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Effect of irrigation rate on yield of drip-irrigated seedless watermelon in a humid region
Scientia Horticulturae 113 (2007) 155–161 www.elsevier.com/locate/scihorti
Effect of irrigation rate on yield of drip-irrigated seedless watermelon in a humid region Ian McCann *, Ed Kee, James Adkins, Emmalea Ernest, Jeremy Ernest University of Delaware Research and Education Center, 16483 County Seat Highway, Georgetown, DE, United States Received 15 September 2006; received in revised form 9 March 2007; accepted 13 March 2007
Abstract Seedless watermelon can be a profitable crop in humid regions such as the Delmarva Peninsula in the Mid-Atlantic region of the USA. Production using drip irrigation under plastic mulch is increasingly common but, although this is potentially an efficient production system, it is also complex in terms of irrigation scheduling. Experiments were conducted in 2004, 2005 and 2006 in Georgetown, Delaware to measure the effect of irrigation rate on yield of seedless watermelon grown with drip irrigation under plastic mulch. Relative irrigation rates were used to provide water amounts that ranged from low to high. The irrigation rates in all years included 50%, 100% and 150% of nominal crop water use, with additional rates of 0% and 250% in 2005 and 2006. Data from a nearby weather station were used to estimate reference evapotranspiration. Volumetric soil water content in the 50%, 100% and 150% treatments was measured in the center of each mulched bed using multi-sensor capacitance probes with sensors at depths of 10 cm, 20 cm, 30 cm, 50 cm and 70 cm. Yield of seedless melons per unit area of land ranged from about 55–95 tonnes/ha, depending on the year and the irrigation rate, but within each year the yield differences due to the irrigation rate were not significant ( p = 0.10) despite the large range in rate. Differences in quality, as measured by sugar content and the incidence of hollow heart, was also not significant. This lack of response to irrigation may be due to the ability of the root system to use infiltrated rainwater from outside the mulched bed. Growers in the region tend to apply more water than may be required as a form of risk avoidance. This study indicates that irrigation amounts could likely be reduced without having an adverse effect on yield. # 2007 Elsevier B.V. All rights reserved. Keywords: Rainfall; Mulch; Soil water measurement; Roots; Capacitance probe; ET
1. Introduction Irrigation management in humid regions is more complex than in arid regions because of the significant but unpredictable contribution that rainfall may make to the soil water balance in the root zone. Supplemental irrigation in humid regions can be profitable because it enables water stress to be avoided during periods of drought, thereby increasing and stabilizing yields. Sprinkler systems such as center-pivot are commonly used for field crops such as corn, but drip irrigation under plastic mulch has become an important production system for vegetables and other high value crops, including watermelon. However, drip irrigation under plastic mulch in a humid region is especially complex. In contrast to sprinkler irrigation, water is applied from a line of point sources to only part of the field, while the
* Corresponding author. Tel.: +1 302 856 7303x586; fax: +1 302 856 1845. E-mail address: [email protected] (I. McCann). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2007.03.008
plastic mulch suppresses evaporation but also sheds rainfall to the edge of the mulched row from where it may infiltrate and perhaps contribute to crop water requirements. Further complexity arises from direct infiltration of rainwater through the mulch via the planting holes and any other holes or tears in the plastic that develop over time. Using reference evapotranspiration (ET0), expressed in units of depth such as mm, to determine the volume of irrigation water required per unit length of drip-irrigated row is not straightforward. Determining how much time a drip system should be operated for requires an assumption about the wetted width and depth of the soil relative to the volume of irrigation water. The crop can extract water from the volume of soil wetted by irrigation and, if the roots extend far enough, may also be able to use water that falls outside the mulch and infiltrates into the row middles. Good irrigation management under such a system is challenging. The Delmarva Peninsula, located in the Mid-Atlantic region of the US and comprising the state of Delaware and parts of
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Maryland and Virginia, is a humid region. Watermelon, especially seedless varieties, is an important economic crop on the Delmarva Peninsula. In Delaware, watermelon production during the 2000–2004 seasons averaged 36.1 million kg (79.5 million lbs) from 874 Ha (2160 ac), with a value of $6.3 million (National Watermelon Promotion Board, 2005), corresponding to $7162 per Ha ($2898/ac). The regional market for seeded watermelons, such as the pollenizer variety required for seedless production, is variable. The prices are usually low, and often growers will not even harvest them. There have been more studies on watermelon production in arid and semi-arid regions than in humid regions. Erdem and Yuksel (2003) found that yield increased significantly with crop evapotranspiration (ET) in a semi-arid region of Turkey, as did Simsek et al. (2004). Wang et al. (2004) found that supplemental irrigation with mulch in northwest China increased yield in a dry year (126 mm seasonal rainfall) but not in a wetter year (223 mm seasonal rainfall). Xie et al. (2006) measured root density patterns in the same study, and found that irrigation and rainfall and the resulting soil water content patterns affected both horizontal and vertical root development, as would be expected. Moderate soil water deficits increased root length. In a humid region, Clark et al. (1996) conducted a study in Florida with three relative irrigation amounts based on ET0. The base rate received irrigation amounts corresponding to fractions of ET0 ranging from 0.1 to 0.3 as the crop developed, while the other two rates were twice and three times the base rate. These fractions are analogous to a crop coefficient (Kc), as in the FAO method of estimating crop water use (Allen et al., 1998). Differences in yield between the irrigation rates were not large, with the highest yields being under the base rate or twice the base rate. They observed that the base rate, which was only 0.3 ET0 at the most even though the maximum Kc, at mid crop stage for watermelon is 1.0 (Allen et al., 1998), did not produce any visible signs of wilting even when the canopy fully covered the area between rows. They suggested that under humid conditions the root system may be sufficiently aggressive to use residual water from rainfall. A practical consideration in watermelon research is the logistics of growing and harvesting plots. The lack of mechanization and the large size of the fruit tends to limit individual plot size. Clark et al. (1996) used four replications of plots that were 11 m long, with 12 plants per plot. Maynard and Sidoti (2003) conducted variety trials in Florida using three replications of plots that were 7.3 m long with eight plants per plot. Similarly, variety trials in North Carolina (Schultheis et al., 2001) used four replications of plots that were 6.1 m long with 10 plants per plot, while in Indiana the plots in variety trials were 16.8 m long with 11 plants per plot replicated three times (Gunter et al., 2001). The relatively low number of plants per plot often necessitated in watermelon research and evaluation does make it more difficult to detect statistical differences between treatments. Monitoring soil water content over time within the root zone can yield information on the relative rates of irrigation and crop water use. For example, if soil water content is decreasing over
time it is being consumed faster than it is being replaced by irrigation or rainfall. Near-continuous measurements of soil water content at a number of depths, such as the measurements enabled using multi-sensor capacitance probes (MCPs), can be particularly useful. Examples of the use of MCPs are reported in Starr and Paltineanu (1998), Paltineanu and Starr (2000), Fares and Alva (2000), and Starr and Timlin (2004). Regardless of the hydrologic complexity of mulched drip irrigation in a humid climate, growers in such regions still have to make decisions on how frequently to irrigate and how long to run their system each time. Without good irrigation management information many growers tend to over-irrigate to eliminate the risk of under-irrigation and the resulting crop water stress. Over-irrigation results in a waste of water and the energy required to provide it, and it also leaches soluble nutrients such as nitrogen into the environment. Over-irrigation is a form of risk insurance that would not be necessary if better information was available. Anecdotal observations of grower management in the region confirm that excess irrigation is common. The objective of this study was to compare watermelon yield with irrigation amounts ranging from deficient to excess. This information can help growers in humid regions develop an irrigation strategy, and is necessary for the future development of guidelines to help them improve irrigation management for their own profitability and to also protect the environment. 2. Materials and methods Field studies were conducted in 2004, 2005 and 2006 at the University of Delaware Research and Education Center in Georgetown, Delaware, USA (388380 N, 758320 W, www.rec.udel.edu). The soil texture at this site is predominantly sandy loam or loamy sand in the top 30–60 cm, and in places overlies either a heavier textured sandy clay loam or a coarser sand that may contain some gravel. Approximate values for field capacity and wilting point are 18% and 10% by volume, resulting in an available water content of 8% by volume. However spatial variability in soil texture and the effects of tillage in the upper soil profile make it difficult to use these values as more than guidelines. Each year, approximately 5-week old seedlings of seedless watermelon (cv. ‘Millionaire’) were transplanted during the third week of May at a spacing of 91 cm (3 ft) in plastic mulched rows 2.44 m (8 ft) apart (2.23 m2/plant, 24 ft2/plant), corresponding to typical production practices in the area. For every two rows of seedless watermelons, a seeded variety was planted as a pollenizer. The row length was 9.15 m (30 ft), while the width of the raised bed covered by the plastic mulch was approximately 76 cm (30 in.). Based on pre-season soil tests, fertilizer was incorporated into the beds preplant to provide sufficient potassium and phosphorus for the season, according to University of Delaware (2006) recommendations. Nitrogen was also added preplant to provide an initial rate of 56 kg/ha (50 lb/ac), with an additional 112 kg/ha (100 lb/ac) of nitrogen applied during the season using fertigation, for a total nitrogen rate of 168 kg/ha (150 lb/ac).
