Controlling irrigation water flow to mole drains

Subsurface drainage in irrigated agriculture presents an interesting challenge in that rapid drainage is required after an irrigation or rainfall event to alleviate any waterlogging, but during irrigation events drainage is undesirable. High rates of drainage during an irrigation event will lead to poor application efficiency. Also, when using mole drains, which rely entirely upon soil cohesion to retain their structure, high rates of water flow can lead to their rapid collapse. This paper compares various methods of reducing rates of irrigation water flow into mole drains and the subsequent effect on mole stability. Localized crack sealing methods using a trailing wedge behind the mole plough, followed by tractor wheeling or rotovation, were unsuccessful. However, generalized surface tillage to create a fine surface tilth about 100 mm deep was successful in reducing mole how rates and improving stability. Preventing discharge from mole drains during irrigation was particularly effective in reducing mole flows and was not found to reduce mole stability compared to high rates of drainage throughout an irrigation. Sprinkler irrigation was found to result in lower mole flow rates than flood irrigation, and improved mole stability. Positioning the mole away from areas where water is ponded on the soil surface for irrigation, such as furrows, also resulted in lower hows and improved stability.