Determining water consumption in olive orchards using the water balance approach

Agricultural Water Management 55 (2002) 15±35

Determining water consumption in olive orchards using the water balance approach M.J. Palomo, F. Moreno, J.E. FernaÂndez*, A. DõÂaz-Espejo, I.F. GiroÂn Instituto de Recursos Naturales y AgrobiologõÂa, CSIC, Apartado 1052, 41080 Sevilla, Spain Accepted 30 October 2001

Abstract Ef®cient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature `Manzanilla' olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip's theory. The results were validated for a 3 l h 1 dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coef®cients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997±1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip's theory showed a good agreement with the values * Corresponding author. Tel.: ‡34-95-462-47-11; fax: ‡34-95-462-40-02. E-mail address: [email protected] (J.E. FernaÂndez).

0378-3774/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 1 ) 0 0 1 8 2 - 2

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measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap ¯ow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Olive; Irrigation; Water balance; Hydraulic conductivity; Drainage; Crop evapotranspiration; Sap ¯ow

1. Introduction Olive trees are usually grown in areas where water for irrigation is scarce. This, together with a large increase during the last few years in the amount of irrigated olive cropland, makes it crucial to optimise the use of water in these orchards. This can be done with the water balance approach, applied at the orchard level. The method, though time and labour consuming, can provide estimations of the components of the water balance equation that are accurate enough for optimising the irrigation management. The conventional method for calculating the crop evapotranspiration (ETc, mm) is based on the reference evapotranspiration in the area (ETr, mm), corrected by a crop coef®cient (Kc) and a reduction coef®cient (Kr) related to the degree of orchard ¯oor covered by plant (Orgaz and Fereres, 1999). This method is widely accepted, but the information on Kc and Kr is scarce. There are no speci®c values of Kr for the olive trees, and the values of Kc that are found in the literature cannot be applied to orchards with trees and environmental conditions different from those under which the coef®cients were obtained. Therefore, this method may give inaccurate values of ETc. In addition, it does not provide information on the water losses by drainage, which is essential not only to determine whether too much water is being added but also to optimise the design of the irrigation system. The actual value of ETc can be determined after estimating the values of the other components of the soil water balance equation. Estimations are made for a certain period, ranging from just a few days to months. Thus, Michelakis and Vougioucalou (1988) and Goldhamer et al. (1993) applied the water balance equation for the whole irrigation period, taking into account just the total amount of water supplied by irrigation and the soil water content at the beginning and end of the experimental period. MartõÂn-Aranda et al. (1982) and Moreno et al. (1988), however, frequently monitored the changes in all the components of the water balance equation, obtaining the evolution of ETc in olive orchards under dryfarming conditions and de®cit irrigation. We are not aware of any detailed study of the water balance in an olive orchard irrigated with enough water to meet the crop water demand. Among the several disadvantages of the soil water balance approach, summarised by FernaÂndez and Moreno (1999), is the variability of the hydraulic properties of the soil pro®le, such as the hydraulic conductivity±soil water content relationship (K(y)). Moreno et al. (1983, 1998) and Vanderlinden et al. (1998) state that accurate estimations of the hydraulic properties of the soil are needed to obtain reliable results when working on the soil water balance at the orchard level. The spatial variability of these properties has to be taken into account to establish to what degree local measurement can be extrapolated to the orchard scale. Temporal variability can also be signi®cant. Despite these dif®culties,

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the ETc obtained from the soil water balance can be used for adjusting the values of Kc for a particular orchard, for estimating water losses by drainage, and for validating simulation models dealing with crop water consumption. In addition, the method can provide a reliable ETc value suitable to evaluate other methods for calculating crop water needs. The aim of this work was to determine the components of the water balance in an olive orchard with mature `Manzanilla' olive trees under three water treatments: treatment I, daily drip irrigated trees to supply crop water demand; treatment D, trees irrigated with about 30% of the irrigation amount of treatment I, supplied in three irrigation events during the dry season; and treatment R, rainfed trees. Results from the hydraulic characterisation of the soil were used to validate a model to simulate the size and shape of the wet bulbs created by drippers of different discharge rates. Among other applications, this sort of models can be used for estimating the wetted and dry volumens of the rootzone in orchards with localized irrigation, which is compulsory for applying the water balance approach. 2. Materials and methods 2.1. Orchard site and water treatments The experiments were conducted in an olive orchard at La Hampa, the experimental farm of the Instituto de Recursos Naturales y AgrobiologõÂa, located at Coria del RõÂo near Seville in southwest Spain (378170 N, 6830 W, elevation 30 m). A 0.5 ha plot containing 29year-old olive trees (Olea europaea L. `Manzanilla de Sevilla', from now on `Manzanilla') planted at a spacing of 7 m  5 m was selected for the experiments. The trees were pruned every year, reaching a maximum leaf area index (LAI, m2 m 2) of about 1.7 at the end of the growing season. The ground cover was 34%. The slope of the orchard ranges from 3 to 6%. The soil of the plot is a sandy loam (Xerochrept) of variable depth. A hard limy sandstone pan impedes the penetration of both roots and water at a depth which varies within the orchard from 0.9 to 2 m. From the surface to the pan, the soil texture is quite homogeneous, with average values of 14.8% clay, 7.0% silt, 4.7% ®ne sand and 73.5% coarse sand. Laboratory measurements showed that the volumetric soil water content (y, m3 m 3) is 0.33 m3 m 3 for a soil matric potential (h, MPa) of 0 MPa, and 0.10 m3 m 3 for 1.5 MPa. In the ®eld, the values of y measured closed to the drippers a few hours after irrigation were rarely greater than 0.20 m3 m 3. More details on the soil characteristics are given by Moreno et al. (1983, 1988). The climate of the area is typically Mediterranean, with mild rainy winters and very hot, dry summers. The average rainfall and evapotranspiration values for the area are given in Table 1. The rainfall values recorded during the experimental period was above average. The orchard was divided into three similar parts, and a water treatment established in each one: (a) treatment I was a daily drip irrigation to supply the crop water demand as calculated by the equation ETc ˆ Kr Kc ETr ;

(1)

where Kr is a coefficient related to the percentage of ground covered by the crop (Fereres and Castel, 1981). In our case K r ˆ 0:7, since the trees covered 34% of the orchard floor.