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Fig. 1. Location of MCPs under plastic mulch relative to the drip tape and the plant. The wetted volume is for illustration only. The inset shows the general layout of a replication, with the rows labeled P being pollenizers.
The rows were irrigated using drip tape (T-Tape, TSystems1) with an emitter spacing of 30 cm (12 in.) and flow rates of 164 l h1/100 m (0.49 l h1/emitter, 0.22 gpm/100 ft, 0.13 gph/emitter) in 2004 and 335 l h1/100 m (1.01 l h1/ emitter, 0.45 gpm/100 ft, 0.27 gph/emitter) in 2005 and 2006. If the simplistic assumption is made that the volume of irrigation water applied per unit length uniformly wets only the soil covered by the mulch, the equivalent irrigation rate was 1.9 mm/h in 2004 and 4.4 mm/h in 2005 and 2006. In all years, the base irrigation rate was nominally 100% of crop water use, and so approximated optimal irrigation. Daily crop water use was estimated from ET0 applied to the mulched area and a coefficient based on the fraction of the total area covered by the canopy. I 100 ¼ ET0 K c CF W m 100 m
(1)
where I100 is the irrigation per 100 m of crop row (l 100 m), ET0 the potential evapotranspiration (mm), Kc the crop coefficient at full canopy (assumed to be 1.0), CF the fraction of area covered by canopy and Wm is the width of the mulched row (m), measured as 0.76 m. As an example, if ET0 was 10 mm and there was full canopy cover (CF = 1.0), the irrigation water required per 100 m of plot would be 760 l. The operating time required to apply this amount of water depends on the flow rate of the drip tape. The drip tape used in 2005 and 2006 (which had a flow rate of 335 l h1/100 m) would require 2.27 h (760 l 100 m (335 l h1/ 100 m)). There were 24 irrigations in 2004 and 2005, and 21 1 Trade names are used in this publication to provide specific information. Mention of a trade name does not constitute a guarantee or warranty of the product or equipment by the University of Delaware or an endorsement over other similar products.
irrigations in 2006. The average irrigation durations for the 100% irrigation treatment in 2004, 2005 and 2006 were 4.18 h, 1.87 h and 2.33 h respectively, resulting in corresponding irrigation volumes of 685 l/100 m (9.0 mm), 625 l/100 m (8.2 mm) and 780 l/100 m (10.3 mm) respectively. Other irrigation treatments received a fixed percentage of this base rate. For each irrigation treatment there were four replications in 2004 and 2005, and three replications in 2006. In all years the replications consisted of adjacent blocks each of which contained single row plots of seedless watermelon (9.15 m long and 2.44 m apart) that were randomly assigned an irrigation treatment. In 2004 the irrigation treatments consisted of relative rates of 50% (a low rate representing deficient irrigation) and 150% (a high rate representing excessive irrigation) in addition to the base rate of 100%. In 2005 and 2006 the irrigation treatments were the same as in 2004, but two additional irrigation treatments were added to increase the range of irrigation rates. One of these additional treatments was 0% (no irrigation), while the other additional treatment was 250% (very high irrigation). The pollenizer rows received the 100% rate. Data from a nearby automatic weather station were used to estimate daily ET0 using the Penman–Monteith method (Allen et al., 1998). Each of the irrigation rates was supplied by a separate water line equipped with a manual valve. The rates were imposed by manually turning water on or off to these supply lines, thereby varying the run time of the drip tape connected to each supply line. For example, if it was determined that an irrigation of 2 h duration was needed for the 100% treatment, the 50%, 150% and 250% treatments received water for 1 h, 3 h and 5 h respectively. While the run time for the 100% treatment varied during the season, the 50%, 150% and 250% ratios were always maintained. The 0% treatment received no irrigation except that associated with the fertigations that all plots received.
