Comparison of sprinkler, trickle and furrow
irrigation ef®ciencies for onion production
M.S. Al-Jamal, S. Ball, T.W. Sammis
*Department of Agronomy and Horticulture, Box 3003, Dept. 3Q, New Mexico State University, Las Cruces, NM 88003, USA
Accepted 7 February 2000
Abstract
In the Mesilla Valley of southern New Mexico, furrow irrigation is the primary source of water for growing onions. As the demand for water increases, there will be increasing competition for this limited resource. Water management will become an essential practice used by farmers. Irrigation ef®ciency (IE) is an important factor into improving water management but so is economic return. Therefore, our objectives were to determine the irrigation ef®ciency, irrigation water use ef®ciency (IWUE) and water use ef®ciency (WUE), under sprinkler, furrow, and drip irrigated onions for different yield potential levels and to determine the IE associated with the amount of water application for a sprinkler and drip irrigation systems that had the highest economic return.
Maximum IE (100%) and economic return were obtained with a sprinkler system at New Mexico State University's Agriculture Science Center at Farmington, NM. This IE compared with the 54± 80% obtained with the sprinkler irrigation used by the farmers. The IEs obtained for onion ®elds irrigated with subsurface drip irrigation methods ranged from 45 to 77%. The 45% represents the nonstressed treatments, in which an extra amount of irrigation above the evapotranspiration (Et) requirement was applied to keep the base of the onion plates wet. The irrigation water that was not used for Et went to deep drainage water. The return on the investment cost to install a drip system operated at a IE of 45 was 29%. Operating the drip system at a IE of 79% resulted in a yield similar to surface irrigated onions and consequently, it was not economical to install a drip system. The IEs at the furrow-irrigated onion ®elds ranged from 79 to 82%. However, the IEs at the furrow-irrigated onion ®elds were high because farmers have limited water resources. Consequently, they used the concept of de®cit irrigation to irrigate their onion crops, resulting in lower yields. The maximum IWUE (0.084 t haÿ1mmÿ1 of water applied) was obtained using the sprinkler system, in which
water applied to the ®eld was limited to the amount needed to replace the onions' Et requirements. The maximum IWUE values for onions using the subsurface drip was 0.059 and 0.046 t haÿ1mmÿ1
of water applied for furrow-irrigated onions. The lower IWUE values obtained under subsurface
*Corresponding author. Tel.:1-505-6463405; fax:1-505-6466041.
E-mail address: tsammis@nmsu.edu (T.W. Sammis).
drip and furrow irrigation systems compared with sprinkler irrigation was due to excessive irrigation under subsurface drip and higher evaporation rates from ®elds using furrow irrigation. The maximum WUE for onions was 0.009 t haÿ1mmÿ1of Et. In addition, WUE values are reduced by allowing the onions to suffer from water stress. # 2001 Elsevier Science B.V. All rights reserved.
Keywords:Evapotranspiration; Irrigation ef®ciency; Irrigation water use ef®ciency; Water use ef®ciency
1. Introduction
Irrigation engineers when designing an irrigation system try and maximize irrigation efficiency (IE), defined by the American Society of Civil Engineers (ASCE) on-farm irrigation committee (ASCE, 1978), as the ratio of the volume of water that is taken up by the crop to the volume of irrigation water applied. Drip irrigation has the potential to increase IE, because the farmer can apply light and frequent amounts of water to meet crops Et needs. The IEs ranged from 80 to 91% when the crop was grown in fields using a surface drip system (Battikhi and Abu-hammad, 1994; Chimonides, 1995). IEs ranged from 54 to 80% (Chimonides, 1995; Zalidis et al., 1997) with a sprinkler irrigation system and IEs under furrow irrigation were between 50 and 73% (Oster et al., 1986; Battikhi and Abu-hammad, 1994; Chimonides, 1995; Zalidis et al., 1997).
