Water use and productivity of Cannabis sativa L., KwaZulu-Natal Midlands, South Africa.

Aims: The South African National Water Act (No. 36 of 1998) mandates the regulation of land-based activities that reduce streamflow by declaring them streamflow reduction activities (SFRAs). Hemp (Cannabis sativa L.) is commonly known as a water-intensive crop, yet no published journal articles providing measurements of its evapotranspiration (ET) or crop factor (Kc) exist in South Africa, and there is limited information on hemp ET and Kc internationally. Therefore, its impact on streamflow reduction cannot be assessed. In the context of this research, the term water use was used synonymously with ET, and refers to the combined soil evaporation and transpiration from the Cannabis sativa L. crop (and when present, weeds or grasses in the interrow), which is the overall water use associated with growing the crop.

Methods: This study provides ET data to determine if irrigated hemp should be investigated further as a potential SFRA by determining its ET and water productivity. An eddy covariance (EC) system was utilised in a hemp field trial. Standard microclimatic variables, volumetric soil water content, plant height, and Leaf Area Index (LAI) were measured.

Results: Total ET from the hemp crop over the measurement period (7 December 2022 to 15 April 2023) was 377 mm. The average daily ET was 28.4 L/tree, or 2.94 mm/plant irrigation depth. The crop coefficient varied between 0.73 and 0.77, and the water productivity was 0.96 kg of fresh bud per m^- 3 of water. Hemp had a high water use and low water productivity compared to international hemp studies due to a low planting density (2000 plants/ha).

Conclusions: These results provide the first field-based measurements of water use and crop coefficient estimates of hemp in South Africa and contribute to the very limited data available internationally. In South Africa they will be critical to assess the streamflow reduction activity of hemp.