Greenmount agricultural catchment study (Queensland) 1976-1990
Description
This catchment study near Greenmount on the eastern Darling Downs 15km south of Toowoomba was established to examine the influence of soil surface conditions on runoff and soil erosion. The study was established in 1976 on a soil representative of one of the major soil types (Black Earth – Irving clay). Management at the paddock scale is likely to strongly influence water quality for the whole catchment, thus the basic unit of hydrology and erosion, a contour catchment was choosen as the focus for this study. This project examined the influence of rainfall depth and intensity, runoff rate and ground cover on suspended sediment moving through a flume at the exit from a contour bay and the effects of contour banks on sediment delivery to streams were evaluated. Key findings included: Surface cover was effective in reducing soil movement and runoff from contour bay catchments. 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. F ield observations were able to be reproduced within reasonable agreement using the Howleaky water balance model.
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The catchment study's configuration was based on an existing soil conservation plan with an established grassed waterway and contour banks (graded) (see aerial photograph above). While "tillage treatments" were applied, the aim was to create a range of soil surface conditions (soil cover, tillage) by applying five arbitrary but contrasting tillage/crop regimes. This resulted in soil cover ranging from very low (stubble burnt) to very high (zero tillage), with soil water increasing during summer fallows. A summer crop was grown in one catchment (bay) which has a declined soil water and increasing green cover. Summer fallow treatments were rotated or moved to different bays each year in order to attempt to reduce results being attributed to a bay position rather than a soil condition. Implicit in this design was that any long term impacts of a "treatment" were ignored or avoided, given the study has no replication and such subtle treatment effects are were unlikely to be observed if they did exist. This was the design philosophy - to create a range of soil conditions, not compare treatments per sec, although data is generally summarised by treatment. Tillage, planting and harvest operations were carried out by the cooperating farmer (Mr Terry Cuskelly) with his equipment and by Queensland Government staff and equipment. Treatments Four catchments were devoted to winter crop (wheat) with intervening summer fallow and a fifth to summer crop. Throughout the summer fallows, a variety of stubble management and fallowing practices provides a range of soil conditions (cover, tillage). Treatments were described as: • winter crop, stubble burnt, bare soil summer fallow • winter crop, disc tillage, stubble incorporated summer fallow • winter crop, blade or chisel tillage, stubble mulched Summer fallow • winter crop, no tillage (herbicide weed control), zero tillage summer fallow • summer crop, chisel tillage, winter fallow Measuring runoff through H flumes (left) and an 90oV notch weir (right). water level was recorded using either Stevens recorder local direct float-pen recorders. Water was sampled using rising stage samplers and in some cases, automatic pumping samplers. 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. Runoff was measured through recorded gauge height using weirs at 5 minute time intervals and summed to daily (9am to 9am) values in this data collection. Runoff water was sampled using rising stage and in some cases timed samplers at the weir outlet to determine suspended sediment concentrations. Total soil movement, as described by Freebairn and Wockner (1986a) was measured in this study by estimating the volume of rills and sediment deposits after each major runoff event.