Water table management to save water and reduce nutrient losses from agricultural fields: 6 years of experience in North-Eastern Italy

To evaluate the performances of a controlled drainage system in optimizing water use and reducing nutrient losses from agricultural fields, an experimental facility was set up in north-eastern Italy in 1996. Water table management was tested in combination with surface (open ditches) and subsurface (pipe) drainage systems. Data were collected from 2007 to 2013 on: water table depth, drained volumes, nitrogen and phosphorus concentrations in groundwater and in drainage water. Nitrogen in groundwater showed higher concentrations when controlled drainage was combined with open ditches system, with a median of 13.43 mg L-1 for NO3-N and 18.68 mg L-1 for total N. Drainage water showed an opposite trend: subsurface pipes with free drainage provided highest concentrations due to extensive leaching (a median of 20.7 mg L-1 for NO3-N and 24.0 mg L-1 for total N). Phosphorus concentrations showed notable differences in drainage water, with higher values in the controlled drainage – open ditches system due to surface runoff (a median of 0.190 mg L-1 for PO4-P and 0.536 mg L-1 for total P). In general, the most hazardous period for surface water pollution was autumn-winter, due to rainy weather and fertilizer application on bare soil. Overall, water table management reduced total water discharge by 81% compared to free drainage. On average, with controlled drainage annual nitrogen losses were lowered by 92% (from 29 to 2 kg NO3-N ha-1) and annual phosphorus by 65% (from 0.14 to 0.05 kg PO-4P ha-1). Free drainage with subsurface pipe was the worst combination from the environmental point of view: annual nitrogen and phosphorus losses were 46 kg NO3-N ha-1 and 0.10 kg PO4-P ha-1 respectively. Water table management clearly proved to be a reliable tool to improve both water usage and quality.