Overland Flow Variant

Occurrence and Hydrological Flow Pathway

Overland flow is the flow of water that occurs when excess rainwater, meltwater, or water from other sources flows over the surface of the land. It transports water, solutes dissolved in water, and particulates from the land surface to a surface water body faster than any other hydrological pathway and results in the highest risk to water quality.

There are a number of reasons why overland flow occurs. The most common type is when the soil is saturated to full capacity and no more water can infiltrate into the soil. This is known as saturation excess overland flow and is most common over the wetter months when evaporation rates are low and the groundwater table is high. Overland flow can also occur when rain falls at a higher intensity than the soil can infiltrate. This type of overland flow is known as infiltration excess and occurs most commonly in fine textured soils with slow permeabilities or where soils are impacted from pugging and compaction. Overland flow also occurs because the land surface is predominantly impervious, and water cannot infiltrate the land surface. This occurs most commonly in rocky alpine environments with seasonal meltwater and in our built up urban environments where we have paved over the land surface. Overall sloping land is most likely to generate overland flow, while flat land may result in surface ponding.

Landscape Characteristics

As overland flow has minimal interaction with the landscape, contaminant retention by filtration and adsorption or removal by chemical processes, such as denitrification, and plant uptake is limited. It is also the primary pathway for soil erosion by water. The majority of contaminant losses from a property can occur over a small number of these runoff events during the year.

For overland flow to pose a risk to water quality it requires a contaminant source from land use. Due to our land use activities, nutrient-rich sediment and microbes are often concentrated at the land surface and within the upper soil horizon ready for transport if overland flow occurs. Large load contributions occur when there has been a long time between events and concentrations have built up at the land surface, especially over the drier summer months. Subsequent events may not transport as much contaminant as the first flush. Areas that are prone to overland flow, can deliver frequent discharges of relatively smaller contaminant load contributions.

In natural state areas, the load transported has a minimal effect on water quality as it contributes to the background concentration and load of the waterway. Sediment lost from these environments may be large in volume but is relatively unreactive in the receiving environment due to the low nutrient status of the sediment. Pest management may be required to keep microbial contamination to a minimum in some areas.

The role of landscape in regulating contaminants when overland flow occurs. 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.
Overland Flow Variant	Contaminant pathway (dominant hydrological pathway)	How the landscape regulates water quality contaminants					Risk to receiving environment
Overland Flow Variant	Contaminant pathway (dominant hydrological pathway)	Dilution	Resistance to erosion	Filtration and adsorption	Attenuation: N-Reduction	Attenuation: P-Reduction	Risk to receiving environment
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

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 when the overland flow pathway is active. When the pathways is active, the variant risk supersedes the risk for the 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. The contaminants have been colour coded red, orange, and yellow for high, moderately high, and moderate risk, respectively.
Overland Flow Variant	Nitrogen			Phosphorus		Sediment	Microbes
Overland Flow Variant	Nitrate & Nitrite	Ammoniacal	Organic (Dissolved & Particulate)	Particulate	Dissolved Reactive	Particulate	Particulate
Overland flow	Low	High	High	High	Low	High	High

Key Actions

To mitigate for overland flow in lowland areas, artificial drainage is often installed in areas where soils are poorly drained. This aids in reducing the likelihood of overland flow occurring and reduces the highest amount of contaminant loss but also can create other issues requiring careful land management to minimise the impact on water quality.

Mitigation activities should focus on reducing the likelihood for runoff to occur by good land management practices and where it does occur preventing runoff reaching a waterway. Maintaining good soil structure and reducing the time the land is left fallow is critical to minimising runoff and contaminant risk. Vegetated buffer strips, sediment traps, detainment bunds, and riparian planting are just some of the ways of intercepting runoff and minimising contaminant loss. Retiring land or converting to wetlands may be the best option in the most difficult of landscape settings.