4. Water Balance

• Introduction
• Water balance
• Direct Rainfall
• Catchment Runoff
• Evaporation
• Wind Setup
• Other water level factors
• What the balance means
• Water balance in the Lake Illawarra

Introduction

Information Sheet No.3 looked at the influence of ocean tides on Lake Illawarra water levels. This information sheet examines other factors which affect water levels and flows including:

• direct rainfall on the lakes surface
• catchment runoff
• evaporation from the water surface
• wind setup
• wave overtopping of entrance dunes and
• seepage through entrance dunes.

Water balance

When added together, all the inflows and outflows determine the quantity of water in an estuary (ie. its water balance).

Direct Rainfall

Along the NSW coast the average yearly rainfall varies from over 1.6m on the Far North Coast to under 0.8m on the Far South Coast. The average annual rainfall for the Illawarra is approximately 1.2m. Clearly all rain which falls directly on an estuary becomes part of the estuary waters. However, most fresh water enters estuaries from the catchment as runoff via rivers, creeks and drains.

Catchment Runoff

Catchment runoff is heaviest during and just after major storms although not all rain which falls on a catchment flows into the estuary. Rain is retained in the catchment on leaves, in puddles, soaking into the soil, etc., and it is only after the catchment is fully wet that significant runoff commences.

The volume of the runoff is related to the amount of rain, the size of the catchment and its characteristics such as soil types, slopes, vegetation and the extent of clearing for development such as agriculture and housing. Most runoff is surface water but some groundwater flows through the soil before entering the estuary.

Evaporation

Evaporation is dependent on the Sun's energy and other factors such as exposure to winds, shade, proximity to other waterways, etc. Annual rates vary from around 1.0m to 1.6m along the NSW coastline and can often exceed the direct rainfall component.

Wind Setup

Wind blowing across water causes the surface water to move in the direction of the wind. This causes the water to `pile up' against the shore line, creating an effect known as `wind setup'. A corresponding drop in levels also occurs along the up wind shore. During major storms in large, deep estuaries the difference in levels can be over 0.5m although the effect is more usually around 0.1m to 0.2m.

Other water level factors

Wave overtopping of sand dunes across a partly closed estuary mouth can be a significant source of water for small estuaries with intermittent entrances. Most wave overtopping occurs during high tides, immediately after entrance closure when the entrance dune is low.

Seepage into or out of an estuary through the beach barrier is usually small when compared to other factors. Estuary water levels are generally not much higher than mean sea level. Consequently there is rarely sufficient `head' or pressure to generate large seepages.

What the balance means

The size and makeup of the balance tells us much about an estuary. Estuaries with permanently open entrances have large water balances which are dominated by tidal flows. This dominance plays a key role in the estuary's water quality and biological character. Coastal lagoons and other small estuaries which have entrances that are only open for a few weeks each year generally have much smaller water balances. In these estuaries both tidal and freshwater flows can be significant.

Water balance in Lake Illawarra

Lake Illawarra is a relatively large estuary by NSW standards being 35 square kilometres in size. The average lake depth however is only 1.9m. It has a comparatively small catchment, in relation to the size of the lake, of 235 square kilometres. The catchment is mainly farmland and urban development, with only a small section of forested land remaining along the escarpment and adjacent foothills.

The lakes entrance is unstable, opening and closing intermittently, limiting tidal flows and their flushing effects. Thus the expulsion of pollutants and nutrients from the lake is restricted.

For an average year, direct rainfall into the lake is estimated to be around 33 GL per annum and evaporation 36 GL per annum. Catchment runoff is estimated to be 86 GL per annum and ocean inflow is estimated at 670 GL per annum.  Flow out from the lake, including seepage, is estimated to be around 753 GL per annum. The volume of the lake is approximately 60GL.

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