Agricultural Water Management, 11 (1986) 145- -158 145 Elsevier Science Publishers B.V., Amste rdam -- Printed in The Nether lands

WATER BALANCE AND PATTERN OF SOIL WATER UPTAKE IN A PEACH ORCHARD

E. G A R N I E R , A. B E R G E R and S. R A M B A L

Centre Emberger (CNRS), Route de Mende, BP 5051, 34033 Montpellier-Cedex (France)

(Accepted 17 Sep tember 1985)

ABSTRACT

Garnier, E., Berger, A. and Rambal , S., 1986. Water balance and pat te rn of soil water up- take in a peach orchard. Agric. Water Manage., 11 : 145--158.

Five-year-old peach trees were irrigated at 50% and 100% of calculated max imum evapotranspira t ion (MET) in order to de te rmine the inf luence of water stress on the pat- tern of water uptake f rom the soil and on the actual evapotranspira t ion (AET) o f the crop. A simplified water balance m e t h o d based on the relat ionship be tween the drainage c o m p o n e n t and the soil water con ten t averaged over the soil profile has been used to esti- mate A E T f rom periodic neu t ron probe measurements .

Max imum water up take is f rom the upper 60 cm of soil when trees are well-watered. Decreased soil water con ten t induces a shift in the soil water uptake towards deeper layers, which can be due ei ther to upward f luxes of water or to an increased water uptake by deeper roots.

AET in the 50% MET regime is reduced f rom July to September , compared to the 100% MET regime, part ly because of s tomatal closure. There is no drainage in the 50% MET t rea tment f rom June to Sep tember ; it is about 1 mm day- ' in the 100% MET regime unti l the end of August and ceases in Sep tember when the soil dries.

INTRODUCTION

The water balance of the root zone of a crop in a given period can be simply expressed as the difference between the input and the output of water during the same period. The input (precipitation and irrigation, if any) are discontinuous events whereas one of the most important outputs, evapo- transpiration, is a continuous phenomenon occurring essentially during the day. In this system, the soil can be considered as a buffer receiving water intermit tent ly and releasing it continuously through evapotranspiration.

As the soil dries, the water status and physiological processes of the plant are modified. These modifications can be used to determine when irrigation is required (e.g. Shmueli, 1967; Stegman et al., 1976), so long as they are sensitive enough to indicate early water stress.

The amount of water to be applied by irrigation is generally taken as the water consumption of the crop which can be determined using soil water balance methods (see Campbell and Campbell, 1982).

0378-3774/86/$03.50 © 1986 Elsevier Science Publishers B.V.

146

In a study concerning peach tree irrigation, we tried to find sensitive physiological indicators of plant water stress (Gamier and Berger, 1985) to determine irrigation times. Together with these measurements on the plant, we moni tored soil water content in order to determine: (1) the water con- sumption of the orchard; and (2) from which soil layers the water was with- drawn.

MATERIAL AND METHODS

Experimental site and plant material

The research was conducted in an experimental orchard of the Ecole Nationale Sup~rieure Agronomique de Montpellier (ENSAM), 8 km south- east of Montpellier, France. (43 ° 34'N, 3 ° 57'E) from February to September 1984. Peach trees (Prunus persica (L.) Batsch cv. Springcrest on GF 305 rootstock) were planted in January 1981 at 4.5 m spacing in rows 6 m apart. The soil is alluvial with a gravelly layer at 50 cm depth. It is classified as cal- cic-luvisol (FAO classification). Main roots are located in the upper (0--50 cm) clay-silt layer, with some minor ones extending to 60 cm.

Meteorological data

Short-wave radiation (Rg), Piche evaporation (Ep) and rainfall (P) were kindly provided by MontpeUier-Fr~jorgues airport meteorological station, 1.7 km south-west of the orchard. These data were used to calculate Brochet- Gerbier potential evapotranspiration (PET) in the following way:

PET = mRg + nEp (1)

where PET and E p are in ram, and Rg is in cal cm -~; m and n are two coeffi- cients depending on latitude and time of the year (Brochet and Gerbier, 1972, 1974).