Government policy makers are usually interested in achieving the greatest yield for a unit of water applied, and consequently, they are more interested in irrigation water use efficiency, IWUE (t haÿ1
mmÿ1
), defined as the ratio of the crop yield (t haÿ1
) to seasonal irrigation water (mm) applied, including rain (Howell, 1994). The IWUE values are affected by: reducing the irrigation water lost to drainage, canopy interception, soil type, cultural and management practices, and variety choice. Both IE and IWUE can be increased by practising deficit irrigation in parts of the field receiving the minimum water application depth. The most economical deficit irrigation level depends on the uniformity of application of the irrigation water and the associated cost of the irrigation water, any cost of remediation treatment on the drainage water, and the value of a unit of the crop (Wu, 1988, 1995) Sammis and Wu (1986) reported that IWUE increased under soil moisture stress for tomatoes. However, the percentage of marketable tomatoes decreased. Previous research shows a higher IWUE for subsurface drip (from 0.0283 to 0.227 t haÿ1mmÿ1), surface drip (from 0.0235 to 0.127 t haÿ1mmÿ1) and sprinkler systems (from 0.0044 to 0.0659 t haÿ1mmÿ1) compared with furrow irrigation (from 0.0086 to 0.056 t haÿ1
mmÿ1
) (Sammis, 1980; Bogle et al., 1989; Lamm et al., 1995). When growing onions Ellis et al., 1986 has also shown higher IWUE values using surface drip (0.052 t haÿ1
mmÿ1
), and sprinkler irrigation (0.049 t haÿ1
mmÿ1
) compared with furrow irrigation (0.044 t haÿ1
mmÿ1
). In only one study was a higher IWUE value (0.104 t haÿ1
mmÿ1
) obtained using furrow irrigation (Ells et al., 1993).
Crop breeders try to maximize water use efficiency (WUE) when breading for new varieties. However, WUE has been defined as the ratio of dry matter produced per unit area (t haÿ1
and Wardlaw, 1976), and as the ratio of photosynthesis per unit of water transpired (Fischer and Turner, 1978; Sinclair et al., 1984). Consequently, care should be taken when comparing different WUE values.
Jensen and Sletten (1965) reported an average WUE for sorghum as 0.0039 t haÿ1mmÿ1 of Et. Abdul-Jabbar et al. (1983) studied alfalfa under sprinkler irrigation and reported a range in WUE values from 0.009 to 0.014 t haÿ1mmÿ1of Et. Howell et al. (1996) reported WUE values for corn ranging from 0.0124 to 0.0147 t haÿ1mmÿ1of Et. Howell et al. (1990) listed four methods to increase WUE: increase the harvest index, reduce the evapotranspiration to transpiration ratio, reduce the root dry matter, and decrease the transpiration. A buried drip system on deep rooted crops has the potential to decrease the evaporation loss and thus decrease the Et/t ratio.
Farmers are usually interested in the economic return per unit of the water applied. Consequently, deficit irrigation is not economical at current water cost in US (McGuckin et al., 1987), but changing from furrow to drip irrigation which can increase both IE and IWUE is economical for high value crop. Farmers also would like to plant varieties that have high WUE values.
In order to develop best management practices (BMPs), information is needed pertaining to differences between crops and soils as well as application rates and irrigation timing. Therefore, our objectives were to determine the irrigation efficiency (IE), irrigation water use efficiency (IWUE) and water use efficiency (WUE), under sprinkler, furrow, and drip irrigated onions for different yield potential levels and to determine the IE associated with the amount of water application for a sprinkler and drip irrigation systems that had the highest economic return.
2. Materials and methods
Three 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 (15.2 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. Seven irrigation treatments were used on each side of the line source. Amounts of applied water (including rainfall) ranged from a high of 51 cm 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. 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.
Catch-cans for measuring applied irrigation water were installed above the crop canopy in the center of each plot. In 1986, two neutron probe access tubes were installed in each plot to a depth of 1.37 m to measure changes in soil water over time. In 1987, two neutron probe access tubes were installed at a depth of 1.07 m in each irrigation plot except the irrigation plot that received the highest irrigation. The depth of the access tubes in these plots were 1.67 m. Neutron probe measurements were taken at 15 cm increments.
