The Water Cycle Under Grazing

#Four numbers, and only one of them survives

Here are four figures that circulate constantly in South African regenerative material, usually presented as measured local results:

High stock density gives 22 mm/h infiltration against 41 mm/h at low density.
Light-to-moderate grazing retains about 75% of ungrazed infiltration; heavy grazing about 50%.
Trampling raises bulk density by 8–17% and cuts macroporosity by 57–83%.
Karoo crusted soils infiltrate at 40 mm/h on shale and 83 mm/h on dolerite.

Now the provenance. The first does not exist — it appears nowhere in the South African thesis it is attributed to. The second is Gifford & Hawkins (1978), a North American review, quoted inside that thesis's literature chapter. The third is Singleton et al. (2000) cited via Kurz et al. (2006) — New Zealand and Irish work, from the same literature chapter. The fourth is genuinely South African (Mills & Fey 2004), but it sits behind a paywall and could not be checked at source.

One in four is even arguably local. That is the state of the numbers you are being sold, and it is the reason this lesson refuses to give you an infiltration rate to quote.

#What actually changes the water cycle

Strip out the mysticism and grazing affects hydrology through three things you can see from the bakkie: basal cover, litter, and the condition of the surface millimetres. There is no soil sponge. There is a surface that either takes water or sheds it.

Cover. Basal cover is the proportion of ground occupied by the bases of living plants. It intercepts raindrop energy before it hits bare soil, and raindrop energy is what starts a crust. Falling basal cover is the earliest hydrological warning a farm gives, and it shows long before anyone says the word "drought".

Litter. Dead plant material on the surface does the same job after the plant has been grazed, plus it slows overland flow enough for water to enter. On the Highveld, litter carried through winter is what stands between a bare soil and the first high-intensity spring thunderstorm.

The surface millimetres. This is where South African grazing effects concentrate, and it is measurable: on Bloemdal soils near Bloemfontein, poor-condition veld held 14% less organic carbon and bare soil 26% less than good-condition veld in the upper 25 mm (du Preez et al. 2011). A thumb-joint of soil carries the difference between infiltration and runoff.

#The rainfall you measure is not the rainfall the plant gets

Water is lost between the gauge and the root in four ways: intercepted by canopy and never reaching the ground, run off over a capped surface, evaporated from bare soil, and drained past a root zone that stops at a plinthite layer.

Run the arithmetic on a Highveld farm at roughly 730 mm. Assume — and these are illustrative coefficients for the exercise, not measured South African values — that a capped surface sheds 40% of the rain as runoff and that thickened woody cover intercepts 15% of what remains:

730 mm × 0.60 = 438 mm actually entering the soil
438 mm × 0.85 = about 372 mm reaching the root zone

Look at the number you finished with. Three hundred and seventy millimetres is not Highveld rainfall. That is a Karoo-margin figure. Through cover loss and encroachment, management has moved the farm into a different rainfall class without the climate changing at all.

The interception half of that has a real South African measurement behind it: in a southern African semi-arid savanna, interception loss roughly doubled with a thirteen-fold increase in woody cover (PMC10017313). And at catchment scale, 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 catchments at 8.4% and 7.4% (Le Maitre et al. 2016). Riparian invasions cost far more water per hectare than dryland ones, which is why the wattle jungle in your drainage line is usually the highest-return clearing on the farm.

#The genuine South African result, and it is about rest

Now the good news, and it comes from home ground. Kotzé (2015) sampled communal (continuous), commercial (rotational) and land-reform systems across a clayey grassland and a sandy savanna ecosystem for a University of the Free State doctorate. What that work found:

Soil degradation was less pronounced under rotational than under continuous grazing in the clayey grassland. Rotational camps showed little evidence of degradation overall.
In continuously grazed communal grassland, degradation was driven by aggregate breakdown and associated organic matter loss, plus nutrient loss from low cover and litter input — and it was concentrated in the sacrifice area around the water point.
In the sandy savanna, continuous grazing exhausted plant nutrients specifically near water points.
Aggregate fractionation is a sensitive early indicator of the onset of degradation in the clayey grassland — it moves earlier than bulk soil carbon does.
Reduced grazing pressure stimulated microbial activity in both ecosystems, with a positive feedback between microbially mediated nutrient mineralisation, grass cover and biomass.
Clayey grassland soils showed short-term resilience because rest periods under rotational grazing compensated for high grazing pressure in a way continuous grazing could not. Sandy savanna soils were more resilient over the long term.

That last point is the mechanism the rest of this course is built on. It is not density. It is rest. Recovery is what lets a soil absorb a hard graze; without it, the same hoof pressure is simply damage.

#The piosphere is where your soil actually fails

Put the Kotzé findings together and the conclusion is uncomfortable for anyone who has spent a season arguing about mob size. In both ecosystems, in both systems, the damage lived around the water point. Even the well-run rotational camps showed early deterioration of aggregate structure near the trough.

The piosphere is the ring of trampled, dunged, over-grazed and structurally damaged ground that radiates out from every permanent water point. Grazing pressure decays with distance from water, so the trough is the one place on the farm that is grazed hard on every rotation, in every season, in every year, regardless of your grazing chart.

