Understanding the hydrological impacts of replacing commercial forestry with macadamia orchards: A case study from a groundwater-driven South African catchment

Código

I-EBHE0057

Autores

Mlungisi Shabalala, Michele Toucher, Alistair Clulow

Tema

WG 1.04: From local to large scale human-water dynamics

Resumo

In an attempt to reduce water demand, while maintaining livelihood, in the water-stressed Lake Sibayi catchment (Maputaland Coastal Plain, KwaZulu-Natal, South Africa), a 29 000 ha commercial forest plantation will be gradually replaced by macadamia nut orchards. This land use conversion is a response to physical water scarcity in the area, which has been observed through a drastic decline in the lake storage level since 2000. Lake Sibayi is the country?s largest freshwater lake and is the main source of water for domestic requirements for its immediate community. Furthermore, it has an economic value through tourism as it forms part of the iSimangaliso Wetland Park (a World Heritage Site). Commercial forestry in the lake?s catchment area, which is the main source of income for the local communities, has been highlighted as a large consumer of water resources. As a result, the local community has been searching for alternate land use activities which will sustain/improve the local economy, while maintaining a lower water footprint. Dryland macadamia production has been selected as the replacement land use activity, with 100 ha being planted in 2019. This study investigates the hydrological and socioeconomic implications of this land use conversion through measuring the water use (evapotranspiration) of macadamia trees, commercial eucalyptus plantations and natural grassland. Water use was measured using the eddy covariance (macadamia and grassland) and surface renewal (eucalyptus) methods over a two-year period, i.e. 2020-2022. This paper focuses on the water use of macadamia and grassland. Evapotranspiration from macadamia trees (ETMac) was consistently lower than the grassland site (ETGrass) during the period of comparison, which is unusual for evergreen crops. The water use was higher during the summer season for both grassland (735 mm) and macadamia (607 mm). Daily macadamia ET ranged from 1.1 to 6.9 mm.day-1 in summer and 0.4 to 3.5 mm.day-1 in winter. The relatively low ET in the macadamia orchard was due to the following; (i) the sandy nature of the soil resulted in low soil moisture content throughout the monitoring period, limiting root water uptake; (ii) the rainfall received during the monitoring period (575 mm) was lower than rainfall from other regions where macadamia is successfully grown under non-irrigated conditions (>1 000 mm); (iii) the midday vapour pressure deficit (VPD) was relatively higher (>2.5 kPa) than the reported threshold for transpiration (i.e. 0.8 kPa) in irrigated orchards, and (iv) maximum air temperatures often exceeded 30oC, particularly in the summer season, which is above the recommended optimal growing temperature. Findings from this study suggest that irrigation is required to supplement rainfall in young, non-bearing macadamia trees grown in a subtropical region predominated by sandy soils. Therefore, growers should consider irrigation before implementation of the next phase of the land use conversion.