Stranger things: invasive alien plants’ coup of water-catchment areas

The Western Cape loses a dam’s worth of water due to invasive alien plants each year. Plant water-use experts, Drs Mark Gush and David le Maitre of the CSIR, share insight on this topic and how it affects fruit farmers. By Esté Beerwinkel.

Every year the Western Cape loses ±38 million m³ of water from its major water supply system — the equivalent to the whole Upper Steenbras Dam — due to invasive alien plants.

Dense stands of exotic tree species such as pines, wattles, eucalyptus, poplars, Australian acacias, hakea, and the like are responsible for a broadly estimated 1 000 m³/ha/year of water loss in the Western Cape. This is because invasive alien plants (IAPs) have increased evapotranspiration rates relative to the indigenous vegetation such as fynbos.

Drs Mark Gush and David le Maitre of the Council for Scientific and Industrial Research (CSIR), say that, based on dedicated studies, water loss caused by following species are:

• Pines (Pinus pinaster /radiata) = 430–2 000 m³/ha/year
• Riparian eucalypts (Eucalyptus camaldulensis) = 2 000 m³/ha/year
• Riparian wattles (Acacia mearnsii) = 450–1 350 m³/ha/year
• Riparian poplars (Populus canescens) = relatively low by comparison. 200 m³/ha/year.

Gush says riparian areas — areas next to rivers and streams — have greater water loss estimates compared to upland areas. Tree size and density also have an influence.

“The water extracted from the soil by IAPs is generally lost to the catchment — transported by wind to distant areas or the ocean — and is consequently expressed in the form of streamflow reductions. The aftermath of these reductions to catchment yield or useable water in Western Cape Water Supply System (WCWSS) is substantial — approximately 38 million m³ of water per year.”

But how can the agricultural community help relieve this issue? Gush suggests never wasting a good crisis.

“Agriculture uses both surface water (rivers and dam) and groundwater. The current drought created direct competition for the remaining water between domestic and industrial use on the one hand and irrigated agriculture on the other. Therefore, to secure more water sources organised agriculture needs to lobby for increased investment in effective clearing of invasive species — prioritising mountain catchments, riparian zones, and aquifers.”

So, what is the impact of this on fruit production — if one considers that 22 000 ha of apples will have an approximate water requirement of 132 million m³ (working with an average of 6,000 m³ ha including young orchards)?

“Streamflow reductions and lower dam levels have led to water restrictions, and difficult decisions for fruit producers. Less orchard irrigation is one such consequence. Clearing IAPs from water-catchment areas that supply dams can help top up water supplies.

“Using the average estimates stated above, every six hectares of IAPs cleared could yield enough water for one hectare of apples — 6 000 m³/ha/ year. In all cases, clearing riparian areas would yield a greater response (>3 times more water per unit area cleared). Clearing densely invaded areas would also be of proportionally greater benefit.”

Dr le Maitre notes that the current levels of IAP clearing aren’t keeping pace with the rates of spread.

“IAPs are estimated to be increasing by at least 5% annually. Even more concerning is that if clearing of IAPs were stopped, we could expect a reduction in streamflow of 130 million m³ per year by 2045 — the equivalent of the Berg River Dam will be lost annually.”

According to research by Gush, Le Maitre and the CSIR team, the solution lies in replacing IAPs with lower water using indigenous species — and curbing takeover of water-source areas by IAPs.

Indigenous species such as fynbos allow most of the rainwater to run off into our rivers and reservoirs, and also help to keep water in high-lying wetlands, slowly releasing it throughout the year. Fynbos catchments cover 8,47 million hectares or 6,7% of the entire country, and contribute 13% of the country’s surface water. The stats are:

• Fynbos catchments yield a mean annual runoff of 6 628 million m³ per year.
• Water-source areas cover 2.16 million hectares or 26% of the total fynbos catchment area.
• These areas produce a mean annual runoff of 5 032 million m³ per year or 76% of the total fynbos catchment runoff.
• This is the equivalent of 2 323 m³/ha/year — an unmatched six times the national average runoff per unit area.

“Relatively small water-source areas therefore produce disproportionally greater volumes of water and are critical for water supply.”

Gush and Le Maitre reiterate that although effective IAP-clearing programmes are expensive, the benefits far outweigh the costs.

“IAP-clearing programmes must be properly staffed, managed, and resourced, with skilled staff in oversight roles. The removal of IAPs has the potential to release an additional 1 000 m³ per year for every hectare cleared and rehabilitated to fynbos. This process will cost an estimated R80 million/year to maintain clearing within the WCWSS and R200 million/year across the whole province, but every cubic metre released improves water security and sustainability.”

What can growers do to help?

Landowners can assist by clearing their farmland of IAPs. Start high-up and work your way down — target dense infestations, particularly in streamside areas.

Clearing operations and suggestions

Further research and identification of increasingly effective control measures and technological advances, such as using drones to identify IAPs, can help with more cost-effective clearing.

Clearing operations in place include Working for Water, Cape Nature, Land-user Incentives, and LandCare.

Image supplied by Anna Mouton.