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Influence of Preferential Flow on Surface Runoff FluxesT. J. Gish1, W.P. Dulaney1, C.S.T. Daughtry1, and K.-J. S. Kung21 USDA-Agricultural Research Service, Beltsville, Maryland 20705, and the 2 Department of Soil Science, University of Wisconsin-Madison, Madison Wisconsin INTRODUCTIONDetermining the interaction between surface runoff and subsurface hydrology has been hindered by our inability to characterize the subsurface stratigraphy on a watershed scale. To evaluate the subsurface stratigraphy and it's ultimate impact on surface runoff, a 20 ha research site in Beltsville Maryland has been intensively instrumented using innovative protocols. The site contains four adjacent watersheds (about 4 ha each) that drain into a riparian wetland (Fig. 1). Although the watersheds are similar in many regards, the surface fluxes are not. In 1999, the nitrate flux on one of the watersheds ("C") was 18 times larger than the other three watersheds. Preliminary results demonstrate that:
Site Description
Fig. 1. Aerial color infrared photograph of the research site showing watershed designations and boundaries (dotted green lines), soil moisture monitoring stations (blue stars), surface runoff flumes (red diamonds), and forested riparian wetland into which each watershed drains. The riparian wetland contains a first-order stream with five in-channel weir monitoring stations. SURFACE RUNOFFNitrate Runoff Fluxes Each of the four watersheds is equipped with a 1.5' H flumes, flow meter bubbler, and water sampler for evaluating surface runoff. During both 1998 and 1999 no runoff was observed during the growing seasons. However, after plant senescence in 1999 a number of major storms came through the area generating significant runoff over a two month period, "Hurricane Floyd" being one of them. During this time the water flux was three times greater on watershed "C" than on the remaining watersheds, while the nitrate flux was 18 times larger (Fig. 2).
Water Flux Patterns Water runoff fluxes were similar for three of the four watersheds. In 1999, between 700,000 L to 966,000 L of surface runoff had exited watersheds "A", "B" and "D", while 2,678,000 L had exited watershed "C". In addition, watersheds "A", "B", and "D" generally had runoff events that were of short duration, typically ending within a few hours after precipitation ceased (Fig. 3). However, runoff on watershed "C" would continue for several days or even weeks after a significant rain event. Scouting the small watersheds revealed that watershed "C" was the only watershed which contained seepage zones.
Nitrate Runoff Temporal Patterns As the surface runoff water in watershed "C" became increasingly more dominated by seepage flow, the nitrate levels increased dramatically from 3 to 24 mg/L while the nitrate concentrations in watersheds "A", "B" and "D" were consistently in the 1 to 3 mg/L range (Fig. 4).
WATERSHED COMPARISONSTo facilitate direct comparisons between adjacent watersheds, their similarities and differences were determined by evaluating: (1) over 1,700+ soil cores; (2) landscape geophysical properties using GPR, EM-31, EM-38, and multi-depth and multi-frequency EM; (3) surface hydrology using intensive real-time soil moisture monitoring (36,000 volumetric water contents each day), meteorological data, and surface runoff fluxes of water, nitrate and pesticides; (4) and crop physiology and yield parameters using remotely sensed data, leaf area indices, chlorophyl content, and a GPS yield monitor. Surface Soil Slopes Surface topography was evaluated in a geographic information system (GIS) framework, so that the percentage of each watershed as a function of slope could be determined, Table 1. Table 1. Watershed Area and Slope.
Spatial Distribution of Soil Properties In February 1999, six to eight soil cores (8.0 cm2 x 0.3 m) were extracted from each of 274 sampling locations. Laboratory analyses for a number of variables were performed including, organic matter and clay content were conducted on all bulked soil samples. Ordinary block kriging was used to interpolate 10 m x 10 m block estimates (Fig. 5). For many of the soil physical properties the watersheds are similar. For example, watersheds "A" and "D" have nearly identical surface (0-10 cm) organic matter distributions, with a mean of 2.7%, while watershed "B" and "C" have mean organic matter contents of 3.2%. Watersheds "B" , "C" and "D" have mean clay contents between 14 to 15%, while watershed "A" has a mean clay content of 17%. The depth to the first continuous clay lens (as identified with GPR) is also similar, with depths ranging from 0.9 m to 3.5 m (Fig. 7).
