Riparian Ecosystem Management Model Research Articles

A comparison of riparian buffer designs incorporating short‐rotation Salix viminalis to mitigate surface water pollution in the Dunk River watershed on Prince Edward Island

AbstractRiparian buffers protect surface water from diffuse pollutants, mitigating sediment, nutrient, and chemical losses from agricultural landscapes. Prince Edward Island, Canada, has legislated 15‐m riparian buffers, yet stream contamination from agriculture remains widespread. The Soil and Water Assessment Tool and Riparian Ecosystem Management Model were used to simulate 12 years of nutrient and sediment loads from five potato Hydrologic Response Units in the Dunk River watershed on PEI and evaluate water quality impacts of 30 m three‐zone buffers incorporating shrub willow relative to existing 15 m grass and forest buffers and assess optimal buffer width. Willow buffers (30 m) removed 49.9 T ha−1 year−1 sediment, 18.5 kg ha−1 year−1 total N (TN) and 7.8 kg ha−1 year−1 total P (TP). This was 5.6% and 4.1% more sediment and TP than existing 15 m grass buffers and 7.5%, 12.6%, and 16.8% more sediment, TP, and TN than existing 15 m forest buffers. Willow buffers removed significantly more particulate contaminants than forest and significantly more soluble than grass. Optimal willow buffer width downslope of potato fields was 40 m. Overall, incorporating shrub willow and/or riparian widening can have considerable water quality benefits, particularly in regard to sediment retention, and maximize both soluble and particulate pollutant removal on PEI.

Impact of Riparian Buffer Zone Design on Surface Water Quality at the Watershed Scale, a Case Study in the Jinghe Watershed in China

This study was conducted to evaluate the impact of riparian buffer zones on water quality in the Jinghe watershed, China. To evaluate the effectiveness of riparian buffers in reducing sediments and nutrients in surface runoff, we employed two validated models: the agricultural non-point source pollution model (AnnAGNPS) and the riparian ecosystem management model (REMM). The AnnAGNPS was used to divide the catchment into homogeneous drainage areas and generate upland loadings for the REMM. The REMM model was then utilized to assess the impact of different riparian buffer designs on sediments and nutrient reduction in surface runoff. We tested five designs, including the recommended standard design by the United States Department of Agriculture (USDA). This design with 20 m herbaceous perennials next to the field (Zone 3), followed by a 20 m wide harvestable deciduous forest in the middle (Zone 2), and a 10 m wide non-harvestable deciduous forest adjacent to the river (Zone 1). We also evaluated alternative designs, such as removing Zone 3, removing Zone 2, and reducing the widths of the buffer zones further. For the entire Jinghe watershed, we calculated, compared, and analyzed the annual totals of water inflow, sediment yields, and dissolved nitrogen in surface runoff into and out of Zone 1, 2, and 3 for all the designs. The analysis indicated that the removal efficiency of sediments ranged from 85.7% to 90.8%, and the removal efficiency of dissolved nitrogen in surface runoff ranged from 85.4% to 91.9% for all the designs. It is also indicated that riparian buffer zones are highly effective in reducing sediments and nutrients in agricultural runoff, even with reduced buffer widths. This finding underscores the importance of implementing riparian buffer zones as a valuable approach in the agricultural intensive watershed with constraints for allocating for the creation of standard riparian buffers.

Global sensitivity analyses of key riparian nitrogen models

Riparian zones can effectively reduce excess nitrogen loading to streams. Modeling nitrogen retention in riparian zones is useful, especially at larger scales. We evaluated select riparian nitrogen models for their robustness in representing hydrology, vegetation, soils, nutrients, and channel morphodynamics. We used global, time-varying sensitivity analyses of REMM (Riparian Ecosystem Management Model) and SWAT+ (Soil and Water Assessment Tool+) to identify the most influential parameters for calculating water table depth and nitrogen processes. Both REMM and SWAT+ were sensitive to topographic and soil parameters such as slope and soil layer thickness, although spatial and temporal scale and hydroclimatic conditions affected parameter sensitivity. Neither model was sensitive to stream channel depth, which is known to affect riparian hydrology and nitrogen cycling. It is necessary to incorporate stream morphodynamics like channel change into both riparian-scale and watershed-scale nitrogen models to provide useful management tools for addressing excessive nitrogen loading in stream networks.

