Crop Coefficients Specific to Multiple Phenological Stages for Evapotranspiration-based Irrigation Management of Onion and Spinach

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  • 1 Texas A&M University, Texas AgriLife Research, 1619 Garner Field Road, Uvalde, TX 78801
  • 2 USDA-ARS Agricultural Systems Research Unit, 2150 Centre Avenue, Building D, Suite 200, Fort Collins, CO 80526
  • 3 Texas A&M University, Texas AgriLife Research and Extension Center, 6500 Amarillo Boulevard West, Amarillo, TX 79106
  • 4 Texas A&M University, Texas AgriLife Research, Vegetable and Fruit Improvement Center, 1619 Garner Field Road, Uvalde, TX 78801

Weighing lysimeters are used to measure crop water use during the growing season. By relating the water use of a specific crop to a well-watered reference crop such as grass, crop coefficients (KC) can be developed to assist in predicting crop needs using meteorological data available from weather stations. This research was conducted to determine growth stage-specific KC and crop water use for onions (Allium cepa L.) and spinach (Spinacia oleracea L.) grown under south Texas conditions. Seven lysimeters, consisting of undisturbed 1.5 × 2.0 × 2.2-m deep soil monoliths, comprise the Texas AgriLife Research–Uvalde lysimeter facility. Six lysimeters, weighing ≈14 Mg, have been placed each in the middle of a 1-ha field beneath a linear low-energy precision application irrigation system. A seventh lysimeter was established to measure reference grass reference evapotranspiration. Daily water use for onion and spinach was measured at 5-min intervals. Crop water requirements, KC determination, and comparison with existing Food and Agricultural Organization (FAO) KC values were determined over a 2-year period for each crop. The KC values determined over the growing seasons varied from 0.2 to 1.3 for onion and 0.2 to 1.5 for spinach with some of the values in agreement with those from FAO. It is assumed that the application of growth stage-specific KC will assist in irrigation management and provide precise water applications for a region of interest.

Abstract

Weighing lysimeters are used to measure crop water use during the growing season. By relating the water use of a specific crop to a well-watered reference crop such as grass, crop coefficients (KC) can be developed to assist in predicting crop needs using meteorological data available from weather stations. This research was conducted to determine growth stage-specific KC and crop water use for onions (Allium cepa L.) and spinach (Spinacia oleracea L.) grown under south Texas conditions. Seven lysimeters, consisting of undisturbed 1.5 × 2.0 × 2.2-m deep soil monoliths, comprise the Texas AgriLife Research–Uvalde lysimeter facility. Six lysimeters, weighing ≈14 Mg, have been placed each in the middle of a 1-ha field beneath a linear low-energy precision application irrigation system. A seventh lysimeter was established to measure reference grass reference evapotranspiration. Daily water use for onion and spinach was measured at 5-min intervals. Crop water requirements, KC determination, and comparison with existing Food and Agricultural Organization (FAO) KC values were determined over a 2-year period for each crop. The KC values determined over the growing seasons varied from 0.2 to 1.3 for onion and 0.2 to 1.5 for spinach with some of the values in agreement with those from FAO. It is assumed that the application of growth stage-specific KC will assist in irrigation management and provide precise water applications for a region of interest.

Agricultural water users must plan an annual water budget in arid to subhumid areas where water use is regulated as a result of ecological protection programs, limited resources, and competitive demand (Barrett, 1999). Determining crop water requirements specific to each crop is key in providing growers with information to 1) select which crops to grow; and 2) determine the timing and quantity of irrigation events.

The Wintergarden region of Texas, which is located on the South Texas Plains, receives ≈660 mm/year of precipitation and has a growing season of ≈214 to 275 d. In 2000, growers in this region irrigated 40,000 ha (Texas Water Development Board, 2001). From preliminary studies carried on at the Texas AgriLife Research Center, it is estimated that ≈62 million to 74 million m3 of groundwater could be conserved each year by implementing proper irrigation techniques and scheduling. To optimize irrigation events, crop water requirements throughout the growing season must first be determined.