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Table 1 Comparison between the rainfall amounts and the reference evapotranspiration (mm) registered in the experimental farm during the experimental hydrological years (September to August) and the average in the areaa Hydrological year

Rainfall

Reference evapotranspiration

September to December

Rainfall

Reference evapotranspiration

1996±1997 1997±1998 Average for the last 25 years

719 718 484

1479 1377 1442

1996 1997 Average for the last 25 years

479 464 235

320 311 314

a The values for the wetter period, from the beginning of the rainy season in September to the end of the year, are also given. The values of reference evapotranspiration were calculated from the FAO-Penman equation, the most suitable for the area.

Kc is the crop coefficient that FernaÂndez and Moreno (1999) showed as adequate for our orchard conditions (0.70 in March; 0.65 in April and October; 0.60 in May and September; 0.55 in June; 0.50 in July and August). ETr (mm) is the reference evapotranspiration (grass reference), as calculated by the FAO-Penman equation (Doorenbos and Pruitt, 1977), which Mantovani et al. (1991) evaluated as the most suitable for this area. The weather variables needed for calculating ETr were measured with an automatic weather station located next to the experimental orchard. The 30 min averages of global solar radiation, photosynthetically active radiation, wind speed, rainfall, air temperature, and relative humidity were recorded. The irrigation system consisted on a single pipe placed on the soil surface in each tree row, with five 3 l h 1 drippers per tree, 1 m apart. Each season, irrigation was applied from mid March to the beginning of October (Table 2). (b) Treatment D was a deficit irrigation in which water was applied at the three phenological stages when olive trees seem to be most sensitive to water deficit: before flowering, at pit hardening and some 15 days before harvest (Table 2). Each time, water was applied for approximately a Table 2 Water supplies (mm) for the three treatments, during the experimental periods of 1997 and 1998a Treatment

Period

Irrigation

I

24 March to 13 October 1997

418

D

31 March to 7 April 1997 9 to 23 June 1997 27 August to 1 September 1997

I, D, R

24 March to 13 October 1997

I

24 March to 28 September 1998

D

24 March to 6 April 1998 3 to 9 June 1998 21 to 26 August 1998

I, D, R

24 March to 28 September 1998

a

Rainfall

26 38 36 138 389 28 33 50 175

Details on the fully irrigated (I), deficit irrigated (D) and rainfed (R) treatments are given in the text.

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week, until the soil of the wet bulbs was at field capacity. The total amounts of water applied during 1997 and 1998 were 24 and 29%, respectively, of the water applied by irrigation in treatment I. The irrigation system was similar to that of treatment I, but with 6 l h 1 drippers. (c) Treatment R was dry-farming, with rainfall as the only means of water supply. The two irrigation treatments were applied to trees belonging to a part of the orchard which had been irrigated regularly since they were young. The non-irrigated trees were in another part of the orchard, which had been maintained under dry-farming conditions from the beginning. All the trees were fertilized to cover the crop nutrient needs, and they were managed in a similar way, following the practices recommended for the area. Rather than replicating the treatment plots, detailed measurements were made within each of them to determine the water balance components accurately. The measurements are detailed in Sections 2.2 and 2.3, and the water balance approch used in the treatment plots is described in Section 2.4. 2.2. Soil water measurements Access tubes for the neutron probe (Troxler 3300, Research Triangle Park, NC, USA) were installed in the soil of three trees per treatment. In one of the three trees, four access tubes were installed at distances of 0.5, 1.5, 2.5 and 3.5 m from the trunk along the tree row. In addition, a set of ®ve mercury tensiometers, at 0.30, 0.60, 0.85, 1.30 and 1.50 m depth, was installed 1 m away from the trunk close to the irrigation pipe. Soil matric potential (h, MPa) readings were taken every 4±7 days throughout the irrigation period. In the other two trees, two access tubes were installed at 1.5 and 2.5 m from the trunk, along the tree row. The y measurements were made every 0.1 m, from 0.2 m down to: 2 m in treatment R; 1.2 m in treatment D and to 1 m in treatment I, according to the rootzone depth at each location. The neutron probe was calibrated for the soil of the orchard at the beginning of the experimental period. In the top 0.2 m of soil, y was measured by gravimetry. Soil water pro®les were recorded every 10±15 days throughout the irrigation period. During the rainy season, measurements of y were made every 25±30 days, and always after a rain event. These values were used to calculate a depth equivalent of water, expressed as the level of relative extractable water (REW, mm) de®ned by the equation (Granier, 1987) REW ˆ

R Rmin ; Rmax Rmin

(2)

where R (mm) is the actual soil water content, Rmin (mm) the minimum soil water content measured during the experiments, and Rmax (mm) is the soil water content at field capacity. The values of Rmin and Rmax measured in the orchard were 269 and 388 mm, respectively. 2.3. Hydrodynamic characterisation of the orchard soil The hydraulic properties of the soil were characterised to obtain the water dynamics within the vadose and to estimate drainage capacity. The hydraulic conductivity (K, mm s 1) of the soil was determined in situ by the internal drainage method (Hillel et al., 1972). A 1.5 m access tube for the neutron probe was installed at the centre of two

