James D. Gregory

EFFECTS OF CONTROLLED DRAINAGE ON THE HYDROLOGY OF DRAINED PINE PLANTATIONS IN THE NORTH CAROLINA COASTAL PLAIN

Abstract This paper presents results of a 5 year study to characterize the hydrology (rainfall, interception, evapotranspiration (ET), soil water storage, drainage rate, lateral seepage, and water table fluctuations) of three identical drained, pine-forested watersheds in Carteret County, North Carolina. During the 2 year calibration period (1988–1989), all three watersheds were operated in conventional drainage mode with the weirs in the outlet ditch approximately 1.0 m below the soil surface. About 17% of the total rainfall was intercepted and subsequently evaporated and 53% was removed by transpiration and evaporation from the soil during this period. Drainage removed about 28% and the remaining 3% was lost by lateral seepage. During the 3 year controlled drainage treatment period (1990–1993), drainage in Watershed 2, managed for tree growth, was reduced to 21% of gross rainfall as compared with 30.5% for Watershed 1 under free drainage. Watershed 3, managed to minimize offsite impacts, yielded 26% of gross rainfall as drainage. Interception loss accounted for about 14.5% of the gross rainfall. ET amounts computed as the residual in a water balance, were 50%, 60%, and 55% of total rainfall for Watersheds 1, 2, and 3, respectively. The effects of controlled drainage on water table depths, drainage and ET were demonstrated for seasonal and year-to-year variation in rainfall. The controlled drainage treatments affected both drainage volumes and daily peak outflow rates. The treatment in Watershed 3 was more effective in reducing peak outflow rates.

Evaluation of a watershed scale forest hydrologic model

Abstract A watershed scale hydrologic model (DRAINWAT) for drained forested lands was developed by coupling DRAINLOB, a field scale forestry version of DRAINMOD and the ditch and channel routing model section of FLD and STRM. The simulation model was tested with 5 years (1988–1992) of data collected on a 340 ha watershed located near Beaufort in eastern North Carolina. Testing of the model included comparison of observed and simulated daily, monthly, and annual outflows and hourly event hydrographs by three different evapotranspiration (ET) methods. Two of which (Teskey form and GS HR form) are based on the Penman-Monteith method and the third one on the Thornthwaite method. The average absolute deviation in observed and predicted daily outflows for a 5 year period was 0.94 mm day −1 , when the Penman-Monteith methods were used to predict ET. The average absolute deviation in cumulative outflow when ET was predicted by the Thornthwaite method was, respectively, 23% and 50% higher compared with the values obtained with both forms of the Penman-Monteith method. Based on coefficient of determination (R 2 ), coefficient of efficiency ( E ), and root mean square error (RMSE), Teskey and GS HR forms of the Penman-Monteith method performed better than the Thornthwaite method in predicting both daily and monthly outflows. However, the average daily deviations by all three methods were not significantly different at 5% level. Prediction errors in simulating monthly outflows were reduced compared with daily outflows. The predicted mean annual outflow volumes when the GS_HR and Thornthwaite methods were used for ET were in closest agreement with observed data. Statistics showed that errors resulting from use of the Thornthwaite method, with correction factors, were usually within acceptable limits given the large input data required by the Penman-Monteith ET methods. Model prediction of event hydrographs was satisfactory based on different statistical and graphical comparisons. Deviations in predicted and observed results are attributed to errors in both. Errors in the measured outflows occurred for some larger events due to weir submergence. Errors in the simulations resulted from errors in rainfall inputs, and from uncertainties in drainable porosity, hydraulic conductivity and estimates of ET due to a number of factors including approximations of leaf area index (LAI) and stomatal conductance parameters. The model performance as a whole was satisfactory given the complexity of the model, limitations of input data for the watershed, measurement errors in outflow and rainfall, and the fact that the model was not calibrated.