The use of on-site microclimatological data and crop coefficients enable growers the determination of crop water use in a reliable, usable, and affordable format. The concept of “crop coefficient” (KC) was introduced by Jensen (1968) and further developed by the other researchers (Allen et al., 1998; Burman et al., 1980a, 1980b; Doorenbos and Pruitt, 1977). KC is the ratio of the evapotranspiration of the crop (ETC) to a reference crop (ETO) (Allen et al., 1998). ETO may be measured directly from a reference crop such as a perennial grass (Watson and Burnett, 1995) or computed from weather data using temperature models (Doorenbos and Pruitt, 1977; Thornthwate, 1948), radiation models (Doorenbos and Pruitt, 1977; Hargreaves and Samani, 1985), or combination models (Allen et al., 1998). Weighing lysimeters are used to measure ETO and ETC directly by detecting simultaneous changes in the weight of the soil/crop unit (Marek et al., 2006; Schneider et al., 1998). Weather data are used to compute ETO by equations such as the ASCE Penman-Monteith (ASCE-EWRI, 2005). Once KC values are determined, all that is needed to provide growers with real-time irrigation recommendations (ETC) are local weather stations to determine ETO and therefore solve the following simple equation:

DE1

According to Allen et al. (1998), crop type, cultivar, and developmental stage affect ETC. The objective of this multiyear research is to determine crop water use (ETC) and develop KC specific to multiple phenological stages for important vegetable crops such as onion and spinach.

Materials and Methods

Lysimeter facility.

The lysimeter facility at the Texas AgriLife Research Center in Uvalde, TX (long. 29°13′ N, lat. 99°45′ W; elevation 283 m), includes seven weighing lysimeters constructed between 2001 and 2006. Construction details and resolution are described by Marek et al. (2006). Each lysimeter is 1.5 × 2.0 m (length × width) and 2.2 m deep. The surface area (3 m2) of the lysimeters accommodates common row spacings used in the region. The soil monoliths in the lysimeters contain a silty clay soil (fine-silty, mixed, hyperthermic Aridic Calciustolls with a pH of 8.1).

Microclimatological data were collected by a standard Campbell Scientific, Inc. (Logan, UT) weather station every 6 s with 15-min outputs. These include solar radiation, wind speed, air temperature, dew point temperature, relative humidity, precipitation, and barometric pressure (Dusek et al., 1987; Howell et al., 1995). The weight of each lysimeter was sampled every 1 s with 5-min outputs. Weight changes in the lysimeters were measured in mV·V−1 output of the load cell attached to a scale (Avery Weigh Tronix: model HSDS 6060, Fairmont, MN) beneath each lysimeter. The calibration of mV·V−1 output to weight changes represented asmillimeters of water is described in Marek et al. (2006). The load cell signal was composited to 30-min means and the lysimeter mass resolution was 0.01 mm. Daily ETO measured with the lysimeters (Lys ETO) was determined as the difference between the lysimeter mass losses (evaporation and transpiration) and lysimeter mass gains (irrigation, precipitation, or dew) divided by the lysimeter area (9 m2). A pump (–10 kPa) provided vacuum drainage and the drainage effluent was weighed by load cells (drainage rate data are not reported here). ET for each 24-h period was divided by 1.02 to adjust the lysimeter area to the midpoint between the two walls (10-mm air gap; 9.5-mm wall thickness; 9.18 m2 area instead of the 9.00-mm lysimeter surface area) according to Howell et al. (2004).

Lysimeter field data.

A tall fescue grass (Festuca arundinacea Schreb.) seed (cv. Emerald III; Sharp Bros. Seed Co., Healy, KS) was hydromulched in late Fall 2001 on the weather station plot after completing installation of a lysimeter located in the center of ≈1.0 ha, a field that had a subsurface drip irrigation system. The grass height was ≈0.1 m after mowing and varied from 0.12 to 0.15 m before mowing.

Two vegetable crops, onion cv. Texas Legend and spinach cv. DMC 16, were grown during 2002 to 2005 in the crop lysimeter fields, each located in the center of ≈1.0 ha, which were used in the determination of KC (Table 1). All field operations were performed using standard 1.0-m wide row-crop field equipment, except inside each lysimeter where hand-cultural methods were applied. Fertility and pest control practices were uniformly applied following standard production practices in the Wintergarden (http://aggie-horticulture.tamu.edu/extension/vegetable/cropguides). Every year fields were furrow-diked (dike spacing at ≈1.5 m) to minimize field runoff and rainfall and irrigation redistribution. Irrigation was provided with a three-span lateral move sprinkler system (Lindsay Manufacturing Co., Lindsay, NE). The system was equipped with gooseneck fittings and spray heads (Senninger Super Spray 360E, Clermont, FL) with medium grooved spray plates on drops located ≈1.5 m above the ground and 1.0 m apart. The drops could be converted to low-energy precision application (LEPA) heads placed ≈0.3 m above the ground. Fields were managed under full irrigation, which was scheduled based on measured daily crop water use (ET).

Table 1.