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concentric rings of 0.4 and 1.6 m diameter, in a site of the orchard with average soil conditions. Outside the inner ring a set of six mercury tensiometers was installed at 0.2, 0.4, 0.6, 0.9, 1.2 and 1.4 m depth. Continuous measurements of y and h were made during the in®ltration, redistribution and evaporation processes, as described by Moreno et al. (1983). In the top 0.2 m of soil, y was measured by time-domain-re¯ectrometry (TDR) using a Tektronix cable tester (Model 1502C, Beaverton, OR, USA) and two TDR waveguides, each having three parallel stainless steel rods, 2 mm in diameter and 0.15 m long. The TDR data were analysed in a similar way to that of Baker and Allmaras (1990). The tension disc in®ltrometer (Perroux and White, 1988) was also used to obtain in situ determinations of K and sorptivity (S, mm s 1/2) in the range near saturation. These measurements were made in three representative sites within the plot. In each site, the measurements were carried out on the undisturbed soil surface and at 0.2, 0.5 and 1.0 m depth. A disc in®ltrometer of 125 mm radius was used, and the pressure potentials (C0, mm of water) chosen ranged from 120 to 0 mm. The values of K and S for these pressure potentials were obtained using the mono-disc multiple-head method described by Ankeny et al. (1991). This method is based on Wooding's equation for the steady-state asymptotic ¯ux (Wooding, 1968). From the measurements with the tension disc in®ltrometer, a soil structure index can be described by the frame-weighted mean pore size (lm, mm) lm ˆ

s …y0 yn †K ; rg bS 2

(3)

where s (Pa) is the surface tension of water, r (Mg m 3) the density of water, g (m s 2) the acceleration due to gravity, yn the initial volumetric water content, and y0 is the volumetric water content at the imposed C0. Usually, the parameter b for a field soil can be taken to be 0.55 (White and Sully, 1987; Warrick and Broadbridge, 1992). This parameter defines a mean characteristic width of the pores that are hydraulically functioning at the imposed water pressure potential C0. The disc infiltrometer also allows for calculating the gravity time or time from which the gravity is the dominant factor that controls the infiltration (tgrav, min). According to Philip (1969), during one-dimensional infiltration tgrav is given by  2 S : (4) tgrav ˆ K 2.4. Water balance The value of ETc can be deduced from the water balance equation ETc ˆ R ‡ I

DS

D

Rs ;

(5)

where R (mm) is the rainfall, I (mm) the water applied by irrigation, DS (mm) the change in the water storage in the soil profile exploited by the roots, D (mm) the water lost by drainage and Rs (mm) is the surface water runoff. In our case the water table was too deep to be considered in the equation, and the fraction of R intercepted and lost by the leaves was negligible. All these parameters were determined during 1997 and 1998. R-values were measured with the pluviometer of the weather station next to the orchard, and I was measured with the water meter of the irrigation system. The neutron probe and the

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gravimetric readings described in Section 2.2 were used to monitor DS. The drainage component was estimated using Darcy's law D ˆ q Dt ˆ

K…y† grad H Dt;

(6)

where q is the mean volumetric flux density (mm per day) during the period of time Dt (days), K(y) the hydraulic conductivity (mm per day) corresponding to the water content y (Section 2.3) at the maximum depth reached by the roots (zr, m), and grad H is the hydraulic head gradient at the same depth. We established the depths of 1.0, 1.2 and 2.0 m as the drainage depths for treatments I, D and R, respectively. These depths were chosen taking into account the depth of the hard pan, the shapes of the soil water profiles and the results on root distribution and root activity reported by FernaÂndez et al. (1991) for the experimental orchard. Rs-values for the different rainfall events were determined as explained by Moreno et al. (1988). With this approach, Rs is calculated from the water balance equation considering ETc ˆ ETr , which is assumed to be true in the periods of low evaporative demand, as is the case of the rainy periods. Eq. (5) was applied to the periods between two y measurements, for the 19 experimental months. In the case of the irrigated treatments, the water balance components were estimated using a model that separated the soil areas affected by irrigation from the non-affected ones, as described by Moreno et al. (1988). The model includes a weighting method for balancing the values of DS and D in each of the two areas mentioned above. The average area affected by irrigation was established according to the soil water content pro®les measured at different distances from the tree trunks. This was about 10 m2 per tree, which accounts for 2/7 of the total surface area of plant spacing (35 m2). Therefore, Eq. (5) for the plots with localised irrigation can be rewritten as ETc ˆ R ‡ 27 …I

DSi

Di †

5 7 …DSni

‡ Dni †

Rs ;

(7)

where the subscript `i' corresponds to the areas affected by irrigation and `ni' to the areas not affected by irrigation. 2.5. Modelling the size of the wet bulb Estimates of the size of the wet bulbs created under the drippers are of interest for the correct design and management of the drip irrigation system. In addition, estimating the wetted and dry areas of the rootzone of trees with localized irrigation is needed for applying the water balance approach, as explained in Section 2.4. Revol et al. (1996) demonstrated that the existent analytical theories describing three-dimensional in®ltration can be used to predict the size of the bulb from the hydraulic characteristics of the soil and the drip discharge rate (Qd, l h 1). Currently there is no analytical theory able to explain the capillary ¯ow around the dripper and the steady regime due to gravity. Raats' (1971) steady-state theory can be extended to provide an approximate analysis of the transient pattern of wetting around a point source (Revol et al., 1997). We simulated the movement of the wetting front, both in the horizontal and vertical directions, under the type of dripper used in treatment I. The calculations were based on the Philip's (1984) theory and on the values of y measured in the experiment described below, as well as on the data obtained in the orchard with the tension disc in®ltrometer,

