations makes it difficult to apply the FAOmethod to locations where no experimental infor-mation exists. An alternative approach to deter-mine olive ET is to calculate its two components,transpiration (Ep) and evaporation from the soilsurface (Es), independently, with an Ep modelbased on the equation of Penman–Monteith(Monteith, 1965) and an Es model like the oneproposed by Ritchie (1972). The Ep model re-quires a parameterization of canopy conductance(Gc) as a function of environmental variables (e.g.Stewart, 1988) which has to be based on accuratemeasurements of evaporation at short time steps(e.g. Dolman et al., 1988).

The objectives of this work were (a) to developa joint Es–Ep model to determine evapotranspira-tion of intensive irrigated olive orchards, and (b)to analyze temporal variations in the crop coeffi-cient.

2. Materials and methods

The experiments were performed in a 6 hadrip-irrigated olive (Olea europaea L.) cv. ‘Picual’orchard located at the Agricultural Research Cen-ter of Cordoba, Spain (38°N, 4°W, altitude 110m). Plant spacing was 6×6 m. The trees had anaverage leaf area index (LAI) of 1.5 in May 1996and 1.2 in May 1997, as determined with a PlantCanopy Analyzer (model LAI-2000, Li-Cor Inc.,Lincoln, NE) following the procedure of Villalo-bos et al. (1995). Mean tree height was 4 m andground cover was 40%.

Hourly weather data were determined over anirrigated grass (Festuca arundinacea L.) plot of 1.5ha, located 400 m northwest of the olive orchard.

2.1. Experiment 1

Sensible (H) and latent heat flux (LE) weremeasured using the eddy covariance methodabove and below the trees from day of year(DOY) 162 (11 June) to DOY 181 (30 June),1997. Transpiration (LEp) was computed as thedifference between LE above (ET) and LE below(Es). The fluxes were measured with a single-axissonic anemometer (model CA27, Campbell Scien-tific, Logan, UT) and a krypton hygrometer(model KH20, Campbell Scientific). The sensorsabove were set at a height of 5 m with a horizon-tal separation of 0.3 m, while the sensors belowwere separated 0.13 m at a height of 0.4 m.Sampling frequency was 10.67 Hz (inverse of 6/64s). Fetch was 170 m in the west direction, whichmight be considered adequate as it contributed90% of the measured fluxes according to thefootprint analysis of Schuepp et al. (1990). Cor-rection to the LE fluxes due to sensible and latentflux (Webb et al., 1980), oxygen absorption (Tan-ner et al., 1993) and sensor separation (Villalobos,1997) were applied. Fluxes were calculated andstored using a datalogger (model CR10X, Camp-bell Scientific) for 10-min periods and then aver-aged for hourly periods.

2.2. Experiment 2

Energy balance and evaporation measurementswere performed from March 1996 to June 1996.LE and H were determined above the canopy asin Section 2.1. Net radiation (Rn) was measuredabove and below the canopy using four net ra-diometers (model Q-7, Radiation and Energy Bal-ance Systems, Seattle, WA) in locations ofmaximum (above tree) and minimum groundcover (middle point among four trees). Measure-ment heights were 5 and 0.3 m for sensors aboveand below the canopy, respectively. Soil heat fluxwas determined at the same two locations ofmaximum and minimum ground cover. The com-bination method was applied with soil tempera-ture measured using a copper-constantanthermocouple at 0.025 m and heat flux measuredwith a soil heat flux plate (model HFT3.1, Radia-tion and Energy Balance Systems) at 0.05 m. Soil

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163 157

heat flux plates and thermocouples were locatedbeside the net radiometers below the canopy.Measurements of Rn and G were performed at60 s intervals while 10 min averages were storedusing a datalogger (model CR10X) and thenaveraged for hourly periods. Net radiation abovethe orchard was regressed on solar radiationmeasured using a pyranometer (see below). Soilheat flux was regressed on net radiation belowthe canopy. These relationships were later usedto run the evaporation model using weatherdata.

The performance of the eddy covariance systemwas assessed by regressing (LE+H) on (Rn−G)for 24-h periods. The intercept (5.9 W m−2) andthe slope (0.88) were not different from 0 and 1,respectively, at the 0.95 probability level. Theregression forced through the origin yielded aslope of 0.91 indicating an underprediction of thefluxes of less than 10% which is similar to thatreported by other authors (e.g. Rochette et al.,1995) using the eddy covariance technique. Dailyevaporation data were corrected by dividing bythe ratio (LE+H)/(Rn−G).

