First should know the quality of the soil, sand soil, loam soil or clay soil. Different soil have different water diffusion rate. And the water discharge rate is another factor. Clay soil has a shallow diffusion rate, but the lateral diffusion is wide; loam soil has a deep diffusion rate, but the lateraldiffusion scale is small; sand soil has the deepest diffusion rate and the smallest lateral diffusion scale.
type of soil lateral diffusion scale
Rough sand soil 15cm～46cm
Fine sand soil 30cm～90cm
Loam soil 90cm～130cm
Clay soil 160cm～180cm
Water discharge rate will affect the lateral diffusion scale. When the water discharge rate is big, the lateral diffusion scale is big. The reason is that: there will be a saturation zone below the emitter, it will accelerate the lateral diffusion.
The distance between the emitter and the plant will affect the absorption of water, nutrient and salt.
Usually the distance is small during seedling stage. for most of the conditions, it is proper for 45cm. It can be adjusted according to the quality of soil and water discharge rate. For example, when it is sand soil, the distance is proper for 30cm. When it is grown plant or grown trees, the distance can be increased, because the root system is developed and the plant can absorb the water and nutrient from a further distance.
When designing the drip irrigation, the distance has big difference between the seedling trees and the grown trees. And for the vegetables, because the line space is small, growing time is short, so the distance between the emitter and the plant does not need to change throgh different growing stages.
