The Impact of Drought on Water Quality (nutrients) in Agricultural Ditches
Summary
Surface waters in the Netherlands are being polluted by excess nitrate (NO3- ) and ammonium (NH4+) compounds which mostly originate from farming (including livestock). Droughts have shown to alter NO3- and NH4+ concentrations in surface water in previous studies and this could depend on the soil type. The Minerals Policy Monitoring Programme (LMM) in the Netherlands was established to monitor the quality of waters impacted by agriculture. The Netherlands has been divided into 11 Minerals Policy Monitoring Programme (LMM) soil type districts. Thus, this thesis aimed to analyse the impact of droughts on NO3- and NH4+ concentrations in ditch water in the 11 LMM soil type districts. Additionally, an increase in ditch water level was observed which could have been a water management strategy to retain more water during future dry summers in the Netherlands. The impact of this water management strategy on NO3- and NH4+ concentrations in ditches in the 11 LMM soil type districts is unclear and was also investigated. Based this, it was determined whether the water management strategy should be used in the future. Statistical tests were performed such as the Spearman’s correlation test and Superposed Epoch Analysis (SEA). A literature review was conducted to identify whether increasing ditch water height would be appropriate management strategy in the future. It cannot be concluded that droughts affect NO3- and NH4+ concentrations in ditches in the LMM districts but a relationship regarding the extreme drought event was observed. The main findings (of the SEA analysis) reveal a significant increase in NO3- concentrations compared to the long term mean in all LMM district in ditches during the winter of 2019 which was one year after the extreme drought event (occurring in 2018 till early/mid 2019). This could have been observed because the NO3- was not taken up by crops and flowed towards ditches in the precipitation event (in 2019) after the extreme drought. The literature review showed changes in NO3- and NH4+ concentrations depending on fluctuations in surface water depth varies. Therefore, it could not be concluded whether ditch water level management would be an appropriate strategy. Further studies were advised. Further research was advised to generate a better understanding the differences between districts and other factors affecting the results such as the contribution of tile drains under agricultural lands to the NO3- and NH4+ concentrations in ditches.