Randall G. Williams

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.

RIPARIAN ECOSYSTEM MANAGEMENT MODEL (REMM): II. TESTING OF THE WATER QUALITY AND NUTRIENT CYCLING COMPONENT FOR A COASTAL PLAIN RIPARIAN SYSTEM

The Riparian Ecosystem Management Model (REMM) was used to simulate nitrogen (N), phosphorus (P), and carbon (C) cycling and transport in a Coastal Plain riparian buffer system near Tifton, Georgia. The riparian buffer consisted of zone 3 (grass next to a row crop field); zone 2 (mature pine forest downslope from zone 3); and zone 1 (hardwood forest downslope from zone 2, adjacent to a stream). Uncalibrated simulation results for a five-year period were compared to measured values for the same time period at the research site. In general, simulated water table nutrient concentrations were within one standard deviation of observed values on an annual basis. Surface runoff loads exiting zone 3 for most N and P forms were simulated within one standard deviation of the observed. In contrast zone 2, surface runoff loads for inorganic N species were an order of magnitude lower than observed. Although some of the surface runoff differences (observed vs. simulated) were large in relative terms, the overall trends within the riparian buffer were generally well-represented and differences were not large in absolute terms. Simulated values for one of the most important processes responsible for effectiveness of riparian zones — denitrification, were within the range of those observed. Much of the temporal dynamics of the observed data were also captured in the REMM simulations. Certain constraints of the model use are discussed, but REMM appears to be useful for representing many of the specific processes and general trends in riparian ecosystem buffers.

Nutrient Concentrations in the Soil Solution and Shallow Groundwater of a Liquid Dairy Manure Land Application Site

Land application of liquid animal manures offers the potential for recycling large volumes of slurries by using the nutrients available in the manure for plant growth in place of conventional inorganic fertilizers. A study was initiated to determine environmentally and economically sustainable liquid dairy manure application rates on a year-round forage production system. Treatments based on nitrogen application rates of 200, 400, 600, and 800 kg N ha–1yr–1 were established. This work reports on nutrient concentrations in the soil solution of the vadose zone and in shallow groundwater after three years of land application. A 96-instrument network of high tension soil solution samplers was installed at 0.5, 1.0, 1.5, and 2.0 m depths and used to collect biweekly samples from June 1991 through September 1994. A network of 72 shallow groundwater monitoring wells was installed at 3 and 6 m depths and used to collect biweekly samples from May 1991 through September 1994. Statistically significant NO3-N treatment effects were observed at the 0.5, 1.0, 1.5, and 2.0 m depths. NO3-N treatment effect were not observed at the 3.0 or 6.0 m depths or at any depth for NH4-N, TKN, TN, PO4-P, and TP. Mean annual NO3-N soil solution concentrations ranged from a low of 1.45 mgL–1 to a high of 22.70 mgL–1. Concentrations of NH4-N and TKN were low for all depths while PO4-P and TP concentrations were nearly always below detection limits. After three years of study, treatment effects were clearly observed in the vadose zone. If not for very low subsoil permeability, it is likely that treatment effects would have been observed below 2.0 m.

Hydrologic Analysis of a Riparian Buffer Enrolled in Conservation Reserve Enhancement Program in North Carolina Using Riparian Ecosystem Management Model (REMM)

Conservation practices such as the installation of riparian buffers along streams are known to reduce the impacts of non-point source pollutants such as nitrogen and phosphorus. A riparian buffer enrolled in the Conservation Reserve Enhancement Program in North Carolina, located in the Tar-Pamlico Watershed, has been monitored for hydrology and water quality since 2005. This buffer site received nitrogen and phosphorus loads from an upland source of inorganic fertilizer for crops like cotton, peanut and soybean. Results from the field studies have shown that the buffers are able to reduce nitrate-nitrogen loads to the streams. The hydrology is a driving factor for biogeochemical reduction of pollutants in the buffers, which often leads to highly variable performance in terms of nitrate-nitrogen reduction. The Riparian Ecosystem Management Model (REMM) was calibrated and will be validated to enhance the understanding of surface and subsurface hydrologic patterns at this buffer. The results showed the simulated water table depths in good agreement with field measured water table depths in block 1 of the research site. The average absolute error (AAE) was 92 mm for the field edge buffer, 120 mm for mid-buffer and 137 mm for the stream buffer zone over a three year period (2005-2007). Characterizing the hydrology at the site was the first step in our attempt to model nitrate losses from the buffer system.