The relationship between concentrations of nitrogen and phosphorus and catchment area and length of agricultural ditches in Latvia

The hydrological network in agricultural areas in Latvia consist of subsurface drainage systems and agricultural ditches. The excess water along with nutrients mainly in the soluble forms on nitrogen and phosphorus is collected from agricultural areas by subsurface drainage systems and discharged into agricultural ditches. It is known that the amount and quality of water leaving agricultural areas depends of multiple factors, e.g., meteorology (precipitation and air temperature) and catchment specific (catchment area, land use distribution, topography, soils, crops). This study aims to assess the concentrations of nitrogen and phosphorus in relation to the catchment area and length of the selected agricultural ditches. The outcomes of this study will support decision makers to evaluate potential criteria for implementation of buffer strips of different width along agricultural ditches in Latvia.

For the purpose of this study 112 agricultural ditches, which are evenly distributed in the territory of Latvia and among all four geomorphological regions of minimum runoff, were selected for detailed investigation. The analysis of geospatial data showed that the ditches are in the length from 325 m to 1524 m, while the catchment area ranges from 0.37 to 10.98 km². In 107 of 112 ditches water samples were collected and analyzed for total nitrogen (TN) and total phosphorus (TP) concentrations in the accredited laboratory according to the national standards. It was not possible to collected water samples in 5 ditches due to lack of water.

The results of this study showed that 108 of the catchment areas of the selected agricultural ditches were smaller than 2 km², while the length of the same ditches varied from 325 m to 1475 m. The linear regression analysis indicated that there is a weak and positive relationship between the catchment area and length. The observed concentrations of TN varied in the range from 0.49 to 13.95 mg l^-1, while TP concentrations were in the range from 0.003 to 0.911 mg l^-1, where neither catchment area nor length were detected as the factors affecting TN or TP concentrations. The results of this study indicate that the catchment area and length of the agricultural ditches cannot be directly applied as the parameters to define different widths for buffer strips as these parameters does not directly affect water quality.

This study was funded by the Ministry of Agriculture of the Republic of Latvia within the scope of the research project “The assessment of hydrological conditions and water quality during the vegetation period in agricultural ditches in Latvia”, the decision No. 10.9.1-11/25/1545-e.