Irrigation

The trees were sprinkler-irrigated and two treatments were applied: (1) Wet t reatment: eight trees were irrigated at 100% of maximum evapo-

transpiration (MET) when half of the estimated readily available soil water between 0 and 70 cm was depleted. MET was calculated f rom the Brochet- Gerbier equation, corrected by crop coefficients depending on the develop- ment stage of the trees: 0.6 from 1 April to 24 May (stone hardening); 0.9 from 25 May to 23 June (harvest); and 0.6 from 24 June to 30 September (end of irrigations).

(2) Dry t reatment: six trees were irrigated on alternate occasions as above,

cal = 4.1868 J (def).

147

with the same amount of water per irrigation as in the wet treatment. Inte- grated over a complete cycle of drying and irrigation, this represents 50% MET.

A water-meter was used to control the quantities of water applied with each irrigation.

Soil water content

This was measured weekly from 12 April to 24 September 1984 with a neutron probe (Pitman Instruments, Wallingford, model 225) in five perma- nent access tubes in each treatment. The tubes were located 3, 1.5 and 0.75 m from the trees between rows, and 2.25 and 0.5 m from the trees within a row (Fig. 1). Measurements were made every 10 cm from 10 to 190 cm depth. A hemispheric polyethylene reflector (Moutonnet et al., 1969) was used to correct the 10 cm depth measurement.

6m

4.5m t • 5 (l 0)

4(91 2.25 "l(G) 2(7) 3(8) x 0.Sm/ X I X • )

O.75m

l.Sm

3m

Fig. 1. Position and number of the neutron probe access tubes, o, tree; ×, access tubes (1--5, wet treatment, 6--10, dry treatment)

The neutron probe calibration was established from soil samples at the Centre d 'Etudes Nucl~alres in Cadarache, France, following a technique pro- posed by Couchat et al. (1975). To use this technique, the dry bulk density of the soil must be known. This has been deduced from measurements of wet bulk density made with a sub-surface gamma-ray gauge (Campbell Pa- cific Nuclear Corp., model 501).

The accuracy of the soil water content measurement is a function of the count number of the neutron probe, and can be estimated to within 1%.

Soil water balance

The general equation of the soil water balance can be writ ten as:

A S = P + I - R - A E T - D (2)

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where AS is the change in stored water, P and I are precipitation and irriga- tion, respectively, R is surface runoff , AET actual evapotranspimtion, and D flow of water beneath the root zone, all expressed in mm day -1.

A low wall was erected around the test areas to prevent surface runoff , which was therefore neglected.

To solve Equation (2) for AET, we used a simplified method of soil water balance determination proposed by Rambal {1983, 1984). Several of the underlying hypothesis used in this method are tested and discussed by Jones and Wagenet (1984).

Drainage character is t ic d e t e r m i n a t i o n

Several authors (e.g. Black et al., 1969, 1970) showed that drainage at a given depth z could be related to the average water content (0) over 0--z cm. Thus, equation (2) can be written as:

AS = z AO = P + I - AET - f(O) (3)

where AO is the average soil water content variation (cm 3 cm -3) over z, and f(O) is a function relating D to the water content averaged over z, called the drainage characteristic.

The determination of f(O) requires a period when P, I and AET are equal to 0. In this case:

d S / d t = - D = z dO/dr (4)

In February 1984, an area of 9 m × 6 m was covered with a plastic sheet to prevent any water exchange between the soil and the atmosphere. During this time of the year, the transpiration is negligible because the trees are leaf- less. Water was ponded on this area until water content indicated that a steady-state water flow condit ion had been established to 120 cm depth. The average volumetric water content over z (taken here as 120 cm) during the days following the saturation can be written as (see Rambal, 1983; 1984):

= a exp(-b Log t) (5)

and thus

dO/dr = - a b exp(-(b + 1) Log t) (6)

Combining (4) and (6) leads to:

- D = - a b z exp(-(b + 1) Log t) (7)

Eliminating t between (5) and (7) gives:

D = f(O) = exp(c + d Log 0) (8)

with c = - (Log a) / (b ) + Log(bz) and d = b + 1 /b .

A specific drainage characteristic has been determined for each tube posi- tion, and Table 1 gives the values of a, b, c and d for six tubes.