2.3. Calculations

Hourly canopy conductance (Gc) for daylightconditions was derived from the Penman–Mon-teith equation:

G c−1=

�DRna+rCpVPDLEp

−D−g� 1

gGa

(1)

where D is the slope of saturation vapor pressureon temperature, g the psycrometric constant, r airdensity, Cp specific heat of air at constant pres-sure, VPD vapor pressure deficit and Ga is aero-dynamic conductance (see below). Absorbed netradiation (Rna) was estimated as the product of Rn

above the orchard and the fraction of interceptedphotosynthetically-active radiation (PAR), whichwas in turn calculated using a model of PARinterception by olive orchards (Mariscal, 1998).

The sensitivity of LEp to changes in Gc and Ga

where calculated according to Raupach andFinnigan (1988).

2.4. Experiment 3

The objective of this experiment was to deter-mine the relationship between aerodynamic con-ductance over the olive orchard and wind speedover a grass (reference) plot.

A three-dimensional sonic anemometer (modelCSAT-3, Campbell Scientific) was installed at a 5m height over the olive orchard at the sameposition where the sensors were located duringmeasurements in 1996. The sensor was monitoredat 10.7 Hz using a CR10X datalogger. The mea-surements were performed from 28/04/97 to 05/05/97 (DOY 118-DOY 125). Fluxes werecalculated and stored for 10-min periods and thenaveraged for hourly periods.

Simultaneous measurements of wind speed at 2m height (Ug) using a propeller anemometer(Young 05103) were performed over the irrigatedgrass plot.

Aerodynamic conductance over the olive or-chard was calculated as:

Ga=u�

2

Uo

(2a)

u�=−w %u % (2b)

where u� is friction velocity (m s−1), Uo averagehorizontal wind speed (m s−1), u % and w % areinstantaneous departures from the average hori-zontal and vertical wind speed, respectively, andhorizontal bar indicates an average. Empiricalrelationships between Ga and Ug and between u�and Uo were derived.

2.5. Transpiration model

We fitted an empirical model (e.g. Stewart,1988) to estimate canopy conductance (Gc) in theorchard described above as a function of irradi-ance (Rs), vapor pressure deficit (VPD) and tem-perature (T):

Gc=Gcm f1(VPD)f2(Rs)f3(T) (3)

f1(VPD)=1−KDD 0BDBDc (4a)

f1(VPD)=1−KDDc D\Dc (4b)

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163158

f2(Rs)=Rs(1000+KR)1000(Rs+KR)

(5)

f3(T)=(T–TL)(TH–T)a

(KT–TL)(TH–KT)a (6)

where a= (TH−KT)/(KT−TL). In order to mini-mize the number of unknown parameters (Gcm,KD, Dc, KR, KT, TL, TH) we assumed that thelower and upper limiting temperatures were TH=40°C and TL=0°C, which are the values sug-gested by Stewart (1988) for pine trees.

The model was combined with Ritchie (1972)model as modified by Bonachela et al. (1999) tocalculate Es. The combined model was testedagainst the data of Section 2.2 (spring 1996). Themodel was then used to calculate the daily cropcoefficient (Kc, ratio of evaporation to grass ET)of the orchard, (case A, typical of intensive plan-tations) using weather data of Cordoba from 1964to 1986, as Kc is a commonly used parameter forcalculating irrigation requirements. The modelwas also applied to a traditional orchard (case B)with the same LAI and ground cover of 30%,assuming that maximum Gc was 80% of that ofthe intensive orchard, due to the lower radiationinterception. The value of 80% corresponds to theratio of intercepted radiation between the twoorchards. Although future work should be di-rected to the scaling-up of olive canopy conduc-tance our assumption is supported by the general

link between canopy conductance, and assimila-tion (e.g. Wang and Leuning, 1998) and the rela-tion between assimilation and radiationinterception of olive trees shown by Mariscal etal. (2000).

3. Results and discussion

3.1. Radiation balance

Net radiation above the tree was not statisti-cally different from net radiation above the alley,thus data from the two net radiometers abovewere averaged. Hourly net radiation was regressedon solar radiation:

Rn=0.81Rs−69 (W m−2), r2=0.99 (7)

Regression of Rn against Rs for 24-h periodshad an intercept not different from 0 and a slopeof 0.6.

