Nitrate hysteresis in a small catchment of the Ukrainian forest-steppe zone

The paper aimed to determine the relationship between water discharge and the concentration of nitrate nitrogen compounds in a river during spring and rainfall floods. The study was carried out in a small experimental catchment of the Boguslavka River in the forest-steppe zone of Ukraine, most of which is used for agricultural production. Analysing the hysteresis of nitrate compounds (hysteresis is the phenomenon of lagging system response to the action of a factor associated with the «memory» of its previous action) allowed us to obtain new information about the patterns of nitrate nitrogen compounds inflow during runoff-forming phenomena. During the spring flood, the highest concentrations of nitrate compounds in water were observed before the snowmelt began. A sharp increase in water discharge led to a decrease in the N–NO3-content,which began to increase on the downward limb of the runoff hydrograph. During the spring floods of 2017-2018, a counterclockwise hysteresis was formed, indicating that the main supply of nitrate compounds came from remote sources. In 2017, the frozen topsoil did not contribute to the flow, whereas in 2018, prolonged flooding led to the washing of the coastal zone soils. During the summer floods, N–NO3- concentrations in the river water during the decline phase of the hydrograph were caused by significant nitrogen emissions and exceeded the concentrations before the event. The hysteresis loop was directed clockwise, indicating a higher concentration of nitrate compounds in the coastal zone. During the rain with wet preliminary conditions, the hysteresis shape was complex and resembled an 8-like shape. In this case, the increase in concentrations in the stream began at the limb of the hydrograph rise. When snowmelt was combined with rainfall or prolonged rains in the summer, a two-peak hydrograph was formed, and the hysteresis index of the second one was much lower than the first. The area of the hysteresis loop was significantly affected by the conditions of preliminary moistening.