Teoría y práctica de la soberanía

Export of nutrients through waterways in the Peel Region continues, particularly PO4

(Figure 21). PO4 export in the North Dandalup River has not changed significantly in the short term, and has a high level of variance throughout the year (Figure 22). Nutrient data are not available for the Peel Main Drain.

Estimated PO4 efflux is variable and peaks in late winter (Figure 22). Breaking PO4 efflux into quickflow and baseflow components, there is a large baseflow contribution to total efflux, particularly from August to December. Baseflow accounts for more than half of the total PO4

efflux (Figure 23).

Figure 21. Total PO4 (mg/L) at the MRRE01 sample site on the North Dandalup River, sampled monthly

Figure 22: Estimating monthly PO4 load in the Peel Main Drain using four year average values from North Dandalup River at site MRRE01 and average Peel Main Drain flows – 1976-2000

Figure 23: Baseflow and quickflow components of estimated monthly PO4 load in the Peel Main Drain using four year average values from North Dandalup River at site MRRE01 and average Peel Main Drain flows – 1976-2000

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

PO4(kg/month)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

PO4 kg/month

baseflow quickflow

5 Discussion

5.1 Water

There are two key issues relating to water quality within the Peel-Harvey Region identified in this study. These are groundwater decline (Figure 7) and continued nutrient efflux (Figure 22, 23), which supports previous studies (e.g. Kelsey et al. 2011). Rainfall quantity has not changed significantly over the last 30 years (Figure 8) in the north-western area of the Peel Region where the Peel Main Drain is located, and although this does not match

findings of reports on regional scale trends in rainfall by CSIRO (2009b), it is possibly because it is a measurement from a single location.

Over this period, groundwater levels have significantly declined (R²=0.67, P<0.01, Figure 7).

Groundwater decline is likely to continue over the long term as reduced rainfall in the southwest will result in decreased groundwater recharge (CSIRO 2009a). As rainfall has remained relatively unchanged at this location, the decreasing groundwater level is likely related to landuse in the Swan Coastal Plain, as well as a reduction in groundwater recharge at a landscape level, particularly as there are broad groundwater flows across the Swan Coastal Plain in response to the potentiometric surface of the surficial aquifer (CSIRO 2009a).

In addition to the decline in groundwater level (Figure 7), high nutrient levels persist in wetlands and waterways in the Peel Regions (Figure 22) (Kelsey et al. 2011). Estimation of nutrient load using North Dandalup River values may give an indication of loads in the Peel Main Drain. Using this method, PO4 peaks in August to September, and in this model is

influenced by baseflow contribution (Figure 22,23). Baseflow contribution provides more than half of the PO4 of the total P efflux. Nutrient export in drains and rivers continues in the Peel Region, and nutrient intensive land uses such as agricultural and residential land uses occur in this landscape, in an area which has sandy soils and low nutrient retention capacity (Kelsey et al. 2011; Summers et al. 2002). Sediment in drains and rivers may have a high nutrient load which will continue flushing for some time. The high water solubility of fertilisers used in the Peel-Harvey Region has also contributed to the water quality issues (Keipert et al. 2008) and this has resulted in eutrophication of waterways and associated ground surface (Figure 20). Nutrients travel relatively quickly into groundwater, and into the drains and rivers. The flat landscape and saturated soils leads to rapid overland flow represented by spikes in the Folly Rd hydrograph (Figure 15) which is mentioned as one of the factors contributing to high nutrient efflux in the area (Gerritse and Schofield 1989).

The Peel-Harvey area is subject to rapid urbanization, and much of the landscape is low, made up of a number of older coastal dune swales (Brearley 2005). Lower areas within the landscape often are areas which in the recent or more distant past used to be wetlands, and these areas may be prone to flooding (Figure 20).

There is a significant amount of literature describing the movement of nutrients through soil in the Peel Region (Gerritse and Schofield 1989; Haygarth and Jarvis 2002; Hodgkin et al.

1980; Rivers et al. 2011; Summers et al. 2002), and a range of options exist to manage nutrient movement. Nutrient retention of the soil may be increased by site engineering, including amending the soil with clay (Summers et al. 2002) and controlling drainage (Ayars et al. 2006; Borin et al. 2001; Kröger et al. 2011; Wesström et al. 2001). Nutrient retention

may also be improved through changes in fertilizers used (Keipert et al. 2008). Catchment water-quality may also be improved through changes in land management practices, including changes in fertilizer use, which could become more effective through education and information targeting land managers (Keipert et al. 2008). Application of clay to the soil surface can improve nutrient retention (Summers et al. 2002), as clay has far lower particle size than sand, and therefore a much greater surface area. Clay is less permeable than sand, which may increase the residence time of water in the soil.

5.2 Controlled Drainage

Groundwater level in the study area, from Hope Valley to Karnup Rd varies by about one metre throughout the year, and during the winter months, the Peel Main Drain intersects the groundwater surface (Figure 20) (Brooks 2009). Baseflow, which is derived from groundwater entering the drain, is a major component of total flow in the Peel Main Drain, occurring over a 3-4 month period, with base flows of up to 100 ML/

dayat the Folly Rd Station (Figure 15). Controlled drainage in the Peel Main Drain would decrease baseflow (Sharpley et al. 2007).