Seasonal sulfate deposition and export patterns for a small Appalachian watershed

Sulfate deposition and exports from 1988–92 were analyzed for a small headwater catchment in north-central West Virginia. Annual sulfate inputs, estimated by applying throughfall-adjusted ratios to bulk deposition values, and outputs were approximately equal for the five years. Annual mean throughfall-adjusted deposition and export loads were 55.78 and 55.48 kg ha-1, respectively. While these results indicate the watershed has reached sulfate equilibrium relative to current deposition levels, seasonal sulfate accumulations and deficits were evident. Deposition and exports averaged 5.61 and 2.49 kg ha-1 mo-1, respectively, during the growing season, and 3.69 and 5.22 kg ha-1 mo-1 during the dormant season. Sulfur accumulated within the soil during the growing season because inputs of wet and dry sulfur deposition were high while outputs were negligible. The latter was due largely to the lack of runoff resulting from high evapotranspirational demands. By contrast, net sulfate losses occurred during dormant seasons, primarily due to high runoff, even though inputs declined during this season. Researchers working on other watersheds have interpreted similar input/output patterns to mean that sulfate accumulated during the growing season is the source of sulfate exported during the dormant season. However, radioisotopic evidence from a companion study on this watershed showed that some labeled sulfate applied to the watershed more than a year earlier was still present in the organic and mineral soil layers at the point of application (with some as soluble sulfate), and in soil water dispersed throughout the watershed. Its presence indicates that dormant season exports can originate from sulfate deposited over longer periods than just the previous growing season or even previous year. Volume-weighted concentrations in soil leachate suggest that dormant-season sulfate losses resulted from progressive depletion of the anion through the soil profile. During the fall and early winter, soluble sulfate was depleted in the upper soil horizons; in later winter, depletion occurred in the lower horizons.

Simulating the water budgets of natural Carolina bay wetlands

Wetland restoration projects attempt to recreate the hydrology found in natural wetlands, but little is known of the water budgets associated with wetlands in their natural state. The objective of this study was to compute the water budgets of three natural Carolina bay wetlands in Bladen County, North Carolina, USA. DRAINMOD models of various locations in the bays were calibrated with measured water table depths over a 2-yr period using inputs of rainfall, air temperature, and soil physical properties. The models were successful in simulating water table depths at all well locations during the calibration period with average absolute deviations between simulated and measured water table depths of approximately 4 cm. Measured and simulated data revealed very shallow (< 0.1 m) water table depths at all of the bays. Groundwater inflow was a significant component of the water balance at locations near the perimeters of the bays, ranging from 3%–26% of the total water input for these sites during the study period. A semi-confined aquifer below one of the bays was likely the source of groundwater inflow for that bay. Meanwhile, locations near the centers of the bays did not have groundwater inflow as an input to their water budgets. Groundwater outflow for the centers of the bays ranged from 2%–21% of rainfall. Areas near the perimeters of the bays were recharge, discharge, or flow-through wetlands depending on hydrologic conditions at the sites. Areas near the centers of the bays exhibited characteristics of recharge wetlands only. These results were consistent across the three Carolina bays studied, and can be used to better understand the hydrology of natural Carolina bays, improving the success of restoration projects of similar sites.

Linking Plant Ecology and Long-Term Hydrology to Improve Wetland Restoration Success

Although millions of dollars are spent restoring wetlands, failures are common, in part because the planted vegetation cannot survive in the restored hydrology. Wetland restoration would be more successful if the hydrologic requirements of wetland plant communities were known so that the most appropriate plants could be selected for the range of projected hydrology at the site. Here we describe how hydrologic models can be used to characterize the long-term hydrology of wetland plant communities, and we show how these results can be used to define wetland design criteria. In our study, we quantified differences in long-term (40-year) hydrologic characteristics of the pond pine woodland (PPW), nonriverine swamp forest (NRSF), high pocosin (HP), and bay forest (BF) plant communities native to the North Carolina Coastal Plain. We found that the median water level was 8 cm below the land surface in PPW and 9, 2, and 8 cm above the land surface for NRSF, HP, and BF, respectively. When the land surface was inundated, the median duration of inundation was 91 d year-1 for PPW and 317, 243, and 307 d year-1 for NRSF, HP, and BF, respectively. Our models suggested that the PPW received an average of 15% of its water input from groundwater inflow, whereas the other communities we modeled did not appear to receive groundwater inflow. Using these results and soil organic layer thickness, we developed and propose design criteria linking soil, vegetation, and hydrology parameters that should contribute to improved restoration success.

