The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period of watermelon are shown in the figure below.
This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.
he crop can deplete soil water to a soil water tension of over 2 atmospheres without the yield being affected. Irrigation should take place when, depending on the level of evaporation, the soil water has been depleted some 50 to 70 percent of available soil water. In dry climates with moderate evaporation and little rain the watermelon produces an acceptable yield (15 ton/ha) with one heavy irrigation in the beginning of the growing period when soil water over the full root depth is brought to field capacity.
The growth periods of a 80 to 110 day watermelon are: the establishment period (0) of 10 to 15 days; the vegetative period (1) of 20 to 25 days, including early (1a) and late vegetative growth (vine development, 1b); the flowering period of 15 to 20 days; yield formation (fruit filling, 3) of 20 to 30 days and ripening (4) 15 to 20 days. The crop usually has 4 fruits per plant, which is controlled by pruning practices, and harvest date depends on the number of fruits per plant and on uniformity of ripening.
The relationship between relative yield decrease and relative evapotranspiration deficit is given in Figure 50. Water deficit during the establishment period (0) delays growth and produces a less vigorous plant. When water deficit occurs during the early vegetative period (1a), less leaf area is produced which causes yield reduction. The late vegetative period (vine development, 1b), the flowering period (2) and the yield formation period (fruit filling, 3) are the most sensitive periods to water deficit. During the ripening period (4) a reduced water supply improves fruit quality. Yields are little affected by water deficits immediately prior to harvest.
Whereas under limiting conditions some water savings may be made during vegetative (1) and ripening (4) periods, water supply should be directed toward maximizing production per ha by meeting full crop water requirements rather than extending the cultivated area under limited supply.