The effect of rainfall variability on Nitrogen dynamics in a small agricultural catchment

Abstract. Throughout history, extreme storms and droughts have had serious impacts on society and ecosystems globally. Rainfall variability in particular has been identified as a primary performance of climate change. However, so far little has been done to explore the effect of rainfall variability on water quality. This study is aimed at investigating the effect of rainfall variability on nitrogen (N) dynamics and its potentially negative influence on water quality. The transport of water and nitrate was simulated for a small agricultural catchment in Central Germany using the fully coupled surface-subsurface model HydroGeoSphere. Rainfall time series with specific climatic characteristics were generated using a stochastic rainfall generator. N transformation and transport were compared for four scenarios (with high, normal, low annual precipitation amounts, and low annual precipitation amounts coupled with reduced plant uptake, respectively) in order to identify the impact of inter-annual rainfall variability on N dynamics. The results suggest that higher annual precipitation amounts can enhance the transformation and transport of nitrogen. Lower annual precipitation amounts are conducive to nitrogen retention. Nonetheless, when vegetation suffers from drought stress, the retention capacity will decline markedly, suggesting that vegetation plays a vital role in N dynamics under extreme droughts. The linear regressions between selected parameters of the rainfall generator and N loads / fluxes were analyzed to elucidate the impact of intra-annual rainfall variability on N dynamics. The results indicate that wet / dry conditions and different dry-wet patterns caused by the distribution of storm durations and inter-storm periods over the course of a year can significantly affect N loads and in-stream nitrate concentration, respectively. In the warm season, droughts prompt the accumulation of SON, but drying-wetting cycles can enhance the extensive transformation of SON. In-stream nitrate concentration dramatically elevates during the rewetting period after the drought. High mean rainfall intensity contributes not only to the transformation of N when mineralization is not limited by low temperatures, but also to the plant absorption of inorganic nitrogen in the growing season. There is merely small effect of mean rainfall intensity on stream water quality. Overall, the study clarifies the effect of rainfall variability on N dynamics in a small agricultural catchment, which provides theoretical support to formulate fertilization strategies and protect aquatic ecosystems under climate change in the future.