149

T A B L E 1

Parameters for the d e t e r m i n a t i o n of t he dra inage charac te r i s t i cs for t h e upper 120 cm of soil

Parameter T r e a t m e n t

100% ME T 50% MET

Tube 1 Tube 3 Tube 4 T u b e 6 T u b e 8 Tube 9

a 0 .269* 0 .258 0 .250 0 .263 0 .253 0.251 b 0 . 0 1 0 0 " 0 .0102 0 .0112 0 . 0 1 3 4 0 . 0 1 8 9 0 .0189 r - - 0.77 0.94 0 .82 0 .96 0.88

c 1 3 3 . 7 8 " 135.24 126.37 102 .45 75 .84 76 .26 d 1 0 0 . 0 1 " 98.05 89 .30 74 .64 52 .93 52 .93

r = co r re l a t ion coef f ic ien t of the regress ion b e t w e e n ~- and t ime (*, ex t r apo la t ed values because of b r o k e n access tube for n e u t r o n probe) .

Using the value D obtained from (8) enables the solution of (2) for AET from periodic field measurements of the soil water content . Following an iterative procedure proposed by Rambal (1984), D was calculated and AET adjusted using a time increment of 1 day.

R E S U L T S

Seasonal course o f soil water c o n t e n t

We established the seasonal evolution of the average soil water content for three soil layers: 0--60 cm, 60--120 cm and 120--190 cm. Results are pre- sented for the farthest (Fig. 2A) and the nearest (Fig. 2B) tubes from the tree for both treatments.

(1) Layer 0--60 cm. From Fig. 2, it clearly appears that rain or irrigation water mainly affects the first 60 cm of soil in both treatments: the progres- sive drying of this layer was s topped by any water supply.

After a similar evolution in the two treatments during April, the average soil water content in the wet t reatment f luctuated be tween the estimated field capacity (0.26 cm 3 cm -3 for tube 1 and 0.25 cm 3 cm -3 for tube 4) and the lower limit of the readily available water (70% of field capacity) reached in September. In the dry treatment, this lower limit was reached in the middle of June for tube 9 and at the beginning of July for tube 6. The average soil water content f luctuated around this value for the rest of the season.

Table 2 shows that the soil water reserve used (i.e. storage at field capacity minus minimum storage observed) in the wet t rea tment represents 80% of the reserve used in the dry one (calculated with the five tubes of each treat- ment).

1 5 0

A

0.255

'., o.2t5

--~ 0.t75

W--. 0.26'

0.22

--' 0.18

0.tT

0.t3

0.09

z Q

" llt --,.. ,,',, " ~ % . / * • .

Le---*....

6 ~ 1 2 0 c m " " " . . . . . . . * . . . . . . . . . . . . . r '

I I I I l

120-190cm I I I I

APRIL IJ . . . . . . . . i. ,I I .,,,

MAY JUNE AUG SEPT'

i I ; I

, I JULY

0.24

E 0.20 u

0.2 3 ~" §

0.~9

o 0.~5

,~ 0.19

0.15

5oI-

ol g -

o ill L I I 1 i * ~ " * J J ~ . r ~ I

- - . 0 - . o

6 0 - 1 2 0 ¢m ' " • . . . . . . . . . . ._ I I I I I

1 2 0 - 1 9 0 cm "1 " • . . . .

I I l I

H . . . . . . . . . . ; ,I , , , ,i t

~APRIL ' MAY ' JUNE JULY AUG SEPT

Fig. 2. Seasonal course o f the average soi l water c o n t e n t for d i f ferent soil layers. (A) T u b e s 1 (+ +, w e t t r e a t m e n t ) and 6 ( e . . . . . , dry treatment) . (B) Tubes 4 (+ t, w e t t r e a t m e n t ) and 9 ( . . . . . . , dry treatment) . The b o t t o m o f the f igure s h o w s natural rainfall ( ), irrigation on b o t h t rea tments ( - - - - - - ) and on w e t t rea tment o n l y ( . . . . . . . ).

151

(2) Layer 60--120 cm. In the wet t reatment , the participation of this layer to the water consumption of the crop was very low until the beginning of September when slight drying occurred.

After a similar evolution in the dry treatment until the end of May, this soil layer progressively dried until the end of the season. Water uptake from this layer became important when the soil water content in the upper 60 cm reached 76 and 80% of field capacity for tubes 6 and 9, respectively.