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particularly the slope of the K(h) curve and Qd. In order to evaluate the position of the wetting front during the in®ltration of the water applied by the dripper, we measured the changes of the diameter of the wet bulb in the soil surface and the time required by the wetting front to reach a certain depth. Six sites were selected in the orchard, each one equidistant from two trees and close to the irrigation pipe. In these places the water supplied by the irrigation system of the orchard did not affect the soil. In each site we installed a 3 l h 1 dripper, and a tensiometer at the depth of 0.3 m. The average discharge rate of the six drippers measured was 3:12  0:26 l h 1. The experiment was carried out in June 1999, when h ˆ 0:06 MPa at 0.3 m. The evolution with time of the wet bulb diameter in the soil surface of the six selected sites was measured in and perpendicular to the pipe direction. We measured the time (td, min) between the beginning of the irrigation and h ˆ 0:01 MPa, and assumed this was the time needed for the wetting front to reach the tensiometer depth. Measurements of y in the top 0.15 m of soil and at a distance of 0.15 m from the dripper were carried out by TDR, before the irrigation (yi) and at td (yf). 2.6. Plant measurements Both the water status and water consumption were monitored throughout the irrigation season of 1998 in representative trees of treatments I and R. Leaf water potential was assumed to be equal to the xylem pressure potential at the petiole, as measured with a pressure chamber (Soilmoisture Equipment Corp, Santa BaÂrbara, CA, USA). The measurements were made once per month, from March to August, on clear-sky days. Each leaf sampled per tree was 1.6±1.9 m above ground, in six trees per treatment. We choose a sunny and healthy twig, and sampled the fourth or ®th leaf from the apex. The leaf water potential was measured just before dawn (Cpd, MPa) and in the central hours of the day, when the minimum values were recorded (Cm, MPa). We also estimated the daily water consumptions (Ep, l per day) of one tree per treatment, in which we measured the sap ¯ow velocities with the compensation heat-pulse technique (Green and Clothier, 1988). Details on the experimental gear and calibration coef®cients are given by FernaÂndez et al. (2001). On the nights of 14 and 15 July, we inserted three sets of heat-pulse probes into the trunks of both trees, at the height of 0.58 m for the I tree and 0.65 m for the R tree. With each set of probes we measured the sap ¯ows at 5, 12, 22 and 35 mm below the cambium, once every half hour. With these data and following the procedure described by FernaÂndez et al. (2001) we calculated the values of Ep for each experimental tree. Sap ¯ows were monitored in the trunk of both trees during three periods of 4±6 days, in the middle of July, and in the beginning and middle of August. Those were periods of variable water atmospheric demand, in which ETr varied from about 5 to 10 mm per day. 3. Results and discussion 3.1. Soil water The seasonal evolution of REW in the soil of the three treatments during the experimental periods of 1997 and 1998 is shown in Fig. 1. The trends for each of the treatments

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Fig. 1. Seasonal evolution of the relative extractable water calculated from the soil water measurements carried out in the experimental orchard during 1997 and 1998. Measurements were made for the fully irrigated (I), deficit irrigated (D) and rainfed (R) treatments. Details on the treatments and measurements are given in the text. The periods between two arrows are the irrigation seasons.

were similar in both years, and it can be clearly seen that there were major differences in soil water content between treatments. The values of REW for treatment I were between 0.8 and 1 for most of the time during both irrigation seasons, indicating that the soil of the irrigated bulbs was constantly kept close to ®eld capacity. In fact, the average y values measured in that period were 0.19 m3 m 3. In treatment R, however, the water uptake by the trees caused a continuous decrease of soil water content, until very low values of REW were recorded at the end of the summer, when the average y for that treatment was 0.14 m3 m 3. The effect on REW of the three water supplies by irrigation carried out each dry season in treatment D can be clearly seen in this ®gure. FernaÂndez et al. (1997), after plotting the REW data against predawn leaf water potentials (Cpd, MPa) measured in the trees of our orchard, concluded that a value of REW 0.4 can be considered as a threshold for soil water de®cit. A REW threshold of 0.3± 0.4 seems to be a general feature of water stress for many tree species (BreÂda et al., 1995). Fig. 1 shows that, in treatment R, REW was less than 0.4 from the end of June, both in 1997 and 1998. This indicates that the R trees likely suffered from signi®cant water stress during the rest of the dry season. It is also true that, although the soil water content in the R treatment was lower than in the irrigated treatments, the root system of the R trees is

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supposed to explore a greater volume of soil than the irrigated trees (FernaÂndez et al., 1991). This allows them to take up more water when y is low. The results of h measured during the two experimental years (data not shown) are in agreement with the y values. 3.2. Hydraulic characteristics of the orchard soil The K(y) and h(y) values measured in the orchard are shown in Fig. 2. The set of K(y) data obtained during the internal drainage experiment ®ts well to the same curve, the equation is given in this ®gure. A similar h(y) relationship was found at all the depths explored. This is in agreement with the low vertical variability of the soil texture in the orchard. The values of K and S in the range near saturation determined from measurements made with the tension disc in®ltrometer are shown in Fig. 3. The two variables decreased with C0. The difference in K between the soil surface and deeper layers is due to the consolidation of the soil below the surface. Actually, a plough pan was found from 0.2 to 0.35 m depth. The average bulk density of that soil layer was 1.50 Mg m 3, while an average value of 1.30 Mg m 3 was found at the soil surface. The K value was signi®cantly higher in the soil surface than in the plough pan, for C0 > 40 mm. At C0 ˆ 120 mm, K was also higher in the soil surface, though not signi®cantly different from the values measured deeper in the soil. Signi®cant differences in K were found at the other depths explored, for all the C0 values. In the case of S, the values in the 120 < C0 < 0 mm range were signi®cantly lower in the plough pan and in the deeper explored layers than in the soil surface. This was due to lower initial values of y in the soil surface than in deeper layers. Lower variability was found, both for K and S, in the soil surface (coef®cient of variation <0.3) than in the plough pan (coef®cient of variation ranged between 0.4 and 0.6).

