Weak Bedrock

Occurrence

The Weak Bedrock Environment is typically identified by rolling to steep topography where shallow soil overlies weak bedrock. This environment also includes unconsolidated material such as windblown silt, sand, pumice or ash deposits that overlies bedrock resulting in the formation of relatively deep well drained soils. The bedrock environments have commonly been referred to as ‘Hill Country’, however the Bedrock Environment also exists across plateaus where shallow soils overlie bedrock without any significant relief. Soils in this environment are typically thin and well drained, relative to other lowland environments.

Nationally 16.5% or 4,346,530 ha of New Zealand is classified as Weak Bedrock Environment. Manawatu, Hawkes Bay, and Gisborne have the largest areal extent. Gisborne and Taranaki have the highest proportion of the region at 45% classified as Weak Bedrock Environment.

Extent of the Weak Environment in New Zealand.
Region Area (ha) Regional (%) National (%)
Manawatu 910,527 41.03 3.46
Hawkes Bay 498,347 35.30 1.89
Gisborne 378,139 45.11 1.44
Taranaki 327,696 45.20 1.24
Southland 300,284 9.65 1.14
Northland 293,158 23.51 1.11
Canterbury 291,913 6.58 1.11
Otago 250,757 8.05 0.95
Tasman 236,585 24.63 0.90
West Coast 228,621 9.88 0.87
Auckland 171,517 34.99 0.65
Waikato 163,609 6.87 0.62
Wellington 149,360 18.63 0.57
Marlborough 136,943 13.10 0.52
Bay of Plenty 4,834 0.40 0.02
Nelson 1,288 3.06 0.00
slope map of new zealand with the parts classified as 'Weak Bedrock' showing.

Water Source and Flow Pathway

The Weak Bedrock Environment typically receives a higher rainfall volume relative to lowland environments. The water source is predominantly local rainfall, except in the subalpine sibling which is hydrologically connected to the Alpine Environment. In the Bedrock Environment, most of the rainfall infiltrates the soil and moves laterally at the contact between the soil and underlying bedrock. Where soils are particularly shallow, rainfall is most likely to runoff as overland flow. As we have cleared native forest or tussock grasslands and developed the land for agriculture and forestry, we have also reduced the ability of the land to store water, resulting in faster more erosive discharges. Volumetrically, a small number of surface runoff events are often responsible the majority of contaminant transport from land to waterways in this environment.

Waterways are often ephemeral, which means they only occur for short period of time after rainfall. The convergence of small streams across the Bedrock Environment results in more permanent waterways. Significant aquifers are uncommon as recharge to an aquifer is limited by the depth to bedrock and the permeability of the geological material which is typically poor in weak bedrock settings. Seeps are also common in this environment.

Although much of the hydrology is similar between both Bedrock environments, the strength of the underlying bedrock results in different water quality outcomes for some contaminants, particularly sediment.

Landscape Characteristics

Heavy or prolonged rainfall in the Bedrock Environment often results in runoff due to the limited ability of the soil to infiltrate and store water. Runoff is able to transport loose sediment, sediment-bound nutrients, and microbes quickly to waterways. This is especially relevant in Weak Bedrock Environments, which are inherently more erosion prone with soil creep, slips, and mass movement generating more easily erodible sediment relative to the Strong Bedrock Environment.

Where water infiltrates the soil, it may pool at contact between thin soils and the poorly permeable bedrock. Here, due to redox reactions, the oxygen content of the water decreases, and dissolved iron and manganese may be generated. Under these redox conditions phosphorus tends to be more soluble and mobile and is commonly elevated in hill country streams. Low oxygen conditions also favour the removal of nitrate nitrogen by denitrification. Any denitrification occurring in the soil zone can produce both the harmless dinitrogen gas, which makes up the majority of our atmosphere, and nitrous oxide, a harmful greenhouse gas.

In subalpine environments or where extensive hill country farming occurs, the impact of land use is hard to detect and quantify due to large volumes of dilute alpine or high country sourced water. While water quality is high in these relatively pristine environments, it is still important to reduce contaminant losses as much as possible to minimise the total load delivered to rivers, lakes, and estuaries. As the nutrient status of the soil increases, so too does the risk of the sediment to degrade waterways. Surface water takes from this and neighbouring environments decreases the dilution potential of the water down catchment as the volume of alpine or bedrock sourced water is reduced.

Overall, stream water quality is best the higher up the catchment, where there is less influence by intensive land uses. Water quality typically declines downstream as small streams converge and land use effects accumulate. Seemly small actions that seek to slow down surface runoff and minimise contaminant loss can have a positive effect on downstream water quality.