Fig. 5. Spatial distribution of surface organic matter (left), clay content corn (right) and variograms for each immediately below. Spatial Distribution of Corn Grain Yields Analysis of the spatial distribution of yield can also be used to compare and evaluate watersheds as plant growth and development integrates soil, climatic, and managerial effects. In 1998 and 1999 corn grain yields were mapped with a differential GPS grain monitor (Fig. 6). Although drought conditions were more severe in 1999 than in 1998, yield maps from both years showed a marked degree of similarity in the spatial pattern of the higher producing regions. Yield data for both years was normalized and combined to generate a map representing the "average" spatial distribution of corn grain yield for drought years (Fig. 6). This latter map was used to evaluate yield spatial structure and to identify various soil or landscape parameters that could influence the spatial variability of corn grain yields.
Fig. 6. Spatial distribution of corn grain yield in 1998 (left), 1999 (center), and mean normalized yield (right) . Mean normalized yield values > 1, indicate corn grain yields greater than the modal value. Notice the spatial location of the highest yielding regions (darker areas) were very similar for the two water limiting years. SUBSURFACE PATHWAYSGround-Penetrating Radar Paramount to identifying subsurface flow pathways is the characterization of the subsurface layering structures. Over 40 km of GPR data provided a continuous image profile of the subsurface soil structures and was used to map subsurface restricting layers, usually clay lenses, that control the magnitude and direction of groundwater movement (Fig. 7).
Fig. 7. Depth to the first continuous restricting layers as identified with GPR. Darker colored regions indicate a greater depth to the subsurface restricting layer. Remote Sensing and Subsurface Flow Pathways
Fig. 8. Color infrared red image showing vigorous vegetation (dark red regions), GPR identified subsurface flow pathways (blue lines), and the consistently higher corn producing regions (dotted black polygons). Watersheds "A" and "B" (left) and "C" (right). The GPR data gave the orthogonal depth from the soil surface to the first continuous restricting layer. It was necessary to take into account the surface topography above the subsurface restricting layers before subsurface convergent flow pathways could be determined. Subtracting the depth to the first continuous restricting layer from photogrammetrically derived surface elevations provided a topographically corrected subsurface restricting layer. Arc/Info was used to run simple hydrologic models on the corrected subsurface restricting layers in order to determine potential subsurface convergent flow pathways (blue lines in Fig. 8). As can be seen from the color infrared imagery, the corn plants appear to be more vigorous around the GPR identified flow pathways. In a GIS framework, between 30 to 40% of watersheds "A" and "B" are drained by these GPR identified flow pathways, while in watershed "C" as much as 72% is drained by "funnel flow". Apparently, as more subsurface water converges in these discrete subsurface flow pathways, the soil profile down slope can become saturated, creating seepage zones. Soil Moisture Profile Near the Discrete Subsurface Flow Pathways
Fig. 9. Volumetric water contents for 1 of 48 soil moisture probes. This particular probe shows a preferential funnel flow plume along a clay lens located at 1.54 m. The probe is located near one of the GPR identified subsurface flow pathways that was identified in the GIS framework. CONCLUSIONSSurface and subsurface flow interactions were studied on four small adjacent watersheds. Although the watersheds are fairly similar, the nitrate surface runoff flux in 1999 was 18 times greater on watershed "C" than on the remaining three watersheds. A protocol was developed using primarily GPR to identify discrete subsurface flow patterns arising from funnel flow. Remote sensing imagery and soil moisture profiles were used to confirm the extent and location of the subsurface flow pathways. In a GIS framework, it was calculated that between 30 to 42% of watershed area in watersheds "A", "B", and "D" would eventually drain into discrete subsurface channels that exited the watersheds near the surface runoff flumes, compared to 72% for watershed "C". As more water converges in these discrete subsurface flow pathways the soil profile down slope can eventually become saturated, creating seepage zones. Once seepage zones occur, the reemerged subsurface waters can flow along the surface topography and exited the runoff flume. This study demonstrates the dramatic impact subsurface stratigraphy can have on surface chemical runoff fluxes, even when soil properties, yield distributions, and climate are similar. CONTACTTimothy J. Gish
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U. S. Department of Agriculture | Agricultural Research Service
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