Calibration and validation of SWAT model for estimating water balance and nitrogen losses in a small agricultural watershed in central Poland

Soil and Water Assessment Tool (SWAT) ver. 2005 was applied to study water balance and nitrogen load pathways in a small agricultural watershed in the lowlands of central Poland. The natural flow regime of the Zgłowiączka River was strongly modified by human activity (deforestation and installation of a subsurface drainage system) to facilitate stable crop production. SWAT was calibrated for daily and monthly discharge and monthly nitrate nitrogen load. Model efficiency was tested using manual techniques (subjective) and evaluation statistics (objective). Values of Nash–Sutcliffe efficiency coefficient (NSE), coefficient of determination (R2) and percentage of bias for daily/monthly discharge simulations and monthly load indicated good or very good fit of simulated discharge and nitrate nitrogen load to the observed data set. Model precision and accuracy of fit was proved in validation. The calibrated and validated SWAT was used to assess water balance and nitrogen fluxes in the watershed. According to the results, the share of tile drainage in water yield is equal to 78%. The model analysis indicated the most significant pathway of NO3-N to surface waters in the study area, namely the tile drainage combined with lateral flow. Its share in total NO3-N load amounted to 89%. Identification of nitrogen fluxes in the watershed is crucial for decision makers in order to manage water resources and to implement the most effective measures to limit diffuse pollution from arable land to surface waters

Monthly Precipitation Patterns in a Region Vulnerable to Climate-Related Hazards—A Case Study from Poland

Precipitation totals at annual and monthly scale are key elements affecting water availability for a number of human activities. Thus, the multi-year variability and probability distribution of precipitation form the basis information for water management and damage risk assessment (caused by drought and heavy rains). The main goal of this paper is to analyse the precipitation patterns and to determine the non-exceedance probability (p, %) of extreme monthly precipitation in the region influenced by lignite mining and dominated by traditional agricultural production. Monthly precipitation totals recorded in 1961–2010 at four weather stations close to the analysed region were examined, including comparative analysis of precipitation patterns between the stations. Different non-exceedance probability (p, %) was determined with focus on extreme precipitation. Annual and monthly precipitation was characterised by a large variation of values. Temporal variability was larger than the spatial variability. The highest average sums were registered in July and the lowest in February. Minimum monthly precipitation (<10 mm) were recorded between January and April, and in October at all four weather stations. Maximum totals, amounting to more than 200 mm, were noted in July. The largest share of absolute minimum and maximum monthly precipitation was recorded at the turn of 20th and 21st centuries. Temporal and spatial distribution of precipitation for very dry, abnormally dry and extremely dry months (p ≤ 20%) were characterised by lower dispersion of values compared with wet months (p ≥ 80%). The precipitation probability distribution at p ≤ 20% may be useful to determine agricultural drought risk assessment in the region. Monthly precipitation at p ≥ 80% may be the background for planning drainage system efficiency and assessment of flooding risk of lignite deposits