Fig. 2. Relationship between the soil hydraulic conductivity and the volumetric soil water content obtained by the internal drainage method (left). These measurements on soil water content have been plotted together with the measurements on soil matric potential made at the same site, represented by the black symbols (right). Data from the fully irrigated (I) and rainfed (R) treatments have also been included in the figure. Details on the measured locations and on the treatments are given in the text.

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Fig. 3. Hydraulic conductivity and sorptivity in the range near saturation, determined from measurements with the disc infiltrometer at three representative sites within the orchard. Measurements were made at the soil surface and at three selected depths. Each point represents the average of three values. Vertical bars indicate twice the standard error.

Table 3 shows the values of lm for the C0 values measured. On the basis of the Student's t-test and the 95% con®dence interval, similar lm values were found at the soil surface for the 120 < C0 < 0 mm range. In contrast, at the depth of the plough pan a slight increase of lm was observed as C0 decreased. The lm was higher, in general, at the soil surface than at the plough pan, but only signi®cantly different for C0 ˆ 40 mm. The decrease of K as C0 decreases (Fig. 3) is in apparent contradiction to the increase of lm, and suggests a lack of interconnected pores (Angulo-Jaramillo et al., 1997). It is also striking that for C0 > 40 mm, the reduction of lm does not follow a decrease in sorptivity. This apparent discrepancy could be explained both by some fraction of the smaller pores becoming hydraulically isolated (Beven and Germann, 1982) and by the deposition of eroded small Table 3 Characteristic mean pore radius (lm, mm) and gravity time (tgrav, min) at different pressure potentials in the range near saturation (C0, mm), determined from measurements with the disc infiltrometer at three representative sites within the orcharda C0

10 40 120

Soil surface

Plough pan

lm

tgrav

lm

tgrav

0.06 a 0.07 a 0.08 a

40.4 a 36.8 a 83.9 a

0.05 a 0.06 b 0.08 a

69.1 a 55.7 a 77.0 a

a The measurements were made at the soil surface and at 0.2 m depths where a plough pan was found. Values in each line for the same parameter followed by the same letter are not significantly different (P < 0:05).

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Fig. 4. Water profiles recorded in 19 August 1997, in one of the fully irigated I trees. Data represent single measurements made with the neutron probe (0.3±0.9 m) and the gravimetric method (0±0.2 m) at the four explored distances from the tree trunk. Details on the treatment and measurements are given in the text.

easily transportable particles. The low tgrav values, also shown in Table 3, clearly indicate that the in®ltration in the orchard soil is controlled by gravity. 3.3. Water balance An example of the soil water pro®les recorded for treatment I is shown in Fig. 4. Values of y close to ®eld capacity were measured within the wet bulbs, up to 2.5 m from the tree trunk. Lower y values were recorded at 3.5 m from the trunk, an area not affected by irrigation. The lower y values observed in the upper layers were likely due to the evaporation from the soil surface and to the root extraction. The greater values of root density found by FernaÂndez et al. (1991) in the drip irrigated trees of the orchard were in the top 0.2 m of soil. At 3.5 m from the trunk, the values of y in the deeper explored layers were quite high. This can be due to the high amount of water stored in the soil caused by the high rainfalls that year. The cumulative values of the water balance components for the three treatments are shown in Table 4. The results are given for three periods, corresponding to the irrigation seasons of 1997 and 1998, and the period between them. Fig. 5 shows the cumulative values of the different components of the water balance equation during the irrigation period of 1998, for the three treatments. The 1997 results were similar to those of 1998 (Table 4). In fact, the results of ETc for 1997 and 1998 were 478 and 493 mm, respectively, for treatment I, and 196 and 204 mm, respectively, for treatment R. The ETc values of treatment D were closer to those of treatment R than those of treatment I (220 mm in 1997 and 245 mm in 1998), due to the small amount of water applied. For the hydrological year 1997±1998, ETc values were 653 mm in treatment I, 405 mm in D and 378 mm in R (Table 4). Moreno et al. (1988) and FernaÂndez (1989) estimated ETc values from a water balance in a plot placed next to ours. A de®cit irrigation was applied, calculated from the total weekly evaporation

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Table 4 Values (mm) of the components of the water balance equation for different periods in the fully irrigated (I), deficit irrigated (D) and rainfed (R) treatmentsa Treatment

Period

P

I

D

I

17 March to 1 October 1997 1 October 1997 to 23 March 1998 23 March to 1 October 1998

138 555 175

408 10 377

61 68 51

D

17 March to 1 October 1997 1 October 1997 to 23 March 1998 23 March to 1 October 1998

138 555 175

100 0 111

R

17 March to 1 October 1997 1 October 1997 to 23 March 1998 23 March to 1 October 1998

138 555 175

0 0 0

DS

Rs

ETc

‡7 8 ‡8

0 345 0

478 160 493

21 49 32

3 2 ‡9

0 348 0

220 160 245

0 3 1

58 ‡33 30

0 345 0

196 174 204

a Details of the treatments are given in the text (R: rainfall, I: irrigation; D: drainage, DS: changes in the soil water storage, Rs: surface water runoff, and ETc: crop evapotranspiration).

