Irrigation runoff from a rolling landscape with slowly permeable subsoils in New Zealand

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 irrigation events we measured up to 8% of the applied water 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. 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 a combination of irrigation intensity and depth have 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.