Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol46. Issue1.Nov2000:
Agricultural Water Management 46 (2000) 29±41
Computing the crop water production
function for onion
M.S. Al-Jamal, T.W. Sammis*, S. Ball, D. Smeal
Department of Agronomy and Horticulture, Box 3003,
Dept. 3Q, New Mexico State University, Las Cruces, NM 88003, USA
Accepted 6 December 1999
Abstract
Onions are a major irrigated crop in New Mexico. An excessive amount of water is generally
applied, because the crop is shallow-rooted and requires frequent irrigation to achieve good yields.
Onions under de®cit irrigation have a decrease in evapotranspiration and yield. Consequently,
farmers need to use the water production function (wpf) for onions to estimate water requirements
at different locations for selected yield goals. The wpf is the relationship between yield and water
applied. The same relation can be expressed in terms of evapotranspiration, in which case the
production function is known as the evapotranspiration production function (Etpf). A gradient
sprinkler line source onion experiment was conducted in 1986 and 1987 at Farmington New Mexico
and a linear Etpf determined. The linear Etpf was expressed as a relative Etpf and the yield response
factor (Ky), which represents the slope of relative Etpf, was calculated for onions at Farmington,
NM and found to be 1.52, compared to 1.5 obtained by [Doorenbos, J., Kassam, A.H., 1986. FAO
Irrig. Drain., Paper 33, Rome, Italy] for onions stressed at the yield formation period.
A second gradient drip line- source irrigation experiment was conducted at Las Cruces, NM,
during 1994±1996 to determine a wpf as related to applied water for drip irrigated onions.
The irrigation treatments were 40, 60, 80, 100, and 120% of calculated nonstressed
evapotranspiration determined from the sprinkler line source experiment. The wpf was curvilinear
because excess water was applied to the different irrigation levels in the experiment in order to keep
the base plate of the onions wet so root growth would continue. The result was that part of the
applied water went to deep drainage rather than to evapotranspiration. The wpf was corrected for
the amount of irrigation water lost as deep drainage and expressed as evapotranspiration versus
yield (Etpf) by using reference evapotranspiration measured at Las Cruces and season crop
coef®cients for selected yield levels measured at Farmington, NM. Maximum onion yield at Las
Cruces under the drip irrigation system was 20% higher than measured at Farmington using the
sprinkler system. The results indicate that high onion yield are achievable using a drip system
compared to a sprinkler system but a larger amount of applied water goes to deep drainage using a
*
Corresponding author. Tel.: 1-505-6463405; fax: 1-505-6466041.
E-mail address: [email protected] (T.W. Sammis).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 0 0 ) 0 0 0 7 6 - 7
30
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
drip system compared to a sprinkler system to achieve maximum yield. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Evapotranspiration; Water production functions
1. Introduction
Onions are one of the most important cash crops in New Mexico with an estimated
value of $45 million in 1996. In New Mexico, most onions are furrow-irrigated, but some
farmers produce onions under drip or sprinkler irrigation. Water must be applied
frequently to avoid crop water stress and adequately recharge the plant root zone (AbdulJabbar et al., 1983). Deficit irrigation results in crop water stress and reduced crop yields
(Sammis, 1981; Abdul-Jabbar et al., 1982).
Variability in the water requirements for onions is a function of location and irrigation
method. Doorenbos and Kassam (1986) reported that the water requirements for optimum
yield (35,000±45,000 kg haÿ1) might vary from 35 to 55 cm of water using furrow
irrigation. Ells et al. (1993) reported that furrow-irrigated onions required 104 cm of
water to obtain a yield of 59,000 kg haÿ1.
Using a sprinkler system, the water requirement for onions was 91 cm, resulting in a
77,300 kg haÿ1 yield (Drost et al., 1996). Wu and Shimabuku (1996) reported a water
requirement of 50 cm for onions grown under a drip irrigation system to achieve a
43,176 kg haÿ1 yield. Feibert et al. (1996) reported that onions grown under a subsurface
drip system require 102 cm of water for a yield of 110,017 kg haÿ1. This yield was
achieved with a 224 kg haÿ1 nitrogen application.
