The assessment of phosphorus concentrations in agricultural ditches during thevegetation period in Latvia

The climatic conditions in Latvia can be characterized as humid, where the annual
precipitation exceeds evapotranspiration. Therefore, sustainable and economically
sound agricultural production is not possible without drainage systems including
subsurface tiles and open ditches. Open ditches not only receive and convey excess
moisture from agricultural fields, but also serve as a pathway to transport nutrients to
downstream waterbodies. The aim of this study was to assess phosphorus
concentrations in the selected agricultural ditches during the vegetation period. The
monitoring results obtained within this study will characterize water quality in the
selected agricultural ditches, which are located in all four geomorphological regions
of minimum runoff as designated by the Regulations on Latvian Building Code LBN
224-15 “Land reclamation systems and hydrotechnical structures”, thus making it
possible to identify the need and importance of establishment of buffer strips along
agricultural ditches to improve water quality.
Overall, 112 agricultural ditches were selected for this study, whose locations evenly
cover the territory of Latvia. The water samples were collected in 107 ditches using a
manual grab sampling approach during one sampling campaign carried out from
September 20 to October 30, 2025, the water flow in 5 ditches had dried out. Water
samples were collected in 0.5 l polyethylene bottles, which were stored in a
refrigerator at 2º – 4º C before transportation to the accredited laboratory. Total
phosphorus (TP) and orthophosphate (PO 4 -P) concentrations were determined
according to the national standards. The geospatial dataset of Corine Land Cover
2018 was applied to extract the information on land use patterns in the catchment
areas of the selected agricultural ditches.
In the case of 70 water samples collected in the agricultural ditches, the estimated
ratio between PO 4 -P and TP was greater than 50% indicating for phosphorus losses
from agricultural fields, where the excess moisture and soluble form of PO4-P are
collected by the subsurface drainage systems and transported to the ditches. By

comparing TP concentrations detected in the selected ditches with the threshold value
of the good water quality relevant for small and slow flowing rivers (0.09 mg l -1 ) as
specified in the River Basin Management Plans in Latvia, it can be concluded that in
the case of 18 ditches the concentrations of TP exceed the threshold value. In these
ditches buffer strips would be a relevant measure to reduce losses of phosphorus from
agricultural fields. No distinct patterns in TP concentrations were observed relative to
spatial representation of the agricultural ditches within the geomorphological regions
of minimum runoff. The analysis of the share of land use and water quality
monitoring results showed that there is a positive, but weak relationship between the
share of agricultural land and TP concentrations in the selected agricultural ditches.
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.