Soil heat flux was related to Rn below thecanopy (Rnb):

G=0.17Rnb−16 (W m−2), r2=0.74 (8)

3.2. Aerodynamic conductance

The relation between friction velocity (u�) andwind speed above the olive orchard is shown inFig. 1. There was an apparent reduction in slopefor Uo greater than 1 m s−1, which could be dueto swaying of the trees. The linear regression was:

u�= −0.026+0.302Uo r2=0.92 (9)

with the intercept different from 0 at the 95%probability level. The regression forced throughthe origin yielded a slope of 0.282. From thelogarithmic wind profile we may calculate:

z−dzo

=exp� k

u�/un

(10)

where k is Von Karman’s constant (0.4), d zeroplane displacement (m), zo roughness length (m),and z is the measurement height (m). Assumingthat d is in the interval 0.5–0.75 h, where h is themean tree height (4 m) then zo would lie between0.48 and 0.61 m.

Fig. 1. Relation between friction velocity (u�) and wind speedabove the olive orchard, Cordoba, Spain, April 1997. Datashown are hourly averages.

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163 159

Fig. 2. Daily course of latent heat flux measured above (total)and below (soil) an olive orchard. Data are the averages ofhourly fluxes for DOY’s 171, 172 and 173, Cordoba, Spain,June 1997.

roughness elements which are concentrated in thetree canopies. Measured aerodynamic conduc-tance yielded values around four times higherthan those of a nearby grass (Festuca arundinacea,L.) plot.

3.3. Soil and plant e6aporation

Fig. 2 presents the daily course of total (LE)and soil (LEs) latent heat flux measured in Section2.1. Data correspond to averages of hourly datafor DOY’s 171, 172 and 173, which are the onlydays when complete 24 h data are available. BothLE and LEs were close to 0 during the night, andreached maximum values of 222 and 54 W m−2,respectively, 2 h after solar noon. The latent heatflux corresponding to transpiration (LE−LEs)reached a maximum of 167 W m−2, but it wasfairly constant during most of the daylight period(09:00–18:00 h) ranging from 131 to 167 W m−2.

Total values of evapotranspiration, soil evapo-ration and transpiration were 3.12, 0.74 and 2.38mm per day, respectively. At the time these mea-surements were made, the soil surface was dry.Nevertheless, soil evaporation contributed 24% tothe orchard ET. The ratio soil evaporation/ETwould probably increase substantially when thesoil is partially wetted by drip irrigation.

3.4. Canopy conductance

Fig. 3 shows the daily course of estimated Gc ona sunny day of June 1997 (DOY 172). A typicalasymmetrical pattern is seen, and it may be ex-plained by the combined effects of irradiance andVPD on stomatal aperture, similar to the responseat the leaf level shown by Fereres (1984). AverageGc increased rapidly from sunrise to its maximumvalue (8.5 mm s−1) 4 h afterwards, and thendecreased throughout the daytime period, with arapid decline until 13:00 h and a slower changethereafter, when the air humidity deficit had al-most reached its maximum. Similar trends of Gc

have been reported for other tree species (e.g.Nothofagus, Schulze et al., 1995; Maritime pine,Gash et al., 1989).

The parameters of the Jarvis–Stewart modelfitted to our data are shown in Table 1. Data for

Aerodynamic conductance (m s−1) over theorchard was regressed against Ug:

Ga=0.0053+0.0496Ug r2=0.85 (11)

with the intercept not different from 0 at the 95%probability level. The regression forced throughthe origin yielded a slope of 0.052, which is simi-lar to the value calculated for Pinus pinaster Ait.(0.056) of 20 m height by Gash et al. (1989). Thus,olive trees show a comparatively higher aerody-namic roughness than pines which may be due tothe highly heterogeneous arrangement of the

Fig. 3. Daily course of canopy (Gc) and aerodynamic (Ga)conductance, Olive, Cordoba, Spain, June 1997.