Onion and spinach crop and seasonal conditions at the Texas AgriLife Research, Uvalde, TX.

Table 1.

Crop coefficient (KC) was calculated using the following equation:

DE2
where ETO was determined either from direct measurement using the lysimeter (Lys ETO) or from calculations using the ASCE Penman-Monteith equation (ASCE-EWRI, 2005) for grass (ASCE ETO) and/or alfalfa (ASCE ETr). KC curves for the three methods (Lys ETO, grass ETO, and alfalfa ETr) were fitted to third-order polynomials. Other studies demonstrate that KC curves can be fitted to third- and up to fifth-order polynomials (Ayars and Hutmacher, 1994; Sammis and Wu, 1985; Stegman, 1988; Wright, 1982). Data were analyzed by paired t test using PROC TTEST, analysis of correlation using PROC CORR as well as PROC REG (SAS version 9.1, Cary, NC). These methods were used to determine statistical differences of the measured lysimeter ETO data from the ASCE Penman-Monteith calculated data.

Results

Onion evapotranspiration and crop co-efficient.

Daily onion ETC over the growing seasons 2002–2003 and 2004–2005 ranged between 1 and 7 mm and peaked at ≈160 d after planting (DAP) in 2002–2003 and ≈120 DAP in 2004–2005 (Fig. 1A). Daily ETC rates were variable depending on seasonal conditions (i.e., planting date, temperatures). Accumulated onion ETC was 362 mm in 2002–2003 and 438 mm in 2004–2005, respectively. These values are smaller than those obtained in Hawaii (Wu and Shimabuku, 1996), New Mexico (Al-Jamal et al., 2000; Sammis et al., 1985), and Utah (Drost et al., 1996). However, our values are within the range of the water requirements (350 to 550 mm) for optimum onion yield (35,000 to 45,000 kg·ha−1) described by Doorenbos and Kassam (1986). The reference ETO during the corresponding crop seasons ranged between 1 and 7 mm for both lysimeter measured ETO (Lys ETO) and calculated ETO using ASCE Penman-Monteith equation for grass (ASCE ETO) (Fig. 1B). The ASCE ETO matched well with Lys ETO with a Pearson's correlation coefficient (r) of 0.81 and root mean square error (RMSE) of 1.13 mm (Fig. 2).

Fig. 1.
Fig. 1.

(A) Onion crop evapotranspiration (ETC); (B) reference evapotranspiration (ETO); and (C) crop coefficient as a function of days after planting (DAP) in 2002–2003 and 2004–2005. A third polynomial equation for each KC is as follows: Lys KC = 0.45 + 1.44·10−3·DAP + 5.09·10−5·DAP2 – 2.76·10−7·DAP3 ASCE Kco = 0.45 + 2.95·10−3·DAP + 3.85·10−5·DAP2 – 2.47·10−7·DAP3 ASCE Kcr = 0.40 + 2.36·10−3·DAP + 3.94·10−5·DAP2 – 2.68·10−7·DAP3.

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.421

Fig. 2.
Fig. 2.

Lysimeter measured evapotranspiration (Lys ETO) versus calculated ETO using the ASCE Penman-Monteith equation for grass (ASCE ETO) during the onion crop seasons in 2002–03 and 2004–05. The dashed line was generated by the fitted linear regression equation.

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.421

Throughout crop development, daily onion KC varied from 0.2 to 1.3 for all lysimeter-based KC (Lys KC), ASCE grass-based KC (ASCE Kco), and KC based on ASCE Penman-Monteith equation for alfalfa (ASCE Kcr) (Fig. 1C). Growth stage-specific KC for onion determined in this study was 0.40 at emergence, 0.90 at bulb development, and 0.70 at dry leaf stage (Table 2). These values were determined based on the third polynomial KC curve (see Fig. 1C). The values are smaller at initial and midgrowth stages than those from the Food and Agricultural Organization (FAO) (Allen et al., 1998).

Table 2.

Onion crop coefficients (KC) determined using the data of two seasons (2002–2003 and 2004–2005) at Uvalde, TX (A) in comparison with those from FAO (Allen et al., 1998) (B).

Table 2.

Spinach evapotranspiration and crop co-efficient.

