J. M. Sheridan

MANAGEMENT EFFECTS ON RUNOFF AND SEDIMENT TRANSPORT IN RIPARIAN FOREST BUFFERS

The effectiveness of mature riparian forests in reducing the impact of agriculture on the quality of the nation’s water resources has been documented, but the impact of forest management practices implemented within riparian forest buffers on their water quality function has not been evaluated. This article examines the effect of forest management within a Coastal Plain riparian forest buffer system (RFBS) on runoff and sediment transport over a four year period. The RFBS, which conformed to USDA-FS and USDA-NRCS best management recommendations, included a narrow strip of undisturbed forest located adjacent to the stream drainage system (Zone 1), a wide managed pine forest downslope from the grass filter (Zone 2), and a narrow grass filter strip immediately downslope from an agricultural field (Zone 3). Forest management treatments evaluated within Zone 2 were mature forest, clear-cut, and selectively-thinned. Significant reductions in runoff and sediment transport were measured under all three forest management treatments. The primary zone of runoff and sediment reduction was within the grass filter portion of the RFBS. These results indicate that riparian forests within a RFBS may be managed for economic return to the land owner without adversely affecting the runoff and sediment reduction function performed by these buffer systems.

RAINFALL CHARACTERISTICS AND SPATIAL CORRELATION FOR THE GEORGIA COASTAL PLAIN

The Georgia Coastal Plain of the United States holds significant agricultural and hydrologic importance. The profitability and sustainability of agriculture in this and other regions are dependent upon climatic patterns and, in particular, rainfall. The temporal and spatial variability of this rainfall play key roles in agricultural management. Developing an understanding of seasonal patterns and individual storm characteristics is critical. Thirty years of rainfall data collected from a dense rain gage network on the 334 km2 Little River Watershed near Tifton, Georgia, were analyzed for this purpose. Storm patterns were characterized by season to establish means and trends. Individual storm characteristics and spatial correlation patterns within storms were quantified. Rainfall patterns, although highly variable from year to year, show rainfall to be greatest in the midsummer months with high intensity, convective thunderstorms. While these summer storms yield relatively low rainfall depths, they occur more frequently than during other seasons. Frontal storms with moderate rainfall amounts are typical of the winter and spring months. The fall months generally have low rainfall totals and storms during this time occur less frequently than during the other months. For larger storm events, defined here as those where at least one rain gage in the network measured 25.4 mm or greater during the event, the mean storm depth weighted over the mean storm coverage of 295 km2 was 20.6 mm, while the mean storm duration was 7.2 h. For these larger events, the summer storms are separated by the least time between events (mean of 116 h) while the fall events are separated by the greatest time (mean of 291 h). For summer events, rain gage depths for individual storm events collected by gages separated by 1.9 km or less are likely to be highly correlated (r iÝ 0.9). This distance increases to 9.2 km for the winter. High correlations (r iÝ 0.9) are expected up to distances of 5 km throughout most of the year, except summer. This analysis establishes the basis for detailed modeling across this and other watersheds in the region.

WATERSHED--SCALE SIMULATION OF SEDIMENT AND NUTRIENT LOADS IN GEORGIA COASTAL PLAIN STREAMS USING THE ANNUALIZED AGNPS MODEL

Sediment and nutrient loadings in the Little River Research Watershed in south central Georgia were modeled using the continuous simulation Annualized Agricultural Nonpoint–Source Pollution (AnnAGNPS) model, part of the AGNPS suite of modeling components. Specifically, nitrogen, phosphorus, sediment, and runoff were predicted over a seven–year period. Land under cultivation makes up approximately 25% of the 333 km2 watershed. Livestock facilities include swine, poultry, dairy cows, and beef cattle. Results from the simulation were compared to seven years of monitoring data at the outlet of five nested subwatersheds and at the outlet of the Little River Research Watershed (LRRW). The average annual predicted runoff in the upper part of the watershed was one–third to half of observed runoff. In contrast, predicted runoff in the lower part of the watershed was close to observed, and was 100% of observed at the outlet of the watershed. Runoff underprediction was attributed to the method of landcover discretization. The extent of forest land in the upper watershed (55% to 63%) and the fragmented landscape that has relatively small fields surrounded by riparian forests and tracts of forest resulted in overestimation of forested area in the watershed. In addition to runoff, sediment and nutrient loads were also underpredicted in the upper part of the LRRW. Two factors are most likely responsible for underprediction. Runoff is underpredicted at these sites, which reduces the carrying capacity of sediment loads. In addition, the overestimation of forested areas at these sites coincides with underestimation of sediment–producing areas, such as cropland. In contrast to the upper part of the watershed, sediment and nutrient loads were overpredicted in the lower part of the watershed. This may have resulted from inadequately simulating nonpoint–source pollution attenuation by the extensive riparian forests and forested in–stream wetland areas found in these watersheds. Prediction results can be improved through better input into the model, as well as modification of the processes within the model to account for forest and riparian conditions.

Estimating the effectiveness of vegetated floodplains/ wetlands as nitrate-nitrite and orthophosphorus filters

Abstract The role of vegetated floodplains/wetlands on the stream water quality of agro-ecosystems in the Coastal Plain region of the southeastern United States is reported. Water-borne nitrate plus nitrite nitrogen budgets and orthophosphate phosphorus budgets from a cropped agricultural area were compared with those of a watershed with alluvial forests below the cropped areas. Analyses were made to determine if observed differences in nutrient concentrations and loads were the result of dilution of cropped area runoff by flows from non-cropped areas. Reductions in the observed levels of nitrate plus nitrite nitrogen and orthophosphate phosphorus between upland cropped areas and watershed outlets exceed reductions that would be caused by dilution effects. Significant portions of the observed nutrients leaving cropped areas were retained, utilized and/or transformed in the vegetated floodplains/wetlands characteristic of these Coastal Plain watersheds.