Riparian Zone Nitrogen Management through the Development of the Riparian Ecosystem Management Model (REMM) in a Formerly Glaciated Watershed of the US Northeast

The Riparian Ecosystem Management Model (REMM) was developed, calibrated and validated for both hydrologic and water quality data for eight riparian buffers located in a formerly glaciated watershed (upper Pawcatuck River Watershed, Rhode Island) of the US Northeast. The Annualized AGricultural Non-Point Source model (AnnAGNPS) was used to predict the runoff and sediment loading to the riparian buffer. Overall, results showed REMM simulated water table depths (WTDs) and groundwater NO3-N concentrations at the stream edge (Zone 1) in good agreement with measured values. The model evaluation statistics showed that, hydrologically REMM performed better for site 1, site 4, and site 8 among the eight buffers, whereas REMM simulated better groundwater NO3-N concentrations in the case of site 1, site 5, and site 7 when compared to the other five sites. The interquartile range of mean absolute error for WTDs was 3.5 cm for both the calibration and validation periods. In the case of NO3-N concentrations prediction, the interquartile range of the root mean square error was 0.25 mg/L and 0.69 mg/L for the calibration and validation periods, respectively, whereas the interquartile range of d for NO3-N concentrations was 0.20 and 0.48 for the calibration and validation period, respectively. Moreover, REMM estimation of % N-removal from Zone 3 to Zone 1 was 19.7%, and 19.8% of N against actual measured 19.1%, and 26.6% of N at site 7 and site 8, respectively. The sensitivity analyses showed that changes in the volumetric water content between field capacity and saturation (soil porosity) were driving water table and denitrification.

Riparian buffer effectiveness as a function of buffer design and input loads.

Although many agricultural watersheds rely heavily on riparian buffer adoption to meet water quality goals, design and management constraints in current policies create adoption barriers. Based on focus group feedback, we developed a flexible buffer design paradigm that varies buffer width, vegetation, and harvesting. Sixteen years of daily-scale nutrient and sediment loads simulated with the Soil and Water Assessment Tool (SWAT) were coupled to the three-zone Riparian Ecosystem Management Model (REMM) to compare the effectiveness of traditional, policy-based buffer designs with designs that are more flexible and integrate features important to local farmers. Buffer designs included (i) 10m grass, (ii) 15m grass, (iii) 15m deciduous trees, (iv) 30m grass and trees, (v) 30m grass and trees with trees harvested every 3 yr, and (vi) 30m grass and trees with grass harvested every year. Allowing harvesting in one zone of the buffer vegetation (either trees or grasses) minimally affected water quality, with annual average percent reductions differing by<5% (p>.05; 76-78% for total nitrogen [TN], 51-55% for total phosphorus [TP], and 68% for sediment). Under the highest input loading conditions, buffers with lower removal efficiencies removed more total mass than did buffers with high removal efficiencies. Thus, by focusing on mass reduction in addition to percent reduction, watershed-wide buffer implementation may be better targeted to TN, TP, and sediment reduced. These findings have important implications for informing flexible buffer design policies and enhanced placement of buffers in watersheds impaired by nutrient and sediment.

