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Evapotranspiration of Flood Irrigated Pecans

T. W. Sammis 1, J. G. Mexal 1 and D. Miller 2

Abstract

Pecan orchards require more irrigation water to maximize yield than any other crop

grown in the Southwest U.S.. This paper reports daily evapotranspiration (Et)

measurements for 2001 and 2002 in a 5.1 ha, mature pecan orchard on the Rio Grande

floodplain, 7 km south of Las Cruces, N.M. The 21-year-old stand had an average tree

height of 12.8 m, diameter at breast height of 30 cm, and tree spacing of 9.7 m by 9.7 m.

Additional pecan orchards surrounded the study orchard. When the tensiometer reached a

suction of 65kPa at the 45 cm depth, the orchard was flood irrigated. Sparling meters

were installed on the pumps and read before and after each irrigation. The total irrigation

amount was 1940 mm in 2001 and 1870 mm in 2002. A walk-up tower was placed in the

orchard’s center to support flux sensors at 16 m height. The instrument package included

a net radiation (Rn), discs for soil heat flux (G), and two sets of one-propeller eddy

covariance (OPEC) sensors. OPEC systems measure sensible heat flux (H) with a

sensitive, vertically oriented propeller anemometer and a fine-wire thermocouple. Latent

heat flux (LE) was obtained as a residual in the surface energy balance LE= Rn-G-H.

The maximum daily evapotranspiration was 8 mm/day, and the yearly cumulative

evapotranspiration averaged for two years was 1420 mm, resulting in a yearly average

irrigation application efficiency of 79%. The crop coefficient (daily measured Et/

reference Penman Et) ranged from 0.2 to 1.1. Increased evaporation due to irrigation was

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detected only for the April 9 irrigation in 2001. The seasonal water use was 4% lower in

2001 and 12% lower in 2002 than previously reported values.

1. Department of Agronomy and Horticulture, New Mexico State University Las

Cruces, NM 88003

2. Natural Resources Management and Engineering Department, University of

Connecticut, Storrs, CT 06269-4087

3. The research was supported by the Agriculture Experiment Station at New

Mexico State University.

Key words, Evapotranspiration , Pecan, water use efficiency, crop coefficient,

irrigation efficiency

Introduction

Pecans are an important crop for irrigated agriculture in southern New Mexico and

western Texas. Pecan water use is great compared with other irrigated crops and was

estimated to be 1310 mm per year for mature pecan trees grown in the El Paso and Las

Cruces Mesilla valley (Miyamoto, 1983). In the southwest, water is a scarce commodity,

so the goal is to maximize irrigation application efficiency (AE) when designing and

operating a pecan irrigation system. Irrigation application efficiency is the ratio of the

water volume used by the crop to the irrigation water volume applied (ASCE, 1978).

Irrigation application efficiency can vary considerably, depending on irrigation

management and the type of system. Measured irrigation application efficiencies for

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flood irrigated pecan orchards in the Las Cruces Mesilla Valley were high (89%) (Al-

Jamal, et al. 2001) compared with typical flood irrigation application efficiencies ranging

from 50 to 73% for other crops (Oster et al., 1986; Chimonides, 1995; Zalidis et al.,

1997).

When water is scarce and the goal is to conserve water, it is advantageous to grow crops

with high water-use efficiency (WUE), variously defined as:

a) dry matter produced per unit area per unit of evapotranspiration (Et)

(t ha–1 mm–1) (Jensen et al., 1981);

b) total dry matter per unit of Et (Begg and Turner, 1976);

c) harvested yield per unit Et (Evans and Wardlaw, 1976); and

d) photosynthesis per unit of water transpired (Fischer and Turner, 1978; Sinclair et

al., 1984).