Sibling and Variations

There are three siblings within the Strong Bedrock Environment:

  • Subalpine identifies bedrock areas that are hydrologically connected to the Alpine Environment and as a result, have a moderately high dilution potential for downstream receiving environments. This sibling may also have snow accumulation over the winter months. The shallow soils and steeper topography also have a runoff risk relative to other bedrock areas. This environment is likely to be highly erodible.
    Extent: 293,764 ha (1.12% of New Zealand)

  • Hill identifies areas of shallow soils with rolling topography (greater than 8 degrees). This sibling has a moderate dilution potential for downstream receiving environments. Due to the rolling topography, runoff risk is typically higher relative to the low relief sibling. This environment is likely to be highly erodible.
    Extent: 3,608,172 ha (13.70% of New Zealand)

  • Low relief maps bedrock areas less than 8 degrees. This sibling has a moderate dilution potential for downstream receiving environments. Runoff risk is typically lower than the other bedrock environments.
    Extent: 444,594 ha (1.69% of New Zealand)

Variants are uncommon in this environment. In MAPS - HYDROLOGY check the Overland Flow map to see how runoff risk varies across this environment. In MAPS – PHYSIOGRAPHIC ENVIRONMENTS check to see if any other variants apply to your location.

The role of landscape in regulating contaminants in the Weak Bedrock Environment. If the landscape function is high it is good at reducing the risk to the receiving environment.
Weak Bedrock Environment Sibling Contaminant pathway (dominant hydrological pathway) How the landscape regulates water quality contaminants Risk to receiving environment
Dilution Resistance to erosion Filtration and adsorption Attenuation: N-Reduction Attenuation: P-Reduction
Subalpine Lateral drainage through the soil zone either to stream or a neighbouring lowland environment. Recharge to the underlying aquifer is limited by the permeability of the bedrock. Overland flow is common due to seasonal wetness. Moderately high Low Moderately low Moderate Moderate Load to surface water
Hill Lateral drainage along contact with bedrock discharging to stream or neighbouring environment. Slope and depth to bedrock controls overland flow risk where the steeper the slope or shallower the bedrock the more likely it is to occur (see Overland flow variant). Minimal aquifer potential. Moderate Low Moderately low - Moderate Moderately high Moderately low Concentration & load to surface water, minor groundwater contribution
Low relief Lateral drainage along contact with bedrock discharging to stream or neighbouring environment. Depth to bedrock controls overland flow risk where the shallower the bedrock the more likely it is to occur (see Overland flow variant). Minimal aquifer potential. Moderate Low - Moderately low Moderately low - Moderate Moderately high Moderately low Concentration & load to surface water, minor groundwater contribution
Hydrological Variant Occurrence (See MAP VARIANTS to check if they apply at your location)
Overland flow Occurs when soils are saturated and/or infiltration is limited. Pathway is active after prolonged or intense rainfall. N/A¹ Low Low Low Low Concentration & load to surface water
Natural soil zone bypass Occurs when soils are cracked (under soil moisture deficit) or jointed. Pathway is active when soils are very dry with the highest risk occurring after prolonged dry periods. Low Low Low Low Low Concentration & load to groundwater

¹ Dilution potential is dependent on the recharge domain of the Physiographic Environment.

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss in the Weak Bedrock Environment.
Nitrogen, phosphorus, and microbes require a source or input for losses to occur. Sediment risk is elevated if nutrient status is also elevated.
Weak Bedrock
Environment Sibling
Nitrogen Phosphorus Sediment Microbes
Nitrate & Nitrite Ammonium & Ammonia Organic (Dissolved & Particulate) Particulate Dissolved Reactive Particulate Particulate
Subalpine Moderately low Moderate Moderate Moderately high Moderate Moderately high Moderate
Hill Low – Moderately low Moderate – High Moderate – High Moderately high – High Moderate – Moderately high Moderately high – High Moderately high – High
Low relief Low – Moderately low Moderate – High Moderate – High Moderately high – High Moderate – Moderately high Moderately high - High Moderately high - High
Hydrological variants
Overland flow Low High High High Low High High
Natural soil zone bypass High High Moderate Low Moderate Low High

Key Actions

In the Bedrock environments, maintain vegetation cover as soils are especially vulnerable to loss on sloping land. Shallow soils are especially vulnerable to phosphorus loss, therefore low solubility fertilisers are advised. Incorporate native vegetation where possible, such as tussocks and grasses, to help increase the carbon content of the soil and the ability of the soil to store water. Native species, such as Pittosporum, are also effective for stabilising land and minimising gully erosion. Small retention dams can also be built to trap sediment.