An assessment of the frequency and cause of concentrated flow on agricultural fields in the Virginia Coastal Plain portion of the Chesapeake Bay watershed
Date Thesis Awarded
Honors Thesis -- Access Restricted On-Campus Only
Bachelors of Science (BS)
Agricultural lands are the largest non-point source of nitrogen, phosphorous, and sediment that is delivered to Chesapeake Bay. There has been an increase in the amount of these nutrients and sediment being delivered to the Bay over the last century, which has caused extensive eutrophication and subsequent anoxic zones in the Bay. The Chesapeake Bay Act attempted to mitigate this problem by mandating 100-foot riparian buffer zones between agricultural fields and perennial streams in the Coastal Plain region of Virginia. Previous studies have shown that riparian buffers increase infiltration of runoff into the groundwater system, where nutrients and sediment can be removed from the water before it discharges into perennial streams. However, riparian buffers require flow to be widely disseminated throughout the buffer area in order for them to be effective. No declining trends have been noted in the amounts of nutrients and sediment being delivered to Chesapeake Bay since 1990, when buffers were required to be maintained in the Coastal Plain region of Virginia. I hypothesized that flow concentration, which can promote channel incision and allow agricultural runoff to bypass riparian buffers, is widespread in the Coastal Plain region of Virginia and may be one reason why reductions of nutrients and sediment in Bay waters have not been as great as expected. Geographic Information Systems (GIS) was used to determine flow accumulation patterns across 74 agricultural fields in the Coastal Plain region of Virginia. The percentage of each field drained by the 5 field margin points of greatest flow accumulation area was used as a proxy for measuring flow concentration. The cell size for points along each field margin was 3 m by 3 m. Further, 4 field indexes that related topographic and soil parameters on a field (wetness index, topographic index, water retention index, sediment transport index) were calculated for each field. These indexes were analyzed to determine if any were correlated with and could be used to easily predict places of flow concentration over a large geographic area, such as the Coastal Plain region of Virginia. Flow concentration occurred on all 74 study fields. On average, 70% of a field was drained through just 5 points along its field margin. The strongest field index relationship existed between the Wetness Index and flow concentration (R2 = 0.323). The field indexes were not good predictors of areas of high flow concentration because for any given value of an index, there was a large range of possible flow concentrations. Of the 6 topographic/soil property characteristics analyzed by these 4 field indexes (specific catchment area, runoff source area, slope, saturated hydraulic conductivity, depth to impermeable subsurface layer, and rainfall erosivity), the field slope showed the strongest relationship with flow concentration (R2 = 0.270). Principle component analysis on these 6 topographic/soil property characteristics had a first component that described 47% of the variance among these 6 variables and displayed the strongest relationship with flow concentration (R2 = 0.403). Rather than being attributable to characteristics of the topography or soil that vary from place to place, flow concentration seems to be a phenomenon that is of widespread occurrence in all areas of the Coastal Plain region of Virginia. I recommend requiring farmers to maintain riparian buffers on their land yearly. Future research should focus on determining how to reduce flow concentration through and increase the effectiveness of riparian buffers in the Coastal Plain region of Virginia.
Stone, Monica H., "An assessment of the frequency and cause of concentrated flow on agricultural fields in the Virginia Coastal Plain portion of the Chesapeake Bay watershed" (2014). Undergraduate Honors Theses. William & Mary. Paper 53.
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