WATER USE REQUIREMENTS OF HIGH DENSITY WALNUTSUNDER LOCALIZED IRRIGATION

D. A. Goldhamer, R. Kjelgren, R. Beede, T. M. Dejong, D. Ramos

ABSTRACT

Water use requirements and diurnal physiological response of second year,

high density Chico walnut trees under low volume sprinkler irrigation

were studied on an experimental block at the Kearney Agricultural Center.

A water balance approach based on knowledge of the weekly changes in soil

water status and amounts of applied water was used to estimate crop

water use (ET). Plant based measurements (leaf water potential,

stomatal conductance) were taken hourly from predawn to sunset on selected

days. Results indicate that water use of young high density walnut

trees exceeds that estimated for other deciduous tree species under dripirrigation, and reached a maximum of 15 gallons per tree per day inmid July. This can be explained by both the rapid canopy development and

the larger surface area wetted with low volume sprinkler (5.4 gph; 9 ft

dia.circular pattern; one sprinkler per tree; twice weekly irrigations).A comparison of mid July ET with pan evaporation indicated a cropcoefficient (Kp) value of 0.32.

Diurnal measurements showed that leaf water potential of well watered

trees taken on July 21 ranged from -1.5 predawn to -11.9 bars midday,with values moving in a narrow range from 1200 to 1600 hours. Stomatal

conductance data revealed a pattern of partial midday stomatal closureand an apparent influence of evaporative demand on stomatal behavior.

OBJECTIVES

This report details the progress of a project begun last year to evaluate

water use from planting to orchard maturity of high and conventionallyspaced walnuts under localized irrigation. Interest in high density

walnut plantings is increasing because it offers the possibility ofobtaining economically viable walnut yields during the early age of an

orchard; leve Is which, in a conventionally spaced planting, aren'tachieved until several years later. An additional potential benefit

involves the use of mechanical rather than hand pruning.

Not surprisingly, increasing the number of trees per acre can signifi-cantly affect soil-water-plant relationships that have been established

for conventional practices. The faster rate of canopy development not

only increases transpiration needs but also results in a larger area of

the orchard floor being shaded. Thus, while the water requirements are

increased, less water is presumably lost through surface evaporation.

This enhanced beneficial use of applied water can lower production costs,

especially in parts of the San Joaquin Valley where water is becomingincreasingly more expensive, and its future availability is uncertain.

Regardless of planting density,

is necessary to achieve optimalinformation currently available

knowledge of tree water requirementsirrigation management. Most of the

to walnut growers evolved from a

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relatively limited amount of research conducted decades ago on older

varieties. In view of the different varieties, horticultural (includingirrigation) practices, and the current interest in plant water stressaffects on tree growth and productivity, data on walnut water use is

both timely and of practical importance.

Therefore, the long range goals of this study are:

1) To establish the relationship between crop water use and canopydevelopment from the time of planting to orchard maturity, as

well as to identify the degree of canopy cover that correspondsto maximum orchard water use.

2) To evaluate the relationships between productivity and water use

of both high and conventional density walnuts. This includesestablishing the functional relationship between water use and

nut yield and quality, and biomass production for mature trees.

The 1983 objectives were:

1) To determine the water requirements (ET) of second year, highdensity, walnut trees.

2) To evaluate the diurnal physiological response of these well wateredtrees.

PROCEDURE

A two year old experimental block of Chico trees equipped with a low

volume sprinkler irrigation was used for this study. This 2.5 acre

orchard which is divided into high (11 x 22 ft) and conventional (22

x 22 ft) sections, is located at the Kearney Agricultural Center in

Fresno County. The sprinklers are set in the tree rows 5.5 ft awayfrom each tree and apply water at a rate of 5.4 gallons per hour over

a circular area 9 ft in diameter. This year two healthy trees wereselected for intrumentation in the high density block with care taken

to insure that all the adjacent trees were of normal size and vigor.Fifteen neutron access tubes were installed on each tree. Four tubes

were installed into the drive at intervals of 2.75 ft with a fifth

tube splitting the difference between the tree and inner most tube(16.5 in). Tubes in the tree row were also installed at distances

of 2.75 ft between the instrumented and adjacent tree. Two additional

tubes were installed 16.5 in away from the instrumented and adjacenttree. Within the right angle formed by these two rows of tubes, fiveadditional tubes were installed to form a grid pattern in order to

adequately characterize one quadrant of the potential root zone.Since the sprinklers were located midway between the trees in the

tree row, this tube arrangement also represented one quadrant of thewetted pattern.

The installation procedureimportance becasue a loosesoil can lead to erroneous

for neutron access tubes is of critical

fit between the tube and the surrounding

results. The procedure involved augering

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a 1.75 in hole with a bucket auger to 10 ft, then pounding the tubeinto the hole. The tubing used was standard 2 in electrical metal

tubing (EMT), and an impact fitting was placed on the end to prevent

damage during installation. Since the diameter of the hole was

smaller than the tube, the advancing tube cut away soil from insideof the hole which collected in the tube. This soil was later removed

with an auger. Finally, bentonite clay was used to seal the smallcrack around the tube at the soil surface to prevent water from

running down the side of the tube.

A neutron probe (Campbell Pacific Model 503) was calibrated to theorchard soil (Hanford sandy loam), and readings taken at one ft inter-

vals in each tube approximately weekly during the growing season.

The volume of water applied by the low volume sprinklers during the

growing season was determined from water meter readings. Additionally,individual emitter output to the instrumented trees was measured for

comparison to the meter readings.

A water balance approach was used to estimate orchard water use.

