Erosion, Wetlands and the Drainage Line

#Two numbers, four orders of magnitude apart

Gauteng has 347 149 hectares classified at high sheet-and-rill erosion risk. Gauteng has 110 hectares of mapped gully.

Both figures come from the same national assessment (Le Roux & Smith, Grain SA, November 2014). They describe completely different things, and a farmer who reads the first as though it were the second will spend his whole erosion budget on the wrong problem.

Gabions, rock packs and check structures are what erosion looks like in the farming press, because dongas photograph well. They are the right answer in the Eastern Cape and Northern Cape. On the Highveld they are somebody else's answer to somebody else's problem, bought with your money.

South African specifics

The two national datasets, and why they must never be added together.

Province	Sheet & rill: area at high risk (ha)	Gully: area affected by mapped gullies (ha)
Eastern Cape	3 860 702	151 759
Northern Cape	1 974 854	160 885
Free State	2 153 343	64 674
KwaZulu-Natal	1 284 975	87 522
North West	379 879	10 782
Gauteng	347 149	110

The left column is modelled susceptibility — where sheet and rill erosion is likely given soil, slope, rainfall and cover. It is not observed loss, so a well-covered farm inside a high-risk polygon is not necessarily eroding. The right column is mapped physical features — over 100 000 individual gullies identified nationally by satellite.

Read the columns against each other and the country divides: the Eastern Cape and Northern Cape have South Africa's donga problem, and Gauteng essentially does not.

#What 12.6 tonnes a hectare actually means

The headline national figure is a mean predicted water-erosion soil-loss rate of about 12.6 t/ha/yr, rising to roughly 13 t/ha/yr under annual cropland — about three times Australia's 4.1 t/ha/yr, against a soil-formation rate the same source puts at under 5 t/ha/yr.

Two honesty notes. The assessment is from 2014, a decade-old picture. And "under 5 t/ha/yr formation" is generous — much of the pedological literature puts actual formation an order of magnitude lower, which makes the deficit worse rather than better. No published South African soil-formation rate could be verified for this course, so read the comparison as "erosion vastly exceeds replacement", not as a precise ratio.

Work it on a 100 ha holding at the national mean:

100 ha × 12.6 t/ha/yr = 1 260 tonnes of soil per year leaving, on the modelled average
less formation at the source's own generous 5 t/ha/yr: 100 × 5 = 500 t/yr replaced
net deficit: about 760 tonnes a year

Seven hundred and sixty tonnes of topsoil off a smallholding in a year, with nobody watching it go and no line for it in the books. It is a modelled mean, not a measurement of your farm, so what it gives you is the order of magnitude — and the invisibility.

#Triage: three forms, three completely different bills

Sheet erosion removes a uniform skim across a whole slope. You cannot see it happening; you notice it when the A-horizon is gone and the veld no longer responds to a good season. Cover fixes it, and only cover.

Rill erosion is the first visible stage: shallow parallel channels a hand-span deep, usually below a track, a gateway or a bare patch. A rill under 30 cm deep is repaired with cover on the catchment above, a brush pack in the channel, and rest. Cheap.

Gully erosion (donga) is a capital project with a head-cut migrating uphill. It is the symptom, not the disease — the runoff that carved it was generated above it.

Do this on the farm

Repair hierarchy, cheapest first. Do not skip a step to get to the exciting one.

Stop the cause. Fix cover and grazing on the catchment above the erosion. A structure built below an unfixed catchment fails, and you pay twice.
Vegetation and brush packing in the gully floor to slow flow and trap sediment.
Packed-stone or rock-pack check structures at intervals in the channel.
Gabions — only where the hydraulic loads justify the cost, and only engineered with proper keying and an apron.
Concrete or earth weirs — last resort, and they need a design.

Two overrides. Where a gully has an active head-cut, armour the head first, then work downstream; working upstream from the mouth wastes money. And check the law before steps 2 to 5 — every one is regulated, from two directions.

Steps 3 to 5 are priced per structure and per cubic metre; step 1 is priced in changed grazing decisions, which is why it comes first. Before any structural spend, phone the provincial Working for Wetlands office — co-funded rehabilitation on private land exists in some catchments (SANBI).

#The law arrives before the machine does

The legal position

Conservation of Agricultural Resources Act 43 of 1983, regulations GN R1048 in GG 9238 of 25 May 1984 as amended:

Regulation 8 — the flow pattern of run-off water. Except on written permission of the executive officer, no land user may divert run-off water from a watercourse to any other watercourse; and no one may create, permit, remove or alter an obstruction to the natural flow pattern of run-off water where that would cause excessive soil loss or resource deterioration. It governs contour banks, waterways, keyline-style earthworks, brush packs and rock packs alike, and is routinely left out of regenerative teaching.
Regulation 13 — restoration and reclamation of eroded land. Every land user shall restore or reclaim land on which excessive soil loss due to erosion occurs or has occurred, and the executive officer may direct further measures in writing. Doing nothing about a donga is not a neutral choice in South African law.
Regulation 7 — vleis, marshes, water sponges and watercourses. No land user may use the vegetation in a vlei, marsh or water sponge, or within the flood area of a watercourse, or within 10 metres horizontally outside that flood area, in a way that causes or may cause deterioration or damage to the natural agricultural resources. Regulation 7(3) forbids draining or cultivating a vlei, marsh or water sponge — or cultivating within the flood area or that 10 m strip — except on written permission.
Regulation 7(2) — the one that surprises everybody. It requires removal of vegetation in a watercourse to the extent that it would otherwise obstruct flood flow and cause excessive erosion. Regulation 7 is not "don't touch". It is "don't degrade, and keep the flood path clear."