Table 2 shows that the total water uptake from this layer is 3.5 times greater in the dry treatment than in the wet one.

T A B L E 2

Soil wa te r charac te r i s t ics for the th ree layers: 0 - -60 cm, 60 - -120 cm and 120 - -190 cm

Layer 100% ME T 50% MET

SFC MOS UR SFC MOS UR ( m m ) ( m m ) ( ram) ( m m ) ( m m ) ( m m )

0- -60 cm 154 109 45 149 93 56 6 0 - - 1 2 0 cm 151 139 12 142 101 41

1 2 0 - - 1 9 0 cm 117 105 12 100 76 24

Means ca lcula ted w i th five t ubes of each t r e a t m e n t . SFC, s torage at field capac i ty ; MOS, m i n i m u m observed s torage; UR, used reserve.

(3) Layer 120--190 cm. The general trend over the season was a progres- sive and slow drying of this layer, slightly more important in the dry treat- ment (see Table 2). The total amount of water supplied by this layer was the same as that of layer 60--120 cm for the wet t reatment and half that of the middle layer for the dry one (Table 2).

The low mean soil water content encountered in this layer (especially in the dry treatment) reflects the presence of a calcareous conglomerate in the subsoil. As its depth is somewhat variable between the tubes (between 120 and 150 cm) and the participation of this layer to the water use of the crop is low (see the discussion), further analysis was done on the upper 120 cm of soil only.

Pattern o f water uptake from the soil

Different patterns of water uptake from the soil were observed: when the soil was well-watered (Fig. 3A, 7 days after a 51 mm irrigation on both treat- ments), maximum water loss was located in the upper 60 cm (roughly 80% in both treatments) with approximately 50% in the first 30 cm of soil. Stock variations are alike in the two treatments and represent between 0.56 and 0.59 PET.

Fig. 3B represents the pattern of water uptake 7 days after a 52 mm irriga-

152

tion on the wet treatment and 20 days after a 51 mm irrigation on the dry one. In the wet treatment, water loss by the different soil layers had not changed. In the dry one, water uptake shifted towards the deeper layers, and 56% of the total variation took place between 60 and 120 cm depth. Together with this shift, water loss was reduced: stock variation represented 0.27 PET in the dry treatment whereas it was still 0 . 5 0 P E T in the well- watered treatment.

E

ae (% mm -I)

4 L

51%

27%

11%

~1%

AS: 20.6 mm

3 2 I 0 2 i J I i

L__

¢00% MET a~

3O

120 ( ~

PET = 3ZOBmm 50% MET

Ae (%rnm -1)

3 4 I I

49 %

32 %

9.5%

9.5%

AS:22:15mm

I/o I ' °' tom-1' 4 3 Z I 0 I 2 3 4 i/o a '°' ram-% L ~ I I I I I A

54% I 30 ~ 3'I °"°

~. 27% 30.9% 60

::z o 9.5 Yo 37.3% 9O

9.5% 18.2% 120

AS: 21.5mm AS: 11.8mm PET : 43.02mm

Fig. 3. Pattern of soil water supply as a function of depth for wet (tube 4, left) and dry (tube 9, right) treatments: (A) between 29 June and 5 July; and (B) between 13 July and 19 July (see text for details). Figures on both sides represent the proportion of water supplied by the different layers compared to the total stock variation (AS).

Monthly water balance

(1} AET in the 100% MET regime (Fig. 4). AET in this treatment in- creased from May (1 m m d a y - ' ) to July (about 3.5 mm day-'} and gradually decreased from July to September {about 2 mm day -~ ).

The increase in the first part of the season is due to an increase in both leaf area of the trees and in PET. The leaf area index o f the crop reached its maximum value in July (see Chalmers et al., 1983) and did not decrease be-

153

fore mid-October. Therefore, the decrease in AET observed in late summer can be at tr ibuted to the decrease in PET and eventually to physiological modifications (e.g. leaf ageing) affecting the transpiration of the trees.

AET/PET ratio (Table 3) reaches its maximum value in August (0.70, 0.77 and 0.86 for tubes 1, 3 and 4, respectively), showing a larger decrease in PET than in AET between July and August.

