In the US over a third of our fresh water consumption is used to irrigate crops. Greenhouse crops can require up to 0.4 gallons of water per square foot per day. As water prices rise and extended droughts put a strain on resources, producers are aware of the need to conserve water and adopt more sustainable practices.
Closed greenhouse operations will always be more water efficient than open air growing environments. Glazing and shade cloth will reduce evapotranspiration, but in doing so reduce the available light. Automated shade curtain systems are probably best in most greenhouse applications and will deliver shade during the hotter parts of the day while maximizing sunlight during cooler times.
Drip irrigation can reduce water usage dramatically, often using 50–70% less water than more conventional methods. When combined with an automated watering system, drip irrigation is highly efficient, but the initial cost may make an automated system prohibitive for smaller operations.
Other sub-irrigation systems offer similar water savings and may be a good option in certain situations. Flood floors and trough systems have the potential to reduce fertigation (injection of fertilizers into an irrigation system) by up to 50%.
Constant pressure systems should always be used. Monitor for inefficiencies and leaks; even the smallest of leaks can waste over 2000 gallons of water a year.
Most soilless mixes are peat- or coir-based, or in the case of living soil, compost-based. These mediums are often very hydrophobic, especially on the surface, as they dry. Using a wetting agent in fertigation can reduce runoff from hydrophobic media.
Water-holding capacity can be tuned by the different percentages of media components used. Different media components have different characteristics with regard to saturation, container capacity, and the volume of water that can be held. Smaller, finer particles will hold more capillary water where larger, coarser particles will be faster draining, with less water-holding capacity. It is important that growers use suppliers that can deliver consistent quality media.
It’s a given that container size affects the volume of water a container can hold when saturated, but container shape is also an important consideration. Gravitational potential is higher at the top of a container and lower toward the bottom of the container. Capillary action, cohesion, and adhesion attract water particles to media particles; this is the water that is usable to the plant. The potential to hold more water is greater toward the bottom of the container.
Each type of media has a perched water table (the saturation point, where the capillary action in the soil is canceled out by the force of gravity that does not change no matter how tall the container). This can be easily understood with a rectangular sponge. When saturated and horizontal, little water will leach from the sponge. When turned vertically more of the water will leach, and the sponge, in that orientation, has less capacity to hold water. If you were to mark the top of the perched water tables on the sponge at both orientations you would find that they are at the same distance from the bottom.
The same holds true for media in containers. In shorter, wider containers, a greater percentage of media will be below the perched water table, reducing the amount of water needed to saturate and, with controlled watering, reducing the amount of water and nutrients that would otherwise become wastewater. Careful consideration must be made as to what containers and media are used. Short containers do not necessarily create optimal conditions. At different stages of development, plants will have different needs. Choosing containers and media solely based on an attempt to reduce water usage could lead to some devastating consequences or require unintended inputs that may create more cost than benefit.
Rainwater, generally speaking, is of exceptional quality. Rainwater harvesting can be applied to any scale operation and reduce dependency on municipal and groundwater resources.
Rainwater catchment systems will work best in gutter-connected greenhouses.
Polyethylene or fiberglass tanks, cisterns, and silos positioned inside the facility can help reduce heating inputs and keep water at a more ideal temperature for irrigation. Most rain harvesting systems will also require a human-made or natural wetland, swale, or other drainage area to catch and slow overflow.
Larger greenhouse operations should consider catchment ponds. Catchment ponds can catch both rain runoff and greenhouse wastewater. The water quality is often reduced in these systems and requires treatment. When run through micron filtration and ozonation systems, the water is exceptionally pure, removing chemical residues, pathogens, and pollutants. Using this exceptionally pure water can reduce pesticide and fungicide use, increase crop yield, and improve product quality. Regulation and watershed concerns vary by location and it is imperative that operators are aware of and comply with local laws and regulations regarding wastewater and rain catchment.