Estimates of the impacts of invasive alien plants on water flows in South Africa

David C Le Maitre,Sebinasi Dzikiti,Mark B Gush,Greg G Forsyth

doi:10.4314/wsa.v42i4.17

Abstract

The adverse impacts of alien plant invasions on water flows have been a prime motivation for South Africa’s Working for Water Programme. The approach used in this study builds on a previous national assessment in 1998 by incorporating factors that limit plant water-use, information from recent research and improved flow reduction models. The total reduction in flows is estimated to be 1 444 million m 3 ·yr −1 or 2.9% of the naturalised mean annual runoff (MAR), less than half of the 3 300 million m 3 ·yr −1 estimated in 1998. Two main factors account for this difference: (a) a decrease in the estimated unit-area flow reduction to 970 m 3 ·ha −1 ·yr −1 compared with 1 900 m 3 ·ha −1 ·yr −1 estimated in 1998, largely due to the new model being based on more representative reduction factors; and (b) the updated estimate of the condensed invaded area of 1.50 million ha (previously 1.76 million ha), although the taxa mapped for this assessment only accounted for 1.00 million of the 1.76 million ha reported in 1998. Reductions due to invasions in Lesotho are estimated to be about 161 million m 3 ·yr −1 and those in Swaziland about 193 million m 3 ·yr −1 . The taxon with the greatest estimated impact was wattles ( Acacia mearnsii , A. dealbata , A. decurrens ) with 34.0% of the total reductions, followed by Pinus species (19.3%) and Eucalyptus species (15.8%). The revised estimate is considered on the low side largely because the extent and impacts of riparian invasions have been underestimated. If the current estimates that 4–6% of Acacia mearnsii , Eucalyptus , Populus and Salix invasions are riparian, are adjusted to a more representative 20%, 50%, 80% and 80%, respectively, the total reductions increase by nearly 70% to ~2 444 million m3·yr−1. Producing these estimates involved a number of assumptions and extrapolations, and further research is needed to provide more robust estimates of the impacts. Keywords: plant water-use, flow reduction, mean annual runoff, riparian invasions, Working for Water Programme

Highlights

Estimates of the impacts of alien plant invasions on surface water runoff in South Africa were, and continue to be, a key factor in motivating for the establishment and continuation of the Working for Water programme (Van Wilgen et al, 1998)
The model used for the first national estimates of reductions in water flows was based on biomass–age relationships for different plant growth-forms and a linear, biomass-based model which estimated reductions in mean annual runoff (MAR) in millimetres (Le Maitre et al, 2002, 2000, 1996; Van Wilgen et al, 1997): Streamflow reduction = biomass (g·m−2) x 0.238 (1)
This weakness was addressed by reworking the model so that the flow reductions were expressed as proportions, based on revised biomass equations (Dzvukamanja et al, 2005; Le Maitre and Görgens, 2003), like the flow reduction models for forest plantations developed by Scott and Smith (1997)

Summary

Introduction

Estimates of the impacts of alien plant invasions on surface water runoff in South Africa were, and continue to be, a key factor in motivating for the establishment and continuation of the Working for Water programme (Van Wilgen et al, 1998). Since additional information has been gathered on the water-use of invasive alien plants (reviewed by Le Maitre et al, 2015) and a new dataset on the distribution, species composition and density of invasions has become available – The National Invasive Alien Plant Survey (NIAPS, Kotzé et al, 2010). These two advances provide the basis for new estimates of the impacts of invasions on both river flows and groundwater resources – termed water flows in this paper.

Methods

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Discussion

Conclusion