The water production function (wpf) represents the relationship between crop yield and
seasonal water applied. The relationship between yield and seasonal evapotranspiration can
be characterized by the evapotranspiration production function (Etpf) (Jensen and Musick,
1960; Jensen and Sletten, 1965; Musick and Sletten, 1966; Hanks et al., 1969; Downey, 1972;
Hillel and Guron, 1973; Power et al., 1973; Stewart and Hagan, 1973; Morey et al., 1975;
Stegman and Olson, 1976; Stegman and Bauer, 1977). The relationship between crop yield
and evapotranspiration (Et) is often linear. The wpf is linear in the deficit irrigation range,
because all the applied water is used as Et and the wpf is equal to the Etpf (Stewart and Hagan,
1973; Hanks, 1974, 1983; Bauder et al., 1978; Hexem and Heady, 1978; Garrity et al., 1982;
Wright, 1982; Kallsen et al., 1984). However, non-linear Etpf relationships have been
reported (Turk et al., 1980; Garrity et al., 1982; Hanks, 1983; Evett et al., 1996). A non-linear
response indicates that not all water was used by the crop, because some went to deep
drainage and the Etpf function is really a wpf function. The wpf becomes curvilinear as more
of the applied water goes to deep drainage. Generally, a curvilinear wpf is expressed as a
second or third order polynomial (Hexem and Heady, 1978).
ETpf's can be useful to determine the capacity of irrigation systems and irrigation
amount and timing, as well as to compare relative water use efficiencies. Daily and
seasonal Et varies according to the climate and irrigation management. The Etpf is not
unique but varies among climate zones and between years, varieties, and crops (Miller
and Hang, 1982; Highstreet, 1987). Clumpner and Solomon (1987) studied 300 Etpf's to
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
31
test their reliability and transferability. They found significant year-to-year and site-tosite differences, as well as crop growth stage effects. Sammis (1981) demonstrated that
the Etpf of cotton (Gssypium hirsutum L.), and to a lesser extent that of alfalfa, varied
among locations. Sammis (1981) also found that the Etpf for alfalfa varied with each
cutting. Thus, determining Etpf for a site-specific location is usually required. Because
the wpf varies according to management skills of the irrigator and the type of irrigation
system, no unique wpf can be determined for a crop.
The objectives of this study were to determine the water production function (wpf)
under drip irrigation on a sandy loam soil and the evapotranspiration production function
(Etpf) for onions, which is independent of the irrigation system and soil type.
2. Materials and methods
Two irrigation experiments were conducted. The first sprinkler irrigation experiment was
conducted over 2 years (1986 and 1987) at the Agricultural Science Center at Farmington,
NM. The soil was a Wall sandy loam (coarse, loamy, mixed, calcareous, mesic, Typic
Camborthid). Onions were row planted in 1.83 m wide beds (eight rows/bed in 1986 and six
rows/bed in 1987) parallel to the sprinkler line-source. Coated onion seed was planted with a
cone-seeder at a rate of 2.8 kg of coated seed haÿ1 in 1986, and 5.6 kg of coated seed haÿ1 in
1987. In 1986, the variety was Golden Cascade F-1 Hybrid, while in 1987 we grew Germains
x-400. Dates of planting were 8 April 1986 and 15 April 1987. The emergence dates were 2
May 1986 and 6 May 1987. Plant populations were 284,170 plants haÿ1 in 1986 and 126,020
plants haÿ1. (35% of desired population because of weak germination) in 1987.
To ensure onion establishment, all plants were irrigated uniformly using a solid-set
sprinkler irrigation system at a rate of 0.254 cm per day from the planting date to 1 June
during each growing season. Subsequently, a single sprinkler line source was operated
(Hanks et al., 1976) at pressures of 310±345 kPa to provide a symmetrical, decreasing
gradient of water application levels from the sprinkler line to the edges of the plot
(15.24 m). Sprinkler heads (Model 30 TNT, Rainbird Co.) were placed 6 m apart in a line.
A different irrigation treatment was applied to each bed.
The plots were replicated twice on both sides of the sprinkler lines source, making a
total of four replications. During the 1987 growing season, seven irrigation treatments
were used on each side of the line source. Amounts of applied water (including rainfall)
ranged from a high of 51 to a low of 28.5 cm. The treatments were located at distances of
1.8, 3.7, 5.5, 7.3, 9.1, 11.0, and 12.8 m from the sprinkler line source. During the 1986
growing season, treatments located at distances of 1.8, 5.5, 9.1, and 12.8 m were
measured. Irrigation was scheduled weekly to maintain soil moisture in the plots adjacent
to the line source at a level near field capacity (approximately 15% by volume in the top
0.914 m). The available water holding capacity was 9 cm mÿ1 and the maximum root
depth was 45 cm. The weekly irrigation frequency was similar to what typical farmers
used to schedule irrigation for onions planted in sandy soils.
Catch-cans for measuring applied irrigation water were installed above the crop canopy in
the center of each bed (16 catch cans in the 1986 growing season and 28 catch cans in the 1987
growing season). In 1986, neutron probe access tubes were installed in each plot to a depth of
32
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
1.37 m (16 access tubes) to measure changes in soil water over time. In 1987, the neutron
probe access tubes (28 access tubes) were installed at a depth of 1.07 m in the low- and
medium -irrigation plots, and to a depth of 1.67 m in the irrigation plots that received the
highest irrigation. Neutron probe measurements were taken at 15 cm increments.
