How much water are we losing to alien trees?

This post provides an interactive visualization of the results of Le Maitre et al. 2016. Estimates of the impacts of invasive alien plants on water flows in South Africa. Water SA Vol. 42 No. 4. It aims to highlight how much water could be reclaimed by clearing alien species from our catchments.

The extreme drought that is gripping Cape Town and surrounds has municipalities desperately seeking options to augment bulk water supply. Unfortunately, local municipalities have thus far only invested in engineering solutions and we are increasingly witnessing negative impacts on the environment due to their hasty implementation.

Invasive alien species are estimated to reduce runoff into major dams supplying Cape Town and surrounds by more than 100 million litres per day (20% of Cape Town’s water requirement), yet we have seen no investment by the municipalities in alien clearing. This represents a major opportunity lost in the response to this drought as invasions (and thus water loss) are only predicted to become worse with time.

The Visualization

Here I’ve visualized the data as a zoomable map where you can toggle layers on and off and switch between basemaps by clicking on the names in the legend. Catchments with no data are clear and show no estimates of the reduction in Mean Annual Runoff (MAR).

The data represent the state of alien invasions and water reduction at the quaternary catchment scale as of 2008. It is generally believed that the extent of invasions has only become worse over time, leading to greater reductions in runoff. Conversely, the water gains from alien clearing are likely to be lower in lower rainfall years. I provide a brief description of the methods below. Note that I still need to add which catchments feed which major dams. If you have those data available please let me know and save me working it out manually!

The Science

The study provides a national assessment of the adverse impacts of alien plant invasions on water flows across South Africa at the quaternary (4th order) catchement scale (4 800 to 180 000 Ha). It builds on a previous assessment in 1998, adding recent research, using more realistic species and site-specific reduction factors, incorporating additional factors that limit plant water-use and using improved flow reduction models. Alien plant impacts are expressed as the reduction in naturalised mean annual runoff (MAR) data for quaternary catchments estimated by Middleton and Bailey 2008 study of Water resources of South Africa (WRC Report TT381/08. Water Research Commission, Pretoria).

Calibrated estimates of the distribution, species composition and density of invasions across most of the country were obtained from the Kotze et al. 2010 National Invasive Alien Plant Survey (Report No. GW/A/2010/21 for the Agricultural Research Council – Institute for Soil, Climate and Water, Pretoria).

Portions of the country not covered by the NIAPS mapping were excluded from the study. This predominantly included the arid interior, i.e. the Desert, Nama Karoo and Succulent Karoo biomes, and the arid portions of the Savanna biome. Transformed areas including cultivated, urban and forest plantation land cover classes were excluded, based on the South African National Land Cover 2000 by Van den Berg et al. (2008, Report No. GW/A/2008/86 for the Agricultural Research Council (ARC) and Council for Scientific and Industrial Research (CSIR), Pretoria).

It necessarily makes use of pragmatic assumptions and surrogates, but these are similar to those used in any water resource assessment. Note that the estimates are considered to be overly conservative, because the extent and impacts of riparian invasions have been underestimated.