In New Mexico WUE can vary from 0.0014 t ha -1mm-1 for cotton fiber (Sammis, 1981)

to 0.009 to 0.014 t ha -1mm-1 for dry alfalfa forage (Adul-Jabbar et al.,1983) to 0.12 t ha -

1mm-1 for sweet onions bulbs (Al-Jamal et al., 1999). Consequently, WUE can vary by a

factor of 100 depending on what crop is being harvested. Based on total biomass

measured by Kraimer (1998) and Et measured by Miyamoto (1983), an initial WUE

estimate for pecans is 0.015 t ha -1mm-1. Pecan WUE, based only on nut yield, is 0.0019 t

ha-1mm-1, which is comparable to cotton. While the WUE may be low, the monetary

value of pecans is high and the economic return per unit of water applied is high

compared with other crops (Libben, 2001). Consequently, in New Mexico, the area under

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pecan production is increasing, and the land area used to grow cotton and other lower-

valued crops is decreasing.

WUE and nut quality decrease when trees are water stressed during the nut filling stage

(Herrera, 2001). Maintaining non-moisture stress conditions is important in pecan

orchards throughout the growing season, but especially during the nut filling stage in

August and September in order to maximize yield, quality and economic return.

Farmers are more concerned with increasing irrigation water use efficiency (IWUE),

defined as the ratio of the crop yield to seasonal irrigation water applied (t ha -1mm-1) that

includes rain (Howell, 1994). IWUE is affected by water lost to drainage, canopy

interception, soil type, cultural practices and plant species. IWUE can be increased by

proper irrigation timing using irrigation scheduling models based soil water balance, soil

moisture or soil water potential measurements, or plant-based measurements like the

Crop Water Stress Index. However, if irrigation management delays irrigation past the

optimal time, then decreased yields, profits and IWUE will result.

Irrigation scheduling models based on a soil water balance compute the daily change in

the soil moisture reservoir and schedules irrigation when the available water in the soil

profile starts to cause a reduction in Et and yield. Et subtract from the soil water balance

is calculated from reference evapotranspiration (Eto) using climatic data and a crop

coefficient to scale reference evapotranspiration to actual evapotranspiration. The crop

coefficient, (Et/Eto) changes throughout the growing season as a function of time or

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growing degree days as the leaf area index of the orchard changes. In the irrigation

scheduling soil water balance model, irrigation water fills the root zone and deep

drainage occurs until the soil moisture reservoir reaches field capacity.

IWUE usually is higher for subsurface drip (0.0283 to 0.227 t ha –1mm-1) or surface drip

(0.0235 to 0.127 t ha -1mm-1) compared with sprinkler (0.0044 to 0.0659 t ha -1mm-1)

or furrow irrigation (0.0086 to 0.056 t ha -1mm-1) systems (Sammis, 1980; Bogle et al.,

1989; and Lamm et al., 1995). An estimated pecan IWUE based on an average yield in

the Mesilla Valley is 0.0016 t ha -1mm-1, which again is low compared with other surface

irrigated crops. IWUE increases in years when pecan high yields are high and decreases

in “off” years when the alternate-bearing characteristics of pecans lower yields.

The research objectives were to measure a mature pecan orchard’s seasonal

evapotranspiration by measuring the daily and seasonal evapotranspiration using an

energy balance -eddy covariance meteorological method; and to determine the crop

coefficient for pecans. Another objective was to determine the flood irrigated pecan

orchard IWUE.

Procedures

A 5.1 ha Western Schley pecan orchard located 7 km south of Las Cruces in the Mesilla

valley was planted in 1970. The climate in the Mesilla valley is semi-arid with an

average rainfall of 234 mm with half of the rainfall occurring during the winter months

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and the other half occurring during the summer monsoon season. The tree spacing was

9.7 m X 9.7 m. In 2001 average orchard height was 12.8 m with an average tree

diameter at breast height of 30 cm. These size trees represented a mature orchard.

Western Schley, is an alternate-bearing variety with an alternate-bearing intensity (I) of

0.66 compared with the 0.57 average for all pecan varieties (Conner and Worley, 2000).

The tendency for alternate-bearing increases with increased yield. Consequently, the

average yield from a mature orchard is 2,500 kg/ha, with an off year yielding 1400 kg/ha

and an on year yielding 3,600 kg/ha (Stahman Farms, 2001).