Do this on the farm

Find and read your piosphere. Half a morning, no equipment.

Walk out from each permanent water point along four lines at roughly right angles to each other.
Mark where the ground changes: where basal cover picks up, where dung density drops off, where the grass stops being short and flat and starts standing. Pace it — you want a radius, not a survey.
At the trough, at your marked edge, and at double that distance, dig a spade face and squeeze a handful of moist soil. Does it break into crumbs, or does it come apart as powder and slabs? That structural difference is the aggregate breakdown Kotzé measured, and it shows before the grass does.
Record the radius and the date. Repeat each season. A growing radius is the earliest whole-farm degradation signal you own.
Then act on it: rotate temporary water points where you can, take permanent troughs off vertic and hydromorphic soils, and accept that moving a trough is a soil intervention, not a plumbing job.

#Measure the difference, not the number

Because there is no trustworthy South African infiltration rate to quote, do the one measurement that needs no literature at all: a paired test.

Take two rings — a 20 cm offcut of pipe pushed a few centimetres into the ground works. Put one on bare, capped ground and one under an adjacent grass tuft, within a few metres of each other so the soil form, slope and rainfall history are identical. Pour one litre into each and time it.

The comparison is the measurement. Do not attach an absolute millimetres-per-hour figure to it; you have not controlled for enough to earn one. What you have is a difference on your own soil, on a dated record, that you can repeat in the same two spots every quarter. If the bare ring ponds while the tuft ring drains, you have a crust — the cheapest thing on the farm to fix and the most expensive to ignore.

Do this for four quarters and you will know more about your own farm's hydrology than any published rate could tell you.

Contested — here is both sides

Does hoof action improve infiltration?

For. On a hard-setting, capped soil, concentrated hoof impact physically breaks the crust and presses litter into contact with the surface, which is exactly what a sealed soil needs. The mechanism is real, the effect is visible within a season, and advocates can point to the well-established plant-side benefit of adequate recovery to argue the whole package works.

Against. Hawkins (2017) reports that two of the three studies on soil infiltration found no effect of high-production grazing relative to continuous grazing, and no significant difference in plant basal cover, plant biomass or animal gain. Note the provenance before you weight it: this is a global meta-analysis of 75 datasets from five countries — Argentina, Australia, Canada, the United States and Zimbabwe — published in an African journal. Only one of the five is African. It is strong evidence about the general claim and it is not home-ground data. Her qualifier cuts mildly the other way — mean annual precipitation and animal density did raise the effect size for plant basal cover only — but not for biomass or animal gain. And on a vertic or duplex soil at field capacity, the same hoof action that breaks a cap makes a pan.

Where this course lands. The direction is well established: heavy grazing compacts, reduces macroporosity and lowers infiltration. The magnitudes most often quoted are North American, New Zealand or Irish, and there is no verified South African rangeland figure. Soil form and moisture status decide whether hoof impact helps or harms on any given day. What the South African evidence does support is that rest matters and that the water point, not the paddock average, is where structure fails.

So put down the infiltration rate you were going to put on a slide. Walk out from your troughs this week, pace the piosphere, dig three faces, and set up one paired ring test you will repeat in three months. The next lesson takes the water that did not infiltrate and follows it downhill, which is where the law starts paying attention.

#Check yourself

4 questions — answers explained as you go

1A consultant shows you a slide claiming high-density grazing lifted infiltration from 22 mm/h to 41 mm/h. What is the single best response?
Why: The figure has no traceable source. Three of the four infiltration and compaction numbers commonly presented as South African are North American, New Zealand or Irish, and this one is not in the cited work at all. Season is a fair question, but provenance comes first — and the honest position is that no verified South African rangeland figure of this kind exists.
2Kotzé (2015) found soil degradation concentrated in one place across both a clayey grassland and a sandy savanna, in continuous and rotational systems alike. Where?
Why: The piosphere is the best-evidenced soil finding in the South African literature reviewed for this course. Even well-run rotational camps showed early aggregate deterioration near the trough. It follows that moving or rotating a water point is a soil intervention, and that a farm arguing about mob size while running one permanent trough per big camp is arguing about the wrong variable.
3Why is a paired ring test — one ring on bare ground, one under an adjacent tuft — better farm practice than quoting a published infiltration rate?
Why: You have not controlled for enough to earn an absolute rate, and you do not need one. What you need is a dated, repeatable difference on your own soil that tells you whether a crust is forming or breaking. It also sidesteps the provenance problem entirely, which is the point.
4Your gauge reads 730 mm for the season but the veld performs like a much drier farm. Which explanation is most consistent with the South African evidence?
Why: Losses between the gauge and the root are interception, runoff, bare-soil evaporation and drainage past a shallow root zone. Interception loss roughly doubled with a thirteen-fold increase in woody cover in a southern African savanna study, and Highveld rainfall arrives as high-intensity storms that cap soils gentle rain would not. Management, not the climate, moved the farm into a drier class.