Streamflow reduction due to Eucalyptus plantations in the St. Lucia Catchment
Abstract
Lake St. Lucia is an internationally recognized, vital estuary lake system that is the core feature of the iSimangaliso Wetland Park, a UNESCO World Heritage Site. Lake St. Lucia is situated within the study area. W32 is a tertiary catchment with extensive afforestation, most dominant in E32G and W32H. Inappropriate management of coastal aquifers may result in irreversible damage and destroy freshwater resources (Vaeret et al., 2009). Eucalyptus tree roots will increase in depth if the soil moisture is more reliable at the increased depth and is not restricted by confining geology. Where the soil is deep and the groundwater table is approximately eight meters below the surface, a three year old Eucalyptus tree is most likely extracting water directly from the groundwater and the total evapotranspiration may likely exceed total precipitation (Benyon et al., 2006; Von Roeder, 2014). Reference evapotranspiration (ET0) and potential evapotranspiration (PET) are the same in the absence of water restrictions, which is the case where trees have tapping roots mining groundwater from an aquifer (Karisson & Pomade, 2004).
The ETₒ rate of the Eucalyptus trees in the Lake St. Lucia study area were calculated using the Shuttleworth-Wallace (S&W) model (1985) as per example by Zhou et al. (2006), using 2014 and 2015 meteorological data. The S&W model quantified viable time variant data comparable to literature data, and with little variance within monthly simulated ETₒ. The maximum average daily ETₒ for 2014 and 2015 months' as simulated using the S&W model, is 6 mm/day, while the minimum average daily ETₒ is 3.4 mm/day. The average monthly total ETₒ for 2014 and 2015 has an 85 % correlation to the monthly ET estimated by the WRC (2016) for quaternary catchment W32H and an 84 % correlation to that of quaternary catchment W32G. The average annual simulated ETₒ exceeds the annual average MAP, by a factor of 1.67.
Drawdown increases at a 3 % greater rate per year during drought in comparison to years with normal MAP. The drawdown continues to increase, without exception for every sequential year - for both normal rainfall and drought scenario’s, correlating to the results from S&W that the annual ET exceeds the annual precipitation.
After six years of afforestation, the impact of afforestation, only considering normal rainfall circumstances, decreased streamflow most by 14.45 m³/day at Zone 8. Zone 8 has afforestation along the whole western streamflow zone boundary. Zone 2 is reduced by 4 m³/day and Zone 7's streamflow is reduced by 3 m³/day when subjected to forestry's water needs - irrespective of receiving the MAP. Zones 4 and 9's streamflow do not reduce when receiving normal rainfall and is subjected to afforestation. Streamflow Zone 4 has no afforestation situated along it. Zone 9 flows out of Lake St. Lucia which is recharge by regional groundwater, precipitation and the inflow from the other streamflow zones; zone 9 does not have reduced streamflow even when subjected to drought and afforestation. The streamflow reduction ascribed to afforestation's impact is generally constant and not significantly greater due to drought, but adds to the steamflow reduction from the drought