of a class A pan, adjusted by a reduction coef®cient of 0.4. This accounted for about half of the irrigation amount applied by us in treatment I. Under those conditions, Moreno et al. (1988) and FernaÂndez and Moreno (1999) showed that the ETc values obtained for several hydrological years (from 1982 to 1988) ranged between 322 and 466 mm. The value of 405 mm obtained by us for treatment D is within this range. We have compared the ETc calculated by Eq. (1) (ETc-eq1) with the ETc obtained with the water balance approach in the plot of treatment I (ETc-wb). For the 1997 irrigation period, ETc -eq1 ˆ 0:93ETc-wb, and for the 1998 irrigation period, ETc -eq1 ˆ 0:90ETc-wb. This data show good agreement between the two methods. In treatment I, the amounts of water lost by drainage during the irrigation periods were 61 mm in 1997 and 51 mm in 1998, which accounts for 13 and 10%, respectively, of the ETc recorded in those periods. The REW values (Fig. 1) and the soil matric potential values (data not shown) recorded throughout both irrigation seasons, indicated that the soil of the wet bulbs was maintained close to ®eld capacity conditions. Therefore, the irrigation system used in the orchard, widely used by the olive growers of our area, showed to be adequate for having high y without an excessive drainage, at least for the soil of our orchard. The water amounts lost by drainage in treatment D during the irrigation seasons of 1997 and 1998 were signi®cantly lower than in treatment I (Table 4), as expected due to the lower irrigation amounts applied to the D trees. The data of Table 4 still show a certain amount of water lost by drainage in treatment D, which can be related to the characteristics of the wet bulb created by the 6 l h 1 drippers used in this treatment. The results of simulation carried out as described in Section 3.4 clearly shows that with a dripper of 6 l h 1 discharge rate, the time of wet-front penetration to a speci®c depth was about half of the time needed with the 3 l h 1 dripper. This means that in the orchard soil, the 6 l h 1 dripper can favour water loss below the root zone. A negligible drainage was recorded in treatment R during the two experimental years. Apart from the lack of irrigation, this can be explained by the fact that the root system of the R trees explored greater soil depths than the I and D trees (FernaÂndez et al., 1991). The pro®les of h recorded in treatments D and R

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Fig. 5. Cumulative values of the components of the water balance equation for the irrigation period of 1998. Results of the fully irrigated (I), deficit irrigated (D) and rainfed (R) treatments are shown. Details on the treatments and measurements are given in the text.

(data not shown) indicated that there was an inversion of the hydraulic gradient in some periods within the soil layers explored by the roots; this produced an upward movement of water. The greatest water losses by drainage were recorded in the autumn and winter of 1997±1998, due to the heavy rainfall during this period, unusually high for the area (60 mm on 2 November, with a maximum rainfall intensity of 11.4 mm h 1; 47 mm on

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17 December, with a maximum rainfall intensity of 24 mm h 1). A substantial part of the rainfall was also lost by runoff. The value of Rs recorded during the rainy season in the experimental orchard was about 345 mm, which means that 62% of the rainfall recorded on that period was lost by runoff (Table 4). Moreno (1986) determined, in a plot close to ours, with similar slope and hydraulic characteristics, that rainfall episodes with an intensity of around 10 mm h 1 produced a surface water runoff of 27±35%. The changes in DS during the two irrigation periods studied were practically insignificant in treatment I (Table 4 and Fig. 5). In treatment R, the tree water uptake caused a depletion of the water stored in the soil depths explored by the roots, recorded as 58 mm for the summer of 1997 and 30 mm for that of 1998. The soil water depletion in treatment D was not as great as in the dry treatment, due to the water supplied by the three irrigation events. Results of Table 4 show that the annual water consumption of our olive orchard irrigated to cover the crop water needs is of approximatly 650 mm. The water irrigation needs of treatment I, with the drip irrigation system we had in our orchard, are about 380 mm. These data were calculated using the ETc and R values shown in Table 4 for the years 1997 and 1998. We have considered an effective rainfall during the irrigation period of 70% of the total recorded R, as is normally done when calculating the irrigation amount applied in a commercial orchard. The experiments on root-signalling carried out by FernaÂndez and Moreno (1999) showed that localised irrigation could limit the amount of water consumed by the olive tree, due to a root-signalling phenomenon. Therefore, these amounts could be greater if the irrigation systems moistened the full soil volume explored by the roots. Our results on ETc are not very different from those obtained by other authors. Dettori (1987) reported ETc values of 620 and 560 mm for the olive crop in areas of ETr around 1200 and 1000 mm, respectively. Fereres (1995) and Villalobos et al. (2000) estimated ETc values of 700±850 mm for olive orchards in southern Spain, where ETr is about 1400 mm. For the same area, Orgaz and Fereres (1999) estimated an ETc of about 590 mm. In any case, the crop water demand depends on the atmospheric demand in the area, and the tree and orchard characteristics. 3.4. Size of the wet bulb The time course of the measured and simulated values of the radius of the wet bulb at the soil surface (r, m) and the maximum depth of the wetting front (z, m) are shown in Fig. 6. The time for the wetting front to reach z ˆ 0:3 m (tz) was 156 min. At this time r ˆ 0:23 m. These results indicate that the penetration of the wetting front in the vertical direction, due to gravity, was approximately 30% greater than the lateral spread observed in the surface. The agreement between the experimental and simulated values of r and z was very good, particularly for z (Fig. 6). This shows that the Philip's (1984) theory is valid for the conditions of the orchard. After this validation exercise, we used the model for simulating r and z for a 6 l h 1 dripper similar to the ones used in treatment D. The results are shown in Fig. 7. In this case, tz ˆ 81 min is about half than that for the 3 l h 1 dripper. The value of r for tz was smaller too (r ˆ 0:20 m). It can be seen from this data that the penetration of the wetting front in the vertical direction was 50% greater than the lateral spread in the surface. These results show that the model is a useful tool to determine the shape and the size of the