Daily weather data was measured 400 m from the experiment. Eto was estimated by using a modified Penman's equation referenced to grass (EtoS/(Sg) Rng/(Sg) Ea,
where S is equal to slope of the vapor pressure versus temperature curve; g the
psychometric constant;Rn the net radiation in equivalent (mm per day) was computed from solar radiation in equivalent of (mm per day); Ea (mm per day) an empirically
derived aerodynamic term (Sammis et al., 1985)). Evapotranspiration was estimated from the water balance equation (EtIRDSmÿDr, where Et is equal to evapotranspiration
(cm);I amount of irrigation water applied (cm); DSm change in soil moisture content
(cm); andDrto 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.
Irrigation efficiency was determined by dividing seasonal Et by the seasonal depth of water applied (I) including rainfall for each irrigation level. The IWUE (t haÿ1
mmÿ1
) was estimated from dividing yield byI. The WUE (t haÿ1
mmÿ1
of Et) was estimated from dry yield divided by Et. Onions bulbs were cut into small pieces, weighted, put in the oven at 688C, and weighed again after 72 h. Thus, dry yields were estimated based on a onion moisture content of 90%.
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 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. Onion harvesting dates in 1986 were 22±23 September (east of sprinkler line) and 2±3 October (west of sprinkler line). The 1987 harvest was on 16 October.
the first experiment (Al-Jamal et al., 1999a) 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 m3haÿ1
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 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. The IE, IWUE and WUE values were estimated as described before.
Table 1
Agronomic information for onion experiments at Farmington (Experiment 1) and Las Cruces (Experiment 2)
Fertilizer name Application date Application rate (kg haÿ1)
Experiment 1
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 poasta 15 June 1986 1.2 l haÿ1
Goala 19 May 1987 1.2 l haÿ1
Brominal-goala 9 June 1987 1.2 l haÿ1
Experiment 2
Year
1994 15 Februaryb 10 Augustc
1995 31 Januaryb 4 Augustc
1996 7 Februaryb 2 Augustc
aPesticides. bPlanting date. cHarvesting date.
Weed and insect control was managed uniformly according to standard practices. Urea nitrogen fertilizer (32±0±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 1994; 5 July 1995; and 7 July 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. 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.
The third furrow irrigation experiment included three different onion fields, which were furrow irrigated by farmers using Elephant Butte Irrigation District water that has 57.1 mg Clÿ
lÿ1
concentrations. Each field varied in size from 5 to 10 ha. The fields are near the cites of Goggin, Marting, and LaMesa, NM. The soils ranged from clay to loamy fine sand.
Producers provided onion yields for each field. These values were used to estimate Et from the evapotranspiration production function (Etpf) (Al-Jamal et al., 1999b). After harvesting, each field was divided into four sections. Two sets of soil samples were taken from the middle third of the field in each section, mixed and combined. Samples were taken in 15 cm increments, from depths of 15±180 cm, using a 7.62 cm diameter bucket auger. Gravimetric soil moisture was computed for each soil sample immediately after collecting the soil samples from each field. The soil samples also were analyzed for Clÿ ions. An above-ground plant sample was taken from a 150 cm40 cm area from each field before harvesting the onions. These samples were taken from the same locations where soil sampling occurred. The planting and harvesting dates for the study fields were determined by the procedures and recommended management practices for New Mexico's Mesilla valley.
The chloride estimate was adjusted for each field by subtracting the amount of chloride taken up by the crop from the total amount of chloride in the water source. The amount of chloride taken up by the crop was calculated by multiplying the crop dry biomass by the plant chloride concentration. Biomass was calculated from onion yield and the harvesting index. The harvesting index for onions was 0.66. The groundwater level was measured during soil sampling, and the water table was 180 cm below some fields. The Lf was used to estimate the seasonal IE, where IE equals one minus the Lf. The Lf in this experiment was estimated using chloride as a tracer (Al-Jamal et al., 1997). Irrigation water use efficiency and WUE were estimated as indicated above. A thorough discussion of the methodology of this study was presented by Al-Jamal et al. (1997).
