Water Management in MJ
A new law in California, SB837, is going to regulate how much water the Cannabis growers can take from creeks in an attempt to preserve forest lands and the flows of streams mainly in the Emerald triangle of the northern part of the state. This is actually par for the course and should not surprise anybody involved in agriculture. River, lake or stream pumping is severely regulated in most states and marijuana growers are now bound by similar rules governing most crops. What may escape many of these growers is that the techniques to reduce water consumption and efficiencies have been around for decades and are widely employed in a varied mixes of combinations across the agricultural and horticultural industry. This is a just a sampling of them as you could write entire books about the gritty details of each one.
Automated, Drip and Micro Irrigation
For the man on the street the understanding of irrigation is often limited to water his potted plants with a hose bib and a wand or to flood the field (which has been the traditional methods in agriculture for more than 5000 years). Both methods are actually equally wasteful of the water resource. Hand watering although fine for a pleasant evening meditation in the backyard takes a very long time and is not particularly uniform. Flooding requires flat, leveled fields and extravagant amount of water. One of the most revolutionary ideas behind precision irrigation is that you simply do not need to wet the entire field and by just bringing water straight into the roots of the plant you save already a large part of the water. The installation of piping, valves, pump also allow precise scheduling of the irrigation to prevent excessive evaporation during the hottest time of the day or allow for all the water to be absorb fully in hard soils. Traditional flood irrigation efficiencies can be boosted from about 30% (30% of the water used actually going through the plant as sap) to over 85-90% efficiency by slowly dripping the water next to the roots or by limiting the wet area to a narrow trench under the row of plants. Growers had to wait until the late 60’s for technology (mostly molded plastic injection) to allow that precise application of the precious liquid. Furthermore, trying to put fertilizers through the lines complicated the matter greatly by clogging the lines and the drippers themselves. Those problems have since been resolved, but illustrate that fertigation (the combination of irrigation and fertilization) requires a careful attention to details.
Similar to the concept of putting the water just where you need it, as described above, reducing the volume of the dirt to be wetted work in similar fashion. Hence, growing in pots, or soiless substrates that are just there to slow the water down just enough for the plant roots to catch most of it. Combined with fertigation this is where most of the modern hydroponic techniques exist. Notice that pure hydroponics don’t even use substrate to contain the roots and rely on a variety of techniques to keep the water on the roots themselves.
A good root zone substrate, in opposition to natural soils, allows most of the water and fertilizer provided to be available to the plant and allow the roots to breathe. It creates synergies with the irrigation to greatly push yields per unit of water used (interestingly the measurable water efficiencies are not noticeably improved).
It may seem odd to list greenhouses as a water saving measure. The whole point of the greenhouse structure (or shadehouse) is actually to reduce environmental stresses on plants to make them grow better (either more yield or higher quality; ideally both) not only to protect them for cold snaps. It turns out, that much of the water consumption of a plant is determined by the amount of light but also ambient temperature and humidity (which can be combined in vapor pressure deficit, VPD). Greenhouses, by their very nature, limit the escape of gases from the structures and keep the canopy environment more humid, which limits transpiration by the plants and effectively reduces their consumption of water. Equipped with mechanical ventilation or vents to modulate the escape of vapors, turns the greenhouse into a water management tool. Furthermore, many structures also provide shade (retractable or not) that can control the amount of light energy received by the crop indirectly reducing the evapotranspiration and hence the water consumption.
One of the sources of inefficiency in irrigation comes from the need to slightly over water to prevent solid accumulations in the soil or substrate. Unless the water is distilled, it carries dissolved minerals and often even suspended solids which will inevitably accumulate in the root zone (even more so if fertilizers are applied alongside the water) causing all kinds of problems. Run-off is one of the simplest forms of management of these accumulations and what better way to reduce water consumption than to capture and reuse that run-off. The challenge is that now the irrigation water carries even more dissolved and suspended solids and managing these accumulations requires rigorous precision and careful water and soil analysis on a regular base. The benefit is that 99% or more of the water will be used in the plants.
A note on recirculation: although not mandated in most jurisdictions in the USA at the time of writing, it is a very common legal requirement for vegetable greenhouse facilities in Canada and most of Europe. Cannabis growers should expect this requirement coming soon, particularly if the same trends and growing techniques are followed.
This is the ultimate in water efficiency, a technique really only available to the most air tight, most high tech greenhouses or warehouses. It consists in condensating the water vapor in the growing space through the clever use of the HVAC systems and channeling that water back to the irrigation system (with the added benefit of that water being effectively cleared of any minerals, i.e. RO or distilled water). Depending on the tightness of the structure and the capacity of the coolers, most of the vapors can be recaptured and the very same molecules of water circulated AD ÆTERNAM. The only small loss of water being the little moisture left in the crop taken out of the structure. Even the largest multi-acre growing facilities can have water consumption figures similar to an average household.
Many of the techniques presented above have been well known and used extensively in horticulture for many decades. The important point however is that they are all additive methods that can be combined in many different ways to squeeze the “Most Crop per Drop”.