Hydrological Systems

Approximately 70% of the Hawkesbury-Nepean Catchment consists of mountainous terrain with a maximum elevation of around 1290 metres. Areas at higher altitudes often receive more rainfall because as air ascends to higher elevations, it cools and condenses, causing moisture to form clouds and precipitation. This process, known as orographic lifting, is common in mountainous regions, where the rising air is forced to cool and release moisture. 

Much of the catchment is also located close to the coast. Coastal regions are close to bodies of water, such as oceans or seas, which serve as a constant moisture source. The evaporation of water from these bodies creates moist air masses. These moist air masses can transport moisture inland and lead to precipitation when they encounter cooling or lifting, such as when the air rises over mountainous terrain. 

The average annual rainfall for the wider Hawkesbury-Nepean Catchment is about 900 mm, but this is a potentially misleading average as it can vary from 500 mm to 1500 mm across the catchment. Most of this rainfall falls between February and May. Precipitation in the catchment is also influenced by climatic factors like Australian East Coast Lows, El Niño, La Niña, and multi-decadal drought and flood cycles. 

The Hawkesbury-Nepean Catchment is one of the largest coastal basins in NSW with an area of 21,400 square kilometres. The distribution of rainfall over this large area of land plays an important role in catchment hydrology. Higher intensities and durations of rainfall lead to increased freshwater inflow, while lower rainfall results in reduced inflow. The state of soil in the catchment prior to rainfall, whether dry or wet, also influences how much of the rainfall generates runoff.

In the Hawkesbury-Nepean region, four main rivers discharge freshwater into the estuary: the Nepean, Grose, Colo, and MacDonald Rivers. Their flow regimes lack a strong seasonal pattern but are influenced by localized weather events, climatic patterns like Australian East Coast Lows, El Niño, La Niña, and multi-decadal drought and flood cycles.

River regulation through dams, weirs and water extraction changes natural flow patterns. The Nepean River, which covers a significant portion of the catchment, would be the dominant freshwater source if not for substantial water extraction for Sydney's metropolitan aided by the construction of dams such as the Warragamba and Nepean Dams. This extraction diverts around 80% of the natural freshwater flow to the estuary, resulting in substantial alterations to the river's hydrology.

The health of rivers is greatly affected by the land use and development in their catchment areas. 

In areas with natural forests, rainwater is filtered and slowed down by the trees and vegetation. This means that when the water flows downstream, it carries fewer sediments and pollutants, resulting in better water quality. However, in urban or agricultural areas, where there's a lack of natural vegetation and a large amount of hard surfaces like roads and buildings, rainwater rushes off quickly. This can lead to erosion, carrying more sediments and pollutants downstream, which can harm the river's water quality.

The type of land use in the catchment area can also change how water flows. In forested catchments, rainwater can seep into the soil, leading to slower, more sustained flows. In contrast, urban areas with impervious surfaces can result in rapid, high-volume runoff, which can cause flash floods.

Historically, the Hawkesbury-Nepean catchment has seen significant land clearing and development, leading to erosion and sedimentation of waterways. Large areas of the catchment are now conservation areas, but urban and industrial development is increasing, especially in the Western Sydney area. Population growth and future land use changes will put additional pressure on the Hawkesbury estuary. As more people move in and urban areas expand, the catchment will be further modified, affecting water inflow patterns and quality.

To maintain the ecological health of the Hawkesbury-Nepean River System, it's crucial to balance development with conservation and sustainable land use practices. Protecting natural areas, like national parks and wild rivers, is essential to preserve the river's condition and ensure it remains a healthy and thriving ecosystem.