Water Ponding in the Rangelands

Rangelands are used for very diverse purposes and have many associated management issues.

They cover 80% of Australia and comprise diverse ecosystems, such as woodlands, tropical savannah, shrub lands, grasslands and deserts. There are many land uses of rangelands, such as mining, tourism, Indigenous use and agriculture. The semi-arid areas are very important for grazing, and $4.4b is made annually from cattle, and a further $1.1b from sheep and wool.

nsw-map-rangelands-higher-rainfall
The rangelands of NSW cover 60% of the state and are located west of the 500mm per annum rainfall zone. The field preparation for this report was conducted in the area around Trangie and Nyngan. SPicture: R Hacker et al., Best management practices for extensive grazing enterprises, NSW Department of Primary Industries, April 2005. 

 

Introduction

Water ponding was used to reclaim scalded soils at two sites in the Nyngan locality. These sites are now 25 and seven years old and a Landscape Function Analysis was carried out on them to compare functionality and pond age, using a scalded site as a baseline.

Landscape Function Analysis

Landscape function analysis (LFA) is used to assess the functionality of rangelands. It uses visual indicators to give an indicator of 11 physical and biological processes to describe three key soil phenomena, and thus the overall functionality of that landscape (Tongway and Hindley, 2004).

indicators
2 Eleven tests are conducted in order to assess three key soil processes that describe landscape functionality.
Picture: Tongway and Hindley, 2004.

An LFA was carried out on ponded sites in order to determine the effects of water ponding on landscape function in terms of stability, infiltration, nutrient cycling, soil carbon, and soil biomass relative to pond age.

Water ponding

I don’t know about you, but when I hear the word ‘pond’ I think of this:

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Water ponding; less to do with the Ponds, more to do with soil scalds. Picture: Bill Israel.

Water ponding, however, is a practice used to reclaim scalded soils, which we’ve talked about before.

These soils develop due to clearing and overgrazing, causing an excess of sodium ions (i.e. >15%) which creates hard, concrete like surfaces called scalds. Neither water nor roots can penetrate these surfaces, leading to an absence of vascular plants in these areas, which in turn further disrupts the site’s hydrology. At this stage the landscape enters a new stable state that is X to agricultural use. A functional landscape experiences short tortuous flows of water through grass tussocks, whereas a dysfunctional has long straight flows around tussocks which loses resources such as water, soil, and seeds.

water flow
3A functional and dysfunctional landscape. Note the differences in plant cover and resulting water flow.
Picture: Tongway and Hindley, 2004.

Water ponding is a highly successful practice that uses to reclaim these soils, a system that uses 0.4ha horseshoe shaped ponds to hold water and encourage plant growth by holding water on the surface. When a site is identified for water ponding, it is surveyed with a laser leveller in order to find potential positions for pond banks. Ponds are then laid out from the highest point in the scald and overlap like fish scales so water is not lost. They must be no more than 10cm deep or only reeds and rushes will grow, and wave action can destroy the banks. In order to ameliorate wave action, tunnelling, and dispersion there is a heightened buffer section between the banks and the pond. As the system is reliant on rainfall, water ponding may be less effective in times of drought.

Results

Two water ponding sites were visited, one 25 years old and one seven years old. An LFA was carried out in four ponds at each site in order to assess the effect of water ponding on landscape function relative to pond age. This was done by laying down 15m transects at the back of ponds and collecting information on the fore mentioned 11 indicators in the patches (areas of vegetation) and interpatches (clear areas). Soil carbon and biomass were also measured.

biomass
4 Biomass results from the two differently aged pond sites compared to a scalded site. 

Biomass was taken from a 30cm2 plot in the middle of the transect, oven dried and weighed.

carbon
5 Soil carbon results from the two differently aged pond sites compared to a scalded site.
stability
6 Soil stability is a result of LFA. This graph shows the results from the two differently aged pond sites compared to a scalded site.

infiltration

7 Infiltration of water into soil is a result of LFA. This graph shows the results from the two differently aged pond sites compared to a scalded site.
nutcyc
8 Nutrient cycling is a result of LFA. This graph shows the results from the two differently aged pond sites compared to a scalded site.

Discussion

The above results show that water ponding has a positive effect on landscape functionality. In every data set, treated sites performed better than the scalded control. In the case of nutrient cycling and infiltration, this difference was particularly significant. These two factors are both dependant on basal cover, litter origin and surface roughness among other things. The scald does not have these properties as it has no vascular plants and the dispersive soils create a smooth, hard crust that does not allow for the build-up of resources such as seeds, litter and water.

Water ponding allows water to sit on the scalds, which restores the swelling and cracking properties of the duplex soils (Thompson, 2008). Cracks allow windblown seeds to get caught in the micro-topography, as well as allowing water and root penetration of the surface. The seeds can then grow, restoring infiltration and nutrient cycling. This was indicated in the results; stability does not significantly increase with pond age, whereas infiltration and nutrient cycling do, suggesting that stability recovers first and is needed for nutrient cycling and infiltration.

There was no biomass on the scald, and differently aged ponds had similar amounts. This is because total biomass increases and plateaus relatively quickly; it can be up to a 20% increase in the first year.

Soil carbon increased over time and in comparison to the scalded site. Soil carbon helps to restore functionality by ameliorating soil structure and available nutrients (CSIRO, 20011).

Conclusion

It can be seen from the results that water ponding is successful in reclaiming scalded soils for use. Functionality of landscapes was seen to increase with age, but not significantly in regards to stability. It can also be concluded that, despite it’s irrelevance to agriculture, Amy and Rory’s story is wonderful and that the Last Centurion and the Girl Who Waited are heart warming in their (literally) undying love for each other.

rory
He waited 2000 years for a woman who ripped Time apart for him, but can he reclaim a scalded soil?