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: tsammis@nmsu.edu (T.W. Sammis).
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 (Abdul-Jabbar 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 haÿ1yield (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 haÿ1yield. 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 haÿ1nitrogen 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).
test their reliability and transferability. They found significant year-to-year and site-to-site 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 haÿ1in 1986, and 5.6 kg of coated seed haÿ1in 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 haÿ1in 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 mÿ1and 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.
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. Eva-potranspiration (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;gto 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);Iis 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)
Application date Application rate (kg haÿ1)
Experiment 1
Fertilizer name
Urea (46±0±0) 20 March 1986 89.5
8±24±20 plus 1% Zn 21 March 1986 35
Urea (46±0±0) 13 June 1986 56
Brominal±goal poast 15 June 1986 1.2a
Goal 19 May 1987 1.2a
Brominal±goal 9 June 1987 1.2a
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 m3haÿ1to 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 haÿ1for 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 ÿ37393:81358:302X R20:96 (1)
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 Kcseasonalvalue exists for each yield level, and these values can be used between locations. The linear relationship (Fig. 2)
Fig. 1. Evapotranspiration production function for onion crop at Farmington, NM.
between Kcseasonaland 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 measure-ment. The Etpf for onions at Las Cruces, based on a Kcseasonaldetermined 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 haÿ1and 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 Kcseasonalvalues 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 ÿ353001224:2X R2 0:97 (2)
whereYis the ungraded onion yield (kg haÿ1).Xis 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:
Y ÿ7809:28693Xÿ1:164X2 R20:90 (3) whereYis the ungraded onion yield (kg haÿ1).Xis the water applied including rainfall (cm).
The wpf varies from a water application of 80 cm and a yield of 40,221 kg haÿ1to 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 ofFusariumdisease incidence (6±17%). The graded compared to the ungraded wpfs calculated for each year and combined years (Table 2) are statically (P<0.05) different based on a comparison of the quadratic regression analysis (Ames and Sammis, 1990), but the difference between the graded and undegreaded wpfs was small for the 1994 growing season, because of the low percentage of disease incidence (4±9%). Lack of crop rotation probably caused theFusariumproblem during the 1995 and 1996 growing seasons. The field site had been planted with onions for the previous 10 years.
The water applied included 20 cm of rainfall in 1994 and 7 cm of rainfall in 1995 and 1996 but did not include the water needed in the first 9±12 days for germination. At planting, a large amount of irrigation water was applied to get the water to sub up into the seed zone. Additional water was applied in the next 3 days to keep the surface of the bed wet and to prevent crusting of the soil surface.
was installed was poor because the beds were rotatilled when the soil was too wet. In 1994 the beds when rotatilled resulted in no clouds and good tilth, consequently only 7.6 cm of water were needed to be applied to keep the seed beds wet. The different amounts of water applied to establish germination indicate the importance of proper seedbed preparation.
The wpf (Figs. 3 and 4) was curvilinear, due to deep percolation water in excess of Et requirements. Yield continued to increase each year as more water was applied with a maximum of 91,170 kg haÿ1occurring in 1996. With a subsurface drip system, sufficient water must be applied to the shallow root system to wet the soil to the surface and wet the full width of the bed. Under sandy soil conditions, the onion yield increased with increasing water applications. However, deep drainage water also increased. Deep drainage loss did not occur using a sprinkler system, because water was applied in light amounts that only filled the root zone.
Comparing the calculated Etpf and the wpf at Las Cruces excess watering above 45 cm went to both Et and deep drainage. Water applied less than 45 cm was all used as Et. The seasonal Et, calculated from the seasonal Kc, contains an evaporation component that may vary between irrigation systems, times and locations. The wpf and Etpf should start from planting, but the presented wpf and Etpf for onions grown at Las Cruces begin at emergence (Figs. 3 and 4). Consequently, an additional 4 cm should be added to the Etpf to account for evaporation loss, during the period from planting to emergence and as discussed an additional 7±31 cm should be added to the drip irrigated wpf.
Comparing the ungraded onion yields obtained during the 1994, 1995 and 1996 growing seasons with the average Dona Ana County yield of 47,580 kg haÿ1(1993 and Table 2
Onion yields as in¯uenced by irrigation treatment during the 1994, 1995, and 1996 growing seasons
Treatment (%) Water applied (cm)a Graded yield (kg haÿ1) Ungraded yield (kg haÿ1)
1994) indicated that higher yields are possible, when the Et is increased from 57 to 101 cm. This data varied from year to year, depending on weather conditions. However, it is still clear that a maximum yield for onions using a buried drip system on sandy loam soil can be achieved by applying 224 cm of water.
Another useful way to express the Etpf, measured at Farmington, NM, is on a relative basis, where yield is divided by maximum yield and Et is divided by maximum Et (Fig. 5). The relationship between relative evapotranspiration (1ÿEt/Etmax) and relative yield (1ÿy/ymax) is always linear if the Etpf is linear. The slope of the relative Etpf is called the yield response factor (Ky), or:
Ky 1ÿ y=ymax 1ÿ Et=Etmax
(4)
where Ky is the yield response factor.Yis the yield level at a particular Et level (kg haÿ1).
ymaxis the maximum yield level (kg haÿ1). Et is the evapotranspiration associated with a particular yield level (mm). Etmax is the maximum evapotranspiration associated with maximum yield obtained under non-moisture stress conditions (mm).
1987 growing seasons. This interpretation was reached by plotting the measured monthly Et for each treatment versus time in days throughout the 1986 and 1987 growing seasons. The slope (Ky) of Etpf for onions obtained at Farmington, (Fig. 1) is 1.52, similar to that (1.5) computed by Doorenbos and Kassam (1986) for onions stressed only at the yield formation period. The Ky value of 1.5 obtained by Doorenbos and Kassam (1986) represents water deficit's effect on onion crop yield for the total growing period. Generally, higher Ky values indicate that the crop will have a greater yield loss when the crop water requirements are not met.
The yield in the Etpf and wpf functions are expressed as ungraded yields because evapotranspiration is related to photosynthesis which in intern is related to yield. Discarding part of the yield due to quality criteria decreases the transferability of the Etpf function from year to year for a given site and the transferability of the calculated Kcseasonal used to calculate Etpf for other sites. The grading index (the ratio of graded yield to ungraded yield) can be used to estimate graded yield, when the onion crop is not infected with diseases such asFusarium. Irrigation management using different irrigation systems not only affects the amount of water that goes to deep drainage, but can affect bulb size and the grading index. However, the grading index at Farmington for sprinkler irrigated onions versus relative Et (Et/Etmax) (Fig. 6) shows that there is no general trend between the relative Et and the amount of marketable onions. The average grading index was 0.79 and 0.83 for the 1986 and 1987 growing seasons, respectively.
4. Conclusions
The results of this study suggest that the use of a subsurface drip irrigation system increase onion yields. Yields increased with increased depths of irrigation water applied. Seasonal Et of onions in the Mesilla Valley of Las Cruces, is about 101 cm, resulting in the highest yield. Even though yield increased, deep drainage also increased and irrigation efficiency decreased. In order to achieve maximum onion yield on a sandy loam soil with a drip irrigation system twice as much water must be applied than is evapotranspired by the crop. Onions yields under sprinkler irrigation were less but irrigation efficiencies were considerable higher than under drip irrigation. Increased
yields under drip compared to the sprinkler system appears to be due to the irrigation system type and operation, but differences in onion varieties and climate conditions at the two sites may have contributed to the yield differences.
Acknowledgements
This research was supported by the New Mexico State University's Agricultural Experiment Station.
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