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163160

Table 1Parameters for calculating canopy conductance of olive treesobtained by optimizationa

PineParameter Olive

Gcmax (mm s−1) 33.415.4KR (W m−2) 2611194

0.0610.059KD (kg g−1)11.8Dc (g kg−1) 10.9

20.020.2KT (°C)

a Data for pine trees (Gash et al., 1989) are also shown.

that of pine. According to the Gc model, theresponse of evaporation to changes in SHD showsa maximum at SHD=0.5/KD, being 8.5 g kg−1

for olives (1.4 kPa).Air temperature for maximum Gc, which is

equal to KT, was 20.2°C for olive. No attempts toderive low (TL) and high (TH) temperaturethresholds were performed because of the limitedrange of temperature during our measurements. Itis important to emphasize the limitations of thistype of analysis as radiation, temperature andhumidity are not varied independently, and moreimportant, there is a direct dependence betweencalculated Gc and the environmental variables (viathe Penman–Monteith equation), which meansthat this type of model of Gc is statistically incor-rect. However, most of our current knowledge oncanopy conductance for different types of vegeta-tion is based on the same model which has beenshown adequate for predicting evaporation inmany cases (e.g. Dolman et al., 1988). Neverthe-less the overall agreement of our olive data withprevious studies on pine indicate that olive Gc

responds to environmental conditions very muchlike coniferous forests despite differences in bothheight and LAI. Further research is needed todetermine the effect of LAI and soil water deficiton olive Gc.

The relative sensitivity of LEp to changes in Gc

was large (Fig. 4), with values around 0.9 duringmost of the daytime period, indicating thatchanges in Gc will cause changes of the samemagnitude in olive evaporation. On the otherhand, the sensitivity of LEp to changes in Ga,which is associated with wind speed, was ex-tremely low with absolute values typically below0.03. These sensitivities indicate that accurate pre-diction of olive evaporation requires a soundmodel of Gc (a function of irradiance, air humid-ity and temperature) and that wind speed willplay a minor role in determining LEp.

3.5. Model test

Calculated and measured daily ET data of theorchard in spring 1996 (Section 2.2) are shown inFig. 5. This data set was used to derive therelationships between net and solar radiation and

pine (Pinus pinaster Ait.) trees determined byGash et al. (1989) for Les Landes forest areshown for comparison. Maximum Gc was 15.4mm s−1, which would correspond to an average0-deficit stomatal conductance of ca. 13 mm s−1

(assuming that gs scales-up linearly with LAI),which is close to maximum values of stomatalconductance observed by Orgaz (pers. commun.)in this orchard. Maximum values of Gc are lowerfor olive trees than for pine (Table 1) which maybe partly explained by a lower LAI (1.2 vs. 2.3).The response of Gc to radiation indicates a sloweropening of olive stomata as radiation increaseswhen compared with pine. For instance, 75% ofmaximum gs will be attained at radiation intensi-ties of 620 and 383 W m−2, for olive and pine,respectively. On the other hand, the response ofolive Gc to specific humidity deficit was similar to

Fig. 4. Daily course of sensitivity of evaporation to canopy((dLE/LE)/(dGc/Gc)) and to aerodynamic conductance ((dLE/LE)/(dGa/Ga)), Olive, Cordoba, Spain, June 1997.

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163 161

Fig. 5. Calculated and measured daily ET of the olive orchard,Cordoba, Spain, 1996.

tween 5 and 10%, which is extremely wide whencompared with commercial orchards (1–5%). An-other possible factor could have been an increasein Gc when irrigation started, although rainfallamount during this spring was probably enoughto avoid any water stress. These results are en-couraging in terms of model validity for condi-tions of uniformly wetted soil. The under-prediction shown after the start of drip irrigationmay be associated with evaporation from the wetbulbs; the study of this association deserves fur-ther research.

3.6. Orchard ET and crop coefficient

The annual course of simulated average decadalET is presented in Fig. 6. Minimum values areclose to 1 mm per day during the winter, whilemaximum values around 3.5 mm per day occurredin June and July. The differences between the twoorchards are negligible in winter and increase asET increases, due to changes in the relative im-portance of Es (Fig. 7), which is the main compo-nent of ET during the winter and decreases to lessthan 10% of ET during the summer. This iscaused by the typical Mediterranean pattern ofrainfall in Cordoba, with a wet season from Octo-ber to April and a dry season during the summer.

Average annual ET’s were 855 and 758 mm, fororchard A and B, respectively, while average an-

soil heat flux and Rn below the canopy, thus it isnot strictly independent, although it is so in termsof the transpiration–evaporation model.

