Agricultural Practices Which Increase the Risk of Erosion

Farmers do not intentionally cause erosion. But sometimes it happens inadvertently, as a by-product of agricultural practices. These can be grouped under three broad headings:

depletion of ground cover;

weakening of soil strength; and

damage to watercourses.

Depletion of ground cover

Opportunities for surface erosion are created, if strong winds or heavy rains follow events such as:

cultivation of paddocks when soil is dry and loose;

burning of stubble after crop harvest;

continued light grazing of pasture during drought, cold, or wet conditions;

heavy grazing of pasture at any time of year;

depletion of tussock grassland, by heavy sheep grazing, frequent burning, rabbit infestation or hawkweed competition.

Windblow, sheetwash and rilling of topsoil occur almost anywhere if ground cover is depleted, even on flat land. Light soils with a high silt or sand content are more susceptible than loams, heavy clays or peats.

Wind Blow

Wind blow

Sheet Wash

Depletion of ground cover also creates an opportunity for tunnel gullies (under-runners) to form in subsoil. This happens on sloping ground covered by sandy or silty soils. On these coarse soils, it is easier for rainwater to seep through than to flow across the surface. Once sub-surface, water easily moves downslope into hollows where its concentrated flow disperses the soil, i.e., detaches individual grains of sand and silt until tunnels are formed. If rabbit holes are present, they hasten the process. Eventually the tunnels collapse, forming shallow gullies.

Rilling

Tunnel Gully

Shallow Gully

On sloping land, shallow gullies some-times form directly from the surface, for example, where surface runoff from vegetated ground flows onto patches of soil that are completely bare. This happens on clay soils and peats, as well as sands or silts. Runoff picks up soil particles, initially rilling the bare surface. With each rainfall the rills deepen, eventually forming gullies which cut back and undermine surrounding, still-vegetated soil.

Very deep gullies can form on hill country if the underlying base is weathered rock rich in clay minerals. The soil is already disturbed by earthflows, exposing patches of bare rock underneath. In winter when water tables are close to the surface, runoff seeps out and cuts channels. With each successive winter, the channels cut deeper into the weathered rock and gnaw headwards.

Weakening of soil strength

Soil failure by mass movement (slipping, slumping or flowing of subsoil) can arise on hill country when heavy rains or protracted wet weather follow:

bush or scrub clearance to create new pasture;

cutting down scattered trees in the course of pasture improvement;

timber-cutting in residual patches of indigenous forest, or in farm woodlots;

undercutting of slopes by farm tracks.

Agricultural practices which involve tree removal are infrequent on farmland today. but such practices created much of the farmland in hill country districts of both Islands. Once cleared, some of the land is permanently susceptible to mass movement. Here are the reasons why.

Tree roots, particularly the lateral ones that spread sideways, greatly reinforce soil’s resistance to downhill movement, a bit like reinforcing mesh in concrete. Most soils have a high inherent resistance to movement anyway - due to factors like physical friction between soil particles, and chemical bonding between them - so the added reinforcement of tree roots is unnecessary. But in soils with weak resistance, removal of tree roots may tip the balance in favour of stresses acting on the soil. The main stresses are the downhill "pull" of gravity, and the pressure of water in saturated soil.

Deep Gully

Soil Slip

Earthflow

Slump

Tree removal also causes the inherent resistance of weathered rock to weaken after a number of years, Soil, and the underlying layer of weathered rock, wets and dries more often under pasture than under forest. This is not a problem in hard rock, but repeated cycles of wettin g and drying cause weakly bonded rock to disintegrate. They also promote chemical changes such as leaching of calcite cement from between rock grains, and swelling of clay minerals.

On geologically stable land these after-effects are not a problem. But slipping, flowing and slumping are greatly increased even now from past tree removal on hill country where subsoil is impermeable (clays and fine silts), or where underlying rocks are deeply weathered (soft marine sediments like mudstones); or where hard marine sediments have been fractured and crushed (argillites and greywackes).

Damage to watercourses

This can happen where farmers graze stock on streambanks in order to:

give access to water;

obtain extra feed;

suppress weed regrowth.

These practices are part of everyday farm management, but if uncontrolled they also cause breakdown of banks by livestock trampling, loss of soil into channels, nutrient enrichment (eutrophication) of water by dissolved chemicals in runoff from adjacent paddocks, and fouling by dung, urine and micro-organisms when stock enter channels. Similar damage occurs when stock crossings and vehicle crossings pass through streams, instead of across bridges or culverts. Where these effects are severe, siltation reduces the flood capacity of channels passing through a farmer’s property, while nutrient enrichment and fouling pollute his water intakes. Their cumulative build-up, from a number of farms in a river’s headwaters, certainly causes similar problems for downstream properties and water users.

Sediment is a particularly widespread contaminant of water in New Zealand. Not only does it cause siltation, clogging of streambeds and destruction of aquatic life, it renders water unsuitable for supply except at the cost of treatment. Sediment particles also carry other contaminants: nutrients, pesticides and chemical residues. So, reducing the discharge of sediment from streambanks can also control a large proportion of the contaminants reaching larger downstream waterbodies such as rivers, lakes and estuaries.

Siltation from bank collapse

Siltation from Bank Collapse