The soil type was a Harkey loam (coarse-silty, mixed, calcareous, thermic Typic

Torrifluvents). The farmer applied 325 kg/ha of urea nitrogen in 2001 and 314 kg/ha in

2002 via the flood irrigation system throughout the growing season. Application of

nitrogen occurred at each irrigation (19 kg/ha/irrigation).

On March 10, 2001, the orchard was equipped with two one-propeller energy balance

eddy covariance systems (OPEC) on a 16.5 m tower placed between two rows of trees in

the pecan orchard’s center to measure the sensible heat flux (H) using eddy covariance,

net radiation (Rn), soil heat flux (G) and estimate evapotranspiration (Et) from the

surface energy balance equation.

Et= Rn-G-H Eq. 1 Net radiation was measured using a Fritschen Q7.1 net radiometer and soil heat flux

using a HFT -3.1 heat flux disk (Radiation and Energy Balance systems Inc). H was

measured using the eddy covariance equation.

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( ' ')pH c w Tρ= Eq. 2

Where

ρ = Air density

pc = Specific heat capacity of air

' 'w T = Covariance between the fluctuations in vertical wind velocity ( w ) and the

fluctuations in the air temperature (T ).

Air temperature was measured using a thermocouple probes TC-BR-3 (Campbell

Scientific) and vertical wind velocity was measured using a propeller anemometer

#27106 ( R. M. Young Inc.) The OPEC underestimates H without a frequency response

correction for the response of the propeller anemometer to changes in vertical wind

speed. The frequency correction function published by Blanford and Gay (1992) was

used to correct H. The frequency correction factor is to increase the sensible heat

calculations by a factor of 1.4 under stable weather conditions when sensible heat transfer

is toward the ground. Blanford and Gay (1992) presented a complete description of the

equipment and theory behind OPEC. Measurements of sensible heat using the eddy

covariance method and latent heat calculated from the energy budget were averaged over

half hours time periods and the results summed for the day.

The OPEC equipment was located at a height of 17.3 m or 4.56 m above the top of the

canopy. Three soil heat flux plats were spaced between the tree rows and buried at the

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surface. The net radiometer was located at the 14.5 m and placed at the drip line to

integrated the net radiation over the trees and open space.

The pecan orchard was surrounded by other pecan orchards in all directions except the

northeast. Northeast of the 5.1 ha field, the land was planted to alfalfa. The predominant

wind direction is from the northwest and southeast and the orchards south of the tower

were irrigated at the same time as the 5.1 ha field. The fetch distance in the predominate

southeast wind direction was more than 5 km.

The orchard was dead level flood irrigated from two wells located where the water is

discharged into the orchard through a high-flow turnout. The farmer irrigated when a

tensiometer placed at a depth of 450 mm and located midway between the trunk and the

tree’s drip line read 65 kPa. Sparling meters were installed on the pumps to measure the

irrigation amounts. A Hobo H8 (Onset Computer Corporation) data logger was connected

to a magnetic switch that recorded when the irrigation gate on the high-flow turnout was

raised and lowered to measure when the water was turned into the orchard and to verify

the Sparling meter readings. The orchard area was determined from aerial photographs

using Arc View software.

A standard Campbell weather station located above a grass cover crop, 5 km southeast of

the test orchard, was used to calculate grass reference evapotranspiration (Eto) using

Penman’s equation (Sammis et al, 1985). Pecan crop coefficients were determined from

the Eto and Et measured using the OPEC system. Growing Degree Day (GDD) were

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calculated using the average air temperature calculated from the maximum and minimum

temperature, minus a base temperature of 15.5 C and no cutoff temperatures. Miyamoto

(1983) determined that the base temperature should be 15.5 C. The farmer measured

yield at the end of the growing season by shaking the trees, gathering the nuts and

weighing the nut yield from that field.

Results and Discussion

Irrigation started on March 3 2001 and March 14 2002 before bud break (Table 1). The

total irrigation water applied to the orchard was 1840 mm in 2001 and 1744 mm in 2002.

