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Automatic measurement of drainage flux and nitrate leaching losses in a heavy clay soil Trevor Hendry, Keith C. Cameron, Hong J. Di, Jim Moir, Neil Smith and Nigel Beale Centre for Soil and Environmental Quality, PO Box 84, Lincoln University, Canterbury, New Zealand. Email: hendryt@lincoln.ac.nz Abstract
Dairy farming is expanding rapidly in Canterbury, New Zealand, and there is increasing concern about the effects that this expansion may be having on water quality in rivers, lakes and aquifers. The Canterbury plains is made up of many different soil types, including soils with a relatively impervious clay layer in the B horizon, usually at around 300 mm depth. Accurate measurement of drainage flux and nitrate leaching losses in these heavy textured soils is essential to determine the effects of dairy farming on water quality in this region. Six plots measuring 20 m x 5 m were constructed, each side by side and hydraulically sealed off from the outside paddock and each other by a vertically installed 700 mm deep polythene sheet. A permeable plastic ‘nova flow’ drainage pipe (100 mm) was installed in the centre of each plot at a depth of 800 mm, and the trench backfilled with gravel to 250 mm depth. The drainage pipe from each plot runs into a common sub-surface drainage chamber where 6 x 2 L capacity tipping buckets are housed to enable the simultaneous measurement and recording of drainage from all 6 plots. Each tipping bucket is connected to a data logger and an automatic water sampler. For each plot one tip of the bucket is equal to 0.02 mm of drainage. Every 25 tips (0.5 mm of drainage) the auto sampler takes a 70 ml sample and stores this in a 1 L bottle. After 10 samples (5 mm of drainage) the sampler will switch to a new bottle to continue collecting the next sample. Each auto sampler has 24 bottles, enough for 120 mm of drainage. All data files can be downloaded via telemetry and rate of drainage, number of samples taken, total amount of drainage (mm) per plot, rainfall (mm) and soil temperature can be viewed on the computer in real time back at the laboratory. Key Words
Dairy farming, drainage, water quality, automatic measurement
Introduction
Dairy farming is a major New Zealand export industry and around 95% of dairy products are exported. The value of exported dairy products (c. NZ$6 billion) represents about 20% of the total export returns for New Zealand, and this contribution is increasing.
In Canterbury dairy farming is growing rapidly. The conversion of a mid-Canterbury mixed cropping farm producing an income of around NZ$300 per hectare can return NZ$5,000 per hectare or more after conversion. Employment has also increased in Canterbury with significant demand for dairy farm workers on the farms that are newly established.
Despite the considerable economic and social benefits that have occurred through dairy conversions there is public concern with the dairying expansion in the South Island. One of the main concerns is the possible threat to the quality of ground and surface waters, particularly by nitrate (N), phosphate (P) and microbial contaminants.
Lincoln University has established a new dairy farm on the same major types of soils that are being converted throughout the South Island of New Zealand. This 650-cow dairy farm has 161 ha that are irrigated using two 400-m long centre pivot irrigators. Production in the 2003-04 season was 422 kg Milk Solids (MS) per cow and 1,684 kg MS per hectare placing the farm in the top 5% in New Zealand.
Objective
The objective of this project was to design and construct large drainage plots and associated automatic recording and proportional sampling systems that can be used to make accurate measurements of drainage water fluxes and nitrate leaching losses in a heavy clay soil.
Pipe drainage measurement and monitoring system
In Temuka clay soils (Gley soil) on the new Lincoln University dairy farm drainage water flows vertically until it reaches the subsoil, and then flows horizontally until it reaches a drainage ditch at the farm boundary and no waterlogging occurs. Therefore, they are suited to a pipe drainage measurement and monitoring system. A ‘nova-flow’ drainage pipe has been installed in each hydraulically isolated drainage plot (each 100 m2) in this soil on the farm (Figures 1, 2). Six plots measuring 20 m x 5 m were constructed, each side by side and hydraulically sealed off from the outside paddock and each other by a vertically installed 700 mm deep polythene sheet. A permeable plastic ‘nova flow’ drainage pipe (100 mm) was installed in the centre of each plot and the trench backfilled with gravel to 250 mm depth.
Figure 1. Environmental monitoring system for measuring the effect of dairying on water quality.
Figure 2. Aerial view of the drainage plots during construction on the Lincoln University dairy farm.
Automatic samplers are used to collect drainage water samples at each outfall in order to collect the drainage water in proportion to the flow rate in the drains. Tipping-bucket sensors are used to determine the drainage flow rate (Figure 3) and this information is used by a computer to schedule the rate that the water samples are collected (Figure 4). The water then flows to waste via a local farm drainage ditch. The drainage pipe from each plot runs into a common sub-surface drainage chamber where 6 x 2 L capacity tipping buckets are housed to enable the simultaneous measurement and recording of drainage from all 6 plots. Each tipping bucket is connected to a data logger and an automatic water sampler. For each plot one tip of the bucket is equal to 0.02 mm of drainage. Every 25 tips (0.5 mm of drainage) the auto sampler takes a 70 ml sample and stores this in a 1 L bottle. After 10 samples (5 mm of drainage) the sampler will switch to a new bottle to continue collecting the next sample. Each auto sampler has 24 bottles, enough for 120 mm of drainage.
Figure 3. Six tipping buckets are used to measure the drainage flux from 6 plots.
Figure 4. Automatic samplers collect sub-samples of drainage water in proportion to the rate of low.
Rainfall and irrigation are recorded at the site using a tipping bucket rain gauge, attached to a data logger. Data from the drainage samplers, rain gauge and other sensors are sent by telemetry to the Centre’s laboratories. This also alerts staff about the need to collect samples.
Figure 5. The drainage plots immediately outside the electric fence are grazed by cows the same as the rest
of the farm paddocks.
Figure 6. Simultaneous measurement of the drainage rate on six plots by the tipping bucket method.
Initial results
The system is working well over a wide range of drainage rates. Figure 6 illustrates the range of drainage rates measured simultaneously by the tipping bucket system for the 6 plots during the period September 2002 to May 2004.
Initial results show that the amount of nitrate leached from these Temuka clay soils was less than 10 kg N/ha/y (Figure 7). These losses are less than the amounts reported to be lost from free draining stony soils in Canterbury where over 100 kg N/ha/y can be leached (Di and Cameron 2002).
This is an on-going research programme that will continue to monitor nitrate leaching under grazed pasture systems until, at least, June 2007. An integral part of the work is the transfer of technology to the New Zealand dairy industry, and as a component, we have an active visitor programme in place.
Figure 7. Cumulative amount of nitrate leached from the Temuka clay soil on the Lincoln University dairy farm.
Acknowledgements
The authors would like to acknowledge Dairy InSight, MAF (Sustainable Farming Fund), Ravensdown Fertiliser Cooperative Ltd and Lincoln University for financing this project.
Reference
Di HJ, Cameron KC (2002) Nitrate leaching and pasture production from different nitrogen sources on a shallow stony soil under flood-irrigated dairy pasture. Australian Journal of Soil Research 40, 317-334. 
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