Irrigation runoff from a rolling landscape with slowly permeable subsoils in New Zealand
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Irrigating to hill slopes introduces a number of complexities that are otherwise not present when irrigating flat land. These include variable soil depth and a greater propensity for water to move laterally in surface and subsurface flow pathways. We monitored soil–water dynamics under a centre-pivot irrigation system on rolling downlands in North Otago. During the 2013 irrigation season, a total of 180 mm of irrigation was applied, of which a total of 15 mm were measured as overland flow exiting the catchment (approximately 1 ha). Runoff occurred rapidly during the irrigation event as a result of saturation-excess conditions that developed at the base of the catchment. No overland flow was measured across the weir in direct response to rainfall during the irrigation season. Modelling of the soil water dynamics suggests the onset of overland flow occurred prior to the soil profile reaching a point of saturation, indicating the presence of preferential flow under irrigation but not in response to rainfall. We propose that the combined effect of both irrigation intensity and depth have resulted in localised areas of water saturation within the matrix of the soil A horizon. This in turn has led to the preferential movement of water along the interface of a slowly permeable B horizon. Water lost in overland flow compromises water-use efficiency and has environmental significance as a vector for mobilising contaminants such as nitrogen. Runoff occurred rapidly during the irrigation event as a result of saturation-excess conditions that developed at the base of the catchment. Modelling of the soil water dynamics suggests the onset of overland flow occurred prior to the soil profile reaching a point of saturation, indicating the presence of preferential flow. However, this was not apparent in response to rainfall. We propose that a combination of irrigation intensity and depth has led to localised areas of water saturation within the soil matrix that have resulted in preferential movement of water laterally along the slowly permeable B horizon. Water lost in overland flow compromises water-use efficiency and has environmental significance as a vector for mobilising contaminants such as nitrogen.
We would like to acknowledge funding from the Sustainable Farming Fund and Irrigation New Zealand for this project, the kind support of John Officer, and the other farmers who were involved in the project. We also acknowledge the technical support provided by Tash Styles, Tom Orchiston, Phil Olyott, and Matthew Brown (GIS).
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