The amount of water available will vary with the depth of the root zone and the texture of the soil. When using a tensiometer, place one at the 8-inch depth in the onion row and irrigation at 40 centibars tension on a fine sandy soil. These soil moisture measurement methods can indicate when the soil becomes too dry or if excess irrigation water is being applied. A good rule to follow in checking for irrigation need is to examine the soil around the root plate of the onion. This soil should be moist but not wet, and never dry.
Onions require light but frequent irrigation.
For successful production, the onion grower needs an ample and readily available supply of water for irrigation throughout the entire onion growing season. Kenneth R. Bolen, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.