|Ayars, James - Jim|
Submitted to: Journal of Irrigation and Drainage Systems
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/12/2004
Publication Date: 5/1/2005
Citation: Hornbuckle, J.W., Christen, E.W., Ayars, J.E., Faulkner, R.D. 2005. Controlled water table management as a strategy for reducing salt loads from subsurface drainage under perennial agriclture in semi-arid australia.. Journal of Irrigation and Drainage Systems. Vol. 19: 145-159. Interpretive Summary: Recent community based actions to ensure the sustainability of irrigation areas and associated ecosystems in the Murrumbidgee Irrigation Area (MIA) of Australia have resulted in the implementation of Land and Water Management plans. These plans aim to improve land and water management within the irrigation area and to minimize downstream impacts associated with subsurface drainage from irrigation. The plan has set objectives to decrease current salt loads generated from subsurface drainage in perennial horticulture within the area from 20,000 tonnes/year to 17,000 tonnes/year. Controlled Watertable Management (CWM) is a possible and Best Management Practice to reduce drainage volumes and salt loads. During 2000-2002 a trial was conducted on a typical 15 ha subsurface drained vineyard. A traditional unmanaged subsurface drainage system was compared with a controlled drainage system utilizing weirs to maintain watertables and changes in irrigation scheduling to maximize the potential crop use of a shallow watertable. Drainage volumes, salt loads and watertable elevations throughout the field were monitored to investigate the effects of controlled drainage on drain flow rates and salt loads. Controlled drainage significantly reduced drainage volumes and salt loads compared to unmanaged systems. However, there were marked increases in soil salinity which will need to be carefully monitored and managed. Controlled drainage will provide opportunities for improved water use efficiency and reducing negative environmental impacts of irrigation.
Technical Abstract: Controlled drainage was implemented in a 7 year old 15 ha vineyard in the Murrumbidgee Irrigation Area of Australia. Two controlled plots containing 3 drainage laterals were compared to a free flowing plot also containing 3 laterals. The lateral installation depth ranged from 1.8 to 2 m and the water table position was controlled a 1 m depth at the lateral outlet. Water table depth applied irrigation volumes and quality was measured for each irrigation over the 2 year study. Soil profile salinity, and drainage flow rates and applied water and runoff from furrow irrigation was measured. The results of the study demonstrated that the water table position was maintained for a longer period of time with controlled drainage than with free drainage. Peak and total drainage flow was reduced with controlled drainage compared to free drainage. Drainage during the study period from the free drainage treatment was 9 mm while only 1 mm of drainage was measured from the controlled sites. The total salt removal from the free drainage treatment was 164 kg/ha compared to 12 and 13 kg/ha for the controlled site. The irrigation water applied approximately 77 kg/ha of salt to the field. This suggest that salt accumulation was occurring in the profile which was confirmed by the soil sampling. There was relatively low rainfall and under irrigation during the study period which probably contributed to the salt accumulation in the controlled plots. While accumulation did occur, the yields were not affected in the controlled plots. For controlled drainage to be sustainable, salt accumulation in the soil profile will have to be managed. This can be done by regulation of the water table position and using winter rainfall or a leaching irrigation early in the growing. Controlled drainage in semi-arid irrigated areas offers opportunity to improve irrigation efficiency and minimize the environmental impacts of irrigation on surface water quality.