Effects of perennial and annual vegetation on a soil water balance and groundwater recharge.

Abstract

The movement of land applied fertilizers, pesticides, and other agricultural chemicals from land surface to groundwater is a major environmental concern, especially in regions of coarse textured soils with shallow unconfined aquifers. A replicated field experiment was conducted on the Anoka Sand Plain, Minnesota, to examine the effects of perennial and annual vegetative cover on the movement of water through the unsaturated zone to groundwater. A Darcian analysis of soil water flow, water table hydrograph analysis, and chemical analysis of a bromide tracer in pore water in the unsaturated and saturated zones were utilized to estimate recharge rates and amounts to a shallow unconfined aquifer beneath four land cover types: corn (Zea mays), well-established prairie, newly-established hay, and bare ground. Soil water storage and precipitation were measured directly. Evapotranspiration estimates were determined by difference in the other water balance terms. Prairie soils to 125 cm were maximally drier than corn by mid-July each season due to greater early season ET demands by prairie than corn, with the maximum difference in soil water storage being 6.3 cm. Hay, prairie, corn, and bare ground recharge estimates from 6/3/2008 through 12/31/2009 were 31.6 +/- 4.5 cm, 37.9 +/- 3.3 cm, 40.2 +/- 3.4 cm, and 43.7 +/- 6.8 cm representing 28 %, 33%, 35%, and 39% of precipitation, respectively. Piston flow model estimates of residence time in the upper 225 cm of the soil profile were 312, 410, 352, and 318 days for hay, prairie, corn, and bare ground respectively. Bromide mass loss as determined for soil pore water 160 cm below land surface in one continuously monitored plot of each treatment resulted in 0.7%, 34%, 34%, and 100% of applied bromide leaching in prairie, hay, corn, and bare ground plots respectively. Peak bromide concentrations in prairie soil water were marginally significantly lower than all other treatments. Bromide was detected in the groundwater of all five replicate plots for hay, bare ground, and corn treatments, but only detected in two of five prairie replicate plots. Results indicate that on coarse soils, well established annually harvested perennial prairies have the potential for reducing inputs of land applied chemicals to groundwater relative to corn through slight reductions in recharge and reductions in solute transport.