Farmers hear the first half of Regulation 7 and conclude the drainage line is a no-go zone. The regulation demands more: manage it, keep it working as a flood path, get written permission before draining or cultivating it.

The legal position

National Water Act 36 of 1998 is the other half of the same decision. Section 21 lists eleven regulated water uses; the ones that catch erosion works are 21(c) impeding or diverting the flow of water in a watercourse and 21(i) altering the bed, banks, course or characteristics of a watercourse. Under section 22 you may use water without a licence only under Schedule 1, as a continuation of an existing lawful use, or under a general authorisation.

And Schedule 1(1)(b)(iii) is not the blanket exemption it is usually described as. It permits taking water on land you own or occupy for the watering of animals — excluding feedlots — which graze on that land within the grazing capacity of that land, from a resource on or bounding it, if the use is not excessive relative to the resource and other users. Four conditions, each able to fail. Overstock and you can lose the exemption, which makes the grazing plan and the water entitlement one decision.

The water-law half is taught in full in Module 8. The trigger you need here: a dam, crossing, weir, gabion or gully structure in a drainage line very likely needs CARA Regulation 8 permission and a National Water Act authorisation — two authorities, two applications, before the TLB arrives.

#The vlei is the most valuable and most wrecked ground you own

Wetlands are the highest-value, most legally protected and most easily destroyed hectares on a South African livestock farm — and in the summer-rainfall interior they are the only green feed in late winter, which is exactly why they get destroyed.

Delineation follows the departmental field procedure using four indicators — terrain unit, soil form, soil wetness (gleying and mottling) and vegetation — with the outer boundary where hydromorphic indicators stop. A Katspruit or Kroonstad form, from the previous lesson, is a wetland flag before you look at a plant. (The guideline's current edition year is inconsistently cited; confirm it with the Department of Water and Sanitation or a registered wetland practitioner rather than quoting a year.)

Get it delineated once, properly, and mark it on the grazing plan.

Contested — here is both sides

Fence the wetland off, or graze it?

Exclude it. Grazing in riparian zones reduces vegetation complexity and therefore the wetland's flood attenuation and sediment-trapping function — precisely the service the state is protecting. Wet soils pug catastrophically, the damage heals slowly, and a fence with a gate that stays shut is the simplest guarantee of compliance.

Manage it. South African industry guidance is explicit that wetlands must be managed, not merely fenced off — the wording in the sugar industry's own sheet is "controlled burning and controlled grazing", alongside no cropping and active removal of alien and invader plants (SASRI; see also the Milk SA buffer guideline). A shut gate does none of those things. Alien clearing in particular does not happen by itself, and Regulation 7(2) requires you to keep the flood path clear whether you graze the vlei or not.

Where this course lands. Manage it, with conditions, and record what you did. The practitioner rules usually given — moderate pressure, growing season, stock off the deeper wetter soils — are practitioner judgement rather than sourced guidance, so monitor them rather than cite them.

#The highest-return hectare on the farm

One more reason the drainage line comes first. Invasive alien plants reduce national surface runoff by about 1 444 million m³ per year, equal to 2.9% of naturalised mean annual runoff, with the worst-affected catchments at 8.4% (Mossel Bay to Tsitsikamma) and 7.4% (Border, Eastern Cape) (Le Maitre et al. 2016). A figure near 2 444 million m³/yr also circulates; it is not a measured upper bound but a sensitivity analysis of what the estimate becomes if riparian invasion is as under-mapped as specialists believe.

The farm-scale point: riparian invasions cost disproportionately more water per hectare than dryland invasions. The wattle and gum in your drainage line are drinking water you paid for and destabilising banks you will otherwise pay to repair.

So before you price a gabion, do three things. Walk the drainage line and count active head-cuts, dated, so you can watch the number fall. Look hard at what your veld surface will be in September, because a bare winter surface meeting the first thunderstorm is where the real loss happens. And find out in writing which permissions you would need before moving a cubic metre of soil.

#Check yourself

4 questions — answers explained as you go

1A Gauteng farmer reads that his province has 347 149 ha of erosion and puts his whole erosion budget into gabions in his drainage line. What has gone wrong?
Why: The two datasets measure different things and differ by more than three thousand to one in Gauteng. The left-hand column is modelled risk, not observed loss, so a well-covered farm inside a high-risk polygon may not be eroding at all. Gabions have their place — in the provinces that actually have dongas, below a catchment already fixed.
2Your farm has a 2 m deep gully with an actively retreating head-cut. Where does the first money go?
Why: A structure below an unfixed catchment gets paid for twice. Then armour the head-cut before anything downstream — working upstream from the mouth wastes an erosion budget.
3Which statement about CARA Regulation 7 is correct?
Why: Farmers hear the prohibition and miss the duty. Regulation 7 is "don't degrade, and keep the flood path clear", and Regulation 13 adds a positive obligation to reclaim land already eroding. Doing nothing is not a safe harbour, and the National Water Act applies alongside CARA rather than instead of it.
4You run 20% more stock than your CARA grazing-capacity figure and water them from a stream that bounds your farm. What is the water-law consequence?
Why: Schedule 1 has four conditions, and grazing within the capacity of the land is one of them, tying your water entitlement directly to CARA Regulations 10 and 11. There is no numeric volume threshold in Schedule 1 — the widely quoted 10 m³/day figure comes from registration practice, not the exemption. Module 8 takes this apart in full.