Daily changes of spinach ETC during 2002–2003 and 2003–2004 seasons were between 0.5 and 5.0 mm (Fig. 3A). Daily ETC rates varied from 0.5 to 3.0 mm during fall (0 to 35 DAP) and seldom exceeded 2.0 mm between 35 and 50 DAP. Accumulated spinach ETC was 160 mm in 2002–2003 and 156 mm in 2003–2004, respectively. To our best knowledge, there is no comparable report on spinach ETC or crop water use. However, our values are less than the vegetable water requirements (250 to 500 mm) described by Doorenbos and Kassam (1986). ETO during the corresponding crop seasons was between 0.5 and 5.0 mm for both Lys ETO and ASCE ETO (Fig. 3B). ASCE ETO matched well with Lys ETO with an r value of 0.76 and RMSE of 0.64 mm (Fig. 4).

Fig. 3.
Fig. 3.

(A) Spinach crop evapotranspiration (ETC); (B) reference evapotranspiration (ETO); and (C) crop coefficient as a function of days after planting (DAP) in 2002–2003 and 2003–2004. A third polynomial equation for each KC is as follows: Lys KC = 0.31 + 0.01·DAP + 6.07·10−5·DAP2 – 9.34·10−7·DAP3 ASCE Kco = 0.45 + 6.12·10−3·DAP + 9.37·10−5·DAP2 – 8.70·10−7·DAP3 ASCE Kcr = 0.18 + 0.01·DAP – 9.40·10−5·DAP2 + 2.55·10−7·DAP3.

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.421

Fig. 4.
Fig. 4.

Lysimeter measured evapotranspiration (Lys ETO) versus calculated ETO using the ASCE Penman-Monteith equation for grass (ASCE ETO) during the spinach crop seasons in 2002–03 and 2003–04. The dashed line was generated by the fitted linear regression equation.

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.421

Spinach KC ranged between 0.2 and 1.5 for all of Lys KC, ASCE Kco, and ASCE Kcr (Fig. 3C). Growth stage-specific KC estimates for spinach based on the third polynomial KC curve were 0.35 at emergence, 1.0 at 13 to 15 leaves, and 1.05 at 19-harvest stages (Table 3). The values are smaller at initial and greater at end growth stages than those from FAO (Allen et al., 1998).

Table 3.

Spinach crop coefficients (KC) determined using the data of two seasons (2002–2003 and 2003–2004) at Uvalde, TX (A) in comparison with those from FAO (Allen et al., 1998) (B).

Table 3.

Discussion

The aim of this research was the determination of KC for two major winter vegetable crops and to determine their plant water use or crop ETC. Once KC are determined, irrigation scheduling can then be improved for applications by private consultants and growers to avoid water overuse and to more precisely meet the crop water demand to produce greater yields, crop quality, and enhanced water productivity. From these studies, accumulated ETC estimates for each crop during the growing season were 400 mm for onion and 158 mm for spinach. The seasonal KC values varied from 0.2 to 1.3 for onion and 0.2 to 1.5 for spinach. On the other hand, the present stage-specific KC values were determined based on KC curves that represent the distribution of KC over time throughout the season. The results in this study showed that some of the KC values were in accordance with FAO, whereas those during crop establishment and early growth were smaller than FAO (Allen et al., 1998).

Research has repeatedly shown that proper irrigation management is key to achieving profitable crop yields. Potential evapotranspiration (PET) network is a group of meteorological stations to acquire weather data to compute PET (Howell, 1998). The PET networks (Brock et al., 1995; Howell, 1998) and crop simulation models (Guerra et al., 2005, 2007; Santos et al., 2000) have proven to be reliable, inexpensive, and effective tools for estimating crop water needs in research settings. Networks of weather stations have been established in many diverse growing regions for the purpose of supporting predictions of crop ET. To support predictions of crop evapotranspiration, generic crop coefficients will not fulfill the need for precise irrigation applications.

The need for regionalized KC is demonstrated by the comparison between the KC described in FAO 56 (Allen et al., 1998) and those obtained at Uvalde, TX. For example, the onion and spinach KC values were smaller at initial growth stages than the KC values from FAO. In addition, the onion KC values are smaller at the midgrowth stage and greater at dry leaf stage than those reported by Al-Jamal et al. (2000) at Farmington, NM. This site has annual average temperature of 11.2 °C, which is cooler than that (20.5 °C) at Uvalde, TX. It is assumed that the differences of KC between the two regions are the result of elevated air temperatures and water vapor pressure deficit over the growing season that caused temporal and transient leaf stomata closure (Baker et al., 2007; Bunce, 1997; Cornic and Massacci, 1996), impeding plants to transpire at its full potential. In addition, different environmental conditions between regions allow variation in variety selection and crop developmental stage, which affect KC (Allen et al., 1998). In the Wintergarden region, the use of KC generalized (Allen et al., 1998) or developed in other regions (e.g., Al-Jamal et al., 2000) will result in overwatering and consequently increased production costs and reduced profits. At the same site, Leskovar and Piccinni (2005) demonstrated that ETC-based irrigation management for spinach can be efficiently applied. They concluded that it is possible to reach a maximum of 25% water-saving in one season using the irrigation scheduling based on crop evapotranspiration.