The two sensible heat flux and Et calculations were similar and consistent throughout the

growing season (Figure 1 and 2). Soil evaporation accounted for most of the Et

measurements from March 11 to March 26 in 2001 and January 1 to March 27 2002

when fifty percent bud break occurred. The soil surface was dry because rainfall from

January 1 to March 17 was only 14.7 mm in 2001 and 32 mm in 2002 when the reference

evapotranspiration during that time period was 379 mm. Consequently during that time

period except following the winter irrigation on 3/3/2001 and 3/14/2002, the soil

evaporation process was in stage two, in which evaporation is controlled by water

availability at the soil surface (ASCE 1990). Stage two soil evaporation in January,

February, and March ranged from 0.1 to 2.0 mm/day. On April 9, 2001, following the

second irrigation, evapotranspiration increased to 72% of reference evapotranspiration

(Eto), indicating stage one soil evaporation where climate controls the evaporation rate.

Normally following winter irrigation, the Et should equal reference evapotranspiration

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except in orchards where some of the solar energy is intercepted by tree branches and is

not available for the soil evaporation process. During the time when leaves are not on the

tree, the branches intercept 14% of the solar radiation. By April 11, stage one soil

evaporation ceased, and stage two soil evaporation had started again. Stage two soil

evaporation decreased to 26% of Eto by April 18. The pecan orchard’s water was used

predominately by transpiration from that point until the next irrigation on May 1, 2001.

No increase in evapotranspiration was observed following the April irrigation in 2002.

The pecan orchard closed canopy sooner in 2002 compared with 2001, due to increased

Growing Degree Days (GDD) in 2002. Consequently, the soil evaporation component for

the April irrigation in 2002 was lower compared with the 2001 irrigation. The increase in

leaf area index and transpiration by April 17, 2002 compared to April 9 2001 resulted in

most of the available energy being used for transpiration following the April 17, 2002

irrigation, and lower amount being used for evaporation. Consequently, the increased due

to evaporation was not detectable by the OPEC system which appears to be able to only

detect changes in soil evaporation of 1 mm/day or greater depending on the amount of

available energy on the day of measurement and the LAI.

The smaller increase in Et due to increase in evaporation may not be detectable because

of the coupling mechanism between the soil evaporation at the orchard floor and the

sensible heat measurements using the energy balance eddy covariance method above the

canopy. Blanford and Gay (1992) compared the OPEC system to a Sonic Eddy

Covariance system and found that after correcting the frequency response of the propeller

anemometer, the sensible heat measured by the two systems were the same.

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Consequently, the OPEC system should be as accurate and sensitive as a Sonic Eddy

Covariance system when used to solve the energy balance equation for Et.

Pecan orchard evapotranspiration increased due to increased leaf area after bud break and

reached an average maximum evapotranspiration rate of 8 mm/day in June. Eto and Et

decreased in July and August, due to increased cloud cover and a decrease in the vapor

pressure deficit from summer thunderstorms. In November, decreased Et was caused by

leaf senescence.

The total seasonal evapotranspiration measured with the OPEC system was 1170-1260

mm, and the monthly Et was a maximum in June at 221 mm/month (Table 2). The daily

Et was estimated using the crop coefficient and the reference Eto during times when

OPEC was not working.

The OPEC system measured an average of 40 mm a month for evaporation during the

non-growing season of November through March, making the annual evapotranspiration

1460 mm in 2001 and 1370 mm in 2002. Evaporation during the winter months will

depend on the climate and the amount of rainfall that occurs.

Miyamoto (1983) estimated a mature pecan orchard’s consumptive use as 1310 mm for

the growing season of April 1 through Oct.15. Miyamoto’s study involved orchards that

were 8 to 35 years old and ranged in trunk diameter from 130 to 450 mm and heights

from 7.4 m to 18.8 m located in El Paso Texas, and Las Cruces N.M. area. El Paso is 72

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km south of Las Cruces N.M. The computed water use for the mature orchards only

included data from the orchards that were planted on 9.1 by 9.1 m spacing and were 13.6

m or taller. Consequently, the orchards measured in 1972 and 1973 by Miyamoto were

similar in cover and size to the current study. The pruning practices have changed since

Miyamoto’s study was conducted with the orchards being currently pruned to open the

orchards to more light penetration. The trees canopy of the current study covered 65-70

% of the area. Functions developed by Fereres (1980) for almonds and Johnson et al.

