Watermelons require regular irrigation of 8 to 10 inches with either rain or irrigation. The highest water requirement occurs just prior to emergence of seedlings, during the bloom time and about ten days before harvesting time. The frequency of watering differs during various stages of growth from once a week during the early stages of growth to daily at times when plants require most water, cites the University of California Vegetable Research & Information Center.
The watermelon is a deep-rooted plant that is well tolerant of drought. However, unless provided water regularly, yield and fruit quality is affected. Low water during early stages of fruit causes smaller sized, misshaped fruit. Low irrigation at the time of flowering leads to a condition called blossom end rot where the flower stops growing and turn leathery and dark. Too much or too little water just before harvesting can cause fruit to split.
The plants may be watered with drip irrigation systems, sprinklers or manually. In any case, take care not to keep the top of the bed wet. Since watermelons grow on the ground, continuous exposure to excessively wet ground is likely to cause fruit rotting and spots. Water early in the morning to allow foliage enough time to dry during the day. This reduces chances of foliar diseases.
After the plants have been started in the ground, either with seed or transplants, water plants only minimally and allow growth for two to three weeks before starting a regular watering schedule. This allows the roots to grow deep into the ground, recommends the UC Vegetable Research & Information Center. Avoid using sprinklers between 7 a.m. and 11 a.m. during the flowering and fruit setting stages as this can affect pollination.