W. G. Knisel

An approximate method for partitioning daily streamflow data

An approximate method was developed for partitioning daily total streamflow volumes into storm runoff and subsurface flow components. The procedure was tested with streamflow data from ten research watersheds, 2.6–1494.43 km2 in size, in the Coastal Plain of Georgia, U.S.A. Estimated volumes obtained on these watersheds were representative of observed stormflow and subsurface flow volumes from the small upland drainage area, 0.34 ha in size. Measured subsurface flow from this upland area was ∼ 80% of the total flow for the 10-yr. period. Results indicate that the approximate method of streamflow partitioning gives reasonable estimates and may be useful where available data are limited to daily values.

SIMULATION OF NITROGEN AND PHOSPHORUS LEACHING IN A STRUCTURED SOIL USING GLEAMS AND A NEW SUBMODEL, “PARTLE”

The potential negative impact of agricultural chemicals on the quality of surface and ground water resources is a worldwide concern. The complexity of factors affecting nonpoint source pollution makes experimental assessment of the environmental consequences of different management strategies laborious and expensive. Therefore, one feasible method is to use the existing research database to validate and modify the computer models, then use the models to simulate the long-term impact of these systems. In this study the hydrologic and nutrient loss data from a subsurface drained, structured clay soil in southwest Sweden was used to examine the applicability of GLEAMS to simulate the drainage discharge and N and P concentrations in the discharge water. The results indicated that GLEAMS was capable of simulating drainage discharge, nitrate-N and dissolved-P losses reasonably well, but there were no algorithms to simulate the particulate-P losses via drain tiles. Therefore, a submodel, “PARTLE” was developed and tested. Our results indicate that the PARTLE submodel, in association with the GLEAMS model, provided reasonable estimates of particulate-P loss via drainage in this soil. The study concluded that considering the impact of preferential flow and the ratio of annual drainage discharge to annual precipitation is necessary for proper predictions of particulate-P in structured soils.

Seasonal Distribution of Rainfall Erosivity in Peninsular Florida

Adequate information on the seasonal distribution of rainfall erosivity required to predict rainfall erosion losses was not available previously for peninsular Florida. In this area, the oceanic and continental climatic influence causes significant seasonal and year-to-year differences in precipitation amount and distribution. Thunderstorms may occur at any time of the year, resulting in high rainfall energy and potentially producing considerable soil detachment by raindrop impact in the absence of adequate plant cover. Daily precipitation data from selected locations in Florida were analyzed to determine seasonal variation in the rainfall erosion index (EI). Long-term seasonal rainfall EI distributions were used to delineate EI-distribution regions, thereby extending applicability of the Universal Soil Loss Equation for erosion modeling in peninsular Florida. Design values for annual total and maximum 1-day EI, and a revised map of average annual rainfall erosivity are presented, also.

Simulating processes in nonpoint source pollution

The paper describes development of a mathematical model to evaluate nonpoint pollution from diffuse agricultural and forestry sources. Although the model includes numerous physical and chemical processes, a generalized flow chart is used to present the entire system along with more detailed components. The nitrogen cycling and pesticide elements of the chemistry components are presented. The interactions of complex processes are described relative to climate, soil, and agricultural management practices.

Coastal Plain Landscape Feature Evolved From Historic Erosion and Deposition

Historical erosion from agricultural fields with subsequent deposition in riparian systems has resulted in a pronounced landscape feature in the upper Coastal Plain physiographic area. Erosion of soil particles and aggregates on cropland with downslope transport by runoff water has resulted in deposition of larger and heavier material that entered riparian areas. This forms terrace-like deposits at the lower edges of fields (Figure a). Continued deposition year-after-year further results in flow restrictions that have to be removed to prevent encroachment into the field. Roadways and/or turn rows are common on these deposits, which require periodic grading or removal.