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Soil water content during the season was monitored in each replication of the 50%, 100% and 150% irrigation treatments in all years, using multi-sensor capacitance probes (EnviroSCAN, Sentek). Each probe had five sensors, located at depths of 10 cm, 20 cm, 30 cm, 50 cm and 70 cm (4 in., 8 in., 12 in., 20 in. and 28 in.). These depths were chosen to enable adequate measurement of the top 30 cm, where soil water content is most dynamic and where root density was likely to be highest, and to also measure soil water content, extraction and movement within the lower part of the root zone down to 70 cm. The probes were installed along the centerline of the mulched bed (the same line as the plants). The drip tape was located approximately 7.5 cm (3 in.) to one side of the centerline. The probe location was selected based on recommendations from the manufacturers of the probes and the drip tape as best representing the wetted soil volume (Fig. 1). In 2004 the probes were located a fixed distance of 15 cm (6 in.) from a plant regardless of the location relative to the emitters. In 2005 the probes were located midway between adjacent emitters at the closest suitable location to a plant. In 2006 the plants were located laterally in line with an emitter and the probes were located midway between emitters. In all years the probes were logged automatically at 10 min intervals. Irrigation was applied as required to replace estimated crop water use and to maintain soil water content in the 100% irrigation plots within the range that would be required to prevent water stress without at the same time over-irrigating and causing percolation below the root zone. Irrigations were applied usually every 1–3 days, using ET0 as a guide. The nearcontinuous measurements of soil water content provided a check as to whether the irrigations were causing water content to remain relatively stable over time in the 100% treatment while increasing or decreasing in the 150% and 50% treatments respectively. The plots were sprayed with fungicide and insecticides during the season as necessary to minimize disease and pest damage. The plots were first harvested in the middle of August and again approximately 2 weeks later. Each marketable watermelon was weighed in the field, and a subsample of five melons from each plot was cut open to measure sugar content using a hand held refractometer to measure the Brix value. If hollow heart was observed its severity was measured as the width of the internal separation. The incidence of poor color was also noted qualitatively. Yields were analyzed on a gross area basis, in which only 2 rows out of 3 (2/3 of the gross area) produced seedless melons. 3. Results and discussion Monthly rainfall totals and ET0 in 2004, 2005 and 2006 are shown in Fig. 2, along with long term averages. Monthly rainfall from May to July was below the long term average in 2004, but ET0 was also lower than it was in 2005 and 2006. In 2005 rainfall was about the same as the long term average from May to July, but significantly lower in August. In 2006 rainfall in May and August was low, while in July it was about average and in June was very high due to two large rainfall events
Fig. 2. Monthly estimated potential evapotranspiration (ET0) and rainfall during the 2006 (top), 2005 (middle) and 2004 (bottom) growing seasons in Georgetown, DE. The vertical lines show long term average rainfall.
(101 mm and 134 mm) that significantly increased the monthly total and caused ponding in the field. Long term estimates of ET0 are not available due to lack of some of the data required, such as humidity and solar radiation. However, ET0 for the 7 years from 1999 to 2006 shows monthly averages of 123 mm, 131 mm, 134 mm, and 116 mm in May, June, July and August respectively. The coefficients of variation (CV) of ET0 for the same 7 years period were 16.0%, 9.5%, 12.0% and 20.6% in May, June, July and August respectively. In contrast, the CVs of rainfall for the same period were much higher, at 56%, 83%, 40% and 60% in May, June, July and August respectively. In drip irrigation the volume of water applied can be calculated by multiplying the flow rate of the drip tape by the time for which the system is turned on, but expressing this volume as an equivalent depth of water requires assumptions about the shape of the wetted soil volume. In this case we made the simple assumption that irrigation uniformly replenished the soil under the mulch, so that the equivalent depth of water was simply the volume applied per unit length of tape divided by the width of the mulched soil (76 cm). The ratio of the mulched area to the total area (calculated as the width of mulched bed divided by row spacing) is 0.31, which is comparable to the 0.30 ET ratio used by Clark et al. (1996) at full canopy development.
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Fig. 3. Cumulative irrigation applied to the 100% irrigation treatment in 2004, 2005 and 2006 (a), with the irrigation schedule in 2006 (b), 2005 (c) and 2004 (d).
Fig. 4. Example of measured average hourly soil water content in the top 30 cm (a) and from 30 to 70 cm (b) for a 3-week period during the 2005 season for relative irrigation rates of 50%, 100% and 150%. Rainfall and the 100% irrigation rate during the period are shown in (c).
The cumulative irrigation applied to the 100% irrigation treatment in each of the 3 years is shown in Fig. 3, along with the amount and timing of irrigations. The 50%, 150% and 250% treatments received corresponding cumulative totals. It can be seen that in the 2 months period from 20 June to 20 August (which encompasses most of the season), the total depth of irrigation applied was approximately the same each year, about 200 mm. In 2004 the irrigation amounts were greater earlier in the season than in 2005 and 2006, but irrigations had essentially ceased by the last week of July. In 2005 and 2006 the irrigation patterns were similar, with irrigations continuing into the middle of August. In 2004 and 2005 the crop had achieved full canopy by 15 July, while in 2006 full canopy was achieved about 1 week later, perhaps due to the ponding caused by the two large rainfall events. In all years, the canopy grew rapidly in the 3 weeks prior to full development, during which time flowering and early fruit set was initiated. Fig. 4 shows an example of hourly measured soil water content in the top 70 cm. The data are averaged across the four replications for a 20 days period in 2005, for the 50%, 100% and 150% irrigation rates. It can be seen that soil water content is dynamic, particularly for the 100% and 150% irrigation rates. The increases in soil water content due to irrigation and the subsequent decreases due to crop water use can be clearly seen. Being able to measure soil water dynamics enables short and
longer term changes in soil water content to be seen, although such data is subject to spatial variability in soil properties and wetting patterns as well as differences due to the individual sensors and the relatively small volume of soil that is measured. Soil water content under the 50% irrigation rate was lower and declined over time compared to the two higher rates. However, the short term dynamics of soil water content can make longer trends over time difficult to discern. To identify longer term trends, the average weekly measured volumetric water content at 6 a.m. in the top 70 cm, from the middle of June to the end of July, is shown in Fig. 5 for the 3 years. In 2004 we were not able to make measurements beyond the middle of July, but by that time the irrigation season was almost finished anyway. In all years, soil water content reflected the relative irrigation rates once irrigations commenced, with the 50% rate being the lowest and 150% the highest throughout the season, as would be expected. The trends in soil water content over time can be seen most clearly in 2005. In the 50% irrigation treatment soil water content generally declined over the season, indicating that irrigation was not sufficient. In contrast, soil water content in the 150% treatment generally increased over time, indicating excess irrigation. Soil water content in the 100% treatment was approximately constant, indicating that irrigation approximately replaced crop water use from the center of the bed. In
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Fig. 5. Weekly average measured soil water content at 6 a.m. in top the 70 cm in 2006 (a), 2005 (b) and 2004 (c) for the 50%, 100% and 150% irrigation treatments. The error bars show the standard deviation of measurements between replications.
2004 the same trends of declining, stable and increasing soil water content in the 50%, 100% and 150% treatments respectively can be seen during the measurement period. In 2006 soil water contents early in the season were relatively high due to the high rainfall in June. The 100% treatment however did decline during the first 3 weeks of July, indicating that irrigation might not have been sufficient to meet crop water requirements. Soil water content in the 150% treatment was more constant, indicating that perhaps it better reflected crop water use. The differences in soil water content between the 100% and 150% treatments were approximately similar in 2005 and 2006.
Fig. 6. Total yield of seedless watermelon in 2004, 2005 and 2006 as a function of relative irrigation rate. Error bars show standard deviation of yield.
