Drip irrigation is widely used in viticulture and other field crops. However, there has been little evidence to date regarding the net effects of diurnal factors on drip irrigation and whether or not irrigating early morning compared to in the afternoon changes the water use efficiency. A field experiment was undertaken to investigate the diurnal factors and to quantify any effects on irrigation efficiency. It showed that, for soil depths >15 cm, drip irrigation efficiency is not affected by the time of day of watering—there were no diurnal effects and the results did not support a long-held belief that watering in the heat of the day should be avoided. At shallow depths (from 0 to 15 cm), afternoon irrigation actually resulted in a higher moisture content in this layer, with some of that difference persisting over the following 72 h. This finding also has the potential to be applied to irrigation scheduling for salad vegetables and similar shallow-rooted crops where, by irrigating in the afternoon rather than early morning, water usage could be reduced while at the same time potentially increasing the yield.