T A B L E 3

Mean m o n t h l y values of A E T / P E T ra t io m e n t s for t he same six tubes as Fig. 4

for dry (50% MET) and wet (100% MET) t reat-

Tubes 1 and 6 (3 m b e t w e e n rows)

Tubes 3 and 8 Tubes 4 and 9 (0 .75 m b e t w e e n rows) (0 .50 m wi th in rows)

50% 100% 50% 100% 50% 100% MET MET MET MET MET MET

May 0 .50 - - 0 .50 0 .26 0.45 0.21 J u n e 0 .60 - - 0 .50 0.48 0 .60 0.52 Ju ly 0 .48 0.55 0.55 0.52 0 .50 0.64 Augus t 0.47 0 .70 0.58 0.77 0 .63 0.86 S e p t e m b e r 0 .53 0 .66 0.44 0.72 0.31 0.53

(2) AET in the 50% MET regime. In May, AET in this t reatment was higher than in the wet one for the three tubes (Fig. 4). This may be due to differences in the leaf area of the two trees caused by an aphid attack, more severe on the 100% MET tree than on the 50% MET one. Morphological dif- ferences induced by this attack progressively disappeared thereafter.

From late June to September, the low soil water content in the dry treat- ment (see Fig. 2) induced a reduction in the AET of the crop, compared to the wet treatment. Maximum reduction occurred in August when AET in the 50% MET treatment was approximately 70% of AET in the 100% MET regime.

(3) Drainage component . In the dry treatment, drainage was noticeable only in May (0.2--0.4 mmday- l ) . When the average soil water content de- creased under 0.23, 0.21 and 0.20 cm a cm -3 for tubes 6, 8 and 9, respective- ly, which is the case from June to September, equation (8) gives zero drain- age values.

In the wet t reatment, drainage is important during the whole season, ex- cept in September when drying occurs (see Fig. 2). It fluctuates between 1.6 and 0.5 mm day -1 from May to August, except for tube 4 where the value for August is 0.1 mm day -1.

The different drainage patterns obviously reflect the differences in the average soil water content over 120 cm between the two treatments.

154

~ 7

E" E 5 - F--

~ 4

2

I

,2o "1

TUBE I (100% MET) TUBE 6 (50% MET) (3 m bet,een rows )

o I - ~

MAY LJUNE JULY AUG ~SEPT 0___~1 I j ~

TUBE 3 (100% MET) TUBE 8 ( 50% MET) (075m bet,een fo,s)

MAY aJUNEIJULY ~AUG jSEPT

TUBE ~ (100% MET) TUBE 9 (50%MET) (050m ,;Thin o to*)

MAY I JUNEIJULY j AUG SEPT

Fig. 4. Mean monthly values of AET and drainage for wet (+ I t u b e s 1, 3 and 4) and dry (e ....... -*, t ubes 6, 8 and 9). The upper curves show the mean monthly values of PET.

DISCUSSION AND C O N C L U S I O N

Rooting pattern

Because the water transfers between the different soil layers and evapo- ration from the upper layer have not been taken into account in this study, the pattern of soil water uptake can only be used to obtain a gross estimate of the rooting pattern of the crop (see Pearson, 1974) . At high soft water content, the major resistance to water uptake is located inside the root (Taylor and Klepper, 1975). As a consequence, water uptake is propor- tional to the rooting density (Taylor and Klepper, 1975; Nnyamah and Black, 1977). The pattern of water uptake from the different soft layers at high soft water content (Figs. 3A and 3B for the wet treatment) suggests that the maximum density of roots is located in the upper 60 cm of soft. Direct observations of soft profiles confn-m this result. Gras (1962) reports rooting depths o f peach trees between 51 and 59 cm depending on soil texture, and Chalmers et al. (1983) found that the major part of the root system was located in the upper metre of soft. Such a shallow root distribu- t ion seems to be linked with unfavourable subsoil physical properties, since other authors found very deep roots (down to 5 m) for peach trees growing in deep softs (Chalmers et al., 1983). This seems to be the case in our or- chard where a gravelly layer at 50 cm prevents further extensive root pene- tration.