Modelled nutrient loads in the Peel Main Drain indicate that about 50% of PO4 efflux is due to baseflow (Figure 23).

At peak flow rates, modelled water level is below bank height (Figure 17). This model is limited by the removal of pools and basins within the drain, which provide more elasticity in the drain network, and actual peak levels are likely to be lower than modelled levels.

One of the risks associated with controlled drainage is flooding in the Peel-Harvey Region (Figure 20), due to the low and flat landscape. This risk can be mitigated though the use of variable height weirs or other structures which can be altered, such as drop board weirs, regulating water levels in the drain. Managing flood risk associated with controlled drainage by using variable control structures would be an ongoing expense, and this would limit the application of controlled drainage to areas where there would be substantial benefit, including areas where wetlands are being drained (Figure 1). Groundwater level is declining in the Peel-Harvey Region (Figure 7), and controlling drainage will reduce the decline in groundwater and retain water for wetlands (Sharplet et al. 2007). Rainfall and groundwater level are forecast to decline further over the next 20-30 years as the south-western Australian climate changes (CSIRO 2009b; Petrone et al. 2010), and retaining water for wetlands will become increasingly important.

5.3 Further Study

Further study can be carried out by measuring nutrient levels in the Peel Main Drain throughout the year, to determine the proportion of nutrients carried on eroded soil particles and in solution. This would aid in determining the effect of controlled drainage on nutrient retention, and the importance of sediment transport of P, particularly during rare high flow events. If transport of P in sediment is high during rare high flow events, this may reduce the effectiveness of controlled drainage. This study was constrained to examining mechanisms of controlled drainage for improving water quality, and the hydrologic component of feasibility for controlled drainage in drains in the Peel-Harvey Region, focusing on the Peel Main Drain. Application of groundwater level interpolation can also be applied for larger scale flood modelling.

Using tracers, including non-toxic dyes and radioactive isotopes to study the movement of groundwater into the drain could also be useful, to aid in identifying when and where groundwater intersects with the Peel Main Drain.

6 Conclusions

From observation of algae and aquatic plant growth in the Peel Main Drain (Figure A1, A2, A3), water quality issues persist within the drain, and P efflux continues in drains and rivers in the Peel Region (Kelsey et al. 2011) (Figure 22), though nutrient data for the Peel Main Drain were unavailable for this project. The occurrence of large algal blooms in the Peel-Harvey Estuary has been reduced through a range of management methods,

including changes in landuse practices, and engineering approaches including soil amendments and the construction of the Dawesville Channel, which increased flushing of the Peel-Harvey Estuary (Rivers 2004). Nutrient efflux and water quality issues persist in the

drains and rivers which feed in to the estuary. This issue is compounded by a second issue, predicted long term decline in rainfall, and declining groundwater levels, due to groundwater extraction and a decline in groundwater recharge.

Controlled drainage is an additional management option to reduce nutrient efflux and increase water retention within the Peel-Harvey Region, particularly where there is groundwater interaction with the drain network. Groundwater contributes to drain flow (Figure 15) within the drainage network in the Peel-Harvey Region, and many wetlands are drained in the Peel Region (Figure 19, Figure 20). In the Peel Main Drain area, Controlled drainage would reduce baseflow (Figure 23) and nutrient efflux, and the main considerations include flooding and cost. Controlled drainage can retain groundwater and decrease nutrient efflux, although there may be an associated flood risk, which could be managed through the use of a variable height weir or other variable flow barrier. Controlled drainage in the Peel Main Drain is a viable management option, but flooding risk, and cost of mitigating this risk limit the number of locations where this approach can be applied.

This work in this thesis can be expanded on by determining nutrient loads throughout the year in the Peel Main Drain to determine the load contribution of baseflow and quickflow for N and P. Studying the proportion of P transported in soluble form and bound to soil particles would also be useful, particularly for estimating the importance of sediment movement in nutrient efflux into the Estuary. Sediment movement (and therefore nutrient movement) may be high during rare peak flow events, and may reduce the effectiveness of controlled drainage over the long term.

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Appendix A. Images of the Peel Main Drain in February 2012

Figure A1. Aquatic plant growth in the Peel Main Drain, Karnup Rd Station.

Figure A2. Aquatic plant growth in the Peel Main Drain, Karnup Rd Station.

Figure A3. Aquatic plant growth in the Peel Main Drain, Folly Pool.

Appendix B. Hydrograph station numbers and locations

Table B1. Hydrograph Stations

AWRC Name AWRC Number

Hope Valley 614013

Marmanup Pool (Outflow) 614100

Marmanup Pool (Inflow) 614099

Dog Hill Rd 614098

Thomas Rd 614076

Spectacles SSP 614095

Folly Rd 614096

Karnup Rd 614121

In document Número 18 - Octubre de 2011 (página 140-144)