Comparison of reactive transport and non-equilibrium modeling approaches for the estimation of nitrate leaching under large water application events

Abstract

In agricultural ecosystems, nitrate (NO3–) leaching is the most widespread loss pathway and non-point source of nitrogen (N) to surface water and groundwater. NO3– leaching in rain-fed and irrigated agricultural systems has been extensively modeled using different numerical approaches of varying complexity. Most numerical modeling studies use simplified zero-order or first-order kinetics when optimizing N budgets. Few studies consider the impacts of temperature, soil moisture and other environmental conditions on biogeochemical reaction rates. In this study, we simulate NO3– leaching and biogeochemical processes in two soil columns under large water application events (groundwater recharge events) using a 1D non-equilibrium (e.g. mobile-immobile) reactive transport HP1 (HYDRUS-1D and PHREEQC) model. We compare this calibrated model to several other calibrated models representing simpler HYDRUS nitrate leaching modeling approaches (uniform flow, non-reactive, zero- and first-order kinetics). Results show that the incorporation of conditional environmental factors such as temperature, soil moisture, and a proxy for oxic/anoxic conditions (percent pore-space filled) results in superior model performance representing the timing and magnitude of important biogeochemical processes when estimating cumulative NO3– leached from the shallow vadose zone. In addition, using a physical non-equilibrium (i.e. dual-porosity type, mobile-immobile solute transport) approach improves model performance when estimating residual NO3– in the soil profile after water application events.