Daily weather data was measured at a site, about 400 m from the experiments. Evapotranspiration (Etr.) was estimated by using a modified Penman's equation referenced to
grass [EtrS/(Sg) Rng/(Sg) Ea, where S is equal to slope of the vapor pressure
versus temperature curve; g to psychometric constant; Rn net radiation in equivalent mm
per day was computed from solar radiation in equivalent of mm per day and Ea equal to
an empirically derived aerodynamic term in mm per day (Sammis et al., 1985)]. Et was
estimated from the water balance equation [EtIRDSmÿDr, where Et is equal to
evapotranspiration (cm); I is to amount of irrigation water applied (cm); DSm to change
in soil moisture content (cm); and Dr is equal to deep percolation water (cm)]. The
amount of irrigation water applied to the highest irrigation water treatment was limited to
the onion consumptive use demand. Consequently, percolation was assumed to be zero.
Weed and insect control was uniformly managed according to standard management
practices. The herbicides and fertilizers used, with rates and dates of applications, are
presented in Table 1. The fertilizers were broadcast applied to the crop. Onions were
harvested by hand from the six center rows of the four plots in 1986 and from the four
Table 1
Agronomic information for onion experiments at Farmington, (Experiment 1) and Las Cruces, (Experiment 2)
Experiment 1
Fertilizer name
Urea (46±0±0)
8±24±20 plus 1% Zn
Urea (46±0±0)
Ammonium nitrate
Urea (46±0±0)
Ammonium-phosphate-sulphate
Urea (46±0±0)
Urea (46±0±0)
Urea (46±0±0)
Pesticide
Goal
Poast
Brominal±goal poast
Goal
Brominal±goal
Application date
Application rate (kg haÿ1)
20 March 1986
21 March 1986
13 June 1986
1 July 1986
1 August 1986
9 April 1987
28 May 1987
23 June 1987
22 July 1987
89.5
35
56
107.5
56
53.5
96.5
45
28
19 May 1986
27 May 1986
15 June 1986
19 May 1987
9 June 1987
1.2a
2.4a
1.2a
1.2a
1.2a
Experiment 2
Year
Planting date
Harvesting date
1994
1995
1996
15 February
31 January
7 February
10 August
4 August
2 August
a
In units of l haÿ1.
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
33
center rows of the seven plots in 1987; for each year, the harvested plot was 30.5 m long.
Yield was determined from the USDA Standards for grades of Bermuda-Granex-Grano
type onions (USDA-Agricultural Marketing Service, 1962). Onion harvesting dates in
1986 were 22 and 23 September (east of sprinkler line) and 2 and 3 October (west of
sprinkler line). The 1987 harvest was on 16 October.
The second subsurface drip irrigation experiment was conducted for 3 years (1994,
1995 and 1996) at the Fabian Garcia Research Center in Las Cruces. Five different
irrigation applications of 40, 60, 80, 100, and 120% of the calculated non stress Et were
applied to onions. Non stress Et was calculated using a crop coefficient determined from
the first experiment (Al-Jamal et al., 1999) and climate data measured at the site to
determine reference Et. Onions were planted in four rows on beds 0.40 m wide and 18 m
long. The soil is classified as a Glendale loam (mixed, calcareous, thermic, Typic
Torrifluvent), but the top 60 cm at the research plot is a sandy loam soil.
Standard cultural practices for onions were used. A single line of 15 mil thick drip (T-tape)
tape with outlets every 0.2 m was installed at 0.08 m below the surface of each bed. BUSAN
1180 (methane sodium) was applied at the rate of 0.561 m3 haÿ1 to control onion soilborne
diseases in 1995 and 1996. Triple-super-phosphate (0±46±0) was broadcast at a rate of
280 kg haÿ1. Two onion varieties (Armada in 1994 and 1996; and Vega in 1995) were sown at
a rate of 3.5 kg haÿ1 for a final plant density of 400,000 plant haÿ1 (Table 1).
Prior to starting irritation treatments, irrigation was applied at intervals of 2±3 days for
the first week and every 4±7 days thereafter, until the plants reached the established stage.
Irrigation treatments started on 2 May 1994; 4 May 1995; and 24 April 1996. Subsequent
applications were applied every other day. The length of irrigation was controlled by the
computer based on the non stress computed Et. The amount of water applied was
measured using a water meter. Rainfall and other weather parameters were collected
using a Campbell Scientific CR-10 weather station.
