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

Abstract

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 losses 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.