301: Graywater Irrigation Principles
Graywater 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 significantly different to the movement of water through soil when the garden is irrigated twice per week with potable water.
Capillary Effect and Evapotranspiration (ET) calculations are the most important factors in creating an efficient graywater irrigation system.
Understanding how plants respond to near constant soil moisture levels will help you select the most cost effective planting stocks.
When graywater is applied every day, the top 4 - 6" of soil reaches constant and wide area moisture. For landscape beds this means we water the soil, instead of the plant. Dripperlines are installed to water the soil, not the plants directly.
Evapotranspiration tells us how much water is required per month to grow a reference crop of alfalfa. Plant / Crop coefficients are published comparing their water needs with alfalfa.
Crop coefficients are misleading for turf / lawns, because they define the amount of water required for full growth. Recent studies by Texas A & M indicate green lawns can be maintained with 50% less water, if irrigation is managed smartly, as is done with the IrriGRAY controller.
301.3 What Plants Really Want >
Plants would prefer to keep their roots in the topsoil where most nutrients are located, IF the topsoil is constantly moist. Graywater gives us constant moisture, and with roots in the topsoil, plant growth to maturity is much fast than with traditional irrigation.