Integrated flow and transport processes in subsurface -drained agricultural fields

Nathan W Haws, Purdue University

Abstract

Describing water and solute movement in subsurface-drained agricultural fields is challenging due, in part, to an array of macropores that network the soil matrix. Using the hypothesis that subsurface-drained fields integrate the effects of spatial heterogeneity, effective parameters was calibrated for a flow and transport model (HYDRUS-2D) using drain outflow data. Though some success in simulating water flux was achieved using a dual-porosity representation, the effective parameter simulations failed to predict the rapid solute flux response at the field-scale. In order to better understand this rapid flux response, the internal transport processes of subsurface-drained fields were then studied in batch, column, and field systems. Batch-scale diffusion studies indicate that inter-ped fissures act as preferential conduits for solute diffusion into the soil matrix. Column-scale experiments further confirm the importance of inter-ped diffusion and give evidence of two preferential flow networks whose relative dominance of the transport response depends on the degree of water saturation. A multi-solute field-scale experiment, conducted on two replicate field plots, dramatically illustrates the spatial extent of these preferential flow networks with the simultaneous arrival of reactive and nonreactive solutes applied 5 meters from the drain. However, the inter-ped diffusion processes trap most of the solute in immobile regions near the soil surface. Finally, numerical experiments indicate that spatial variability of the macropore network at the field-scale leads to a greater degree of nonequilibrium than might be predicted assuming a homogeneous medium and using soil properties measured at the soil surface.

Degree

Ph.D.

Advisors

Rao, Purdue University.

Subject Area

Hydrology|Environmental engineering|Environmental science

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