155

Pattern o f water uptake from the soil

The progressive drying of the 60--120 cm layer in the dry treatment (Fig. 2) together with zero drainage at 120 cm depth (Fig. 4) suggests that the drying of the 0--60 cm layer induces a shift in the pattern of water use by the crop (see also Fig. 3B for the dry treatment). This shift can be due either to an upward flux within the soil caused by an increased potential gradient between the two soil layers or an increase in water uptake by deeper roots at low density (Katerji et al., 1984). Such an uptake of water from the deeper soil layers when drying occurs has been reported for herbaceous crops (Maertens and Cabelguenne, 1971; Taylor and Klepper, 1973; Maertens et al., 1974), woody crops (Levin et al., 1972; Chalmers et al., 1983), forests (Nnyamah and Black, 1977) and natural communities (Sala et al., 1981; Rambal, 1984).

The method used to estimate AET in the orchard supposes a priori that a drainage component existed in the water balance, but did not take into ac- count possible upward fluxes from soil layers deeper than 120 cm. This is certainly an acceptable assumption in the 100% MET regime where drainage occurs during the major part of the season (Fig. 4). Indeed, this method has been shown to give good results when applied to periods when the soil is well-watered (Van Bavel et al., 1968).

However, upward fluxes from deeper soil layers increase when the upper soil layers dry (Daudet and Valancogne, 1976; Reicosky et al., 1977; Katerji et al., 1984). This is probably the case in our 50% MET treatment where no drainage occurred from June to September. We can evaluate the magnitude of these upward fluxes if we admit that they take place only from the 120-- 190 cm layer, i.e. at a maximum of 130 cm beneath the main root zone. In this case, they represent 24 mm (Table 2) between June and September (Fig. 2), that is 0.2 mm day -~ which must be added to the AET values pre- sented on Fig. 4.

Actual evapotranspiration o f the orchard

Table 4 summarizes different values of water use by peach trees reported in literature. Results of CNABRL (1976) and Worthington et al. (1984) were obtained with lysimeters, whereas a soil water balance method was used by Chalmers et al. (1983). The table shows a wide discrepancy between the dif- ferent results. Except at the beginning of the season (aphid attack together with a cold spring), our data fit well with CNABRLs' which was also estab- lished in the south of France on an early peach tree cultivar. However, these values are quite low compared to those of Worthington et al. and Chalmers et al. This is possibly due to: (1) differences in the climates of the study areas -- comparisons are difficult because the 'evaporative demand' was either measured with a class A pan or calculated from a PET equation; (2)

156

TABLE 4

Actua l e v a p o t r a n s p i r a t i o n (AET) o f peach t rees ob t a ined in four s tudies : the po t en t i a l e v a p o t r a n s p i r a t i o n (PET) was e i the r eva lua ted wi th a class A pan (Cha lmers et al., 1983, and W o r t h i n g t o n et al., 1984) or by PE T f o r m u l a ( C N A B R L , 1976, and presen t s t u d y )

Chalmers et al. Worthington et al. CNABRL (1976) Present study (1983) (1984)

AET PET AET AET PET AET AET PET AET AET PET AET

(mm day-') PET (mm day-') PET (mm day-') PET (ram day-5 PET

May 3.5 4.7 0 .74 - - - - - - 2.9 3.8 0 .76 1.8 3.8 0.47

J u n e 6.3 6.4 0 .98 -- -- -- 3.9 5.1 0 .76 2.9 5.0 0 .52

July

6.7 6.3 0.94 7.0 8.3 0.84 3.5 5.4 0.65 3.5 5.8 0.60

August

7.2 5.7 1.26 6.0 9.1 0.66 2.6 4.5 0.58 3.4 4.3 0.78

September

-- -- -- 5.1 7.3 0.70 1.8 2.9 0.62 2.0 3.2 0.64

differences in the management, age and leaf area of the orchard; (3) the low level of fertilizers used in our orchard, which could have prevented optimal growth of the trees.

Water consumption values found by Chalmers et al. and Worthington et al. are quite close, but they were obtained at differing values of 'evaporative demand' . Therefore, the determination of cultural coefficients from such differing results is impossible. However, it seems that the values obtained in this s tudy between July and September 1984 can be used as correct esti- mates of the water consumption of peach orchards in the south of France.