Weed and insect control was managed uniformly according to standard practices. Urea
nitrogen fertilizer (3±20±0) was injected into the drip system during each irrigation at a
rate of 30 ppm (resulting in application of 344 kg haÿ1 at the high irrigation treatment
and 144 kg haÿ1 at the lower irrigation treatment). The last application of nitrogen
occurred on 11 July in 1994; 5 July in 1995; and 7 July in 1996. Onions were harvested
by hand in August (Table 1). Yield was determined after grading the onions using USDA
standards for Bermuda-Granex-Grano type onions (USDA-Agricultural Marketing
Service, 1962). Yields were estimated from the total weight of onions in a 3 m section
of row (18 m row) in the middle of each treatment.
3. Results and discussion
The Evapotranspiration production function (Etpf) in Farmington measured using a
sprinkler line source and the water balance equation and assuming drainage was zero was
linear as reported in the literature for most Etpf functions (Fig. 1):
Y ÿ37393:8 1358:302X
R2 0:96
where Y is the ungraded onion yield (kg haÿ1). X is the Et (cm).
(1)
34
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
Fig. 1. Evapotranspiration production function for onion crop at Farmington, NM.
Drainage was zero because the amount of irrigation water applied to the highest
irrigation water treatment was limited to the onion consumptive use demand. If drainage
had not been zero then the calculated Etpf would have not fit a liner function but would
have fit a curvilinear function.
The Etpf slope will vary from one location to another but was the same for both years
at Farmington. Location variation can be attributed to reference evapotranspiration's (Etr)
variability, which depends on many climate factors including solar radiation, wind, and
most important vapor pressure deficit. Growing season Etr at Farmington, NM averaged
118 cm and at Las Cruces, averaged 137 cm.
In order to transfer Etpf from one location to another a seasonal Kc can be determined
by dividing the seasonal Et by the seasonal Etr. A unique Kcseasonal value exists for each
yield level, and these values can be used between locations. The linear relationship (Fig. 2)
Fig. 2. The relationship between seasonal Kc and ungraded yield for onion grown at Farmington, NM.
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
35
Fig. 3. Water and evapotranspiration production functions for an onion crop based on water applied and
estimated Et (Las Cruces).
between Kcseasonal and ungraded onion yield, based on data from Farmington has a 0.94
coefficient of determination. Because deep drainage occurred in the second experiment
using drip irrigation at Las Cruces, the Etpf could not be determined by direct measurement. The Etpf for onions at Las Cruces, based on a Kcseasonal determined for selected
yield level and seasonal Etr is shown in Fig. 3 along with the applied water production
function derived from the drip irrigation experiment. In order to test the transferability of
a seasonal crop coefficient from one location to another, a sorghum seasonal crop
coefficient was computed for Artesia, NM (0.68) for a yield level of 4500 kg haÿ1 and
used to compute the seasonal Et at Cloves, NM. The computed seasonal Et was 870 mm
compared to the measured Et using a lysimeter of 877 mm for a yield of 5797 kg haÿ1
(Sammis et al., 1985). The seasonal crop coefficient for alfalfa, using the same study,
determined for Artesia was 0.875 for a yield level of 13450 kg haÿ1. The computed Et for
alfalfa at Las Cruces was 1657 mm compared to the measured value of 1687 for a yield of
12100 kg haÿ1. (Sammis et al., 1985). This analysis shows the high reliability of using
Kcseasonal values to compute Et at different locations for the same yield level.
The Etpf at Las Cruces is linear because it was derived from the Farmington Etpf and
can be given by:
Y ÿ35300 1224:2X
R2 0:97
(2)
ÿ1
where Y is the ungraded onion yield (kg ha ). X is the Et (cm).
The slope of the line decreased compared to the Etpf for Farmington because the
evaporative demand of the air is greater in Las Cruces resulting in a higher Etr and
calculated Et for each yield level.
The measured water production function (water applied versus yield) obtained at Las
Cruces, using a drip irrigation system is curvilinear in nature (Figs. 3 and 4) and is
computed using a second order polynomial:
R2 0:90
(3)
Y ÿ7809:28 693X ÿ 1:164X 2
ÿ1
where Y is the ungraded onion yield (kg ha ). X is the water applied including rainfall
(cm).
36
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
Fig. 4. Water production function for an onion crop, based on seasonal water applied (including rainfall) after
emergence.
