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Using GIS in Simulation of Nitrate Leaching from Heterogeneous Unsaturated Soils

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521;

Andrew D. Ward, professor

Dep. of Agricultural Engineering, The Ohio State Univ., Columbus, OH 43210;

Stephen R. Workman, assistant professor

Dep. of Biosystems and Agricultural Engineering, Univ. of Kentucky, Lexington, KY 40546-0276.

^* Corresponding author (joanwu{at}mail.ucr.edu).

ABSTRACT

Solute transport within and through unsaturated soils are important processes that can affect agricultural systems and groundwater supply and contamination. Nitrate (NO^–₃) leaching from three 10-ha experimental plots under different agricultural practices was studied. A Geographic Information System (GIS) model, ARC/INFO was used to help decompose the three hydrologic environments into 34 subenvironments. Each subenvironment was discretized vertically as three modeling units. Within each modeling unit, soil physical and chemical properties were described by a random multivariate normal (MVN) vector with conditional probability. A one-dimensional, field-scale, continuous flow model, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS), was then stochastically applied to each subenvironment by using generated MVN realizations for every modeling unit. Observed NO^–₃ leaching below 1-m depth was estimated with a mass balance approach based on suction lysimeter data. The observed NO^–₃ loads were then compared to predictions from the stochastic simulation. Statistical analysis of the effects of agricultural practice and soil type on NO^–₃ leaching was conducted based on modeling results. Observed and predicted results showed (i) no significant difference between mean values of observed and predicted mean NO^–₃ leaching below the 1-m depth; (ii) agricultural practice and soil type as well as their interaction had an effect on NO^–₃ leaching; and (iii) a large amount of spatial variability with respect to NO^–₃ leaching. The results are important for better understanding solute leaching from heterogeneous agricultural fields and useful in decision making.

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