(2000) for peaches showed that trees covering 65% of the of the ground area have the

same Et as trees covering greater than 65% of the ground area. Consequently, the current

study still represents a closed canopy orchard.

The OPEC system measured higher water use in April and May and lower monthly

values in June, July and August, compared with water use reported by Miyamoto. The

seasonal measured evapotranspiration was 4% lower in 2001 and 11% lower in 2002 than

the amount Miyamoto measured in 1972, 1973 and 1981. Averaged over the three years,

Miyamoto calculated the grass reference evapotranspiration, using Penman’s (1963)

equation from April 1 to Oct. 15, to be 1380 mm. The grass reference evapotranspiration

for Las Cruces for the same period using Penman’s modified equation (Sammis et al,

1985) was 1320 mm in 2001 and 2002, indicating that the water use of pecans should

have been 5% less during the current study compared with the time covered in

Miyamoto’s study.

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Because the exact equation used by Miyamoto to calculate reference Et was not given, it

is possible that the different methods might have created the difference in computed

reference evapotranspiration between the two studies time periods. In order to evaluate

this possibility Samani’s reference evapotranspiration equation (Samani and Pessarakli ,

1986) that only uses temperature data were run for the two study time periods. The

climate data needed to run Penman’s equation was not readily available for the 1972-

1973 time period. Samani’s equation was calibrated to Penman’s modified equation for

the current study period. The reference Et to grass by Samani’s method was 2% higher in

El Paso in 1972-1973 compared to Las Cruces 2001-2002. Consequently, Penman’s

equation calculated a difference of 5 % and Samani’s equation a 2% difference indicating

that the pecan Et during the two time periods should be between 2 to 5 % less in the

current study compared to Miyamoto’s study.

The difference in the monthly Et values can be attributed to the difference in the Et

measurement methods in addition to the slight differences in the climate conditions.

Miyamoto used a soil moisture depletion method with a correction for drainage. This

method can underestimate drainage during the summer months when frequent irrigations

occur and, consequently, overestimate monthly summer Et values. The energy balance

eddy covariance method only measure the energy fluxes above the canopy and does not

involve the errors inherent in the soil water balance approach to calculating Et. However,

the energy balance eddy covariance method does include errors in measuring the energy

components of the energy balance.

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The yearly irrigation application efficiency (water evapotranspired for the year/water

applied by irrigation) was 65% in 2001 and 63% 2002. Total yearly rainfall was 74 mm

in 2001 and was 168 mm in 2002. Normal rainfall for Las Cruces, NM is 234 mm. Both

years of the study represented dry, hot years compared with average conditions. When

rainfall is added to the applied irrigation water, irrigation application efficiency decreases

to 63% for 2001 and 57% for 2002. These values were less than the 89% irrigation

application efficiency measured by Al-Jamal et al (2001) for pecans using the chloride

tracer method which calculates leaching fraction. However, the yield from these study

orchards was less than from the current study field. The fields in Al-Jamal’s study were

probably irrigated less frequently resulting in decreased yield and leaching fraction.

The crop coefficient (Kc) is defined as the ratio of measured evapotranspiration (Et)/

potential evapotranspiration (Eto) referenced to grass. At the year’s start when leaves

were not present on the tree, evaporation from the dry soil resulted in a crop coefficient

of 0.18 in 2002 (Figure 3). The OPEC instruments were not installed early enough to

measure soil evaporation and crop coefficient in the winter months of 2001. The crop

coefficient and evapotranspiration increased until maximum leaf area occurred in mid-

July. The crop coefficient of 1.1 does not change until leaf senescence begins in

September or October depending on the year (Figure 3). By the end of the growing

season, the crop coefficient decreased to 0, after leaf drop due to senescence on Nov. 1 in

2001 and Nov. 20 in 2002, due to the first freeze. No rainfall was recorded since the last

irrigation on Oct. 22, 2001, so the soil surface was dry, and soil evaporation was nil.