Fig. 6 shows the yield response to the irrigation rates (three rates in 2004 and five rates in 2005 and 2006). Surprisingly, there was little difference between treatments within the same year. The highest yields were in 2005 and the lowest in 2006, but within each year there were no statistically significant differences in yield. The coefficient of variation (CV) between replications ranged from 2% to 20%, with an average across all years for the 50%, 100% and 150% treatments of 9.3%. The CV in the 0% irrigation treatment was high in both years (2005 and 2006), averaging 27%. In 2004, 2005 and 2006 the highest yields were in the 150%, 100% and 50% irrigation treatments respectively. In 2005 and 2006, yields in the 0% irrigation treatment were surprisingly high considering that the only water inputs came from rainfall, and that much of this was shed to the edge of the bed by the mulch. In all years, as for total yield, there were also no significant differences in number of melons and average melon weight. Sugar content values ranged from 11.3 to 12.5, but again there were no significant differences within a year and no consistent trend observed. Similarly, the incidence of hollow heart was generally low with no significant differences. Larger plots might have reduced the variability between replications, but it is likely that results would still be similar in terms of the relatively small differences between treatments. This lack of significant yield differences over a large range in irrigation amount was unexpected, especially considering the clear measured difference in soil water content. The lack of yield response is however an important result with implications for irrigation management. Soil water content in the center of the bed can be controlled by the grower through irrigation, but outside this relatively limited area soil water content is determined by rainfall (over which the grower has no control). The lack of a significant yield difference between the 100% treatment and the higher rates may be because soil water content was at least adequate in these treatments. The excess water may drain below the root zone in sandy soils, perhaps also removing nitrogen. However, soil water content in the 50% irrigation treatment as measured in the center of the bed was clearly declining for much of the season in all years, indicating that the crop was using more water than was replaced. The root zone is apparently extensive enough in a humid climate that the net extraction of water from the center of the bed is not sufficient to cause water stress and significant yield reduction. We did not measure soil water content in the 0% treatment in 2005 and 2006, but it was likely to be even lower than in the 50% treatment. Although yield was lowest in the 0% treatment, it was still higher than we expected considering that the only source of water was rain. The crop must have been extracting water from locations outside the center of the bed or from deeper in the profile. The two primary sources of water (irrigation and rainfall) have very different distribution and dynamics within the field. Under the mulch there is a wetted volume (‘‘onion’’) under each dripper along the dripline, and this volume is frequently replenished by irrigation and depleted by transpiration. Its shape is determined by the dripper flow rate and the relative rate of movement vertically and horizontally (Fig. 1). Outside the
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mulch the soil is wetted directly by rainfall and, near the edge of the mulch, by runoff. Wetting is much more uniform spatially than it is under the mulch. However, outside the mulch water can freely evaporate if the soil surface is sufficiently wet. Evaporation is reduced once the soil surface layer dries sufficiently to inhibit water transport to the surface from deeper in the soil. Thus, small rainfall events that only wet soil near the surface will likely be lost to evaporation. Larger rainfall events will wet the soil below the evaporation layer, and this water potentially will be available to plant roots and, to some extent, to move horizontally towards the center of the bed. Thus, rainwater will be available to the crop depending primarily on whether the rainfall events significantly rewet the soil below the evaporation layer, and on whether roots are present to take advantage of it. Apparently rainfall in the 3 years of this study was sufficient to compensate for the lack of sufficient replenishment of the irrigated volume under the 50% and the 0% irrigation rates. The years in this study were not very dry, and perhaps there will only be a significant response to irrigation in a dry year (similar to an arid or semi-arid climate). If this is the case, in many years growers in humid regions could significantly reduce their irrigation without adversely impacting yield. Currently, growers tend to over-irrigate, with rates closer to the 150% and 250% treatments of this study and so they should be able to safely reduce irrigation to the 100% rate in all years. Lower rates than 100% are probably safe in many years. An appropriate irrigation management strategy might be to apply sufficient water in the early part of the season to ensure good crop establishment, and then to apply irrigation amounts that are less than crop water use as long as rainfall events have been of sufficient magnitude and timing to keep the soil profile wet outside the mulch. In the event of a long dry period, the irrigation can be increased to more closely match crop water use (the 100% rate). However, additional research will be necessary to support this hypothesis by documenting the lateral extent, depth and water uptake of watermelon roots as a function of irrigation and rainfall patterns. Although we only studied the yield response of a seedless watermelon variety, it is likely that seeded varieties would have a similar response under similar weather conditions. 4. Conclusions Watermelon yield and quality in 2004, 2005 and 2006 were not significantly affected over a large range in irrigation rate, and this has implications for irrigation management in humid regions. It is likely that in many years growers could rely more on rainfall to provide water than they currently do, and so significantly reduce irrigation without reducing yield.
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Additional studies will be needed to quantify the interactions between irrigation amount and rainfall on root distribution and crop water use in humid regions. Acknowledgements We are grateful for the help of Dr. Jim Starr, USDA ARS, Beltsville, MD, USA, and his team, with the instrumentation. We also acknowledge the help and support given by T-Systems International, San Diego, USA and Sentek Pty, Stepney, Australia for advice and equipment. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56. Food and Agriculture Organisation, Rome, 299 pp. Clark, G.A., Maynard, D.N., Stanley, C.D., 1996. Drip-irrigation management for watermelon in a humid region. Appl. Eng. Agric. 12 (3), 335–340. Erdem, Y., Yuksel, A.N., 2003. Yield response of watermelon to irrigation shortage. Sci. Hortic. 98, 365–383. Fares, A., Alva, A.K., 2000. Evaluation of capacitance probes for optimal irrigation of citrus through soil moisture monitoring in an entisol profile. Irrig. Sci. 19 (2), 57–64. Gunter, C.C., Lang, M., Nowaskie, D., Thompson, A., 2001. Seedless watermelon cultivar trials for southwestern Indiana. Southwest Purdue Agricultural Program, Purdue University. www.hort.purdue.edu/hort/ext/veg/ MWVVT2001/html/INGunter2001SeedlessWM.htm. Maynard, D.N., Sidoti, B.J., 2003. Triploid watermelon cultigen evaluation. University of Florida Gulf Coast Research and Education Center. Research Report BRA-2003. National Watermelon Promotion Board, 2005. Watermelon Reference Book. National Watermelon Promotion Board, Orlando, FL. Paltineanu, I.C., Starr, J.L., 2000. Preferential water flow through corn canopy and soil water dynamics across rows. Soil Sci. Soc. Am. J. 64, 44–54. Schultheis, J.R., Adams, D., Holmes, G., Adams, M., 2001. North Carolina Hybrid Triploid and Diploid Watermelon Cultigen Trial-red Flesh. North Carolina State University, Department of Horticultural Science. Simsek, M., Kacira, M., Tonkaz, T., 2004. The effects of different drip irrigation regimes on watermelon [Citrullus lanatus (Thunb.)] yield and yield components under semi-arid climatic conditions. Aust. J. Agric. Res. 55 (11), 1149–1157. Starr, J.L., Timlin, D.J., 2004. Using high-resolution soil moisture data to assess soil water dynamics in the vadose zone. Vadose Zone J. 3, 926–935. Starr, J.L., Paltineanu, I.C., 1998. Soil water dynamics using multisensor capacitance probes in non-traffic interrows of plow- and no-till corn. Soil Sci. Soc. Am. J. 62, 114–122. University of Delaware Commercial Vegetable Production Recommendations, 2006. Extension Bulletin 137. Cooperative Extension Service, College of Agriculture Sciences, Newark, Delaware. Wang, Y., Xie, Z., Li, F., Zhang, Z., 2004. The effect of supplemental irrigation on watermelon (Citrullus lanatus) production in gravel and sand mulched fields in the Loess Plateau of northwest China. Agric. Water Manage. 69, 29–41. Xie, Z., Wang, Y., Wei, X., Zhang, Z., 2006. Impacts of a gravel-sand mulch and supplemental drip irrigation on watermelon (Citrullus lanatus [Thunb.] Mats. and Nakai) root distribution and yield. Soil Tillage Res. 89, 35–44.