In conclusion, the development of regionally based KC is critical to assist growers optimizing irrigation management and to further provide precise water applications in those areas where high irrigation efficiencies are achieved by center pivot with LEPA systems or subsurface drip irrigation systems.

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Contributor Notes

This study is a partial outcome of the Precision Irrigators Network (PIN) project funded by Texas Water Development Board (TWDB: Project No. 0603580596), Rio Grande Basin Initiative (RGBIRGBI: Grant No. 2005-34461-15661), and partial funding from USDA CSREES No. 2004-34402-14768 (“Designing Foods for Health”).

We thank the Texas Water Resources Institute (TWRI) for administrative project assistance. Current address: Monsanto Company, 700 Chesterfield Pkwy West, Chesterfield, MO 63017.

Research Agronomist.

To whom reprint requests should be addressed; e-mail Jonghan.Ko@ars.usda.gov.

  • View in gallery

    (A) Onion crop evapotranspiration (ETC); (B) reference evapotranspiration (ETO); and (C) crop coefficient as a function of days after planting (DAP) in 2002–2003 and 2004–2005. A third polynomial equation for each KC is as follows: Lys KC = 0.45 + 1.44·10−3·DAP + 5.09·10−5·DAP2 – 2.76·10−7·DAP3 ASCE Kco = 0.45 + 2.95·10−3·DAP + 3.85·10−5·DAP2 – 2.47·10−7·DAP3 ASCE Kcr = 0.40 + 2.36·10−3·DAP + 3.94·10−5·DAP2 – 2.68·10−7·DAP3.

  • View in gallery

    Lysimeter measured evapotranspiration (Lys ETO) versus calculated ETO using the ASCE Penman-Monteith equation for grass (ASCE ETO) during the onion crop seasons in 2002–03 and 2004–05. The dashed line was generated by the fitted linear regression equation.

  • View in gallery

    (A) Spinach crop evapotranspiration (ETC); (B) reference evapotranspiration (ETO); and (C) crop coefficient as a function of days after planting (DAP) in 2002–2003 and 2003–2004. A third polynomial equation for each KC is as follows: Lys KC = 0.31 + 0.01·DAP + 6.07·10−5·DAP2 – 9.34·10−7·DAP3 ASCE Kco = 0.45 + 6.12·10−3·DAP + 9.37·10−5·DAP2 – 8.70·10−7·DAP3 ASCE Kcr = 0.18 + 0.01·DAP – 9.40·10−5·DAP2 + 2.55·10−7·DAP3.

  • View in gallery

    Lysimeter measured evapotranspiration (Lys ETO) versus calculated ETO using the ASCE Penman-Monteith equation for grass (ASCE ETO) during the spinach crop seasons in 2002–03 and 2003–04. The dashed line was generated by the fitted linear regression equation.

  • Al-Jamal, M.S., Sammis, T.W., Ball, S. & Smeal, D. 2000 Computing the crop water production for onion Agr. Water Mgt. 46 29 41

  • Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. 1998 Crop evapotranspiration: Guidelines for computing crop water requirements Proc. of the Irrigation and Drainage Paper No. 56. Food and Agricultural Organization, United Nations Rome, Italy

    • Search Google Scholar
    • Export Citation
  • ASCE-EWRI 2005 The ASCE Standardized Reference Evapotranspiration Equation ASCE-EWRI Standardization of Reference Evapotranspiration Task Comm. Report <http://www.kimberly.uidaho.edu/water/asceewri/>.

    • Export Citation
  • Ayars, J.E. & Hutmacher, R.B. 1994 Crop coefficients for irrigating cotton in the presence of groundwater Irrig. Sci. 15 45 52

  • Baker, J.T., Gitz, D.C., Payton, P., Wanjura, D.F. & Upchurch, D.R. 2007 Using leaf gas exchange to quantify drought in cotton irrigated based on canopy temperature measurements Agron. J. 99 637 644

    • Search Google Scholar
    • Export Citation
  • Barrett, M.E. 1999 Complying with the Edwards aquifer rules: Technical guidance on best management practices/prepared for Field Operations Division, Austin, Texas Texas Natural Resource Conservation Commission 1 RG-348

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