Hydraulic Gradients and Flow Rates of a Shallow Coastal Plain Aquifer in a Forested Riparian Buffer

Water table gradients were measured and saturated flow rates estimated for a hillslope consisting of a tilled upland field and a downslope riparian forest buffer system located in the Gulf-Atlantic Coastal Plain Tifton-Vidalia Uplands. Three years of water table measurements and estimates of saturated hydraulic conductivity were used to evaluate and quantify saturated water flow gradients, directions, and rates. Forest treatments consisting of clear cutting, thinning, and no cutting were examined. The gradient of the water table from the top of the landscape to the bottom varied from 0.9 to 0.2%, less than the 1.5% land slope. The direction of groundwater flow generally followed the land slope. However, during summer months the hydraulic gradient within the forested buffer reversed direction. Water table data indicate the riparian area was saturated from January through March. During this time, flow direction in the shallow aquifer is from the top of the field to the stream bottom. During summer months, high rates of forest evapotranspiration created large water sinks in the shallow subsurface and large local hydraulic gradients. Examination of water table elevations indicates the seasonal water demand of the forest shifts the direction of shallow subsurface aquifer flow. During these periods flow direction within the riparian buffer was from the lowest landscape position to the riparian forest, reversed from winter months. Total subsurface flow within the hillslope was calculated as 35 mm yr–1, 3% of average annual precipitation. Average groundwater linear velocity was calculated as 1.4 mm h–1. Evapotranspiration loss was estimated as 67% of average annual precipitation.

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.

RIPARIAN ECOSYSTEM MANAGEMENT MODEL (REMM): I. TESTING OF THE HYDROLOGIC COMPONENT FOR A COASTAL PLAIN RIPARIAN SYSTEM

The Riparian Ecosystem Management Model (REMM) was used to simulate shallow groundwater movement, water table depths, surface runoff, and annual hydrologic budgets for a Coastal Plain riparian buffer system near Tifton, Georgia, USA. The riparian buffer consisted of zone 3 (grass downslope from a row-crop field); zone 2 (mature pine forest downslope from zone 3); and zone 1 (mature hardwood forest downslope from zone 2, adjacent to stream). Measured surface runoff and shallow groundwater movement from the adjacent agricultural field were used as the hydrologic input to REMM. Uncalibrated simulation results for a five-year period were compared to measured values for the same time period. The overall error in zone 2 and zone 1 mean water table depths was about 0.07 m, although absolute errors were higher. The water table dynamics simulated by REMM were similar to observed although lags were observed in the response of the simulated water table to large rainfall events. Mixed results were obtained from observed versus simulated surface runoff comparisons, primarily due to large variability in observed runoff depths along the riparian transect. Simulated surface runoff depths for zone 3 were within one standard deviation for four out of the five years. For zone 2, surface runoff depths could only be simulated within one standard deviation for two out of the five years. Simulated seasonal total depths of surface runoff did not always agree with observed values but usually followed both similar temporal and spatial patterns. Annual hydrologic budgets produced total streamflow comparable to those estimated for the riparian buffer site. These results provide an adequate basis for subsequent testing of other REMM model components including water quality and nutrient cycling.

Hydrograph Time Parameters for Flatland Watersheds

Available procedures for estimating storm hydrograph time parameters performed poorly in applications on low-gradient drainage systems of the Coastal Plain and Flatwoods regions of the southeastern United States. Existing empirical relationships generally underpredicted observed hydrograph time parameters on nine Coastal Plain and Flatwoods watersheds, with the standard error of estimate ranging from 63 to 132% of observed means. Hydrograph time parameters from flatland study areas were related to watershed physical characteristic and geomorphic data. The simple parameter, length of main channel, proved superior to all other simple or complex watershed characteristics for explaining observed variations in watershed time-of-concentration and hydrograph time-to-peak. Simple empirical relationships developed for estimating hydrograph time parameters for flatland areas provide needed information for watershed scale hydrologic design and environmental resource modeling applications on low-gradient drainage basins similar to those of the coastal regions of the southeastern United States.

Nutrient Movement in Streamflow from Agricultural Watersheds in the Georgia Coastal Plain

ABSTRACT CHEMICAL load of nitrate-plus nitrite-nitrogen and orthophosphate-phosphorus in runoff from a complex cover agricultural watershed in the Georgia Coastal Plain was found to be less than the rainfall input of these chemicals. Chloride load in runoff exceeded chloride load in rainfall. Measured stream flow nutrient losses showed a range in nitrate-plus nitrite-nitrogen load from 0.14 to 0.24 kg/ha/yr and a range in the orthophosphate-phosphorus load from 0.141 to 0.144 kg/ha/yr.

Surface Flow Sampler for Riparian Studies

A low-impact surface flow sampler was developed for riparian studies conducted in the Coastal Plain region of the southeastern United States. The device consists of two primary components, a splitter and a collector, which were used for unattended sampling of surface flow in riparian buffer study areas. This low-cost device provides a composite event sample at selected locations within experimental areas. The quantity of sample is adequate for laboratory analyses of dissolved and suspended constituents for both large and small flow events, and permits estimation of the volume of surface flow at the sampling location. Installation and operation of the device requires little disturbance to the riparian buffer ground surface and vegetation, or to surface flow within experimental areas.