Preliminary Study on the Effects of Surface Microtopography on Tracer Transport in a Coupled Overland and Unsaturated Flow System

Abstract

Surface microtopography influences both spatial and temporal distributions of a series of hydrologic processes including infiltration, surface runoff, and unsaturated flow. Importantly, surface roughness and depressions also affect the fate and transport of pollutants throughout surface and subsurface environments. The objective of this study was to experimentally investigate the effects of surface microtopography on overland flow, infiltration, and tracer bromide (Br) runoff, and leaching into the subsurface system under simulated rainfall. Both rough and smooth surfaces were created by using sieved soil and scanned by an instantaneous-profile laser scanner to acquire high-resolution digital elevation models (DEMs) of the soil surfaces. A mold surface, featuring a number of depressions of varied sizes and relationships, was used to create the rough soil surface. The Windows-based puddle delineation (PD) software was applied to characterize surface microtopography, determine flow directions and accumulations, and compute maximum depression storage (MDS) and maximum ponding area (MPA). Two overland flow and tracer transport experiments were conducted using the smooth and rough soil surfaces. Runoff water and soil samples were analyzed for Br. The experimental results indicated that the smooth surface seemed to have earlier and greater water and tracer runoff than the rough surface. Water discharge and tracer loading from the rough surface with varying depressions exhibited a unique, microtopography-controlled threshold behavior. It was found that spatial distributions of the tracer in the subsurface system matched the surface microtopographic features (e.g., depressions, peaks/mounds, and ridges). Enhanced infiltration and tracer leaching under depressions of the rough surface were observed, which resulted in higher Br levels in deeper soil. In contrast, the dominant surface runoff and tracer transport along the horizontal direction for the smooth surface yielded higher Br concentrations in shallow soil. This preliminary study highlighted the important role of surface microtopography in tracer transport in a coupled overland and unsaturated flow system, and demonstrated that surface microtopography influenced the location and timing of ponding, runoff, and infiltration. Increased understanding of the effect of soil microtopography on hydrologic processes is expected to yield improved ability to simulate soil water and solute movement.