The wpf varies from a water application of 80 cm and a yield of 40,221 kg haÿ1 to an
application of 224 cm and a yield of 91,170 kg haÿ1 (Fig. 4). This function represents the
ungraded onion yield. The harvestable graded onion yields in 1995 and 1996 were lower
than expected due to the percentage of Fusarium disease incidence (6±17%). The graded
compared to the ungraded wpfs calculated for each year and combined years (Table 2) are
statically (P
Computing the crop water production
function for onion
M.S. Al-Jamal, T.W. Sammis*, S. Ball, D. Smeal
Department of Agronomy and Horticulture, Box 3003,
Dept. 3Q, New Mexico State University, Las Cruces, NM 88003, USA
Accepted 6 December 1999
Abstract
Onions are a major irrigated crop in New Mexico. An excessive amount of water is generally
applied, because the crop is shallow-rooted and requires frequent irrigation to achieve good yields.
Onions under de®cit irrigation have a decrease in evapotranspiration and yield. Consequently,
farmers need to use the water production function (wpf) for onions to estimate water requirements
at different locations for selected yield goals. The wpf is the relationship between yield and water
applied. The same relation can be expressed in terms of evapotranspiration, in which case the
production function is known as the evapotranspiration production function (Etpf). A gradient
sprinkler line source onion experiment was conducted in 1986 and 1987 at Farmington New Mexico
and a linear Etpf determined. The linear Etpf was expressed as a relative Etpf and the yield response
factor (Ky), which represents the slope of relative Etpf, was calculated for onions at Farmington,
NM and found to be 1.52, compared to 1.5 obtained by [Doorenbos, J., Kassam, A.H., 1986. FAO
Irrig. Drain., Paper 33, Rome, Italy] for onions stressed at the yield formation period.
A second gradient drip line- source irrigation experiment was conducted at Las Cruces, NM,
during 1994±1996 to determine a wpf as related to applied water for drip irrigated onions.
The irrigation treatments were 40, 60, 80, 100, and 120% of calculated nonstressed
evapotranspiration determined from the sprinkler line source experiment. The wpf was curvilinear
because excess water was applied to the different irrigation levels in the experiment in order to keep
the base plate of the onions wet so root growth would continue. The result was that part of the
applied water went to deep drainage rather than to evapotranspiration. The wpf was corrected for
the amount of irrigation water lost as deep drainage and expressed as evapotranspiration versus
yield (Etpf) by using reference evapotranspiration measured at Las Cruces and season crop
coef®cients for selected yield levels measured at Farmington, NM. Maximum onion yield at Las
Cruces under the drip irrigation system was 20% higher than measured at Farmington using the
sprinkler system. The results indicate that high onion yield are achievable using a drip system
compared to a sprinkler system but a larger amount of applied water goes to deep drainage using a
*
Corresponding author. Tel.: 1-505-6463405; fax: 1-505-6466041.
E-mail address: [email protected] (T.W. Sammis).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 0 0 ) 0 0 0 7 6 - 7
30
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
drip system compared to a sprinkler system to achieve maximum yield. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Evapotranspiration; Water production functions
1. Introduction
Onions are one of the most important cash crops in New Mexico with an estimated
value of $45 million in 1996. In New Mexico, most onions are furrow-irrigated, but some
farmers produce onions under drip or sprinkler irrigation. Water must be applied
frequently to avoid crop water stress and adequately recharge the plant root zone (AbdulJabbar et al., 1983). Deficit irrigation results in crop water stress and reduced crop yields
(Sammis, 1981; Abdul-Jabbar et al., 1982).
Variability in the water requirements for onions is a function of location and irrigation
method. Doorenbos and Kassam (1986) reported that the water requirements for optimum
yield (35,000±45,000 kg haÿ1) might vary from 35 to 55 cm of water using furrow
irrigation. Ells et al. (1993) reported that furrow-irrigated onions required 104 cm of
water to obtain a yield of 59,000 kg haÿ1.
Using a sprinkler system, the water requirement for onions was 91 cm, resulting in a
77,300 kg haÿ1 yield (Drost et al., 1996). Wu and Shimabuku (1996) reported a water
requirement of 50 cm for onions grown under a drip irrigation system to achieve a
43,176 kg haÿ1 yield. Feibert et al. (1996) reported that onions grown under a subsurface
drip system require 102 cm of water for a yield of 110,017 kg haÿ1. This yield was
achieved with a 224 kg haÿ1 nitrogen application.