30

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Fig. 6. Time course of the measured and simulated values of the radius of the wet bulb at the soil surface (r) and of the maximum depth of the wet-front (z) under a 3 l h 1 dripper. Each point represents the average of the measurements made at six selected sites in the orchard. Vertical and horizontal bars represent twice the standard error. The value of tz was 156 min (t ˆ time after the beginning of the infiltration process; tz ˆ time at which the wet-front reached a fixed depth of 0.3 m).

wet bulb produced by a dripper with a given discharge rate, provided the hydraulic properties of the soil are known. Therefore, the model can be used to decide the optimum number of drippers per tree and their location in respect to the trunk, and so to improve irrigation management practices. Although in our validation exercise we have run the model for 0.3 m depth only, simulations for any depth needed can be made to have a clear picture of the bulb shape. This information could be used for estimating the areas affected and non-affected by irrigation (Eq. (7)). 3.5. Plant response to the water treatments In this section we analyze the water response of the well watered I trees as compared to the rainfed R trees, for the 1998 irrigation season. The measurements of leaf water potential (Fig. 8) showed that the level of stress reached by the R trees during the dry months was

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Fig. 7. Time course of the simulated values of the radius of the wet bulb at the soil surface (r) and of the maximum depth of the wet-front (z) under a 6 l h 1 dripper. We used the same model as in Fig. 6.

relatively low, as compare to what it is usual in the area. This may be due to the unusually high rainfall recorded from september 1997 to the beginning of June 1998 (Table 1). Likely this heavy rains recharged the soil of the orchard to a level that avoided severe water stress in the R trees during summer. The values of Cpd shown in Fig. 8a indicate that the R trees reached a greater water stress than the I trees in July and August. The minimum average Cpd value registered in treatment R in August ( 0.57 MPa) was, however, little more negative than the average value of 0.46 MPa found by FernaÂndez et al. (1997) when the soil water depletion in the orchard started to cause a signi®cant level of water stress in rainfed trees. Measurements carried out in 1995, a very dry year (234 mm of rainfall were recorded from September 1994 to June 1995), showed minimum Cpd values in the rainfed trees of the orchard lower than 1.6 MPa, in July and September (FernaÂndez et al., 1997). Fig. 8b shows no signi®cant differences on Cm between the R and I treatments, as a consequence of the high recharge of the soil during the previous rainy season. The measurements of Cm carried our in 1995 by FernaÂndez et al. (1997) showed clear differences between two treatments similar to the I and R treatments. They reported a minimum average Cm value of 2.47 MPa for irrigated trees, registered in September; this is close to the minimum average Cm value of 2.63 MPa observed in the I trees (Fig. 8b). For rainfed trees, however, they reported average values as low as 3.63 MPa in July and 3.62 MPa in September; these values are much more negative than the minimum average Cm value of 2.87 MPa found in the R trees (Fig. 8b). Greater differences between treatments were observed in terms of water consumption. The Ep values of the I tree were greater than those of the R tree, for the three studied periods

32

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Fig. 8. Time courses of (a) predawn leaf water potential and (b) minimum leaf water potential measured on one day of each month throughout the irrigation season of 1998. Measurements were made in the fully irrigated (I) and rainfed (R) treatments. Each point represents the average of six values per treatment. Vertical bars indicate twice the standard error. Details on the treatments and measurements are given in the text.

Fig. 9. Daily values of transpiration determined from sap flow measurements caried out in on tree of the fully irigated (I) treatment and in one tree of the rainfed (R) treatment. Measurements were made during July and August of 1998, in three periods of variable reference evapotranspiration (daily values shown at the top of the figure). Details on the treatments and measurements are given in the text.