Effect of irrigation rate on yield of drip-irrigated seedless watermelon in a humid region Ian McCann *, Ed Kee, James Adkins, Emmalea Ernest, Jeremy Ernest University of Delaware Research and Education Center, 16483 County Seat Highway, Georgetown, DE, United States Received 15 September 2006; received in revised form 9 March 2007; accepted 13 March 2007
Abstract Seedless watermelon can be a profitable crop in humid regions such as the Delmarva Peninsula in the Mid-Atlantic region of the USA. Production using drip irrigation under plastic mulch is increasingly common but, although this is potentially an efficient production system, it is also complex in terms of irrigation scheduling. Experiments were conducted in 2004, 2005 and 2006 in Georgetown, Delaware to measure the effect of irrigation rate on yield of seedless watermelon grown with drip irrigation under plastic mulch. Relative irrigation rates were used to provide water amounts that ranged from low to high. The irrigation rates in all years included 50%, 100% and 150% of nominal crop water use, with additional rates of 0% and 250% in 2005 and 2006. Data from a nearby weather station were used to estimate reference evapotranspiration. Volumetric soil water content in the 50%, 100% and 150% treatments was measured in the center of each mulched bed using multi-sensor capacitance probes with sensors at depths of 10 cm, 20 cm, 30 cm, 50 cm and 70 cm. Yield of seedless melons per unit area of land ranged from about 55–95 tonnes/ha, depending on the year and the irrigation rate, but within each year the yield differences due to the irrigation rate were not significant ( p = 0.10) despite the large range in rate. Differences in quality, as measured by sugar content and the incidence of hollow heart, was also not significant. This lack of response to irrigation may be due to the ability of the root system to use infiltrated rainwater from outside the mulched bed. Growers in the region tend to apply more water than may be required as a form of risk avoidance. This study indicates that irrigation amounts could likely be reduced without having an adverse effect on yield. # 2007 Elsevier B.V. All rights reserved. Keywords: Rainfall; Mulch; Soil water measurement; Roots; Capacitance probe; ET
1. Introduction Irrigation management in humid regions is more complex than in arid regions because of the significant but unpredictable contribution that rainfall may make to the soil water balance in the root zone. Supplemental irrigation in humid regions can be profitable because it enables water stress to be avoided during periods of drought, thereby increasing and stabilizing yields. Sprinkler systems such as center-pivot are commonly used for field crops such as corn, but drip irrigation under plastic mulch has become an important production system for vegetables and other high value crops, including watermelon. However, drip irrigation under plastic mulch in a humid region is especially complex. In contrast to sprinkler irrigation, water is applied from a line of point sources to only part of the field, while the
* Corresponding author. Tel.: +1 302 856 7303x586; fax: +1 302 856 1845. E-mail address: [email protected] (I. McCann). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2007.03.008
plastic mulch suppresses evaporation but also sheds rainfall to the edge of the mulched row from where it may infiltrate and perhaps contribute to crop water requirements. Further complexity arises from direct infiltration of rainwater through the mulch via the planting holes and any other holes or tears in the plastic that develop over time. Using reference evapotranspiration (ET0), expressed in units of depth such as mm, to determine the volume of irrigation water required per unit length of drip-irrigated row is not straightforward. Determining how much time a drip system should be operated for requires an assumption about the wetted width and depth of the soil relative to the volume of irrigation water. The crop can extract water from the volume of soil wetted by irrigation and, if the roots extend far enough, may also be able to use water that falls outside the mulch and infiltrates into the row middles. Good irrigation management under such a system is challenging. The Delmarva Peninsula, located in the Mid-Atlantic region of the US and comprising the state of Delaware and parts of
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Maryland and Virginia, is a humid region. Watermelon, especially seedless varieties, is an important economic crop on the Delmarva Peninsula. In Delaware, watermelon production during the 2000–2004 seasons averaged 36.1 million kg (79.5 million lbs) from 874 Ha (2160 ac), with a value of $6.3 million (National Watermelon Promotion Board, 2005), corresponding to $7162 per Ha ($2898/ac). The regional market for seeded watermelons, such as the pollenizer variety required for seedless production, is variable. The prices are usually low, and often growers will not even harvest them. There have been more studies on watermelon production in arid and semi-arid regions than in humid regions. Erdem and Yuksel (2003) found that yield increased significantly with crop evapotranspiration (ET) in a semi-arid region of Turkey, as did Simsek et al. (2004). Wang et al. (2004) found that supplemental irrigation with mulch in northwest China increased yield in a dry year (126 mm seasonal rainfall) but not in a wetter year (223 mm seasonal rainfall). Xie et al. (2006) measured root density patterns in the same study, and found that irrigation and rainfall and the resulting soil water content patterns affected both horizontal and vertical root development, as would be expected. Moderate soil water deficits increased root length. In a humid region, Clark et al. (1996) conducted a study in Florida with three relative irrigation amounts based on ET0. The base rate received irrigation amounts corresponding to fractions of ET0 ranging from 0.1 to 0.3 as the crop developed, while the other two rates were twice and three times the base rate. These fractions are analogous to a crop coefficient (Kc), as in the FAO method of estimating crop water use (Allen et al., 1998). Differences in yield between the irrigation rates were not large, with the highest yields being under the base rate or twice the base rate. They observed that the base rate, which was only 0.3 ET0 at the most even though the maximum Kc, at mid crop stage for watermelon is 1.0 (Allen et al., 1998), did not produce any visible signs of wilting even when the canopy fully covered the area between rows. They suggested that under humid conditions the root system may be sufficiently aggressive to use residual water from rainfall. A practical consideration in watermelon research is the logistics of growing and harvesting plots. The lack of mechanization and the large size of the fruit tends to limit individual plot size. Clark et al. (1996) used four replications of plots that were 11 m long, with 12 plants per plot. Maynard and Sidoti (2003) conducted variety trials in Florida using three replications of plots that were 7.3 m long with eight plants per plot. Similarly, variety trials in North Carolina (Schultheis et al., 2001) used four replications of plots that were 6.1 m long with 10 plants per plot, while in Indiana the plots in variety trials were 16.8 m long with 11 plants per plot replicated three times (Gunter et al., 2001). The relatively low number of plants per plot often necessitated in watermelon research and evaluation does make it more difficult to detect statistical differences between treatments. Monitoring soil water content over time within the root zone can yield information on the relative rates of irrigation and crop water use. For example, if soil water content is decreasing over
time it is being consumed faster than it is being replaced by irrigation or rainfall. Near-continuous measurements of soil water content at a number of depths, such as the measurements enabled using multi-sensor capacitance probes (MCPs), can be particularly useful. Examples of the use of MCPs are reported in Starr and Paltineanu (1998), Paltineanu and Starr (2000), Fares and Alva (2000), and Starr and Timlin (2004). Regardless of the hydrologic complexity of mulched drip irrigation in a humid climate, growers in such regions still have to make decisions on how frequently to irrigate and how long to run their system each time. Without good irrigation management information many growers tend to over-irrigate to eliminate the risk of under-irrigation and the resulting crop water stress. Over-irrigation results in a waste of water and the energy required to provide it, and it also leaches soluble nutrients such as nitrogen into the environment. Over-irrigation is a form of risk insurance that would not be necessary if better information was available. Anecdotal observations of grower management in the region confirm that excess irrigation is common. The objective of this study was to compare watermelon yield with irrigation amounts ranging from deficient to excess. This information can help growers in humid regions develop an irrigation strategy, and is necessary for the future development of guidelines to help them improve irrigation management for their own profitability and to also protect the environment. 2. Materials and methods Field studies were conducted in 2004, 2005 and 2006 at the University of Delaware Research and Education Center in Georgetown, Delaware, USA (388380 N, 758320 W, www.rec.udel.edu). The soil texture at this site is predominantly sandy loam or loamy sand in the top 30–60 cm, and in places overlies either a heavier textured sandy clay loam or a coarser sand that may contain some gravel. Approximate values for field capacity and wilting point are 18% and 10% by volume, resulting in an available water content of 8% by volume. However spatial variability in soil texture and the effects of tillage in the upper soil profile make it difficult to use these values as more than guidelines. Each year, approximately 5-week old seedlings of seedless watermelon (cv. ‘Millionaire’) were transplanted during the third week of May at a spacing of 91 cm (3 ft) in plastic mulched rows 2.44 m (8 ft) apart (2.23 m2/plant, 24 ft2/plant), corresponding to typical production practices in the area. For every two rows of seedless watermelons, a seeded variety was planted as a pollenizer. The row length was 9.15 m (30 ft), while the width of the raised bed covered by the plastic mulch was approximately 76 cm (30 in.). Based on pre-season soil tests, fertilizer was incorporated into the beds preplant to provide sufficient potassium and phosphorus for the season, according to University of Delaware (2006) recommendations. Nitrogen was also added preplant to provide an initial rate of 56 kg/ha (50 lb/ac), with an additional 112 kg/ha (100 lb/ac) of nitrogen applied during the season using fertigation, for a total nitrogen rate of 168 kg/ha (150 lb/ac).
