Assessing the energy and water balance dynamics of rain-fed rooibos tea crops (Aspalathus linearis) under changing Mediterranean climatic conditions

Abstract

Rooibos (Aspalathus linearis), a herbal tea crop and a member of the plant family Fabaceae, grows in a narrow ecological range under Mediterranean conditions in South Africa's fynbos biome. There is growing uncertainty on the sustainability of rooibos production under rain-fed conditions in a region which is expected to get warmer and drier in future. No accurate quantitative information currently exists on how the water use, growth and yield of this crop is affected by environmental factors, especially climate and soils. This paucity of data reduces the ability of farmers to develop effective strategies to cope and adapt to the negative effects of climate variability and change. The aim of this study was to investigate the seasonal changes in the energy and water balance of rooibos fields and to establish how this relates to crop yield. An open path eddy covariance system was used to quantify the actual evapotranspiration (ETa) and energy balance components over two growing seasons while sap flow was monitored using stem heat balance sap flow gauges. These data were used to develop and test a simple ETa and yield model which was applied to estimate changes in crop water use and yield under the predicted future climatic conditions. More than 36% of the annual total ETa (∼ 338 mm) was consumed during the first three months of the growing season (September to November) driven by residual soil moisture from the winter rains. Crop transpiration accounted for only 16% of ETa early in the growing season rising to almost 100% during the summer dry months. Daily peak transpiration at maximum canopy cover barely reached 1.0 mm suggesting that this crop uses very little water. Six climate change models were used to simulate crop water use and yield for the far future under the RCP 8.5 emission scenario. All the models predicted increases in both the annual ETa and yield by between 8% and 16% in the period 1961–2099. This study reveals that implementing water conservation and weed management practices early in the growing season can save substantial amounts of soil moisture which can sustain rooibos production under low rainfall conditions.