3. Results and discussion
IEs for onions were high, because the farmers have limited water resources and they used the concept of deficit irrigation to irrigate their onion crops. However, yields were 50% of maximum yield obtained on the subsurface drip irrigated plots, Fig. 2.
Fall onions in Las Cruces are irrigated with a combination of surface and ground water. During the winter months, the surface irrigation system is shut down and the Ec of the ground water is 2.4 dS mÿ1. The surface irrigation system is turned on in March and the Ec of the surface water is 0.93 dS mÿ1. Leaching is required to maintain a low salt levels in the root zone. A Lf of 0.2 is required for a average surface and ground water salinity level of 1.7 dS mÿ1
in the irrigation water near Las Cruces (Sammis and Herrera, 1994). If the Lf is not sufficient, salt will begin to build in the root zone, resulting in lower yields and onions are sensitive to soil salinity (ECe) (Doorenboss and Kassam, 1986).
Fig. 1. The relationship between irrigation ef®ciency and the amount of water applied for onion crops under different irrigation methods and soil types.
Fig. 2. The relationship between irrigation ef®ciency and onion yield under different irrigation methods and soil types.
Consequently, for surface and sprinkler irrigation systems that have one dimensional flow regimes, a IE higher than 0.8 in the Las Cruces area would cause yield reduction due to salt stress.
Onion IEs, grown using subsurface drip irrigation, ranged from 45% for non-stressed treatments to 77% for stressed treatments (Fig. 1) with yields at this stress level comparable to the furrow irrigated yields. However, the maximum yield of 91,170 kg haÿ1 obtained from the subsurface drip system resulted in an IE or 45% (Fig. 2). The large amount of excess water was needed to wet the soil profile to the full width of the beds using the subsurface irrigation system. When the outer two row of onions received water at the root plate, which is near the surface, the onions root system developed fully resulting in the largest yield levels.
Reasonable IE values can be obtained under a subsurface drip system by stressing the onion crops outer two rows and getting lower yields (Fig. 2). Another solution may be to place the drip tubing at a shallower depth or to place the drip laterals on the soil surface. Producers in fields with coarsely textured soil under subsurface drip irrigation could install two laterals per bed instead of one, and bury these laterals at a shallow depth. Installing two laterals per bed will reduce the amount of applied water necessary to water-up the bed and, consequently, minimize the deep percolating water.
An economic analysis (Buchanan, 1997) on converting 4.04 ha from flood irrigation to drip irrigation operated at an IE of 45% resulted in a yearly net cash inflow (revenues less down payment, operating cost, loan payments, and taxes) of US$ 4166 haÿ1per year for a system of inline pressure compensating emitters that was designed for a life of 15 years. The return on the investment was 27%. If two drip lines are installed and the IE raised to 90% then the return on investment is 23% because the net cash year inflow decrease to US$ 3660 haÿ1per year. The net cash inflow would be the same if low cost biwall tubing were installed instead of inline pressure compensating emitters, because the biwall tubing would have to be replaced every 2 years compared to the 15 year life expectancy of the in line emitter system. Putting in a second drip line to increase the IE is not cost effective at the current cost of water. This economic analysis does not include a cost for producing drainage water and assumes the yearly cost of irrigation water of only US$ 0.42 haÿ1
cm haÿ1
which is the cost to pump ground water or the cost of surface water without a surface storage system. If the drip system is operated at an IE of 0.77 then the yield decreases to the level achieved under surface irrigation and the installation of a drip system in not economical.
Deciding what is the proper depth of drip tubing placement is a major concern with any drip system. This decision is based on the soil type and structure as well as the depth of the roots. The amount of water applied to wet the soil surface at planting depends on many factors, including: the contact between buried drip line and depth of the drip line; the soil hydraulic conductivity. The amount of water applied by the drip irrigation system varied from 8 to 43 cm during the establishment stage because of difference in soil tilth at planting.