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Fig. 1. Location of MCPs under plastic mulch relative to the drip tape and the plant. The wetted volume is for illustration only. The inset shows the general layout of a replication, with the rows labeled P being pollenizers.
The rows were irrigated using drip tape (T-Tape, TSystems1) with an emitter spacing of 30 cm (12 in.) and flow rates of 164 l h1/100 m (0.49 l h1/emitter, 0.22 gpm/100 ft, 0.13 gph/emitter) in 2004 and 335 l h1/100 m (1.01 l h1/ emitter, 0.45 gpm/100 ft, 0.27 gph/emitter) in 2005 and 2006. If the simplistic assumption is made that the volume of irrigation water applied per unit length uniformly wets only the soil covered by the mulch, the equivalent irrigation rate was 1.9 mm/h in 2004 and 4.4 mm/h in 2005 and 2006. In all years, the base irrigation rate was nominally 100% of crop water use, and so approximated optimal irrigation. Daily crop water use was estimated from ET0 applied to the mulched area and a coefficient based on the fraction of the total area covered by the canopy. I 100 ¼ ET0 K c CF W m 100 m
(1)
where I100 is the irrigation per 100 m of crop row (l 100 m), ET0 the potential evapotranspiration (mm), Kc the crop coefficient at full canopy (assumed to be 1.0), CF the fraction of area covered by canopy and Wm is the width of the mulched row (m), measured as 0.76 m. As an example, if ET0 was 10 mm and there was full canopy cover (CF = 1.0), the irrigation water required per 100 m of plot would be 760 l. The operating time required to apply this amount of water depends on the flow rate of the drip tape. The drip tape used in 2005 and 2006 (which had a flow rate of 335 l h1/100 m) would require 2.27 h (760 l 100 m (335 l h1/ 100 m)). There were 24 irrigations in 2004 and 2005, and 21 1 Trade names are used in this publication to provide specific information. Mention of a trade name does not constitute a guarantee or warranty of the product or equipment by the University of Delaware or an endorsement over other similar products.
irrigations in 2006. The average irrigation durations for the 100% irrigation treatment in 2004, 2005 and 2006 were 4.18 h, 1.87 h and 2.33 h respectively, resulting in corresponding irrigation volumes of 685 l/100 m (9.0 mm), 625 l/100 m (8.2 mm) and 780 l/100 m (10.3 mm) respectively. Other irrigation treatments received a fixed percentage of this base rate. For each irrigation treatment there were four replications in 2004 and 2005, and three replications in 2006. In all years the replications consisted of adjacent blocks each of which contained single row plots of seedless watermelon (9.15 m long and 2.44 m apart) that were randomly assigned an irrigation treatment. In 2004 the irrigation treatments consisted of relative rates of 50% (a low rate representing deficient irrigation) and 150% (a high rate representing excessive irrigation) in addition to the base rate of 100%. In 2005 and 2006 the irrigation treatments were the same as in 2004, but two additional irrigation treatments were added to increase the range of irrigation rates. One of these additional treatments was 0% (no irrigation), while the other additional treatment was 250% (very high irrigation). The pollenizer rows received the 100% rate. Data from a nearby automatic weather station were used to estimate daily ET0 using the Penman–Monteith method (Allen et al., 1998). Each of the irrigation rates was supplied by a separate water line equipped with a manual valve. The rates were imposed by manually turning water on or off to these supply lines, thereby varying the run time of the drip tape connected to each supply line. For example, if it was determined that an irrigation of 2 h duration was needed for the 100% treatment, the 50%, 150% and 250% treatments received water for 1 h, 3 h and 5 h respectively. While the run time for the 100% treatment varied during the season, the 50%, 150% and 250% ratios were always maintained. The 0% treatment received no irrigation except that associated with the fertigations that all plots received.
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Soil water content during the season was monitored in each replication of the 50%, 100% and 150% irrigation treatments in all years, using multi-sensor capacitance probes (EnviroSCAN, Sentek). Each probe had five sensors, located at depths of 10 cm, 20 cm, 30 cm, 50 cm and 70 cm (4 in., 8 in., 12 in., 20 in. and 28 in.). These depths were chosen to enable adequate measurement of the top 30 cm, where soil water content is most dynamic and where root density was likely to be highest, and to also measure soil water content, extraction and movement within the lower part of the root zone down to 70 cm. The probes were installed along the centerline of the mulched bed (the same line as the plants). The drip tape was located approximately 7.5 cm (3 in.) to one side of the centerline. The probe location was selected based on recommendations from the manufacturers of the probes and the drip tape as best representing the wetted soil volume (Fig. 1). In 2004 the probes were located a fixed distance of 15 cm (6 in.) from a plant regardless of the location relative to the emitters. In 2005 the probes were located midway between adjacent emitters at the closest suitable location to a plant. In 2006 the plants were located laterally in line with an emitter and the probes were located midway between emitters. In all years the probes were logged automatically at 10 min intervals. Irrigation was applied as required to replace estimated crop water use and to maintain soil water content in the 100% irrigation plots within the range that would be required to prevent water stress without at the same time over-irrigating and causing percolation below the root zone. Irrigations were applied usually every 1–3 days, using ET0 as a guide. The nearcontinuous measurements of soil water content provided a check as to whether the irrigations were causing water content to remain relatively stable over time in the 100% treatment while increasing or decreasing in the 150% and 50% treatments respectively. The plots were sprayed with fungicide and insecticides during the season as necessary to minimize disease and pest damage. The plots were first harvested in the middle of August and again approximately 2 weeks later. Each marketable watermelon was weighed in the field, and a subsample of five melons from each plot was cut open to measure sugar content using a hand held refractometer to measure the Brix value. If hollow heart was observed its severity was measured as the width of the internal separation. The incidence of poor color was also noted qualitatively. Yields were analyzed on a gross area basis, in which only 2 rows out of 3 (2/3 of the gross area) produced seedless melons. 3. Results and discussion Monthly rainfall totals and ET0 in 2004, 2005 and 2006 are shown in Fig. 2, along with long term averages. Monthly rainfall from May to July was below the long term average in 2004, but ET0 was also lower than it was in 2005 and 2006. In 2005 rainfall was about the same as the long term average from May to July, but significantly lower in August. In 2006 rainfall in May and August was low, while in July it was about average and in June was very high due to two large rainfall events
Fig. 2. Monthly estimated potential evapotranspiration (ET0) and rainfall during the 2006 (top), 2005 (middle) and 2004 (bottom) growing seasons in Georgetown, DE. The vertical lines show long term average rainfall.