The reduction in AET in the dry regime from July to September is the consequence of both reduced soil evaporation as the upper layer dries and reduced transpiration due to partial stomatal closure: on 8 August, mid<iay stomatal conductance values, obtained from porometer measurements, were 0.39 cm s -1 and 0.68 cm s -l in dry and wet treatments, respectively. Such a decrease in the transpiration of plants induced by soil drying is a well known phenomenon (e.g. Jordan and Ritchie, 1971; Turner, 1975).

The experimental design adopted to place the access tubes did not allow us to detect a clear gradient in the water use of the trees, except in July for the wet t rea tment and in August for both treatments (see Fig. 4 and Table 3) when water was preferentially withdrawn near the tree trunks.

Finally, the water uptake by well watered trees seems to be located in the upper 60 cm of soil because of a shallow root system. This may be due to soil conditions and tree age. In these conditions, any fall in water content (or

157

water potential if tensiometers are used) at 60 cm depth can indicate the time when irrigation must begin.

ACKNOWLEDGEMENTS

The authors thank J. Godefroy for his constant support and help during this work, J. Hugard for facilities to work in the experimental orchard of the Ecole Nationale Supdrieure Agronomique de Montpellier (ENSAM), J.P. Luc for advice in soil moisture measurements, and ENSAM staff for valuable assistance in the field. R. Bonhomme provided helpful comments on the manuscript.

REFERENCES

Black, T.A., Gardner, W.R. and Thurtell, G.W., 1969. The prediction of evaporation, drainage and soil water storage for a bare soil. Soil Sci. Soc. Am. Proc., 33: 655--660.

Black, T.A., Gardner, W.R. and Tanner, C.B., 1970. Water storage and drainage under a row crop on a sandy soil. Agron. J., 62 : 48--51.

Brochet, P. and Gerbier, N., 1972. Une methode pratique de calcul de l'~vapotranspira- tion potentielle. Ann. Agron., 23: 31--49.

Brochet, P. and Gerbier, N., 1974. L'~vapotranspiration. Aspect agrom~t~orologique. Evaluation pratique de I'ETP. Monogr. 35, M~tdorologie Nationale, Paris, 95 pp.

Campbell, G.S. and Campbell, M.D., 1982. Irrigation scheduling using soil moisture measurements: theory and practice. In: D. Hillel (Editor), Advances in Irrigation, 1. Academic Press, New York, NY, pp. 25--42.

Chalmers, D.J., Olsson, K.A. and Jones, T.R., 1983. Water relations of peach trees and orchards. In: T.T. Kozlowski (Editor), Water Deficit and Plant Growth. 7, Additional Woody Crop Plants. Academic Press, New York, NY, pp. 197--232.

CNABRL, 1976. Consornmations d'eau hebdomadaires de ref(~rence pour diff~rentes cultures. Document d'aide ~[ la d~cision en irrigation. CNABRL Note, Compagnie Na- tionale d'Am(~nagement du Bas RhSne-Languedoc, Nrmes, 1 p.

Couchat, P., CarrY, C., Marcesse, J. and Le Ho., J., 1975. The measurement of thermal neutron constants of the soil; application to the calibration of neutron moisture gauges and to the pedological study of soil. In: R.A. Schrack and C.D. Bowman (Editors), Nuclear Cross Sections and Technology. NBS SP 425, U.S. Department of Commerce, Washington, DC, pp. 516--519.

Daudet, F.A. and Valancogne, C., 1976. Mesure des flux profonds de drainage ou de remont~es capillalres. Leur importance dans le bilan hydrique. Ann. Agron., 27: 165--182.

Garnier, E. and Berger, A., 1985. Testing water potential in peach trees as an indicator of water stress. J. Hortic. Sci., 60: 47--56.

Gras, R., 1962. Quelques observations sur les relations entre les propri~t~s physiques du sol et la croissance du p~cher dans la vall~e du Rh6ne entre Vienne et Valence. Ann. Agron., 13: 141--174.

Jones, A.J. and Wagenet, R.J., 1984. In-situ estimation of hydraulic conductivity using simplified methods. Water Resour. Res., 20: 1620--1626.