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(Fig. 9). The differences were greater for the second period, corresponding to the ®rst week of August, when the greatest ETr values were recorded. For that period, the average Ep of the I tree (61 l per day) was 41% greater than that of the R tree (43 l per day). This value was 21% for the ®rst period and 30% for the third represented period. The Ep values were determined from the sap ¯ow measurements made in one tree per treatment only. This, together with other sources of variability inherent in the technique (FernaÂndez et al., 2001), bound us to be cautious with the Ep results. Assuming that the Ep values shown in Fig. 9 were close to the actual values, they can be used for estimating the daily evapotranspiration of a single tree in the orchard, together with the relationship between evaporation from the soil surface (Es, mm per day) and the y value for the top 0.08 m soil layer (m3 m 3  102 ), obtained for the orchard conditions by Palomo et al. (1998). They reported that Es ˆ 0:057 e0:2y , on days in which ETr was close to 8 mm per day. For the days represented in Fig. 9, the average y value of the top soil layer was 0.15 m3 m 3 in the areas afected by the localized irrigation applied to the I trees, and 0.05 m3 m 3 in the areas not affected by irrigation, which in the R treatment is the whole soil surface. The soil surface corresponding to each tree was 35 m2, and the average soil surface affected by irrigation in the I trees was 3.9 m2 (each of the ®ve drippes per tree wetted a circle of approximately 1 m of diameter). With these data, the average Es values for the days of the second period represented in Fig. 9 (day of year 212±217), in which the average ETr was 7.9 mm per day, were 9.1 l per day for the I trees and 5.4 l per day for the R trees. Taking into account the average Ep values for that period, the resulting average daily evapotranspiration was 70.1 l per day for the I tree and 48.4 l per day for the R tree. Altough these results must be taken with caution due to both the uncertainties in the estimations of Ep and Es, and to the low number of trees monitored, they can be taken as an indication of the water cosumption at the tree level for the conditions of the I and the R treatments during the experimental period. 4. Conclusions The application of the water balance method in an olive orchard is laborious and time consuming, and requires detailed in situ measurements of the soil hydraulic properties for the soil depth explored by the roots. The spatial and temporal variability of these properties has to be taken into account in order to establish to what degree local measurements can be extrapolated to the orchard scale. The method, however, allows for the determination of the evapotranspiration of the orchard, apart from the drainage and the other components of the water balance equation. This allows for the optimisation of water use in the orchard. For our orchard conditions, the average ETc for the drip-irrigated treatment with enough water to cover the crop demand was 485 mm during the irrigation season and 653 mm for the whole year. Taking into account the actual rainfall in the area, about 3800 m3 ha 1 per year are required for irrigation. The average water losses by drainage during the irrigation period on this treatment amounted to 12% of the ETc. This low value showed that the irrigation system used was appropriate for the orchard conditions. For the de®cit irrigation treatment, the average ETc was 200 mm for the irrigation season, and 378 mm for the whole year. Finally, for the non-irrigated treatment, the average ETc during the period in

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which the other treatments were irrigated was 232 mm, being the average yearly amount 405 mm. After validation, the model to estimate the size of the wet bulb was found to be a useful tool for choosing the most adequate number of drippers per tree and the right discharge rate for each type of soil. Acknowledgements Thanks are due to the staff of La Hampa for managing the orchards, and to J. Rodriguez for help with the ®eld measurements. This study was supported with funds of the Spanish CICYT, project HID96-1342-CO4-01. References Angulo-Jaramillo, R., Moreno, F., Clothier, B.E., Thony, J.L., Vachaud, G., FernaÂndez-Boy, E., Cayuela, J.A., 1997. Seasonal variation of hydraulic properties of soils measured using a tension disk infiltrometer. Soil Sci. Soc. Am. J. 61, 27±32. Ankeny, M.D., Ahmed, M., Kaspar, T.C., Horton, R., 1991. Simple field method determining unsaturated hydraulic conductivity. Soil Sci. Soc. Am. J. 55, 467±470. Baker, J.M., Allmaras, R.R., 1990. System for automating and multiplexing soil moisture measurements by time-domain reflectometry. Soil Sci. Soc. Am. J. 54, 1±6. Beven, K., Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res. 18, 1311±1325. BreÂda, N., Granier, A., Barataud, F., Moyne, C., 1995. Soil water dynamics in an oak stand. Plant Soil 172, 17±27. Dettori, S., 1987. EstimacioÂn con los meÂtodos de la FAO de las necesidades de riego de los cultivos de aceitunas de mesa de CerdenÄa. Olivae 17, 30±35. Doorenbos, J., Pruitt, W.O., 1977. Guidelines for predicting crop water requirements. FAO Irrigation and Drainage Paper no. 24, 2nd Edition. FAO, Rome. Fereres, E., 1995. El riego del olivar. In: Proceedings of the VII Simposio CientõÂfico-TeÂcnico Expoliva'95, p. 18. Fereres, E., Castel, J.R., 1981. Drip Irrigation Management. Division of Agricultural Sciences, University of California, Leaflet 21259. FernaÂndez, J.E., 1989. Comportamiento del olivo (Olea europaea L., var. manzanillo) sometido a distintos regõÂmenes hõÂdricos, con especial referencia a la dinaÂmica del sistema radicular y de la transpiracioÂn. PhD, Department of Agronomy, University of CoÂrdoba. FernaÂndez, J.E., Moreno, F., 1999. Water use by the olive tree. J. Crop Prod. 2, 105±167. FernaÂndez, J.E., Moreno, F., Cabrera, F., Arrue, J.L., MartõÂn-Aranda, J., 1991. Drip irrigation, soil characteristics and the root distribution and root activity of olive trees. Plant Soil 133, 239±251. FernaÂndez, J.E., Moreno, F., GiroÂn, I.F., BlaÂzquez, O.M., 1997. Stomatal control of water use in olive tree leaves. Plant Soil 190, 179±192. FernaÂndez, J.E., Palomo, M.J., DõÂaz-Espejo, A., Clothier, B.E., Green, S.R., GiroÂn, I.F., Moreno, F., 2001. Heatpulse measurements of sap flow in olives for automating irrigation: tests, root flow and diagnostics of water stress. Agric. Water Manage. 51, 99±123. Goldhamer, D.A., Dunai, J., Ferguson, F., 1993. Water use requirements of manzanillo olives and responses to sustained deficit irrigation. Acta Hort. 335, 365±371. Granier, A., 1987. Evaluation of transpiration in a Douglas-fir stand by means of sap flow measurements. Tree Physiol. 3, 309±320. Green, S.R., Clothier, B.E., 1988. Water use of kiwifruit vines and apple trees by the heat-pulse technique. J. Exp. Bot. 39, 115±123. Hillel, D., Krentos, V.D., Stilianou, Y., 1972. Procedure and test of an internal drainage method for measuring soil hydraulic characteristics in situ. Soil Sci. 114, 395±400.

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