(101 mm and 134 mm) that significantly increased the monthly total and caused ponding in the field. Long term estimates of ET0 are not available due to lack of some of the data required, such as humidity and solar radiation. However, ET0 for the 7 years from 1999 to 2006 shows monthly averages of 123 mm, 131 mm, 134 mm, and 116 mm in May, June, July and August respectively. The coefficients of variation (CV) of ET0 for the same 7 years period were 16.0%, 9.5%, 12.0% and 20.6% in May, June, July and August respectively. In contrast, the CVs of rainfall for the same period were much higher, at 56%, 83%, 40% and 60% in May, June, July and August respectively. In drip irrigation the volume of water applied can be calculated by multiplying the flow rate of the drip tape by the time for which the system is turned on, but expressing this volume as an equivalent depth of water requires assumptions about the shape of the wetted soil volume. In this case we made the simple assumption that irrigation uniformly replenished the soil under the mulch, so that the equivalent depth of water was simply the volume applied per unit length of tape divided by the width of the mulched soil (76 cm). The ratio of the mulched area to the total area (calculated as the width of mulched bed divided by row spacing) is 0.31, which is comparable to the 0.30 ET ratio used by Clark et al. (1996) at full canopy development.
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Fig. 3. Cumulative irrigation applied to the 100% irrigation treatment in 2004, 2005 and 2006 (a), with the irrigation schedule in 2006 (b), 2005 (c) and 2004 (d).
Fig. 4. Example of measured average hourly soil water content in the top 30 cm (a) and from 30 to 70 cm (b) for a 3-week period during the 2005 season for relative irrigation rates of 50%, 100% and 150%. Rainfall and the 100% irrigation rate during the period are shown in (c).
The cumulative irrigation applied to the 100% irrigation treatment in each of the 3 years is shown in Fig. 3, along with the amount and timing of irrigations. The 50%, 150% and 250% treatments received corresponding cumulative totals. It can be seen that in the 2 months period from 20 June to 20 August (which encompasses most of the season), the total depth of irrigation applied was approximately the same each year, about 200 mm. In 2004 the irrigation amounts were greater earlier in the season than in 2005 and 2006, but irrigations had essentially ceased by the last week of July. In 2005 and 2006 the irrigation patterns were similar, with irrigations continuing into the middle of August. In 2004 and 2005 the crop had achieved full canopy by 15 July, while in 2006 full canopy was achieved about 1 week later, perhaps due to the ponding caused by the two large rainfall events. In all years, the canopy grew rapidly in the 3 weeks prior to full development, during which time flowering and early fruit set was initiated. Fig. 4 shows an example of hourly measured soil water content in the top 70 cm. The data are averaged across the four replications for a 20 days period in 2005, for the 50%, 100% and 150% irrigation rates. It can be seen that soil water content is dynamic, particularly for the 100% and 150% irrigation rates. The increases in soil water content due to irrigation and the subsequent decreases due to crop water use can be clearly seen. Being able to measure soil water dynamics enables short and
longer term changes in soil water content to be seen, although such data is subject to spatial variability in soil properties and wetting patterns as well as differences due to the individual sensors and the relatively small volume of soil that is measured. Soil water content under the 50% irrigation rate was lower and declined over time compared to the two higher rates. However, the short term dynamics of soil water content can make longer trends over time difficult to discern. To identify longer term trends, the average weekly measured volumetric water content at 6 a.m. in the top 70 cm, from the middle of June to the end of July, is shown in Fig. 5 for the 3 years. In 2004 we were not able to make measurements beyond the middle of July, but by that time the irrigation season was almost finished anyway. In all years, soil water content reflected the relative irrigation rates once irrigations commenced, with the 50% rate being the lowest and 150% the highest throughout the season, as would be expected. The trends in soil water content over time can be seen most clearly in 2005. In the 50% irrigation treatment soil water content generally declined over the season, indicating that irrigation was not sufficient. In contrast, soil water content in the 150% treatment generally increased over time, indicating excess irrigation. Soil water content in the 100% treatment was approximately constant, indicating that irrigation approximately replaced crop water use from the center of the bed. In
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Fig. 5. Weekly average measured soil water content at 6 a.m. in top the 70 cm in 2006 (a), 2005 (b) and 2004 (c) for the 50%, 100% and 150% irrigation treatments. The error bars show the standard deviation of measurements between replications.
2004 the same trends of declining, stable and increasing soil water content in the 50%, 100% and 150% treatments respectively can be seen during the measurement period. In 2006 soil water contents early in the season were relatively high due to the high rainfall in June. The 100% treatment however did decline during the first 3 weeks of July, indicating that irrigation might not have been sufficient to meet crop water requirements. Soil water content in the 150% treatment was more constant, indicating that perhaps it better reflected crop water use. The differences in soil water content between the 100% and 150% treatments were approximately similar in 2005 and 2006.
Fig. 6. Total yield of seedless watermelon in 2004, 2005 and 2006 as a function of relative irrigation rate. Error bars show standard deviation of yield.
