| |
CRC PUBLICATIONS |
Implications of Irrigation Bay Management for Salt Export
Mat Gilfedder,
Luke Connell,
Russell Mein
Publication Type:
Technical Report
This is a publication of the initial CRC for Catchment Hydrology
CRC Program:
Salinity (Previous CRC)
|
Publication Keywords:
Irrigation Water
Drainage
Water Movement
Salt Transport
Groundwater/Surface Water Relationship
Seepage
Salinity Control
|
Abstract / Summary:
Executive Summary
This report presents the results from a detailed field experiment of water movement and salt transport within a border-irrigated bay in northern Victoria, an area characterised by shallow groundwater tables and salinisation problems. The aim of the study was to assess the impact of change in irrigation management on salt and water movement from the bay, including recharge to the region's shallow groundwater table.
An irrigation bay in the Barr Creek catchment was instrumented to monitor irrigation events throughout the 1996/97 irrigation season. The bay was carefully selected to ensure that its physical size, slope and irrigation arrangement were fairly typical of bays in the subcatchment. Surface and subsurface water and salt movement variables and processes were measured, together with supply and drainage volumes and salinities, at an irrigation event scale. These detailed data, from an entire irrigation season, have been used to improve our understanding of irrigation bay behaviour and the implications of irrigation management changes on both the bay's salt export and the bay's soil salinity.
Salt Balance
The monitored irrigation bay was in an approximate surface salt balance for the 1996/97 season. Although the seasonal surface salt balance was close to unity, individual irrigation events showed salt export ratio variability from 50% to 200%.
The bay 'salt balance' was far below the Drain 14 subcatchment salt export ratio of 400% for the same period, indicating that irrigation runoff was only a minor component of regional drain salt load. If irrigation runoff was assumed to be equivalent to supply salt load, then irrigation runoff accounted for only 25% of total subcatchment drainage salt load. Direct groundwater seepage into the deeper regional drains was likely to have been responsible for the remaining 75% of the irrigation season drainage salt load, as well as the large regional salt export ratios. This suggests that improved control of irrigation supply will not decrease the regional salt export ratio, unless it is able to lower the regional groundwater table.
Salt Transport
Salt transport during irrigation events at the monitored bay is dominated by the lateral washoff of salts, mainly from the soil surface. The heavy - clay soils prevented significant washing of salt deeper into the soil profile. The high salinity at the start of drain flow dropped quickly, as a result of the 'first flush' of salt being removed from the bay.
If irrigation volumes were reduced significantly, the lateral removal of washoff salt would be compromised. The low permeability of the soil matrix, combined with the shallow groundwater table and the presence of soil cracks, would remove the opportunity for vertical leaching of soil salt through the profile. Hence, overland flow 'mobilised' salt from the soil surface, and transported it in the surface water. This lateral washoff of soil salt was a significant process in the 'leaching' of bay soil. Significant reduction in irrigation volumes would continue to mobilise soil salt, but would be less effective in removing this salt from the bay in drainage and lead to a build-up of surface salt in the lower part of the bay. These findings highlight the importance of lateral washoff in salt export from irrigation bays, suggesting that reduction in irrigation event volumes is likely to reduce salt export and thus affect the sustainability of irrigation in this area.
Groundwater Recharge
Water movement results showed that the evapotranspiration volume almost wholly explained the soil moisture deficit between irrigation events; deep drainage was negligible. Infiltration was mainly confined to the advance stages of irrigation, with the soil rapidly becoming saturated across the bay, due to the presence of soil cracks. This implies that more efficient management of supply to irrigation bays will not lead to the lowering of the region's shallow groundwater table. The relationship between irrigation practice and regional groundwater appears more complex than previously anticipated, a finding which has important implications for irrigation management.
Re-use to Improve Efficiency
'Re-use' of on-farm irrigation runoff appears to have significant benefits. This 're-use' can allow irrigators to maintain sufficiently high volumes of irrigation supply water, preventing excessive build-up of surface salt in the lower part of the bay, while minimising their contributions to regional drainage. The 're-use' of irrigation runoff would also concentrate flows to the downstream regional drain network, allowing more efficient use of large disposal schemes.
Study Limitations
While this study's research findings have provided insight into the mechanisms operating within irrigation bays, practitioners should be careful when translating these results to other irrigation areas or within the Barr Creek catchment without further validation studies. Particular care should be taken when considering bays with lighter, sandier soil types than those present at the monitored bay.
The report is based on the amalgamation of three manuscripts which have been recently submitted for publication (Section 1 & 2: ASCE Journal of Irrigation and Drainage Engineering; Section 3: Australian Natural Resource Management Journal). The report also reflects a summary of Mat Gilfedder's PhD thesis (Irrigation Bay Processes Leading to Salt Export) which was passed in September 1998.
This report is available for downloading here. Printed copies are available for purchase from the Centre Office.
 |
1 Download(s) |
|
|
|
|
|
|
|
|
Centre Office:
CRC for Catchment Hydrology
Dept of Civil Engineering
Building 60
Monash University Vic 3800
Tel: +61 3 9905 2704
Fax: +61 3 9905 5033
|
|
|
