Field measurement of groundwater recharge under irrigation in Canterbury, New Zealand, using drainage lysimeters

Irrigation using groundwater in Canterbury, New Zealand, is reaching sustainable limits and to assist with water allocation a better understanding of groundwater recharge from irrigated agriculture is required. To help characterise groundwater recharge from irrigated pasture, three sets of three drainage lysimeters were installed in three irrigated dairy farms in Canterbury, New Zealand. Two farms have free draining, shallow, stony soils over gravel and the third site has a deep silt loam. The sites are spread across three landscape positions within the Canterbury Plains-foot-hill, mid plains and coastal plains. Average annual rainfall during the study period (2010-13) at the sites varied between 633 mm (coastal plain) and 891 mm (foothill). Irrigation management varied among the farms. Irrigation applications increased as actual evaporation increased and ranged from 144 to 445 mm/season (September-April). Drainage tended to increase with annual rainfall and most (70%) occurred in the winter (May-August). Drainage from the shallow stony soils and deep silt loams averaged 33 and 18% respectively of total precipitation (irrigation plus rainfall), a similar percentage to those reported from dryland lysimeters studies in this region. However, as the total precipitation on the irrigated sites is greater than rainfall in the dryland studies, irrigated agriculture had more drainage. This implies that irrigation of dryland will result in more recharge, but in much of Canterbury efficient centre pivot irrigators have replaced border dyke flood irrigation that has very high recharge rates, so there may be an overall reduction in recharge.