There have been a limited number of studies comparing the effects of three different levels of temperature of irrigation water on plants. The studies that have been conducted have generally concluded that increasing the temperature of water results in warmer soil. This has a positive effect on some plants but not on others. The change in soil temperature varies with the depth of soil, the amount of water used to irrigate and the type of soil.
Water and Soil Temperature
Cold water causes a drop in soil temperature. Most seeds germinate more quickly in warm, springtime soil, and seedlings grow quickly in warm soil. However, changes in the temperature of the soil caused by irrigation water are short lived. Agronomists at the University of California, Davis, found that the temperature of the soil 4 inches deep returned to its previous temperature within 24 hours of being irrigated by cold water. At 12 inches deep, soil returns to its original temperature in 48 hours. Clay and heavy, poorly drained soils take longer to return to return to their original temperature. Soils returned to their original temperature more slowly at night. Larger amounts of cold water mean that soils generally take longer to return to their original temperature.
Variable Plant Response
Agronomists at the University of California, Davis, report that differences in water temperature vary with the species of plant. Caloro rice likes warm water. Studies show that caloro rice matures only if water temperatures remain above 70 degrees Fahrenheit. In greenhouse experiments, a five-day drop in root temperatures from 77 degrees to 59 degrees Fahrenheit depressed the growth of the rice. However, studies show that lettuce and strawberries all benefit from cold water that lowers soil temperature.
Montana State University agronomists cite a study in which alfalfa was flood irrigated at temperatures of 60, 70, and 80 degrees Fahrenheit. Flooding periods were 4, 8, 12,16 and 20 days. Alfalfa leaves began yellowing 7 to 8 days at 80 degrees Fahrenheit, 10 days at 70 degrees and 14 days at 60 degrees. The agronomists concluded from these data that alfalfa growers who flood irrigate should do so early in the season to take advantage of less vigorous plant growth caused by cooler water and soil temperatures.
Beans and Cucumbers
Agronomists at the University of California, Davis, experimented with different water temperatures on red kidney beans and cucumbers. They irrigated the kidney beans at water temperatures of 50, 59, 68 and 77 degrees Fahrenheit. They irrigated cucumbers at temperatures of 41, 50, 59 and 68 degrees Fahrenheit. They found no significant differences in the yield of beans or cucumbers when cold water was used.
Agronomists at the Cape Peninsula University in South Africa tested the effects of irrigating spinach in greenhouses at water temperatures of 75.2, 79.8 and 82.4 degrees Fahrenheit (24, 26 and 28 degrees Celsius). The spinach showed an increase in the production of leaves when the water temperature increased from 75.2 to 79.8 degrees Fahrenheit, but declined when the water temperature reached 82 degrees. The root growth and the weight of spinach shoots and leaves, however, increased when the water temperature was between 79.8 and 82.4 degrees Fahrenheit.