What influences water quality

Why does water quality vary across the state and from year to year?

A wide variety of factors can influence catchment water quality. These factors can include:

• land cover (the amount, type, and arrangement of vegetation in the catchment and along waterways)
• land use
• land management practices (animal stocking rates, application rates of fertilisers and other chemicals, stormwater management)
• catchment geology, climate and rainfall, topology, and catchment hydrology.

It is known how certain landscape features affect water quality. But there is only limited understanding about how they interact with each other. Thus, it difficult to predict what the water quality will be at certain places at any specific time.

Climate and water quality

Different climatic conditions and seasonality can significantly affect water quality. Large scale weather patterns like El Nino and La Nina can influence water quality. This is because they alter rainfall patterns, rainfall intensity and runoff. Wet and dry periods can have a big impact on water quality.

In Victoria, El Nino typically results in higher summer temperatures, lower rainfall resulting in lower river levels. La Nina typically results in colder temperatures and increased rainfall resulting in higher river levels.

Typical water quality patterns during summer

Unregulated rivers usually have lower flows in summer.

During summer, less material washes in from the catchment. This results in:

• lower nutrient and sediment levels in the water. However, concentrations of nutrients can also increase as water levels drop and nutrients become more concentrated. These conditions can lead to algal blooms.
• warmer water temperatures and improved water clarity are due to the settling of suspended solids onto the stream bed.
• dissolved oxygen levels are lower because of warmer water.
• electrical conductivity (salinity) can increase where groundwater has intruded.

Typical water quality patterns during winter

Victorian rivers usually have higher flows in the winter months.

Rain washes nutrients and carbon (e.g. leaf litter) into rivers. This can lead to poorer water quality, due to higher turbidity and levels of suspended solids. However, increased rainfall can also dilute nutrient concentrations in waterways improving water quality.

Heavy rainfall can greatly increase catchment erosion. Typically, 90% of contaminants are delivered to water systems in heavy rainfall events.

What happens when a catchment is already wet?

If a catchment is dry, rainfall soaks into the soil and will not create surface water runoff to nearby rivers. But when the catchment is wet, more rainfall cannot be absorbed by the soil, and this results in runoff into the nearest river. Under these conditions, runoff can pick up nutrients and sediments and carry them into the rivers.

What is a blackwater event?

Blackwater events can be a natural feature of lowland river systems. These events can occur when organic material, such as vegetation and trees are washed off the floodplain and into rivers. Normally, this movement of organic material from the floodplain to a river channel produces good results by nourishing lowland river ecosystems with carbon, which is a source of food for bacteria, fish, and macroinvertebrates. But if the oxygen consumption by bacteria and microbes eating the leaf litter is faster than oxygen can be replenished from the atmosphere, this can deplete oxygen in the water column, leading to the death of fish and other aquatic life. The black appearance of water is caused by the release of carbon compounds that include tannins as the organic matter decays.