Greywater is generated every day (showers) or every few days (laundry), providing a guaranteed supply of water on a regular basis.
Because of virtually constant moisture levels in the soil, water is retained in the top soil. This is signficantly different to the movement of water through soil when the garden is irrigated twice per week with potable water.
Capillary irrigation and Evapotranspiration calculations are the most important factors in creating an efficient greywater irrigation system.
How greywater moves through soil (capillary effect).
What plants really want