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Water quality in any stream is affected by complex interactions between natural and anthropogenic factors in a given catchment area. Agriculture has been identified as a major contributor of nitrogen and phosphorus inputs to surface waters in the Baltic Sea region. Although decisions regarding agricultural management practices, e.g. crop rotation, tillage, fertilization, have a direct impact on likelihood and magnitude of nitrogen and phosphorus losses from agricultural areas to surface waters, natural factors such as meteorological and hydrological conditions have a triggering role in processes determining transformations, storage, uptake and losses of nutrients. In order to investigate the effects of meteorological (precipitation and air temperature) and hydrological (runoff) conditions on water quality (losses of total nitrogen (TN) and total phosphorus (TP)) the results of the Agricultural Runoff Monitoring Programme collected at three monitoring sites (Berze, Mellupite, and Vienziemite) during the time period of 1995–2020 were summarized and analysed. The pronounced differences in mean annual air temperature and annual precipitation were observed when the meteorological information representing the periods of twenty years was compared indicating for evidences of climate change. In addition, the relationships between seasonal precipitation and runoff was detected. As affected by the hydrological behaviour the losses of TN and TP in agricultural catchments had large variations depending on the intensity of agricultural production and site location. The changes in seasonal and annual patterns of precipitation, air temperature and runoff may increase the risks of nutrient losses from agricultural catchments in the future.

The Baltic Sea is the youngest sea on our planet, the environment of the sea is considered to be unique and fragile. It is affected by various human activities resulting in the impairment of water quality. Riverine nutrient (nitrogen and phosphorus) loads are among the major causes of eutrophication of the Baltic Sea. This study examines temporal trends in water discharge, total phosphorus (TP) and orthophosphate-phosphorus (PO4-P) concentrations and losses from three agricultural runoff monitoring sites in Latvia including Berze, Mellupite, and Vienziemite. The annual datasets of TP and PO4-P concentrations and losses were tested for statistical trends using a nonparametric test - the Mann-Kendall trend test. The timeframe of this study was from 1995 until 2018. The results show a large variety of annual mean concentrations and losses of TP and PO4-P in the study period. No statistically significant trend was detected for TP losses. Meanwhile, statistically significant downward trends were observed for TP concentrations in four out of six study sites and in two study sites for PO4-P concentrations.

Water quality in small agricultural catchments is affected by anthropogenic (crops, field management operations, fertilizer application) and natural (soil texture, topography, meteorological and hydrological conditions) factors. It is important to estimate long-term (1995–2018) trends of nitrogen losses from small agricultural catchments and subsurface drainage fields to surface water bodies in order to evaluate the impacts of anthropogenic and natural factors and to cover management needs such as implementation of national legislation and the EU directives. The content of the Common Agricultural Policy is currently under development, therefore, there is a need to consider existing monitoring results to address water quality goals within the next planning period. This study examines temporal trends in water discharge, total nitrogen (TN) and nitrate–nitrogen (NO3-N) concentrations and losses from three agricultural runoff monitoring sites in Latvia including Berze, Mellupite, and Vienziemite. The yearly data series of TN and NO3-N concentrations and loss were tested for statistical trends using the Mann-Kendall test. The study results show high variations in TN and NO3-N concentrations and losses among monitoring sites both in subsurface drainage field and small catchment scale. For TN and NO3-N losses five out of six monitoring sites showed an upward trend, while for TN concentrations three out of six monitoring sites and NO3-N four out of six monitoring sites showed an upward trend. The results of this study indicate that the upward trends in TN losses are affected by hydrological conditions at the study sites.