Wallumbilla agricultural catchment study 1982-2000 (Queensland, Australia)
Description
A catchment study was established in 1983 on one of the major soil types (a brigalow brown clay) of the western Darling Downs. The objective of this 18 year duration study was to explore changes in soil water, catchment hydrology and water quality associated with different soil surface conditions i.e. tillage, stubble management, gypsum, roughness. The site was chosen to represent clay soils with high water holding capacity in the Maranoa region, where fallowing to store water for later crop growth is an essential risk management tool. Rainfall, soil water, runoff, suspended sediment concentration, cover and all agronomic operations were monitored on four contour bay catchments. Different tillage practices and a pasture were used to create a range of soil cover and roughness conditions in order to explore functional relationships between land management, soil conditions, hydrology and water quality. This study aimed to compliment similar studies at Greenmount and Greenwood on the eastern Darling Downs. This dataset provides benchmark data for further hydrologic and water quality studies. Key findings Daily runoff was determined strongly by soil water content which was strongly influenced by antecedent rainfall, water use and evaporation patterns. Surface cover and roughness had subtle influences on runoff, and a greater effect on suspended sediment concentrations. Runoff and suspended sediment movement was considerably lower under pasture than cropping. Accumulation of soil water in fallows was inefficient, with fallow efficiencies ranging from -7 to 40% due to high evaporation and runoff losses. Runoff and sediment concentration were both inversely related to surface cover and total soil movement was greatly reduced by surface cover. Greater than 75% of the variance in soil movement from single events was explained by surface cover and peak runoff rate. Differences in hydrology and water quality associated with different management (tillage) were more subtle than experienced on two similar sites on the eastern Darling Downs (Greenmount and Greenwood), mainly due to the lower levels of stubble available after winter crops due to the drier climate. Tillage (roughness) was equally influential as cover on runoff.
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Rainfall was measured using a network of three tipping bucket recording rain gauges, a manual rain gauge at each side of the catchment, and a gauge at the farm residence approximately 1 km south. This network provided for some redundancy for equipment failure, and a measure of spatial variability. A Cipoletti or trapezoidal weir design was chosen for its simplicity of construction and installation, ability to be simply adjusted to match differing catchment areas, and sensitivity to measure total runoff and peak discharge with reasonable resolution (~0.5 mm h-1). Samples of runoff water were collected at the weir outlet to determine suspended sediment concentrations. Rising stage samples were collected automatically at 5, 10, 20, 30, 40 and 50 cm stage heights with samples being drawn from mid flow depth. Method of calculating a weighted mean sediment concentration is presented in detail by Freebairn and Wockner (1986b), with concentration at each height weighted by its relative discharge at each stage. Water samples collected on the rising limb of hydrographs provide for a robust estimate of total suspended sediment eroded from the hill slope (Freebairn and Wockner, 1986b). Entrainment of ‘suspended’ material from the graded channel was unlikely due to the low channel slope (0.25%) and consequent low stream power of channel flow. The graded channels were typically aggraded with coarse eroded material from the hill slope. The use of rising stage samples as a reliable estimate of total sediment load assumes that the sediment concentration - discharge relationship for the falling stage is similar to the rising. This assumption rarely holds yet Freebairn and Wockner (1986b) demonstrated that this approach provided a pragmatic and reasonable estimate of total suspended load for most events at this scale of study. Soil conditions Surface cover was measured after each runoff event, after cultivation operations, at time of soil sampling and periodic site visits. Surface roughness was described using a semi-quantitative scale based on a visual assessment made at the same time as soil cover. Soil moisture was determined at nine locations in each bay to a depth of 1.5 m. Soil samples were collected using a hydraulically driven 30 mm diameter steel tube. Each core was separated into intervals of 1-10, 10-30, 30-60, 60-90, 90-120 and 120-150 cm and moisture content determined by weighing and oven drying. A mean of the nine-samples per depth was used in most analyses. Bulk density was measured on several occasions using 10 cm diameter cores, and used to convert gravimetric values to a volumetric basis. Grain yield was determined for each bay using a commercial harvester.
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Queensland Government