Jordan, W.R. and Ritchie, J.T., 1971. Influence of soil water stress on evaporation, root absorption and internal water status of cotton. Plant Physiol., 48:783--788.

Katerji, N., Daudet, F.A. and Valancogne, C., 1984. Contribution des r~serves profondes du sol au bilan hydrique des cultures. D~termination et importance. Agronomie, 4: 779--787.

158

Levin, I., Assaf, R. and Bravdo, B., 1972. Effect of irrigation treatment for apple trees on water uptake from different soil layers. J. Am. Soc. Hortic. Sci., 97: 521--526.

Maertens, C. and Cabelguenne, M., 1971. Influence de l ' irrigation sur les modalit~s d 'uti l isation de l 'eau du sol par diff~rentes cultures annuelles et pluri-annualles. C.R. Acad. Agric. Fr. , 57: 926--937.

Maertens, C., Blanchet, R. and Puech, J., 1974. Influence de diff~rents r~gimes hydriques sur l 'absorpt ion de l 'eau et des ~l~ments min~raux par lea cultures. 1: R~gimes hy- dryques, systbmes racinaires et modalit~s d 'a l imenta t ion en eau. Ann. Agron., 25: 575--586.

Moutonnet , P., Buscarlet, L.A. and Marcesse, J., 1969. Emploi d 'un humidim~tre ~ neu- trons de profondeur associ~ i un r~flecteur pour la mesure de la teneur en eau des sois au voisinage de la surface. Ann. Inst. Tech. Batim. Tray. Publics, 233: 602--606.

Nnyamah, J.U. and Black, T.A., 1977. Rates and pat terns of water uptake in a Douglas fir forest. Soil Sci. Soc. Am. J., 41: 972--979.

Pearson, R.W., 1974. Significance of rooting pat tern to crop product ion and some prob- lems of root research. In: E.W. Carson (Editor), The Plant Root and its Environ- ment. University Press of Virginia, Charlottesville, VA, pp. 247--270.

Rambal, S., 1983. Variabilit~ des propri~t~s hydrodynamiques ~ l'~chelle d 'un versant Karstique. In: Variabilit~ Spatiale des Processus de Transfert dana lea Sols. Lea Col- loques de I ' INRA, 24--25 Juin 1982, Avignon. INRA, Paris, 15: 201--211.

Rambal, S., 1984. Water balance and pat tern of root water uptake by a Quercus coccifera L. evergreen scrub. Oecologia (Berlin), 62. 18--25.

Reicosky, D.C., Doty, C.W. and Campbell, R.B., 1977. Evaporation and soil water move- ment beneath the root zone of irrigated and non-irrigated millet (Panicum miliaceum). Soil Sci., 124: 95--101.

Sala, O.E., Lauenroth, W.K., Patton, W.J. and Trlica, M.J., 1981. Water status of soil and vegetation in a shortgrass steppe. Oecologia (Berlin), 48: 327--331.

Shmueli, E., 1967. The use of physiological indicators for the timing of irrigation. In: Trans. 8th Int. Congr. Soil Sci., Bucharest 1964, pp. 423--431.

Stegman, E.C., Schiele, L.H. and Bauer, A., 1976. Plant water stress criteria for irrigation scheduling. Trans. ASAE, 19: 850--855.

Taylor, H.M. and Klepper, B., 1973. Rooting density and water extract ion for corn (Zea maya L.). Agron. J., 65: 965--968.

Taylor, HMI. and Klepper, B., 1975. Water uptake by cot ton root systems: an examina- tion of assumptions in the single root model. Soil Sci., 120: 57--67.

Turner, N.C., 1975. Concurrent comparisons of stomatal behavior, water status and eva- porat ion of maize in soil at high or low water potential . Plant Physiol., 55: 932--936.

Van Baval, C.H.M., Burst, K.J. and Stirk, G.B., 1968. Hydraulic propert ies of a clay loam soil and the field measurement of water uptake by roots. II: The water balance of the root zone. Soil Sci. Soc. Am. Proc., 32: 317--321.

Worthington, J.W., McFarland, M.J. and Rodrigue, P., 1984. Water requirements of peach trees as recorded by weighing lysimeters. HortScience, 19: 90--91.