The water production function (wpf) represents the relationship between crop yield and
seasonal water applied. The relationship between yield and seasonal evapotranspiration can
be characterized by the evapotranspiration production function (Etpf) (Jensen and Musick,
1960; Jensen and Sletten, 1965; Musick and Sletten, 1966; Hanks et al., 1969; Downey, 1972;
Hillel and Guron, 1973; Power et al., 1973; Stewart and Hagan, 1973; Morey et al., 1975;
Stegman and Olson, 1976; Stegman and Bauer, 1977). The relationship between crop yield
and evapotranspiration (Et) is often linear. The wpf is linear in the deficit irrigation range,
because all the applied water is used as Et and the wpf is equal to the Etpf (Stewart and Hagan,
1973; Hanks, 1974, 1983; Bauder et al., 1978; Hexem and Heady, 1978; Garrity et al., 1982;
Wright, 1982; Kallsen et al., 1984). However, non-linear Etpf relationships have been
reported (Turk et al., 1980; Garrity et al., 1982; Hanks, 1983; Evett et al., 1996). A non-linear
response indicates that not all water was used by the crop, because some went to deep
drainage and the Etpf function is really a wpf function. The wpf becomes curvilinear as more
of the applied water goes to deep drainage. Generally, a curvilinear wpf is expressed as a
second or third order polynomial (Hexem and Heady, 1978).
ETpf's can be useful to determine the capacity of irrigation systems and irrigation
amount and timing, as well as to compare relative water use efficiencies. Daily and
seasonal Et varies according to the climate and irrigation management. The Etpf is not
unique but varies among climate zones and between years, varieties, and crops (Miller
and Hang, 1982; Highstreet, 1987). Clumpner and Solomon (1987) studied 300 Etpf's to
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
31
test their reliability and transferability. They found significant year-to-year and site-tosite differences, as well as crop growth stage effects. Sammis (1981) demonstrated that
the Etpf of cotton (Gssypium hirsutum L.), and to a lesser extent that of alfalfa, varied
among locations. Sammis (1981) also found that the Etpf for alfalfa varied with each
cutting. Thus, determining Etpf for a site-specific location is usually required. Because
the wpf varies according to management skills of the irrigator and the type of irrigation
system, no unique wpf can be determined for a crop.
The objectives of this study were to determine the water production function (wpf)
under drip irrigation on a sandy loam soil and the evapotranspiration production function
(Etpf) for onions, which is independent of the irrigation system and soil type.
2. Materials and methods
Two irrigation experiments were conducted. The first sprinkler irrigation experiment was
conducted over 2 years (1986 and 1987) at the Agricultural Science Center at Farmington,
NM. The soil was a Wall sandy loam (coarse, loamy, mixed, calcareous, mesic, Typic
Camborthid). Onions were row planted in 1.83 m wide beds (eight rows/bed in 1986 and six
rows/bed in 1987) parallel to the sprinkler line-source. Coated onion seed was planted with a
cone-seeder at a rate of 2.8 kg of coated seed haÿ1 in 1986, and 5.6 kg of coated seed haÿ1 in
1987. In 1986, the variety was Golden Cascade F-1 Hybrid, while in 1987 we grew Germains
x-400. Dates of planting were 8 April 1986 and 15 April 1987. The emergence dates were 2
May 1986 and 6 May 1987. Plant populations were 284,170 plants haÿ1 in 1986 and 126,020
plants haÿ1. (35% of desired population because of weak germination) in 1987.
To ensure onion establishment, all plants were irrigated uniformly using a solid-set
sprinkler irrigation system at a rate of 0.254 cm per day from the planting date to 1 June
during each growing season. Subsequently, a single sprinkler line source was operated
(Hanks et al., 1976) at pressures of 310±345 kPa to provide a symmetrical, decreasing
gradient of water application levels from the sprinkler line to the edges of the plot
(15.24 m). Sprinkler heads (Model 30 TNT, Rainbird Co.) were placed 6 m apart in a line.
A different irrigation treatment was applied to each bed.
The plots were replicated twice on both sides of the sprinkler lines source, making a
total of four replications. During the 1987 growing season, seven irrigation treatments
were used on each side of the line source. Amounts of applied water (including rainfall)
ranged from a high of 51 to a low of 28.5 cm. The treatments were located at distances of
1.8, 3.7, 5.5, 7.3, 9.1, 11.0, and 12.8 m from the sprinkler line source. During the 1986
growing season, treatments located at distances of 1.8, 5.5, 9.1, and 12.8 m were
measured. Irrigation was scheduled weekly to maintain soil moisture in the plots adjacent
to the line source at a level near field capacity (approximately 15% by volume in the top
0.914 m). The available water holding capacity was 9 cm mÿ1 and the maximum root
depth was 45 cm. The weekly irrigation frequency was similar to what typical farmers
used to schedule irrigation for onions planted in sandy soils.
Catch-cans for measuring applied irrigation water were installed above the crop canopy in
the center of each bed (16 catch cans in the 1986 growing season and 28 catch cans in the 1987
growing season). In 1986, neutron probe access tubes were installed in each plot to a depth of
32
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
1.37 m (16 access tubes) to measure changes in soil water over time. In 1987, the neutron
probe access tubes (28 access tubes) were installed at a depth of 1.07 m in the low- and
medium -irrigation plots, and to a depth of 1.67 m in the irrigation plots that received the
highest irrigation. Neutron probe measurements were taken at 15 cm increments.