Fig. 6 shows the yield response to the irrigation rates (three rates in 2004 and five rates in 2005 and 2006). Surprisingly, there was little difference between treatments within the same year. The highest yields were in 2005 and the lowest in 2006, but within each year there were no statistically significant differences in yield. The coefficient of variation (CV) between replications ranged from 2% to 20%, with an average across all years for the 50%, 100% and 150% treatments of 9.3%. The CV in the 0% irrigation treatment was high in both years (2005 and 2006), averaging 27%. In 2004, 2005 and 2006 the highest yields were in the 150%, 100% and 50% irrigation treatments respectively. In 2005 and 2006, yields in the 0% irrigation treatment were surprisingly high considering that the only water inputs came from rainfall, and that much of this was shed to the edge of the bed by the mulch. In all years, as for total yield, there were also no significant differences in number of melons and average melon weight. Sugar content values ranged from 11.3 to 12.5, but again there were no significant differences within a year and no consistent trend observed. Similarly, the incidence of hollow heart was generally low with no significant differences. Larger plots might have reduced the variability between replications, but it is likely that results would still be similar in terms of the relatively small differences between treatments. This lack of significant yield differences over a large range in irrigation amount was unexpected, especially considering the clear measured difference in soil water content. The lack of yield response is however an important result with implications for irrigation management. Soil water content in the center of the bed can be controlled by the grower through irrigation, but outside this relatively limited area soil water content is determined by rainfall (over which the grower has no control). The lack of a significant yield difference between the 100% treatment and the higher rates may be because soil water content was at least adequate in these treatments. The excess water may drain below the root zone in sandy soils, perhaps also removing nitrogen. However, soil water content in the 50% irrigation treatment as measured in the center of the bed was clearly declining for much of the season in all years, indicating that the crop was using more water than was replaced. The root zone is apparently extensive enough in a humid climate that the net extraction of water from the center of the bed is not sufficient to cause water stress and significant yield reduction. We did not measure soil water content in the 0% treatment in 2005 and 2006, but it was likely to be even lower than in the 50% treatment. Although yield was lowest in the 0% treatment, it was still higher than we expected considering that the only source of water was rain. The crop must have been extracting water from locations outside the center of the bed or from deeper in the profile. The two primary sources of water (irrigation and rainfall) have very different distribution and dynamics within the field. Under the mulch there is a wetted volume (‘‘onion’’) under each dripper along the dripline, and this volume is frequently replenished by irrigation and depleted by transpiration. Its shape is determined by the dripper flow rate and the relative rate of movement vertically and horizontally (Fig. 1). Outside the
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mulch the soil is wetted directly by rainfall and, near the edge of the mulch, by runoff. Wetting is much more uniform spatially than it is under the mulch. However, outside the mulch water can freely evaporate if the soil surface is sufficiently wet. Evaporation is reduced once the soil surface layer dries sufficiently to inhibit water transport to the surface from deeper in the soil. Thus, small rainfall events that only wet soil near the surface will likely be lost to evaporation. Larger rainfall events will wet the soil below the evaporation layer, and this water potentially will be available to plant roots and, to some extent, to move horizontally towards the center of the bed. Thus, rainwater will be available to the crop depending primarily on whether the rainfall events significantly rewet the soil below the evaporation layer, and on whether roots are present to take advantage of it. Apparently rainfall in the 3 years of this study was sufficient to compensate for the lack of sufficient replenishment of the irrigated volume under the 50% and the 0% irrigation rates. The years in this study were not very dry, and perhaps there will only be a significant response to irrigation in a dry year (similar to an arid or semi-arid climate). If this is the case, in many years growers in humid regions could significantly reduce their irrigation without adversely impacting yield. Currently, growers tend to over-irrigate, with rates closer to the 150% and 250% treatments of this study and so they should be able to safely reduce irrigation to the 100% rate in all years. Lower rates than 100% are probably safe in many years. An appropriate irrigation management strategy might be to apply sufficient water in the early part of the season to ensure good crop establishment, and then to apply irrigation amounts that are less than crop water use as long as rainfall events have been of sufficient magnitude and timing to keep the soil profile wet outside the mulch. In the event of a long dry period, the irrigation can be increased to more closely match crop water use (the 100% rate). However, additional research will be necessary to support this hypothesis by documenting the lateral extent, depth and water uptake of watermelon roots as a function of irrigation and rainfall patterns. Although we only studied the yield response of a seedless watermelon variety, it is likely that seeded varieties would have a similar response under similar weather conditions. 4. Conclusions Watermelon yield and quality in 2004, 2005 and 2006 were not significantly affected over a large range in irrigation rate, and this has implications for irrigation management in humid regions. It is likely that in many years growers could rely more on rainfall to provide water than they currently do, and so significantly reduce irrigation without reducing yield.
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Additional studies will be needed to quantify the interactions between irrigation amount and rainfall on root distribution and crop water use in humid regions. Acknowledgements We are grateful for the help of Dr. Jim Starr, USDA ARS, Beltsville, MD, USA, and his team, with the instrumentation. We also acknowledge the help and support given by T-Systems International, San Diego, USA and Sentek Pty, Stepney, Australia for advice and equipment. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56. Food and Agriculture Organisation, Rome, 299 pp. Clark, G.A., Maynard, D.N., Stanley, C.D., 1996. Drip-irrigation management for watermelon in a humid region. Appl. Eng. Agric. 12 (3), 335–340. Erdem, Y., Yuksel, A.N., 2003. Yield response of watermelon to irrigation shortage. Sci. Hortic. 98, 365–383. Fares, A., Alva, A.K., 2000. Evaluation of capacitance probes for optimal irrigation of citrus through soil moisture monitoring in an entisol profile. Irrig. Sci. 19 (2), 57–64. Gunter, C.C., Lang, M., Nowaskie, D., Thompson, A., 2001. Seedless watermelon cultivar trials for southwestern Indiana. Southwest Purdue Agricultural Program, Purdue University. www.hort.purdue.edu/hort/ext/veg/ MWVVT2001/html/INGunter2001SeedlessWM.htm. Maynard, D.N., Sidoti, B.J., 2003. Triploid watermelon cultigen evaluation. University of Florida Gulf Coast Research and Education Center. Research Report BRA-2003. National Watermelon Promotion Board, 2005. Watermelon Reference Book. National Watermelon Promotion Board, Orlando, FL. Paltineanu, I.C., Starr, J.L., 2000. Preferential water flow through corn canopy and soil water dynamics across rows. Soil Sci. Soc. Am. J. 64, 44–54. Schultheis, J.R., Adams, D., Holmes, G., Adams, M., 2001. North Carolina Hybrid Triploid and Diploid Watermelon Cultigen Trial-red Flesh. North Carolina State University, Department of Horticultural Science. Simsek, M., Kacira, M., Tonkaz, T., 2004. The effects of different drip irrigation regimes on watermelon [Citrullus lanatus (Thunb.)] yield and yield components under semi-arid climatic conditions. Aust. J. Agric. Res. 55 (11), 1149–1157. Starr, J.L., Timlin, D.J., 2004. Using high-resolution soil moisture data to assess soil water dynamics in the vadose zone. Vadose Zone J. 3, 926–935. Starr, J.L., Paltineanu, I.C., 1998. Soil water dynamics using multisensor capacitance probes in non-traffic interrows of plow- and no-till corn. Soil Sci. Soc. Am. J. 62, 114–122. University of Delaware Commercial Vegetable Production Recommendations, 2006. Extension Bulletin 137. Cooperative Extension Service, College of Agriculture Sciences, Newark, Delaware. Wang, Y., Xie, Z., Li, F., Zhang, Z., 2004. The effect of supplemental irrigation on watermelon (Citrullus lanatus) production in gravel and sand mulched fields in the Loess Plateau of northwest China. Agric. Water Manage. 69, 29–41. Xie, Z., Wang, Y., Wei, X., Zhang, Z., 2006. Impacts of a gravel-sand mulch and supplemental drip irrigation on watermelon (Citrullus lanatus [Thunb.] Mats. and Nakai) root distribution and yield. Soil Tillage Res. 89, 35–44.