Measured ET ranged from 2 to 5.5 mm perday, while reference (grass) ET varied from 2.7 to8.5 mm per day. The agreement between observedand measured ET was good up to DOY 145 (24May), when drip irrigation was started. Fromthen on measured ET exceeded estimated ET by0.5–1 mm per day. This difference might be thecontribution of evaporation from the soil wettedby the emitters, which was neither included in themodel nor measured. The wetted area was be-

Fig. 6. Simulated average decadal ET of olive orchards ofLAI=1.4 and 40 (A) or 30% ground cover (B) at Cordoba(Spain), 1964–1986. Vertical bars represent the S.D.

Fig. 7. Simulated average decadal soil evaporation (Es) of oliveorchards of LAI=1.4 and 40 (A) or 30% ground cover (B) atCordoba (Spain), 1964–1986. Vertical bars represent the S.D.

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163162

Fig. 8. Simulated average decadal crop coefficient (ET/grassET=ETo) of olive orchards of LAI=1.4 and 40 (A) or 30%ground cover (B) at Cordoba (Spain), 1964–1986. Verticalbars represent the S.D.

cases. Therefore, accurate prediction of Es is alsorequired to estimate ET of olive orchards.

Crop coefficients varied substantially throughthe year with maximum values close to one duringwinter (Fig. 8) and minimum values around 0.4 inAugust. The variability in decadal Kc’s also de-creased from winter to summer. The high Kc inwinter is the result of enhanced soil evaporationdue to rainfall, which also explains the greatervariability of the Kc. As the season progressed, theprobability of rainfall decreased and so did Es,and consequently, the Kc and its variability. Apartfrom the decrease in Es from winter to summer,increasing VPD from spring to summer and thereduction in the fraction of intercepted radiationled to a relative minimum in the ratio Ep/ETo inthe summer (Fig. 9).

For annual values the crop coefficient was 0.62for case A and 0.55 for case B, well below annualcrop maximum Kc. The variation of the olive Kc

throughout the year and its dependence on VPD,intercepted radiation and Es, and thus, on therainfall pattern, clearly indicates the difficulty inproposing a unique Kc value valid for differentlocations. Even in a given area, interannual vari-ability in rainfall dates and amount will lead tochanges in both winter and spring Kc.

4. Conclusion

Measurements of evaporation above and belowan olive orchard and aerodynamic conductanceallowed the calibration of a transpiration modelof olive trees. The model was combined with asoil evaporation model and tested against an inde-pendent data set, indicating a fair performanceunless a substantial fraction of the soil surface iswetted by irrigation emitters, which is not takeninto account by the model. Crop coefficients ofolive orchards in southern Spain change duringthe year in response to changes in net radiation,air temperature, windspeed, VPD and evapora-tion from the soil surface. The average crop co-efficient is rather low due to the low ground coverand to the enhanced control of canopy conduc-tance by stomatal responses to VPD. These resultsindicate that estimates of olive water requirements

nual ETo was 1373 mm. Annual values of Es werethe same for the two cases (290 and 293 mm)which might be explained by the conservativenature of the Es process, a higher ground cover(case A) leads to lower Es during the first stage(energy limited) after wetting of the soil, leadingto a slower drying of the soil surface which inturn keeps a higher evaporation rate for a longerperiod. The contribution of Es to annual ET washigher in the traditional (39%) than in the inten-sive orchard (34%), but it was substantial in both

Fig. 9. Simulated average decadal transpiration coefficient(Ep/grass ET=ETo) of olive orchards of LAI=1.4 and 40 (A)or 30% ground cover (B) at Cordoba (Spain), 1964–1986.Vertical bars represent the S.D.

F.J. Villalobos et al. / Europ. J. Agronomy 13 (2000) 155–163 163

can not be assessed accurately with the crop co-efficient method, thus an articulate approach, likethe one presented here, is needed.

Acknowledgements

We would like to thank Dr Miguel Pastor forproviding access to the experimental orchard. DrMarcello Donatelli and an anonymous reviewercontributed substantially to improving themanuscript. This work was supported by grantsHID96-1295-CO4-01 of Programa Nacional deRecursos Hidricos and OLI96-2212 of Programade Aceite de Oliva Comision Interministerial deCiencia y Tecnologia, Spain. LT held a fellowshipfrom Fundacion del Olivar de la Provincia deJaen.

References

Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Cropevapotranspiration. Guidelines for computing crop waterrequirements. FAO Irrigation and Drainage Paper 56,Rome, Italy, 300 p.

