Final LDC WQ Report | Page 72

Table 13 . Summary of monitoring outcomes for the 2018-19 , 2019-20 and 2020-21 wet seasons at the Mt Pleasant treatment and control sites . Source : Derived from Bartley et al . ( 2019 , 2020 and 2021 ).

Strathbogie

SBT2 - Treatment

2018-19 2019-20 2020-21

Control ( SBT )

SBT2 - Treatment

Control ( SBT )

SBT2 - Treatment

Site Rainfall ( mm ) 1,352 1,384 439 504 692 652 Hillslope (% cover ) 9 % 90 % 70 % 67 % 94 % 91 % Gully walls (% cover ) 30 % 42 % 44 % 49 % 18 % 35 % Gully floor (% cover ) 30 % 42 % 25 % 40 % 21 % 38 %

Flow weighted annual average TSS concentration ( mg / L )

Control ( SBT )

1,704 2,089 0 * 1,343 3,934 3,934

Total TSS loss ( t / y ) 63.9 28.3 0 * 2.8 5.6 2.7 TSS yield ( t / ha ) 1.7 1.6 0 * 0.14 0.13 0.14

Flow weighted annual average TN concentration ( ug / L )

2,193 1,715 1,778 5,045

Total TN loss ( kg / y ) 82.2 23.2 0 * 2.9 10.6 3.5 TN yield ( kg / ha ) 2.2 1.3 0 * 0.2 0.3 0.2

Note : Catchment area ( ha ) above the monitoring stations is 41.4ha for the treatment and 19.4ha for the control , measured using LiDAR .

1

Long term rainfall is 725 mm ( BOM 1886 – 2016 ). * no runoff due to treatment effects

Vegetation There was a significant difference in the hillslope % cover data between the control and treatment sites in 2019-20 ( p = 0.024 ) and 2020-21 ( p = 0.004 ), with the treatment hillslope having lower percentage cover than the control hillslope . This is likely due to the impact of the earthworks on the treatment hillslope . However , when the data were compared with previous years , this difference was no longer significant ( p = 0.478 ). Hillslope biomass had not changed significantly between control and treatment sites ( p = 0.071 ) which is expected since both sites are in the same paddock under the same grazing regime . In general , this landscape has quite dramatic shifts in cover and biomass between the dry and wet seasons . Overall , however , the land condition at this site is poor ( 76 % ‘ C ’ Condition and 24 % ‘ D ’ Condition ) but it has improved since the initial assessment in April 2017 ( 39 % ‘ C ’ Condition and 61 % ‘ D ’ Condition ).

Similarly , the cover or biomass metrics for both the gully channel and the gully walls were not significantly different between treatment and control gullies following the 2019-20 or 2020-21 wet seasons . The treatment gully had no direct hillslope runoff into this gully over the 2019-20 wet season due to the treatment works on the hillslope .

Water Quality Outcomes In the 2019-20 wet season following the hillslope rehabilitation , there was no measured runoff or water quality samples recorded at the treatment site , but there were flow and samples at the control site ( Figure 29 ). Therefore , the sediment yield from the treatment site during 2019-20 wet season was effectively zero . In 2020-21 , ~ 692 mm of rain fell on the treatment site and ~ 652 mm on the control site at Strathbogie ( Figure 29A ). Rainfall between years has been highly variable with ~ 1,400mm in 2018-19 , resulting in challenges with instrumentation . Despite these challenges , the TSS and TN loads generally followed the rainfall trend with an increase in loads in 2018-19 compared to other years . In general , the % runoff , or amount of rainfall that becomes runoff , was higher at the control site than the treatment site , even before treatment was implemented ( Figure 29 ).

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