Catchment condition related to water quality
A number of landscape characteristics can influence local and catchment scale water quality including groundcover , riparian vegetation extent and vulnerability to soil erosion .
3.1 Ground cover
Ground cover consists of the non-woody plant cover near the soil surface and all litter including woody litter . The level and type of ground cover is important for land management as it affects soil processes including infiltration , runoff and surface erosion ( McIvor et al . 1995 ; Wilkinson et al . 2014 ). In the GBR catchments , low ground cover can contribute to increased sediment loads reaching the GBR ( Queensland Government , 2009 ). Maintenance of ground cover is essential for sustainable production , especially in rangeland environments where rainfall is highly variable . Ground cover also plays an important role in protecting valuable soil resources from wind erosion , and contributes to nutrient cycling and to maintaining biodiversity . The amount and distribution of ground cover can change in response to climate , land or soil type and land management , especially grazing intensity . For example , above-average rainfall can result in above-average ground cover , which helps the soil resist erosion by minimising raindrop impact , improving water infiltration and reducing surface runoff .
Ground cover is measured and reported annually as part of the Paddock to Reef Program using satellite imagery and the fractional vegetation cover method described by Scarth et al . ( 2010 ). The method measures the proportion of green cover , non-green cover and bare ground using reflectance information from late dry season Landsat 5 Thematic Mapper ( TM ), Landsat 7 Enhanced Thematic Mapper ( ETM +) and Landsat 8 Operational Land Imager ( OLI ) satellite imagery . These data are calibrated using field observations . The spatial resolution of Landsat imagery is approximately 30 metres . Total ground cover is given by summing the green and non-green cover fractions ( Queensland Government 2009a ). It is important to note that averaging ground cover across a large area can mask localised areas of lower cover , particularly where there is a strong rainfall gradient . The mean ground cover reported is therefore indicative of general levels of cover within the reporting catchment . It is important to consider the spatial distribution of cover when accounting for its impact on sediment generation . Figure 10 shows the average ground cover in the BBB catchment between 2008 and 2014 . The late dry season ground cover in 2014-15 for BBB was reported as 68 per cent ; however , there were significant areas of low ground cover within the region which were drought declared ( Queensland Government 2015 ). Residual groundcover is an estimate of the difference between expected and observed 10th percentile of average fractional cover 2008-2014 ( dixa2 algorithm ) as developed by Terry Beutel ( DAF ). Higher values indicate areas where the barest areas have more cover than expected given the long term average rainfall ; lower values indicate areas where the barest areas have less cover than expected given the long term average rainfall ( Figure 11 ).
Tree cover ( an element of ground cover ) also has an important role in the cycling of nutrients and water . Trees play a significant role in the water cycle taking up groundwater and releasing it to the environment , often described as ‘ nature ’ s water pumps ’. Marrying rehabilitation of riparian or even larger marginal productive areas with potential tree crops with rising groundwater areas may also be part of the long term solution to rising groundwater levels ( GBRMPA , 2013 ).
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