Economic and ecological impacts of wood chip production in North Carolina: an integrated assessment and subsequent applications

Abstract The North Carolina Wood Chip Mill Study represents an integrated assessment of the economic and ecological impacts associated with production of wood chips at satellite chip mills in the state of North Carolina (NC), USA. Mandated by the Governor of NC, the study was attended by a high degree of public scrutiny. We report principal findings, and describe the processes by which we dealt with uncertainty resulting from limited data availability, methods used to foster public involvement and efforts to reconcile public concerns over forest harvests with our narrower mandate to examine chip mills. We considered the hypotheses that chip mills fostered widespread industrial clearcutting, increased utilization of previously noncommercial timber (especially small hardwoods), depleted future growing stocks of sawtimber, and might create adverse ecological consequences or impair aesthetics important to recreational forest users. NC wood-based industries are a major component of the states economy, but lagged the state in economic growth from 1977 to 1996. Over the same period, the nature-based tourism sector grew rapidly. Forest land losses in North Carolina from 1982 to 1997 totaled more than one million acres. We used an econometric model to adjust timber land base and project timber supply dynamics to 2020. The simulation indicated that softwood removals exceeded growth from 1990 onward. Hardwood removals exceed growth by 2005, causing inventory levels to decline slightly by the end of the projection period. Wood chip mills processed approximately 27% of the states chipwood harvest and 12% of the states total timber harvest. They were statistically correlated with increased timber harvests in the state, especially in the Piedmont and the Mountains. Chip mills have effective storm water management plans and do not show visible signs of adversely affecting water quality. Higher levels of timber harvest alter forest structures in the Coastal Plain and Piedmont, generally creating less habitat for bird, amphibian and reptile species of conservation concern. Fewer species are adversely affected in the Mountains. Public opinion about chip mills is polarized, and controversy exists principally in the western portion of the state. Overall, public acceptance of study findings was favorable, and selected elements of the research findings have been used to support a variety of advocacy positions.

Vascular Flora, Plant Communities, and Soils of a Significant Natural Area in The Middle Atlantic Coastal Plain (Craven County, North Carolina)

Abstract Cool Springs Environmental Education Center (CSEEC), owned by Weyerhaeuser Company, includes a 591 ha State Significant Natural Area. It is located in Craven County, North Carolina, in the floristically rich Atlantic Coastal Plain. A vascular flora inventory documented the occurrences of 567 species and sub-specific taxa and 303 genera in 118 plant families, including populations of the Atlantic Coastal Plain endemics Pondspice (Litsea aestivalis) and LeBlonds Coastal Goldenrod (Solidago villosicarpa). We identified twenty plant community types, including the uncommon Longleaf Pine (Pinus palustris) Woodland, Bald Cypress–Tupelo Gum (Taxodium distichum – Nyssa aquatica) Swamp, a number of small depression wetland communities, and the novel Sand Laurel Oak-Loblolly Pine (Quercus hemisphaerica – Pinus taeda) Woodland. Soils ranged from excessively drained sands to very poorly drained organics. The order of the soil mapping units according to the number of plant taxa they supported per unit area was TaB > PO > Ln > Mu > DO, MM > Se > KuB. Among five of 12 floristic study sites having positive residuals in the regression of log species richness on log area, CSEEC had the third largest residual. There was no relationship between the residuals from regressions of log species on log area and soil drainage heterogeneity on log area. The occurrences of two rare plant species, a species-rich flora, ten natural plant community types, and an assemblage of wet and dry soils in a variety of geomorphic settings are objective factors justifying the recognition of CSEEC as a State Significant Natural Area.