Considerations for Irrigation Management in Watermelon Production – Vegetable Crops Hotline

Considerations for Irrigation Management in Watermelon Production

About half of the watermelon fields in our area are not equipped with supplemental irrigation. Watermelon production in these fields is therefore dependent exclusively on rainfall. In fields where supplemental irrigation is available, drip irrigation under black plastic mulch is the most commonly used system. Overhead irrigation through central pivot is also used in some fields.

Irrigation management is complex in our area because of significant but unpredictable rainfalls during the watermelon production season. Supplemental irrigation is profitable because it avoids water stress during periods of drought, thereby increasing and stabilizing yields. However, there is always a question whether supplemental irrigation is required for watermelon production in our region; if so, how much water should be applied; if not, what would be the consequence if there was an extended drought period. I do not have a straightforward answer to all the questions. But here are a few facts that I think are helpful in guiding water management in watermelon production.

Effects of water stress on watermelon yield. What happens in a dry year?

It is important to understand that effect of water stress on watermelon yield is different at the different growth stages. The majority of yield reductions occur when drought stress happens at the flowering and fruit set stage. The second most sensitive stage is at the fruit expansion stage. The direct consequence of drought stress that happened at the fruit expansion stage is the smaller-sized fruit. During the vegetative period, watermelon is less sensitive to water stress.

Black in 2018, there was an extended dry period from June 20 to July 20, with total precipitation less than 0.5 inches in the one month period recorded at the Southwest Purdue Agricultural Center (Figure 1). we compared watermelon yield from two adjacent fields: one received weekly 1-inch water applied to the mulched area through drip tape; the other field was not irrigated. About a 5% yield decrease was observed in the field without irrigation. There was no difference in fruit numbers, but average fruit weight decreased from 16.6 lb to 15.8 lb.

Figure 1. Irrigation and precipitation events at different stages of watermelon growth during the 2018 watermelon production season at the Southwest Purdue Agricultural Center.

How much water should be applied through irrigation? 

Supplemental irrigation eliminates water stress during the drought period. But it also brings the possibility of overwatering. Overwatering not only results in inefficient use of water, but also leads to fertilizer leaching, and may increase the risk of certain soilborne diseases, like vine decline of watermelons.

A common rule of thumb widely used in vegetable production is a crop at full growth in the middle of the summer uses about 1 to 1.5 inches of water per week. This assumption is based on the average reference evapotranspiration (ETo). If assuming about 1/3 of the field is covered with plastic mulch, and the water applied through drip tape is eventually distributed under plastic, about 9,051-13,577 gallons of water per acre watermelon land per week should be given to the crop. The actual ETo may be obtained from climate centers that can help guide irrigation applications more accurately.

Should the theoretically optimal irrigation rate be followed for watermelon production in a humid area like our region? A three-year study conducted in Delaware that has a similar climate as ours sheds some light on this question. Three irrigation treatments: 100% crop water use based on ETo applied to mulch area (optimal irrigation), 50% (deficient irrigation) and 150% (excessive irrigation) of the optimal level were applied to watermelon fields. Although there was a wide range of differences in water application, surprisingly, watermelon yield was similar across the treatments. The soil water content in the 50% irrigation treatment as measured in the center of the bed down to 2’ deep was clearly declining over the season, indicating the crop was using more water than what was replaced in the center of the beds. But the lack of yield response indicated watermelon roots are extensive enough to extract water either in the deeper soil or outside of the plastic mulch-covered beds.

Without plastic mulch, water on the surface easily evaporates. But the evaporation is reduced once the soil surface layer dries sufficiently to inhibit water transport to the surface from deeper soil. Following large rainfalls, water will wet the deeper soil between the beds, and to some extent, horizontally move to the center of the bed. This water potentially will be available to plant roots if roots are present to take advantage of the water. Plants will need extra energy to grow a more extensive root system, this might disrupt water balance for some crops, and cause detrimental effects to the yield. However, this seems is not the case of watermelons.

What does this mean practically? Using ETo as the guidance for irrigation management is a good starting point. Indiana State Climate Office is working to get the ETo readily available for growers. The National Weather Service provids the forecasts of ETo. Please read the article in this issue Reference Evapotranspiration Forecasts across Indiana to learn how to access the forecast ETo. Using the general rule of thumb (1-1.5 inches of water per week) is also valuable. If growers are currently irrigating over that level, irrigation should be cut back. Lower than optimal irrigation level to 50% probably is also safe in many years based on the study conducted in Delaware. This is especially true if there are large rainfall events that significantly rewetted the deep soil.

Reference:

McCann, I., E. Kee, J. Adkins, E. Ernest and J. Ernest. 2007. Effect of irrigation rate on yield of drip-irrigated seedless watermelon in a humid region. Scientia Horticulturae. 113:155-161.

Erdem, Y. and A. N. Yuksel. 2003. Yield response of watermelon to irrigation shortage. Scientia Horticulturae. 98:365-383.

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