South East Queensland
9.1 Precipitation on surface water

The volumetric value for the line item at the end of the 2011–12 year was 239,397 ML.

This line item consists of precipitation on surface water within the South East Queensland (SEQ) region. As a surface area was not available for surface water, such as rivers, natural lakes or wetlands, precipitation was only provided for the surface water storages listed in line item 1.1 Storages (following table).

The volume of precipitation on surface water was less during the 2011–12 year (239,397 ML) than the 2010–11 year (417,313 ML). This was due to the 2010–11 year being significantly wetter (annual rainfall was 1,748 mm), with major flood events occurring in December 2010 and January 2011. Further information on the rainfall within the region can be found in the Climate overview.

 Notes:

¹ Precipitation calculated using variable surface area

² Precipitation not calculated as surface area of the storage was not available.

³ Precipitation calculated using static surface area

National Climate Centre (NCC) daily climate grids (rainfall); Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; Australian Water Resources Information System (AWRIS) water storage. 

Bureau of Meteorology (the Bureau). 

Monthly precipitation data were produced by the Bureau. It was based on daily data from approximately 6,500 rain gauge stations and interpolated to a 0.05⁰ (approximately 5 km) national grid (Jones et al. 2007).

The precipitation at each waterbody was estimated from the proportionally weighted average of grid points that intersected each water feature. The volume was then estimated using the surface area of each waterbody. The surface area varied dynamically with changing water storage level for water storages where the relationship between storage level and surface area had been derived.

The surface area of most of the storages in the SEQ region was calculated using this dynamic method. Storage rating table data and regularly updated storage level data were not available for some storages, so static surface areas were used. The surface area for Cedar Grove and Mount Crosby weirs was provided by Seqwater. For Lake Cressbrook, Lake Perseverance and Splityard Creek Dam, the AHGF waterbody feature class was used to estimate a static surface area. Surface areas were not available for Bromelton Off-Stream Storage or Wyaralong Dam. For the SEQ region, the precipitation on the surface water store included precipitation on storages only.

• The precipitation estimates were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
• The dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore will not capture changes that occur on a shorter temporal scale.
• The use of the static default waterbody feature class surface area is an approximation only. It represents the storage at capacity and therefore probably results in an overestimation of precipitation on the storage. This default dataset was only used for the small storages (i.e. those where a dynamic surface area could not be determined); therefore, the impact of this approximation is estimated to be small.

The uncertainty estimate was not quantified.

The Bureau recalculated the 2010–11 year volume published in the 2011 Account using the improved dataset with newly available data. Consequently, the volume has been updated from 410,129 ML (as published in the 2011 Account) to 417,313 ML