To date, research evaluating drip laterals has been focused on final yield. For onions, research has shown that drip laterals buried at 0.1 m in a clay loam soil result in high yield (57,000 kg haÿ1
) (Bucks et al., 1981). The highest yield in the Las Cruces drip irrigation experiment on a sandy loam soil, 91,170 kg haÿ1
tubing buried at 0.08 m. The research seems to indicate that maximum yields would be obtained when drip laterals are placed at a shallow depth (0.08±0.1 m) for onions in all soil textures. Trying to place the drip tubing shallower than 0.08 m to increase IE cannot be accomplished because the drip tubing would be caught and destroyed by the farming equipment when the onions are lifted prior to harvesting.
Onion IE values ranged from 88 to 100%, when grown using sprinkler irrigation at Farmington, NM, Fig. 1. A Lf of 0.05 is required to maintain the salt balance for a salinity level of 0.7 dS mÿ1 in the irrigation water near Farmington, which is normally accomplished by winter rains requiring no leaching during the irrigation season. The highest yield was obtained with an IE of 93% because the sprinkler system applied water as a one dimension flow pattern at the surface and excess water was not needed to be applied to cause subbing of water into the onion root plate as was the case in using a drip irrigation system. Using Buchanon's economic model (1997), but substituting the cost of a sprinkler system for the drip system, resulted in a net cash inflow of US$ 6160 haÿ1
per year and a return on investment of 36%. The return on investment is higher for the sprinkler system because the sprinkler system only cost US$ 3833 haÿ1
compared to the drip system cost of 15,500 haÿ1
. The costs include the cost of installing a pump for the irrigation system. The highest economic return on a sprinkler irrigation system will occur when applying 843 mm of water at Farmington and 978 mm at Las Cruces which will result in an IE of 93% on a sandy loam soil. The IE of 93% for the sprinkler system is based on water reaching the ground and not the water discharged by the sprinkler system. Evaporation loss will decrease the IE if the IE is based on water discharged by the sprinklers. The decrease will depend on many climatic variables that effect the evaporation loss and the sprinkler nozzle characteristics. In order for the sprinkler system operation to achieve an IE of 93% and for this IE to be the optimal economical operation level, the sprinkler system uniformity coefficient defined as 100 (1ÿcoefficient of variation) must be 95% Wu (1988).
The amount of water applied to onions depends on many factors, including daily Et and IE. Frequency and amount of application must be timed to prevent stress (Abdul-Jabbar et al., 1983). A good irrigation scheduling program using tensiometers or a climate based water balance model will be needed to achieve a IE of 93% and not stress the crop for moisture. The highest IE under a sprinkler system usually is only 80% in farmers field when no irrigation scheduling procedures are used. The economic analysis for both the drip and sprinkler irrigation systems includes the cost of irrigation scheduling.
The IWUE values give a complete analysis of water resource use so that government regulators and conservationist knows how to influence farmers in the selection of the irrigation system they use and the irrigation management system they apply when making irrigation decisions. The IWUE values under furrow irrigation on farmers fields were low (0.05 t haÿ1
mmÿ1
of applied water) because the crops were stressed for water resulting in a high evapotranspiration/transpiration ratio. Even with the deficit irrigation management system, 20% of the irrigation water went to deep drainage losses and was not used as transpiration to increase yields (Fig. 3). The maximum furrow IWUE value (0.104 t haÿ1
mmÿ1
of applied water) reported in the literature by Ells et al. (1993) also occurred under deficit irrigation, but the high value was achieved on an experimental plot
where small frequent irrigation would have resulted in a high IE and high IWUE. (Fig. 4) not achievable in large farmers fields.
