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Runoff from raised bed crops in south-west Victoria, 2001

Tim Johnston1, Robert White2, Marc Nicolas2 and Bill Slattery3

1 DNRE, PO Box 103, Geelong, Vic 3220. www.nre.vic.gov.au Email Tim.Johnston@nre.vic.gov.au
2
The University of Melbourne, Institute of Land & Food Resources, School of Resource Management, Parkville, Vic 3010. Email robertew@unimelb.edu.au Email m.nicolas@landfood.unimelb.edu.au
3
DNRE, Rutherglen Research Institute, RMB 1145, Rutherglen, Vic 3685. www.nre.vic.gov.au Email Bill.Slattery@nre.vic.gov.au

Abstract

An experiment was conducted in south-west Victoria to investigate the impact of raised bed crops on the quantity and quality of runoff water compared with conventional crops grown without raised beds and pastures. The 2001 season was characterised by above-average rainfall over the growing season, resulting in seven significant runoff events from all plots. Results show that the intensity and length of the rainfall event and the timing of rainfall during the season is a significant contributor to differences in runoff volumes between treatments. During a storm in April 2001, the raised beds released more runoff than the conventionally-cropped and pasture plots. However, there was negligible difference between treatments during lower intensity rainfall events over the winter that are more typical of winter rainfall patterns in southern Australia. Total phosphorus and total nitrogen concentrations for all runoff events were well above levels recommended as satisfactory for Victoria's inland waters.

Key Words

Agricultural runoff, water quality

Introduction

In the high rainfall (500+ mm) regions of southern Australia, raised beds have been shown to reduce the effects of waterlogging and poor soil structure on grain yields (1). However, local authorities and regional communities have concerns about the off-site effects of water flows from raised beds. This paper summarises the preliminary results of runoff flows from raised beds and conventional plots in 2001.

Methods

A field experiment was established in 1999 on a site 30 km west of Geelong, with a randomised block design of three cropped treatments (0.2 ha plots) replicated three times, plus one unreplicated pasture plot (1.5 ha). Average annual rainfall for the district is 525 mm. The treatments were:

1. Crops grown without raised beds (shallow cultivation to 0.08 m depth).

2. Crops grown without raised beds (deep cultivation to 0.15-0.20 m depth).

3. Crops grown on raised beds (deep cultivation to 0.15-0.20 m depth).

4. Pasture plot was on a similar soil type and gradient (1%) as the cropped plots.

In 2001, wheat (cv. Mitre) was sown on 3 August with 23 kg P/ha and 11 kg N/ha as mono-ammonium phosphate. The perennial ryegrass/subclover pasture plot was set stocked with merino wethers at 7 DSE/ha (district average) and topdressed with 9 kg P/ha as single superphosphate early in winter. Runoff volumes were determined using flow measuring flumes connected to a central datalogger. Automatic water samplers either collected water samples at 0.2 mm runoff intervals or following manual activation during flow events. Within 24 hours of collection, water samples were placed in -15oC storage, until they could be analysed for total nitrogen (TN) and total phosphorus (TP).

Results

Rainfall and runoff volume

The first runoff event occurred when 147 mm of rain fell between 21-24 April with rainfall intensities >5mm/hour on occasions. Over the four-day period, the mean volume of runoff from the raised bed crop treatment was considerably higher than from the other treatments (Table 1). The 102 mm of runoff from the raised beds represented 66% of the rainfall received over that period.

Table 1. Mean surface runoff volumes (mm) for 2001.

 

No. of

Runoff volume (mm)

 

runoff events

Flat bed, shallow cult. crop

Flat bed, deep cult. crop

Raised bed crop

Pasture

April

1

71

57

102

55

SE

-

22.5

11.0

15.6

-

June-Sept

6

32

31

38

41

SE

-

4.3

3.4

1.5

-

Total

7

103

88

140

96

SE

-

26.8

14.4

16.1

-

The lower runoff volume observed from the pasture plot in April could be attributed to the extensive cracking of the soil surface (A horizon), providing preferential pathways for water movement into the subsoil (B horizon). However, comparable differences in subsoil moisture have not been identified between the treatments following gravimetric soil sampling during the season. The higher volume of runoff from the conventional, shallow cultivation treatment, compared with the conventional, deep cultivation treatment, could be attributed to pre-existing compaction of the upper layer of the B horizon. This compaction may be limiting water movement below the 0.08 m cultivation depth. A further six runoff events occurred between June to September 2001 and were more characteristic of winter rainfall events in southern Australia. That is, they were of low intensity (<5 mm/hr) and exceeded 10 mm in total. There were no differences in runoff volumes between the crop and pasture treatments in the June-September period.

Runoff water quality

The flow-weighted mean total phosphorus (TP) and total nitrogen (TN) concentrations from the storm event in April differed from later events in the June to September period. In April, all flow-weighted mean TP concentrations were considerably lower than the events in June-September (Table 2).

Table 2. Flow-weighted mean total phosphorus (TP) and total nitrogen (TN) concentrations (mg/L) in 2001.

 

Runoff period

Flat bed, shallow cult. crop

Flat bed, deep cult. crop

Raised bed crop

Pasture

TP

April

0.69

0.43

0.37

0.84

 

Jun-Sep

2.8

2.2

1.5

1.7

TN

April

30

29

29

20

 

Jun-Sep

24

30

38

7.9

The mean TP concentration of runoff from the raised bed treatment in April was lower than the other treatments, possibly due to the dilution effect of the higher flow volumes. However, all concentrations were still well above those recommended as satisfactory for Victoria's inland waters (2). The higher mean TP and TN concentrations of runoff from the crop plots for the June-September period are likely to have come from the fertiliser drilled at sowing.

Conclusion

Raised bed crops have the potential to significantly increase the volumes of water running off agricultural land. Differences in runoff volumes are dependent on the intensity and length of rainfall events and when they occur in the cropping season.

References

(1) Southern Farming Systems Ltd 2001. Trial Results - 2001, Southern Farming Systems Ltd, Geelong, Vic.

(2) Office of the Commissioner for the Environment 1991. State of the Environment Report 1991, Agriculture and Victoria's Environment.

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