Weekly changes in the volume of soil water assessed in the subsurface

quadrant were assumed to represent water status conditions in the

remainder of the potential rooting area. Thus, weekly soil water

storage changes, either positive or negative, subtracted from the

amount of applied water, represented net water loss. Water applica-tions were made twice per week and the duration based on currently

available estimates of ET for deciduous trees that shade 10 percentof the orchard floor.

The water budget c0ncept used assumes that water is lost only by plant

use and surface evaf>uration; deep percolation is neglected. Fortunately.the orchard is underlain by a consolidated sOL 1 layer at approximately

9.5 ft which effectively restricts deep percolation. This assumptionwill be verified with future hydraulic conductivity studies.

Diurnal studies of plant water status, stomatal behavior, and leaftemperature were conducted on the high density trees. Measurementswere hourly from 0500 to 2100. Individual leaves from four random

trees were monitored for leaf water potential using a pressure chamber(PMS Model 600), and eight stomatal conductance measurements were made(Licor Model 1600).

RESULTS

Seasonal water use and pan evaporation for the high density walnuts is

shown in Figure 1. The pan was located nearby in an irrigated pasture

environment. The ET data is presented as mean water use in gallons pertree per day for the two trees, generally averaged over a two weekperiod. Pan evaporation is in inches per day.

The indicated seasonal water use pattern generally follows the typical

shape associated with other crops grown in the San Joaquin Valley.However, the increase in water use from April to mid July and thesubsequent decrease to mid November appear to be steeper than estimates

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for almond trees of similar age. This, no doubt, is due to the laterleaf out and leaf senescence of walnuts. The maximum ET rate of

approximately 15 gallons per tree per day is also slightly higher thanpublished values for second year almonds under drip irrigation. This

difference can be attributed to both larger walnut canopy developmentand the larger surface area wetter by the sprinkler enhancing surfaceevaporation.

The water use rates of any crop depends not only on canopy size and

architecture but on the energy and aerodynamic conditions that prevail.The evaporative demand associated with these environmental factors

is represented by the pan evaporation data shown in Figure 1. As wouldbe expected, ET rates and pan evaporation estimates follow a similarpattern throughout most of the season. It should be noted that

evaporation is expressed as inches per day, while ET is in gallons pertree per day. Thus, a direct comparison between these parameters

requires conversion to equivalent units. For example, ET and Epan

for July 13 are 14.8 gallons per tree per day and 0.31 in per day,respectively. The following expression is used to convert the ET data:

14.8 gal/tree/day f (11 ft x 22 ft) f 0.623 gal/in/ft2 = 0.10 in/day

Therefore, the ratio of ET to Epan, commonly known as the crop coefficient(Kp) for mid July is 0.32 for these second year trees.

Figure 2 present diurnal plant based measurements made on July 21, 1983,a typical midsummer day with a high temperature of 980F and 0.33 in of

pan evaporation. Leaf water potential decreased rapidly from a predawn

value of approximately -1.5 bars to -11.5 bars at 1200. Readings thenmoved in a narrow range (-11.4 to -11.9 bars) through 1600. Thereafter,l~af water potential gradually returned toward predawn levels. It's

interesting to note that plant water status remained relatively constant

over the midday hours. This is not only of academic interest, but is of

practical importance in that it indicates that walnuts apparently satisfyone requirement necessary for the use of leaf water potential as an

indication for scheduling irrigations. It should be noted, however, that

leaf water potential measured diurnally on a high evaporative demandday were more negative even though the soil water status was similar.

This observation is not uncommon with other crops; various techniques

are used to "normalize" the plant based measurements that vary withenvironmental conditions.

The stomatal conductance values shown in Figure 2 have a distinctlydifferent pattern than leaf water potential. They peak at a value of0.83 cm per sec at 0900 and gradually decrease to 0.55 at 1600. There-

after, a small increase occurs until 1900, followed by gradual closure.

This pattern is typical for deciduous trees that undergo partial middaystomatal closure; measurements from nearby almonds under similar environ-

mental conditions had a similar trend. Interestingly, the magnitude

of stomatal conductance monitored on the walnuts under relatively high

evaporative demand was somewhat less than shown in Figure 2, possiblyindicating a functional relationship between orchard energy status andstomatal apeture.

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CONCLUSIONS

It appears that second year high density walnuts have higher water userates than other similarly aged deciduous trees. Estimated ET for 1983

reached a maximum rate of approximately 15 gallons per tree per dayin mid July. Considering the standard USWB Class A pan evaporation

data for this period, this translates into an indicated crop coefficient(Kp) of 0.33. The relatively high water use requirements are predom-inantly due to rapid canopy development.

Diurnal plant based measurements indicate that young well watered walnuttrees undergo partial midday stomatal closure and that stomatal behavior

is apparently influenced by environmental conditions. Leaf water

potential taken in mid July range from a predawn value of -1.5 to midday

values of approximately -11.5 bars. Leaf water potential appears tobe relatively constant between 1200 and 1600 hours.

ACKNOWLEDGEMENTS

Literally hundreds of hours were spent taking the field measurementsnecessary to calculate the water balance used for ET estimates. The

authors express their sincere appreciation to Gilbert Garza and Mark

Moore for their valuable assistance in data collection, and Diana Nixfor her help in data processing and report preparation. Thanks also

go to Bill Dutcher and Bob Melgard who maintained and recorded panevaporation.

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--- -- ---

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Figure 1. Water use estimates based on a water balance approach for second year,

high density Chico walnuts under low volume sprinkler irrigation.

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July 21, 1983Chico Walnuts

2nd Year

Max. Air Temp.: 98 eF

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Ave. Wind Vel.= 0.59 mph

Figure 2. Diurnal measurements of leaf water potential and stomatal conductance taken

July 21, 1983 on well watered, second year Chico walnuts.

700 900 1100

TIME

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