Structural Equation Modeling of Dynamics of Nitrate Contamination in Ground Water

Zhi-Jun Liu
George Hallberg
George Malanson


Structural equation techniques are used to examine the influence of climate, hydrology, and nitrogen management on the dynamics of nitrate in groundwater in the Big Spring Basin, IA. In a structural equation model, the model structure is posited first on the basis of relations by theory, and the model is then tested with the data to determine whether there exists empirical support for the specified structure. A structural equation model consists of a system of equations, each representing possible causal links between an endogenous variable and other endogenous variables as well as exogenous variables. The two most important factors determining the NO sub(3) dynamics are found to be N management practices in agricultural production and climate: the long-term trend of NO sub(3) concentration is related to the N input, but the potential effects of N management are contingent on the variations of climate. The results of structural equation modeling indicate that the variations of climate, especially precipitation, and hydrology have much greater impacts on the NO sub(3) dynamics in the basin than the changes in N input.