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Publication #AE510

Water Use for Seepage-Irrigated Watermelon with Plastic Mulch in Florida1

Sanjay Shukla and Niroj K. Shrestha2

In Florida, watermelon is an important crop that accounts for a significant part of the state’s agricultural water use. Florida ranked first nationally in watermelon production area in 2010, accounting for 19% (10,500 hectares [ha]) of the national watermelon acreage (http://edis.ifas.ufl.edu/pi031). Depending on the type of production system and climate, the water use of this crop can vary. In Florida, watermelon is predominantly grown on raised plastic-mulched beds. To develop improved water management and allocation plans, accurate water use estimates for watermelon are needed.

Seepage irrigation under plastic mulch is a common production system used to produce watermelon in south and northeast Florida where the water table is shallow. Seepage irrigation involves artificially raising the water table to within 18–24 inches, in order to supply water to the crop root zone. This requires applying large volumes of water to the narrow irrigation ditches (between every two to six raised mulched beds). Such shallow water tables for seepage irrigation result in wet row-middles. In seepage-irrigated farms, water tables rise quickly after a rainfall; for south Florida, the water table has been shown to rise a height of 16 times more than the rainfall depth (Jaber et al., 2006). Such conditions cause near-saturation to full saturation of soil in the row-middles’ bare soil area on seepage-irrigated farms. A wet row-middle causes high evaporation from the row-middle and, therefore, higher ETc compared to drier row-middles for drip irrigation.

Plastic mulch alters the rainfall entry and soil temperature of the raised beds and can significantly affect ETc. Evapotranspiration from a mulched production system is different from an open field production system. Covering soil with impermeable plastic reduces soil evaporation and increases transpiration, compared with open field production. The effect of plastic mulch on ETc can also vary depending on climate. For example, crop water use for watermelon grown with plasticulture in drier semi-arid region will be different than a subtropical region with higher humidity. The difference also arises with the irrigation method. There can be large differences in ETc between drip and seepage irrigation systems that are used to grow watermelon in Florida. While ETc for drip-irrigated watermelon has been quantified (see EDIS publication AE 506, Water Use for Drip-Irrigated Watermelon with Plastic Mulch in Florida), there is no information on seepage-irrigated watermelon grown on plastic mulch for subtropical Florida. This publication summarizes the results from a crop water use study for the seepage-irrigated watermelon in south Florida.

Calculating Crop Water Use for Watermelon

For more than 40 years, crop water use or ETc has been calculated with the “crop coefficient” method, which uses a crop-specific coefficient (Kc) and reference ET (ET0) to calculate the actual crop ET (ETc). Reference ET refers to the evapotranspiration (evapotranspiration + transpiration) from a well-watered grass, which can be calculated using commonly available weather parameters. The Florida Automated Weather Network (FAWN) can provide ET0 estimates. This weather station website has a tool to estimate ET0 using the site specific weather data (visit http://fawn.ifas.ufl.edu/). Temperature, humidity, wind speed, and solar radiation are the primary weather parameters that influence ET0. For example, high ET0 occurs on a sunny, dry, windy, and hot day, whereas low ET0 occurs on a cloudy, humid, cold day with little wind.

Crop Coefficients

Different plants use different amount of water under the same climatic conditions. Crop water use for a vine crop such as watermelon will be different than an erect crop (e.g., pepper) because of differences in evaporation and, to an extent, transpiration. Scientists have developed Kc values that relate ET0 with actual crop water use (ETc) for a specific crop of interest. Several studies have been conducted throughout the world to estimate local Kc values, which are then used to translate ET0 (well watered grass) to ETc using the following relationship:

ETc = ET0 × Kc

Where ETc = crop evapotranspiration or crop water use

ET0 = reference evapotranspiration

Kc = crop coefficient

A lysimeter is the most commonly used method for measuring water use for agricultural crops for daily or longer time periods (weekly or monthly). Literature Kc values are available for a wide variety of crops; however, they may not be applicable for all situations. Although crop coefficients have been developed for the few selected plasticulture crops in Florida to allow growers and water managers to accurately estimate crop water needs, they are not available for seepage-irrigated watermelon. Crop water use for drip-irrigated watermelon has been provided in another EDIS publication (Shukla et al., 2014). The wet row-middles of seepage-irrigated fields result in significantly higher ETc compared to drip systems that have drier row-middles. Therefore, despite the use of local (Florida) Kc for drip-irrigated watermelon, it will lead to erroneous estimates of field-scale crop water use for seepage-irrigated watermelon. The large potential for errors in quantifying ETc for seepage-irrigated crop requires the development of Kc values specifically for seepage-irrigated watermelon grown on mulched beds.

Watermelon ET Study

A three-year study was conducted at the UF/IFAS Southwest Florida Research and Education Center, Immokalee, Florida, to measure crop water use and develop Kc for seepage-irrigated watermelon. Two drainage lysimeters (16 × 12 × 4.5 ft.) (Figure 1) were used to estimate ETc using the data collected for three spring seasons (2003, 2004, and 2005). Each lysimeter had two beds (length = 12 ft., width = 32 in., height = 8 in.) that contained six plants (three plants per bed). Using flowmeters, the irrigation, drainage, and runoff were measured. The water balance equation can be written as follows:

ETc = Rainfall + Irrigation – Drainage – Runoff – Change in Soil Moisture Storage

All inflow and outflow from lysimeters are used in the above water balance to calculate ETc as residual of the water balance (inflow minus outflow, including the change in moisture). Using measurements from the lysimeter for three years, ETc was calculated on a biweekly basis (every two weeks). With the known value of biweekly ETc from lysimeters, Kc values were estimated by ratio ETc to ET0, where ET0 was calculated using the FAO Penman-Monteith method (Allen et al., 1998). Based on crop cover, crop coefficient values were developed for four stages: initial stage (0 to 10% of ground cover), development stage (10% ground cover to effective full cover), mid-season stage (effective full cover to start of maturity), and late stage (maturity to harvest).

Figure 1. 

The drainage lysimeter for the watermelon crop water-use experiment


Credit:

Shukla et al. (2014)


[Click thumbnail to enlarge.]

Watermelon ETc and Kc

A three-year average ETc for seepage-irrigated watermelon was 373 mm (14.7 in.), and this value was 34% higher than drip-irrigated watermelon grown at the same farm for the same period (Shukla et al., 2014). The difference in ETc between drip- and seepage-irrigation systems was low during the initial stage. The practice of wetting the fields to make the soil workable for bedding and maintaining high moisture levels for root establishment of the transplants resulted in similar row-middle soil moisture in drip- and seepage-irrigated watermelon and, therefore, similar ETc during the initial stage. The difference in ETc between the drip and seepage irrigation systems was highest during the development stage. This difference was because the vine cover during this stage was less than effective full cover. This lack of vine cover resulted in higher evaporation from the uncovered soil surface and, therefore, ETc for the seepage-irrigated watermelon because of wetter row-middles (compared to drier row-middles of the drip-irrigated watermelons). The three-year average Kc and ETc for different crop stages are shown in Table 1. The initial Kc for watermelon was considerably higher than reported in the literature for agricultural crops due to wet soils in the row-middles during the initial stage.

Table 1. 

Stage-based crop coefficient (Kc), crop ET (ETc), and reference ET (ET0) for watermelon

Crop Stage

Kc

ETc (mm)

ET0 (mm)

Initial

0.64

34.2

53.43

Development

1.00

151.3

150.85

Mid-season

1.28

121.7

94.51

Late-season

1.15

68.3

59.36

1 in. = 25.4 mm

 

The stage-based Kc values in Table 1 are shown graphically in Figure 2 for ease of use. Crop coefficient values from Figure 2 and site-specific ET0 can be used to calculate ETc for a given day during the growing period. Most modern weather stations can provide ET0 estimates for their farms. In the absence of a nearby weather station, you can locate the nearest weather station that is part of the Florida Automated Weather Network (FAWN) (visit http://fawn.ifas.ufl.edu/) and obtain the local ET0 values. Monthly values of reference ET can also be obtained online at https://edis.ifas.ufl.edu/ae481 for major cities in Florida. The example below shows the use of Kc values from the lysimeter study for calculating ETc for seepage-irrigated watermelon.

