South Africa now has its first measured evidence of how much water hemp uses, from a study by researchers at the University of KwaZulu-Natal and Rhodes University that combined a flux tower in the middle of a commercial field near Pietermaritzburg with drones carrying thermal and infrared cameras. The crop used the equivalent of 377 mm of water over its four-month growing season, the team reported in an article first published by The Conversation and carried by Food for Mzansi.

On average, the ground occupied by each plant used about 28 litres of water a day, a figure that includes weeds growing nearby and evaporation from the soil. That puts hemp in the company of large, thirsty plants: a young eucalyptus, known for its heavy water use, takes 30–50 litres a day.

The question matters because hemp, a low-THC form of Cannabis sativa, has been legal to grow for personal use for several years and is being promoted as a "green gold" crop for small-scale farmers, for medicine, clothing, insulation and paper. South Africa's National Water Act can regulate crops shown to reduce the water reaching rivers and dams, as it already does for commercial forestry, but until now regulators had no reliable measure of hemp's draw.

The tower's sensors measured the water vapour the crop released and the wind speed continuously through the day without disturbing the plants; the researchers say this is the first time the method has been applied to hemp, earlier estimates having relied on satellite images or sap-flow measurements. Drone images of leaf temperature, cooler where plants transpire freely and warmer where they are short of water, were turned into water-use maps with the QWaterModel. The team also tracked temperature, wind, sunlight, 452 mm of rain over the season, soil moisture in the root zone and drip-irrigation inputs.

Three findings stand out for growers. Water use varied across the field with the shape of the land, soil depth and storage, so drone maps can show where plants are stressed and where irrigation should be targeted. Much of the water went to weeds between the rows rather than to the hemp itself, which makes weed control a water-saving measure. And the model overestimated total use compared with the tower, probably because of losses from bare soil and weeds.

The authors say the KwaZulu-Natal baseline allows water use to be estimated for other regions before plantations are expanded into areas where water is already scarce, and they urge growers and policymakers to build on it. The equipment is beyond the reach of small-scale farmers today, they add, but as drones and models become cheaper the approach points to where precision agriculture is heading. The project was funded by the Water Research Commission.

Written for The Conversation by Trevor Hill, Alistair Clulow, Richard Kunz and Shaeden Gokool of UKZN and Anthony Palmer and Sukhmani Mantel of Rhodes University, with master's graduate Gary Denton; republished by Food for Mzansi.

Source: Food for Mzansi