Bonachela, S., Orgaz, F., Villalobos, F.J., Fereres, E., 1999.Measurement and simulation of evaporation from soil inolive orchards. Irrig. Sci. 18, 205–211.

Civantos, L., 1997. La olivicultura en el mundo y en Espana.In: Barranco, D., Fernandez-Escobar, R., Rallo, L. (Eds.),El cultivo del olivo. Mundi-Prensa, Madrid, pp. 15–31.

Dolman, A.J., Stewart, J.B., Cooper, J.D., 1988. Predictingforest transpiration from climatological data. Agric. For.Meteorol. 42, 339–353.

Doorenbos, J., Pruitt, W.O., 1977. Crop water requirements.FAO irrigation and Drainage Paper no. 24, Rome, Italy.

Fereres, E., 1984. Variability in adaptive mechanisms to waterdeficits in annual and perennial plants. ActualitesBotaniques 131, 17–32.

Gash, J.H.C., Shuttleworth, W.J., Lloyd, C.R., Andre, J.C.,Goutorbe, J.P., Gelpe, J., 1989. Micrometeorological mea-surements in Les Landes forest during HAPEX-MO-BILHY. Agric. For. Meteorol. 46, 131–147.

Mariscal, M.J., 1998. Intercepcion de radiacion solar y acumu-lacion de biomasa por cubiertas de olivo. Tesis Doctoral,Universidad de Cordoba, Spain. 173 p.

Mariscal, M.J., Orgaz, F., Villalobos, F.J., 2000. Radia-tion use efficiency and dry matter partitioning of ayoung olive orchard (Olea europaea L.). Tree Physiol. 20,65–72.

Mickelakis, N.I.C., Vouyoucalou, E., Klapaki, G., 1994. Plantgrowth and yield response of olive to different levels of soilwater potential. Acta Hort. 356, 205–210.

Monteith, J.L., 1965. Evaporation and environment. Symp.Soc. Exp. Biol. XIX, 205–234.

Orgaz, F., Fereres, E., 1997. Riego. In: Barranco, D., Fernan-dez-Escobar, R., Rallo, L. (Eds.), El cultivo del olivo.Mundi-Prensa, Madrid, pp. 251–272.

Raupach, M.R., Finnigan, J.J., 1988. Single-layer models ofevaporation from plant canopies are incorrect but useful,whereas multilayer models are correct but useless: Discuss.Aust. J. Plant Physiol. 15, 705–716.

Ritchie, J.T., 1972. Model for predicting evaporation from arow crop with incomplete cover. Water Resour. Res. 8,1204–1213.

Rochette, P., Desjardins, R.L., Pattey, E., Lessard, R., 1995.Crop net carbon dioxide exchange rate and radiation useefficiency in soybean. Agron. J. 87, 22–28.

Schuepp, P.H., Leclerc, M.Y., MacPherson, J.I., Desjardins,R.L., 1990. Footprint prediction of scalar fluxes fromanalytical solutions of the diffusion equation. Boundary-Layer Meteorol. 50, 355–373.

Schulze, E.-D., Leuning, R., Kelliher, F.M., 1995. Environ-mental regulation of surface conductance for evaporationfrom vegetation. Vegetatio 121, 79–87.

Stewart, J.B., 1988. Modelling surface conductance of pineforest. Agric. For. Meteorol. 43, 19–35.

Tanner, B.D., Swiatek, E., Greene, J.P., 1993. Density fluctua-tions and use of the krypton hygrometer in surface fluxmeasurements. In: Proceedings of the National IrrigationDrainage and Engineering, 21–23 July, 1993, Park City,UT, ASCE, New York, NY.

Villalobos, F.J., 1997. Correction of eddy covariance watervapor flux using additional measurements of temperature.Agric. For. Meteorol. 88, 77–83.

Villalobos, F.J., Mateos, L., Orgaz, F., 1995. Non-destructivemeasurement of leaf area in olive (Olea europaea L.) treesusing a gap inversion method. Agric. For. Meteorol. 73,29–42.

Wang, Y.-P., Leuning, R., 1998. A two-leaf model for canopyconductance, photosynthesis and partitioning of availableenergy I: model description and comparison with a multi-layered model. Agric. For. Meteorol. 91, 89–111.

Webb, E.K., Pearmen, G.L., Leuning, R., 1980. Correction offlux measurements for density effects due to heat and watervapour transfer. Q.J.R. Meteorol. Soc. 106, 85–100.

..