Impact of rainfall characteristics on the NO3 – N concentration in a tailwater recovery ditch

Abstract

Rainfall characteristics can be a major factor influencing the ability of best management practices to reduce nutrient loss from agricultural lands to receiving waterbodies. The goal of this study was to examine how rainfall characteristics impacted NO3 – N concentrations in a tailwater recovery (TWR) ditch implemented at a farm within the Porter Bayou watershed in Mississippi, USA. We used a methodology that correlates rainfall characteristics (i.e., a combination of variables such as depth, intensity, frequency, duration, and antecedent hydrological conditions before water sampling) and NO3 – N levels measured in the ditch. Subsequently, a hierarchical clustering approach was implemented to classify rainfall events in the context of the NO3 – N concentrations. Results indicate that NO3 – N concentrations observed in the ditch were strongly dependent on antecedent hydrological conditions within the study area, specifically on the (1) duration of rainfall events before sampling and (2) characteristics of next-to-last rainfall events that occurred prior to sampling. Combined variables of total rainfall depth and frequency showed that rainfall classes I and III were likely to have the most impact on in-ditch NO3 – N concentration. Effects of class I rainfall events on NO3 – N levels appear to be magnified under higher depth, intensity, duration, and a shorter time before next-to-last rainfall events. Also, the influence of class III rainfall events on NO3 – N concentrations in the ditch was driven mainly by high-frequency, low magnitude, and dry antecedent conditions. Results suggest that during periods with frequent, consecutive rainfall events, the nutrient reduction potential of TWR ditches may be reduced. Data can be used to improve management of TWR ditches with more frequent pumping when possible, to increase retention times, particularly during the spring and late fall periods.