Water use productivity of the rooibos tea crop in the winter rainfall region, Western Cape, South Africa

Masters of Science

Aspalathus linearis, commonly known as rooibos or red tea, is one of the well-known herbal tea beverages in South Africa. A. linearis is known for its medicinal, economic, traditional and ecological values. This leguminous shrub is endemic to the greater Cederberg Region of the Western Cape Province, South Africa. The area receives winter rainfall. With the Western Cape Province predicted to get drier in future with increasing temperatures and heat waves, the sustainability of range restricted species like A. linearis may be negatively affected by climate change. Given the value of the rooibos crop, there is a need to establish the sustainability of rooibos production to mediate the effects of climate change. Henceforth, this study seeks to determine the water use and yield patterns of rain-fed rooibos crops. In addition, the study sought to test how environmental conditions affect rooibos production and estimate the water use efficiency of A. linearis under present-day (May 2019-June 2020) conditions in a prime rooibos growing area in Porterville in the Western Cape Province. The study's objectives were achieved by determining the micro-climate, soil physical properties, plant attributes and the water use dynamics of cultivated A.linearis. Weather data were obtained from an automatic weather station (AWS). The field leaf area index (LAI) was measured using a LAI-2000 leaf area meter. Plant growth was determined by measuring plant height, width and breath. Soil samples were collected to determine the type and physical properties of the soil at the rooibos field, while soil moisture content was determined in the field using soil moisture probes at 20, 40, 60, 80 and 100 cm depths. Water use of the rooibos crop was quantified using various methods such as micro-stem heat balance sap flow sensors, Penman Montheith equation, open path eddy covariance, FruitLook and the soil water balance method. Lastly, to assess the potential impact of climate change on the rooibos crop, ETo was estimated using two Shared Socioeconomic Pathways (SSPs): SSP1-2.6 and SSP5-8.5. The results show that the study area is dominated by sandy soils, characterised by a low water holding capacity. The rooibos crop's average growth ranged from 1.0 cm in winter to 12cm in spring. LAI increased steadily from 0.28 in June and reached its peak at 1.56 in December.