Daily weather data was measured at a site, about 400 m from the experiments. Evapotranspiration (Etr.) was estimated by using a modified Penman's equation referenced to
grass [EtrS/(Sg) Rng/(Sg) Ea, where S is equal to slope of the vapor pressure
versus temperature curve; g to psychometric constant; Rn net radiation in equivalent mm
per day was computed from solar radiation in equivalent of mm per day and Ea equal to
an empirically derived aerodynamic term in mm per day (Sammis et al., 1985)]. Et was
estimated from the water balance equation [EtIRDSmÿDr, where Et is equal to
evapotranspiration (cm); I is to amount of irrigation water applied (cm); DSm to change
in soil moisture content (cm); and Dr is equal to deep percolation water (cm)]. The
amount of irrigation water applied to the highest irrigation water treatment was limited to
the onion consumptive use demand. Consequently, percolation was assumed to be zero.
Weed and insect control was uniformly managed according to standard management
practices. The herbicides and fertilizers used, with rates and dates of applications, are
presented in Table 1. The fertilizers were broadcast applied to the crop. Onions were
harvested by hand from the six center rows of the four plots in 1986 and from the four
Table 1
Agronomic information for onion experiments at Farmington, (Experiment 1) and Las Cruces, (Experiment 2)
Experiment 1
Fertilizer name
Urea (46±0±0)
8±24±20 plus 1% Zn
Urea (46±0±0)
Ammonium nitrate
Urea (46±0±0)
Ammonium-phosphate-sulphate
Urea (46±0±0)
Urea (46±0±0)
Urea (46±0±0)
Pesticide
Goal
Poast
Brominal±goal poast
Goal
Brominal±goal
Application date
Application rate (kg haÿ1)
20 March 1986
21 March 1986
13 June 1986
1 July 1986
1 August 1986
9 April 1987
28 May 1987
23 June 1987
22 July 1987
89.5
35
56
107.5
56
53.5
96.5
45
28
19 May 1986
27 May 1986
15 June 1986
19 May 1987
9 June 1987
1.2a
2.4a
1.2a
1.2a
1.2a
Experiment 2
Year
Planting date
Harvesting date
1994
1995
1996
15 February
31 January
7 February
10 August
4 August
2 August
a
In units of l haÿ1.
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
33
center rows of the seven plots in 1987; for each year, the harvested plot was 30.5 m long.
Yield was determined from the USDA Standards for grades of Bermuda-Granex-Grano
type onions (USDA-Agricultural Marketing Service, 1962). Onion harvesting dates in
1986 were 22 and 23 September (east of sprinkler line) and 2 and 3 October (west of
sprinkler line). The 1987 harvest was on 16 October.
The second subsurface drip irrigation experiment was conducted for 3 years (1994,
1995 and 1996) at the Fabian Garcia Research Center in Las Cruces. Five different
irrigation applications of 40, 60, 80, 100, and 120% of the calculated non stress Et were
applied to onions. Non stress Et was calculated using a crop coefficient determined from
the first experiment (Al-Jamal et al., 1999) and climate data measured at the site to
determine reference Et. Onions were planted in four rows on beds 0.40 m wide and 18 m
long. The soil is classified as a Glendale loam (mixed, calcareous, thermic, Typic
Torrifluvent), but the top 60 cm at the research plot is a sandy loam soil.
Standard cultural practices for onions were used. A single line of 15 mil thick drip (T-tape)
tape with outlets every 0.2 m was installed at 0.08 m below the surface of each bed. BUSAN
1180 (methane sodium) was applied at the rate of 0.561 m3 haÿ1 to control onion soilborne
diseases in 1995 and 1996. Triple-super-phosphate (0±46±0) was broadcast at a rate of
280 kg haÿ1. Two onion varieties (Armada in 1994 and 1996; and Vega in 1995) were sown at
a rate of 3.5 kg haÿ1 for a final plant density of 400,000 plant haÿ1 (Table 1).
Prior to starting irritation treatments, irrigation was applied at intervals of 2±3 days for
the first week and every 4±7 days thereafter, until the plants reached the established stage.
Irrigation treatments started on 2 May 1994; 4 May 1995; and 24 April 1996. Subsequent
applications were applied every other day. The length of irrigation was controlled by the
computer based on the non stress computed Et. The amount of water applied was
measured using a water meter. Rainfall and other weather parameters were collected
using a Campbell Scientific CR-10 weather station.