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The OPEC calculated crop coefficient fits a Day of the year (DYEAR) date time base

fourth-order polynomial and has a 0.89 coefficient of determination (Figure 3). The

equation when forced through 0 for the 2001 data is:

Kc = -3.19E-10DYEAR4+2.72E-8DYEAR3 +2.22E-5DYEAR2 +2.24E-3DYEAR

[2]

The data for 2002 fits a fifth-order polynomial, has a 0.83 coefficient of determination

(Figure 3) and is:

Kc = -8.01E-12DYEAR5+5.05E-9DYEAR4-1.19E-6DYEAR3 +1.25E-4DYEAR2 –

9.24E-3DYEAR +1.85E-01 [3]

The OPEC calculated crop coefficient based on GDD fits a fourth-order polynomial

equation and was statically the same (P<=0.05) for the two years. The combined equation

has a 0.81 coefficient of determination (Figure 4). The equation is:

Kc = -3.866E-12GDD4 + 1.11E-08GDD3 - 1.08E-05GDD2 + 4.31E-03GDD + 3.34E- 01

Consequently, when a heat unit time base is used, crop coefficients can be combined over

years. When based on a day of the year, the crop coefficients between years are different

due to different development rates caused by different climatic conditions. Miyamoto

(1983) expressed the crop coefficient based on cumulative GDD with no maximum and

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minimum cutoff temperatures and a base temperature of 15.5 oC, using long-term climate

data for El Paso, Texas. Figure 4.

Miyamoto’s reported crop coefficients for the months of April and May were lower than

the current measured values (April, 0.29 compared with 0.5, and May, 0.68 compared

with 0.8, Figure 3), because the temperatures in May during Miyamoto’s study period

were 14 percent cooler than during the 2001-2002 studies. However, when the data were

plotted using GDD, the April and May crop coefficients were the same as the currently

measured crop coefficients. Only later in the growing season, when Miyamoto’s monthly

evaporations values are overestimated due to deep drainage errors, do the two crop

coefficients deviate.

Pecan yield measured by the farmer was 2,349 kg/ha in 2001 and 3,681 kg/ha in 2002.

The measured seasonal WUE, based on nut production, was 0.0018 t ha-1mm-1 in 2001

and 0.0031 t ha-1mm-1 in 2002 compared to a nut WUE of 0.0019 t ha-1mm-1, based on

average yield and Et measured by Miyamoto (1983). The pecan nut yearly IWUE,

including rainfall, was 0.0012 t ha-1mm-1 in 2001 and 0.0018 t ha-1mm-1 in 2002.

Conclusions

The total irrigation applied to the pecan orchard for the two years averaged 1790 mm,

resulting in an AE of 79%, which was high compared with other flood irrigated crops.

However, the figure was lower than the previously reported AE of 89% (Al-Jamal et al

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2001). The maximum daily evapotranspiration was 8 mm/day, and the average seasonal

cumulative evapotranspiration was 1210 mm. An increase in evaporation due to irrigation

was detected only for the April 9, 2001 irrigation, when leaf cover was low. The seasonal

water use was 4% lower than reported values in 2001, 12 % lower in 2002. The crop

coefficients (daily measured Et/ reference Et) ranged from 0.2 to maximum value of 1.1,

which were also lower than the previously reported maximum pecan crop coefficient of

1.4. Pecan yield was 2,341 kg/ha in 2001, which represents an off year for this orchard,

but could be considered an average for the Mesilla Valley. In 2002, pecan yield was

considerably higher than the average for the Mesilla Valley. The crop coefficients

developed in this study can be used in a water balance irrigation scheduling model which

could be used in conjunction with other irrigation scheduling techniques to improve

irrigation management of pecans.

