Level 2 Detail of experimental conditions -Keilambete

Description of study

What?

Runoff and soil movement were measured for three pasture utilisation treatments (high, medium and exclosed) and replicated at the ‘plot scale’ (80-160 m²) and a high and medium utilisation rate for two timbered ‘mini-catchments’ (0.4 - 0.5 Ha) (Figure 1). Treatments were imposed in spring 1994 and the runoff sites were instrumented soon thereafter.

When and Where?

From 1994 to 2000 the investigation occurred at “Keilambete”, Rubyvale, 400km west of Rockhampton (Central Queensland).

Keilambete is predominantly gently undulating with a silver-leaved ironbark (Eucalyptus melanophloia) overstorey and gritty texture contrast red duplex soil. This soil has a shallow A horizon (5 cm) and a 50 cm B horizon overlying weathering granite parent material. Slopes range from 3 to 5%. Dominant pasture species are Bothriochloa ewartiana and Heteropogon contortus. . Keilambete has been regularly grazed for more than 60 years.

How?

Rainfall

Rainfall intensity and volume were recorded using tipping bucket pluviometers (1 minute interval) connected to a data logger. Pluviometers were located adjacent to each plot, while manual rain gauges were located adjacent to each mini catchment to account for the spatial variability across the sites.

Runoff

Runoff plots were replicated for each of the three treatments for a total of six plots per site and two timbered mini catchments. Plots were aligned perpendicular to the slope, with a Gerlach trough (Figure 2) installed at the bottom to collect bed load soil and runoff. Runoff rate and volume were measured via tipping buckets installed at one end of each Gerlach trough. Mechanical counters were attached to the buckets as a backup to measure total runoff tips. Average plot slopes were 3 to 5% and each plot was bounded on all sides to prevent non- contributing surface run on entering the plot area.

For the mini-catchments (0.25 - 0.5 ha), surface runoff was measured using Parshall flumes fitted with capacitance or pressure water level measuring sensors and chart recorders for backup. Artificial boundaries were constructed around each mini-catchment. Average slopes for the catchments ranged from 4.5% to 7.5%.

Time series logger data was processed through in-house Datalog v5.6 package prior to capture in Hydstra. Datalog allowed management and editing of individual logger files; application of a calibration to convert from abstract units (eg. ‘tips’) to real world units of measure (i.e. mm of rainfall and runoff); plus summarisation of data for intervals such as a service period.

Event- based soil movement was determined by measuring the bed and suspended load component of surface runoff.

The division between bed and suspended sediment was unique for each differing Gerlach trough configuration as the deposition of all waterborne sediments is dependant upon the hydraulic characteristics of each structure.

Suspended Sediment

Development and testing of a tipping bucket integrating suspended sediment sampler demonstrated that its performance across the typical discharge range is linear in terms of a flow proportion (Silburn & deVoil, pers comm.). Thus a single suspended sediment sample is flow weighted and representative of the entire runoff event. The suspended sediment sample was accumulated in a container, collected at the end of a service period and then analysed in the NRM&W Emerald laboratory for Total Solids by APHA method 2540B. As this was a discharge integrated sample for the preceding interval runoff event, simple calculation of a sediment load in t/ha was possible by the following formula:

t/ha_suspended = mg/L_suspended * mm_event ÷ 100000

Bedload

At the end of each service period, bed load material was manually removed from the Gerlach troughs and weighed.  Sub-samples were then collected, weighed and oven dried for moisture content correction.  The weight of the oven dry sediment was converted to an equivalent soil loss on an area basis:

t/ha_bedload = kg_bedload ÷ ha ÷ 1000

Total Soil Movement

Simply:

t/ha_total = t/ha_suspended + t/ha_bedload

Total Runoff

Runoff in mm was accumulated for the service period from the logger data.

The mechanical counter total for the service period was also multiplied by the tipping bucket’s static tip volume for a minimum estimate of runoff. This was a precautionary method.

Table 2. Plot characteristics including, utilisation, treatment and plot area (Waters 2004)

*pl – pluviometer installed

Ground Cover/utilisation rate

Projected ground cover was recorded at every site visit which was generally on a monthly basis. Ten replicates were recorded for each plot and averaged to give the percentage ground cover. Detailed species composition and biomass data were also collected annually for each plot.

Utilisation rates (75%, 50%, 0%) were managed by measuring the quantity of standing dry feed in autumn every year and adjusting cattle numbers on each paddock to suite. Therefore cover values were quite variable a times depending on availability of feed in autumn and the amount of rain that fell over the  following year.

Hence ground cover was a better reflection of changes in pasture and soil surface condition then utilisation rate.

Project administration

Site identifier code: na

Principal investigator: David Waters

Principal data manager: -na

Principal organizations: Meat Research Corporation; Department of Primary Industries; Department of Natural Resources, Mines and Water

Data custodian: Department of Environment and Resource Management (DERM)

Key co-operators: Aristida

Data access policy:

Planned pathway for data: completed study, evidence of formal database records (Hydstra)

Data warehousing: for ongoing studies -na

Planned data upload frequency: for ongoing studies -na

Key references and sources of this data synthesis

These data summaries have been extracted from:

1. Grazing management implications and runoff and erosion processes in semi-arid Central Queensland. ISCO 2004 – 13^th International Soil Conservation Organisation Conference
2. Aristida/Bothriochloa pastures under Eucalypt Woodland (Hydstra brief)
3. Interview with David Waters

Level 1, level 3, level 4 and level 5