Weed and insect control was managed uniformly according to standard practices. Urea
nitrogen fertilizer (3±20±0) was injected into the drip system during each irrigation at a
rate of 30 ppm (resulting in application of 344 kg haÿ1 at the high irrigation treatment
and 144 kg haÿ1 at the lower irrigation treatment). The last application of nitrogen
occurred on 11 July in 1994; 5 July in 1995; and 7 July in 1996. Onions were harvested
by hand in August (Table 1). Yield was determined after grading the onions using USDA
standards for Bermuda-Granex-Grano type onions (USDA-Agricultural Marketing
Service, 1962). Yields were estimated from the total weight of onions in a 3 m section
of row (18 m row) in the middle of each treatment.
3. Results and discussion
The Evapotranspiration production function (Etpf) in Farmington measured using a
sprinkler line source and the water balance equation and assuming drainage was zero was
linear as reported in the literature for most Etpf functions (Fig. 1):
Y ÿ37393:8 1358:302X
R2 0:96
where Y is the ungraded onion yield (kg haÿ1). X is the Et (cm).
(1)
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M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
Fig. 1. Evapotranspiration production function for onion crop at Farmington, NM.
Drainage was zero because the amount of irrigation water applied to the highest
irrigation water treatment was limited to the onion consumptive use demand. If drainage
had not been zero then the calculated Etpf would have not fit a liner function but would
have fit a curvilinear function.
The Etpf slope will vary from one location to another but was the same for both years
at Farmington. Location variation can be attributed to reference evapotranspiration's (Etr)
variability, which depends on many climate factors including solar radiation, wind, and
most important vapor pressure deficit. Growing season Etr at Farmington, NM averaged
118 cm and at Las Cruces, averaged 137 cm.
In order to transfer Etpf from one location to another a seasonal Kc can be determined
by dividing the seasonal Et by the seasonal Etr. A unique Kcseasonal value exists for each
yield level, and these values can be used between locations. The linear relationship (Fig. 2)
Fig. 2. The relationship between seasonal Kc and ungraded yield for onion grown at Farmington, NM.
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
35
Fig. 3. Water and evapotranspiration production functions for an onion crop based on water applied and
estimated Et (Las Cruces).
between Kcseasonal and ungraded onion yield, based on data from Farmington has a 0.94
coefficient of determination. Because deep drainage occurred in the second experiment
using drip irrigation at Las Cruces, the Etpf could not be determined by direct measurement. The Etpf for onions at Las Cruces, based on a Kcseasonal determined for selected
yield level and seasonal Etr is shown in Fig. 3 along with the applied water production
function derived from the drip irrigation experiment. In order to test the transferability of
a seasonal crop coefficient from one location to another, a sorghum seasonal crop
coefficient was computed for Artesia, NM (0.68) for a yield level of 4500 kg haÿ1 and
used to compute the seasonal Et at Cloves, NM. The computed seasonal Et was 870 mm
compared to the measured Et using a lysimeter of 877 mm for a yield of 5797 kg haÿ1
(Sammis et al., 1985). The seasonal crop coefficient for alfalfa, using the same study,
determined for Artesia was 0.875 for a yield level of 13450 kg haÿ1. The computed Et for
alfalfa at Las Cruces was 1657 mm compared to the measured value of 1687 for a yield of
12100 kg haÿ1. (Sammis et al., 1985). This analysis shows the high reliability of using
Kcseasonal values to compute Et at different locations for the same yield level.
The Etpf at Las Cruces is linear because it was derived from the Farmington Etpf and
can be given by:
Y ÿ35300 1224:2X
R2 0:97
(2)
ÿ1
where Y is the ungraded onion yield (kg ha ). X is the Et (cm).
The slope of the line decreased compared to the Etpf for Farmington because the
evaporative demand of the air is greater in Las Cruces resulting in a higher Etr and
calculated Et for each yield level.
The measured water production function (water applied versus yield) obtained at Las
Cruces, using a drip irrigation system is curvilinear in nature (Figs. 3 and 4) and is
computed using a second order polynomial:
R2 0:90
(3)
Y ÿ7809:28 693X ÿ 1:164X 2
ÿ1
where Y is the ungraded onion yield (kg ha ). X is the water applied including rainfall
(cm).
36
M.S. Al-Jamal et al. / Agricultural Water Management 46 (2000) 29±41
Fig. 4. Water production function for an onion crop, based on seasonal water applied (including rainfall) after
emergence.
The wpf varies from a water application of 80 cm and a yield of 40,221 kg haÿ1 to an
application of 224 cm and a yield of 91,170 kg haÿ1 (Fig. 4). This function represents the
ungraded onion yield. The harvestable graded onion yields in 1995 and 1996 were lower
than expected due to the percentage of Fusarium disease incidence (6±17%). The graded
compared to the ungraded wpfs calculated for each year and combined years (Table 2) are
statically (P