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Stahman Farms. 2001. Personal Communication, Stahman Farms, Las Cruces, New

Mexico.

Zalidis, G., X. Dimitriads, A. Antonopoulos and A. Geraki. 1997. Estimation of a

network irrigation efficiency to cope with reduced water supply. Irrig. Drain. Syst.

11: 337-345.

22

List of tables

Table 1. Flood irrigations to the pecan orchard in 2001-2002.

Table 2. Monthly evapotranspiration in a mature pecan orchard measured with the OPEC

system during the 2001-2002 growing season. The First hard frosts were Nov. 20, 2001

and Nov. 6, 2002.

List of figures

Figure 1. Evapotranspiration (Et1, Et2, Eto) from the pecan orchard using two OPEC

systems in 2001.

Figure 2. Evapotranspiration (Et1, Et2, Eto) from the pecan orchard using two OPEC

systems in 2002.

Figure 3. Daily crop coefficient for pecans using day of the year as a time base for 2001-

2002.

Figure 4. Daily crop coefficients for pecans using GDD as a time base for 2001-2002.

Table 1. Flood irrigations to the pecan orchard in 2001-2002.

Date amount mm Date amount mm

3/3/01 88.5 3/14/02 115.0

4/9/01 111.8 4/17/02 114.4

5/1/01 118.0 5/5/02 203.6

23

5/14/01 121.0 5/21/02 122.1

5/29/01 118.1 6/3/02 114.3

6/8/01 115.8 6/14/02 125.2

6/18/01 121.9 6/23/02 115.0

6/29/01 123.4 7/04/02 111.3

7/9/01 122.7 7/15/02 96.8

7/19/01 127.5 7/25/02 95

7/27/01 100.0 8/05/02 94

8/6/01 95.3 8/14/02 87.9

8/16/01 99.3 8/23/02 81.3

8/27/01 98.8 8/30/02 94.3

9/7/01 119.6 9/14/02 107.2

9/20/01 82.3 9/23/02 80.4

10/4/01 91.2 10/04/02 112.8

10/21/01 85.1 10/22/02 83.6

Total 1940 1870

Table 2. . Monthly evapotranspiration in a mature pecan orchard measured with the

OPEC system during the 2001-2002 growing season. The First hard frosts were Nov. 20,

2001 and Nov. 6, 2002.

Year April May June July Aug Sept Oct Nov 1 Seasonal mm mm mm mm mm mm mm mm 2001 88 177

202 221 210 185 136 40 1260

24

April May June July Aug Sept Oct Nov 2 Seasonal mm mm mm mm mm mm mm mm 2002 136 176

218 199 198 170 73 3 1170

Year April May June July Aug Sept Oct

15 Seasonal

mm mm mm mm mm mm mm mm 1983 Miyamoto

70 119 225 278 290 239 86 1310

1. November 1-20

2. November 1-6

0

1

2

3

4

5

6

7

8

9

10

2/29 4/19 6/8 7/28 9/16 11/5 12/25

Date

mm

/day

EtoEt1Et2

25

Figure 1. Evapotranspiration (Et1, Et2, Eto) from the pecan orchard using two OPEC

systems in 2001

0123456789

101112

1/0 2/19 4/9 5/29 7/18 9/6 10/26 12/15

Date

mm

/day Eto

Et1Et2

26

Figure 2. Evapotranspiration (Et1, Et2, Eto) from the pecan orchard using two OPEC

systems in 2002.

Figure 3. Daily crop coefficient for pecans using day of the year (DYEAR) as a time

base for 2001-2002.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00Day of year

Cro

p co

effic

ient

year 2001 year 2002

Poly. (year 2001) Poly. (year 2002)

27

Figure 4. Daily crop coefficients for pecans using GDD as a time base for 2001-2002.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 300 600 900 1200 1500 1800Growing Degree Days

Cro

p co

effic

ient

Crop coefficient OPEC system Miyamoto's crop coefficient gdd based Poly. (Crop coefficient OPEC system )