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.

slope map of new zealand with the parts classified as 'Weak Bedrock' showing.

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

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 have a greater 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 to low 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. The risk to the receiving environment is defined as concentration and/or load to surface water, groundwater, or both.
Weak Bedrock Environment Sibling	Contaminant pathway (dominant hydrological pathway)	How the landscape regulates water quality contaminants					Risk to receiving environment
Weak Bedrock Environment Sibling	Contaminant pathway (dominant hydrological pathway)	Dilution	Resistance to erosion	Filtration and adsorption	Attenuation: N-Reduction	Attenuation: P-Reduction	Risk to receiving environment
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 (see Overland flow variant when pathway is active).	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 when pathway is active). 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 when pathway is active). 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.	N/A¹	Moderate	Low	Low	Low	Concentration & load to groundwater

¹ Dilution potential is assessed by the Physiographic Environment recharge domain which is indicative of water source and relative volume. This does not change with the hydrological variant.

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss in the Weak Bedrock Environment. A high susceptibility equals a high risk of loss from agricultural, horticultural, forestry and urban land uses and assumes a source or input of nitrogen, phosphorus, and microbes for losses to occur. Sediment risk is elevated if nutrient status is also elevated. Hydrological variants modify the ability of the landscape to regulate contaminants. When these pathways are active, the variant risk supersedes the risk for the environment. The contaminants have been colour coded red, orange, and yellow for high, moderately high, and moderate risk, respectively. Where the risk is provided as a range, the highest risk is used for the colour.
Weak Bedrock Environment Sibling	Nitrogen			Phosphorus		Sediment	Microbes
Weak Bedrock Environment Sibling	Nitrate & Nitrite	Ammoniacal	Organic (Dissolved & Particulate)	Particulate	Dissolved Reactive	Particulate	Particulate
Subalpine	Moderately high	Moderately high	Moderately high	Moderately high	Moderately high	High	Moderately high
Hill	Low – Moderately low	Moderate - High	Moderate - High	Moderate - High	Moderate – Moderately high	Moderate - High	Moderate - High
Low relief	Low – Moderately low	Moderate - High	Moderate - High	Moderate - High	Moderate – Moderately high	Moderate - High	Moderate - 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.