Use of a finite difference hydrologic model to assess the potential effectiveness of changes in cropping practices to reduce nitrate loading to a shallow aquifer.

Abstract:

Cropping practices, soil & vadose characteristics and water table topography determine how water and nitrogen move from cultivated soils to shallow aquifers. Understanding how these factors interact across space and over time is thus critical to understanding potential effectiveness and timing of crop management changes to reduce nitrogen loading to groundwater. We focus on a strath terrace mantled with 20-100 cm of loess-derived clay loam overlying 5-10 meters of generally unconsolidated gravel in central Montana. Here, a shallow aquifer in the gravel is perched on a relatively impermeable underlying shale. The landform is isolated from mountain front stream recharge, such that all groundwater recharge is derived from infiltration of precipitation through soils on the terrace. Ninety-three percent of the terrace is cultivated with a typical cropping system on a three-year rotation of winter wheat, spring wheat or barley, and fallow (no crops grown to conserve soil water). Each cropping phase represents a different regime of evapotranspiration, recharge, fertilizer application, nitrogen mineralization, and nitrate leaching to groundwater. Discharge from the aquifer feeds springs at the gravel/shale contact exposed at the edges of the terrace. We monitored five to ten ground and surface water locations for three years to calibrate a finite difference model and quantify the flux of water and solutes from the terrace aquifer. A sensitivity analysis of the model allows for hypothesis testing about the relative importance of cropping practice versus soil & vadose characteristics as drivers of nitrate leaching. We assess model performance by comparing model simulations with seasonal variation of solute concentrations, groundwater residence times derived from 3H-3He, and nitrogen gas losses implied by increasing nitrate d18O and d15N along flowpaths. We are developing the model within a broader participatory research effort with local farmers to develop shared understanding of the effectiveness of alternative cropping practices and timing of those effects on local groundwater nitrate concentrations.