The IWUE for subsurface, drip-irrigated onions ranged from 0.059 to 0.04 t haÿ1
mmÿ1
with an average value of 0.047 t haÿ1
mmÿ1
(Fig. 3). The evaporation component of the evapotranspiration process remained similar for all the plots so as additional irrigation water was applied, an increase percentage of the irrigation water went to deep drainage decreasing IWUE. When the drip irrigation yields were similar to the furrow irrigation yields, the IWUE were similar, but as yields increase by applying more water through the drip system, IWUE decreased. Consequently, on a sandy loam field, farmers are financially better off installing a drip irrigation system and managing it for maximum yield, where for water conservation agencies, furrow irrigation run under deficit irrigation conditions results in the best use of the water, because the farmer cannot afford to operate the drip system at a IWUE of 0.059 t haÿ1
mmÿ1
.
Converting from flood irrigation to sprinkler irrigation is the best method to apply the irrigation water based on IWUE calculations and economic analysis if some form of irrigation scheduling is practiced to achieve high IWUE. The IWUE values obtained from
Fig. 3. The relationship between water applied and irrigation water use ef®ciency for onion crops.
the sprinkler system increased linearly as the amount of irrigation water applied increased to 844 mm (Fig. 3). The yield increased as well (Fig. 4) because the percentage of the applied water used as transpiration increase as less deficit irrigation was practiced. The highest IWUE value (0.084 t haÿ1mmÿ1) was observed when the sprinkler system, was operated under non deficit irrigation conditions and IE of 97% (Figs. 3 and 4). The results obtained from this study were similar to those observed by Ellis et al. (1986).
The WUE for onions within a specific location and environment should be linear, regardless of the irrigation method used, because the evapotranspiration water production function generally is a linear model. That is, WUEDry yield/Etb1(b0/Et). The WUE for onions obtained at Farmington by using sprinkler irrigation increased linearly from 0.0019 to 0.009 t haÿ1
mmÿ1
of Et as the Et increased (Fig. 5), because the Etpf (Fig. 6) is linear. The Etpf intercept (Fig. 6) represents the evaporation component. For stressed irrigation treatments, in which most of the applied water was lost as evaporation; the evaporation-to-evapotranspiration component was maximum. Consequently, the WUE
Fig. 5. The relationship between evapotranspiration and water use ef®ciency for onions at Farmington.
Fig. 6. The evapotranspiration production function for onion crop at Farmington based on dry ungraded yield.
was very low (Fig. 5). On the other hand, for the nonstressed irrigation treatments, in which most of the applied water was used for transpiration; the transpiration-to-evapotranspiration component was maximum. Consequently, evaporation losses were small, and WUE was maximum (Fig. 5).
Since WUE can be improved by either increasing dry yield or decreasing Et, these factors might be used by crop breeders to decrease the water use of crops, while maintaining yield and quality. Dry yield is affected by the rate of photosynthesis. Stressing a crop (still a common practice used by some farmers in NM) causes the stomates to close and reduces the rate of photosynthesis and yield potential. Only by breeding, for onion varieties that transpire less water while maintaining photosynthesis rate, can WUE be increased. Irrigation management can decrease the evaporation component. But the most effective way to decrease evaporation is to bury the drip line deep and prevent water from subbing to the surface. However, this is a fatal management decision when growing onions, because for the root of onions to grow, the base plate must be kept wet and this can only happen if the water subs to the surface. Consequently, the potential to increase WUE with a drip system for deep rooted crop does not exist when growing onions.
4. Conclusions
Irrigation efficiency under a drip irrigated onion field are going to be low when using a buried drip irrigation system and operating the system for maximum yield. However, economic analysis indicates that this is the irrigation management practice that should be followed by farmers until the cost of water increases considerable. Using a sprinkler system can increase yield and maintain a high IE compared to furrow and drip irrigation. The IWUE using the sprinkler system was higher compared to the subsurface drip and furrow irrigation methods which indicates that if you are trying to conserve water, then a sprinkler irrigation system should be used with some form of irrigation scheduling. If you are trying to maximize yield , then this will be achieved by using a drip irrigation system. In addition, WUE cannot be improved by letting onions suffer from water stress.
Acknowledgements
This research was supported by the New Mexico State University's Agricultural Experiment Station.
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