Figure 2. 

Crop coefficient (Kc) values for seepage-irrigated watermelon for days after transplant (DAT)


Credit:

Shukla et al. (2014)


[Click thumbnail to enlarge.]

Situation: Watermelon was planted on plastic mulched bed on March 1, 2014, at a farm in Immokalee, Florida. The watermelon was irrigated using seepage system. What will be the ETc for April 30, 2014, which was 60 days after the transplant (DAT)?

Step 1. Obtain daily ET0 in Immokalee using the FAWN weather station (visit http://fawn.ifas.ufl.edu/) at Immokalee, Florida. The ET0 obtained from the FAWN weather station for April 30, 2014, was 0.15 in./day.

Step 2. Read Kc from Figure 2 for 60 days after transplant (DAT) as 1.3.

Step 3. Calculate the crop evapotranspiration (ETc) for April 30 as follows:

ETc = Kc × ET0 = 1.3 × 0.15 = 0.20 in./day

Use of Kc values from Table 1 or Figures 2 provides ETc values for seepage-irrigated watermelon under plastic-mulch production systems with shallow water table conditions. The initial high Kc value was due to shallow water table conditions that resulted in high unproductive losses of water from wet row-middles. Compared to literature (i.e., FAO-56 [Allen et al., 1998], a publication used worldwide as reference for Kc values), a proportional increase in Kc was not observed during the later stages, because the area shaded by the leaves increased with the plant growth to the extent that at effective full growth, almost the entire row middle was covered, which reduced the evaporation (Figure 1). It should be noted that the Kc values presented here represent an average of three-year period. Year-to-year variations in weather and crop parameters may result in Kc values being different than those in Table 1. Watermelon ETc obtained can be used for a variety of applications ranging from constructing water budgets for the farm or watersheds to water allocations. Note that the crop water-use obtained from the use of Kc shown above does not include application and subsurface losses. To obtain water that needs to be pumped for irrigation, divide the ETc by the irrigation efficiency (visit http://edis.ifas.ufl.edu/ch153). Irrigation efficiency is always lower than 100%. Typically, seepage irrigation system will have efficiency of 40% (Stanley and Clark, 1991). Use of Kc values shown here can help improve the accuracy of ETc estimates for seepage-irrigated watermelon in subtropical Florida.

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. Rome, Italy. Food and Agriculture Organization of United Nations.

Jaber, F. H., S. Shukla, and S. Srivastava. 2006. Recharge, upflux, and water table response for shallow water conditions in southwest Florida. Hydrologic Process. 20(9): 1895­­–1907.

Shukla, S., Shrestha, N. K., Goswami, D. 2014. Evapotranspiration and crop coefficient for seepage-irrigated watermelon with plastic mulched in a sub-tropical Florida. Transaction of ASABE 57(4): 1-9.

Stanley, C. D. and Clark, G. A. 1991. Water table management using micro irrigation tubing. Soil Crop Science Society Florida Proceeding 50: 6–8.

Footnotes

1.

This document is AE510, one of a series of the Agricultural and Biological Engineering Department, UF/IFAS Extension. Original publication date August 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Sanjay Shukla, associate professor, Agricultural and Biological Engineering, UF/IFAS Southwest Florida Research and Education Center, Immokalee, FL; and Niroj K. Shrestha, Post-doctoral Associate, Agricultural and Biological Engineering, UF/IFAS Southwest Florida Research and Education Center, Immokalee, FL; UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.