Testing the Riparian Ecosystem Management Model (REMM) on a Riparian Buffer with Dilution from Deep Groundwater

Abstract. The Riparian Ecosystem Management Model (REMM), developed to quantify water quality benefits of riparian buffers, was field-tested using five years (2005-2009) of measured hydrologic and water quality data on a site in the upper coastal plain of North Carolina. This buffer site received nitrate-nitrogen (NO 3 -N) loading from a pasture fertilized with poultry litter. Field results showed reductions in groundwater NO 3 -N concentrations moving through the buffer to the stream for the five-year period; however, further analysis of the groundwater data indicated that dilution was a one of the contributing factors to those observations. Previous testing of REMM was carried out at riparian sites having minimal groundwater dilution. This modeling study is the first attempt to calibrate and validate REMM on a riparian buffer site showing reductions in NO 3 -N concentrations due to groundwater dilution. REMM was calibrated using daily measured water table depths (WTDs) and monthly groundwater NO 3 -N concentrations. Results of model testing showed simulated WTDs and NO 3 -N concentrations in good agreement with measured values. The mean absolute error (MAE) and Willmott‘s index of agreement (d) between simulated and measured WTDs ranged from 18 to 41 cm and from 0.59 to 0.95, respectively. The root mean square error (RSME) and Willmott‘s index of agreement (d) between simulated and measured NO 3 -N concentrations ranged from 1.20 to 8.41 mg L -1 and from 0.11 to 0.73, respectively, for the five-year simulation period. REMM predictions of annual plant nitrogen (N) uptake and denitrification were within the ranges reported in other riparian buffer field studies. A 33-year simulation was carried out using the calibrated and validated model to assess the long-term NO 3 -N removal performance of the buffer. Results showed that the buffer on average reduced groundwater NO 3 -N concentrations from 13Â mg L -1 at the field edge to 1 mg L -1 at the stream edge. In terms of loadings, the NO 3 -N and total N reductions from the field edge to the stream were 81% and 89%, respectively. Results of this modeling study showed that REMM reasonably simulated short-term (five years) and long-term (30+ years) groundwater hydrology and NO 3 -N dynamics for a riparian ecosystem, despite the absence of a groundwater dilution component in the model.

Atrazine Transport Within a Coastal Zone in Southeastern Puerto Rico: a Sensitivity Analysis of an Agricultural Field Model and Riparian Zone Management Model

Agrichemical runoff from farmland may adversely impact coastal water quality. Two models, the Agricultural Policy/Environmental eXtender (APEX) and the Riparian Ecosystem Management Model (REMM), were used to evaluate the movement of the herbicide atrazine to the Jobos Bay National Estuarine Research Reserve from adjacent fields. The reserve is located on Puerto Rico’s southeast coast. Edge-of-field atrazine outputs simulated with the APEX were routed through a grass-forest buffer using the REMM. Atrazine DT50 (half-life) values measured in both field and buffer soils indicated that accelerated degradation conditions had developed in the field soil due to repeated atrazine application. APEX simulations examined both the measured field and buffer soil atrazine DT50 and the model’s default value. The use of the measured field soil atrazine degradation rate in the APEX resulted in 33 % lower atrazine transport from the field. REMM simulations indicated that the buffer system had the potential to reduce dissolved atrazine transport in surface runoff by 77 % during non-tropical storm events by increasing infiltration, slowing transport, and increasing time for pesticide degradation. During a large runoff event due to a tropical storm that occurred close to the time of an atrazine application, the REMM simulated only a 37 % reduction in atrazine transport. The results indicate that large storm events soon after herbicide application likely dominate herbicide transport to coastal waters in the region. These results agree with water quality measurements in the reserve. This study demonstrated the sensitivity of these models to variations in DT50 values in evaluating atrazine fate and transport in the region and emphasizes that the use of measured DT50 values can improve model accuracy.

Field Testing the Riparian Ecosystem Management Model on a Riparian Buffer in the North CarolinaUpper Coastal Plain

AbstractThe riparian ecosystem management model (REMM) was field tested using five years (2005‐2009) of measured hydrologic and water quality data on a riparian buffer located in the Tar‐Pamlico River Basin, North Carolina. The buffer site received NO3‐N loading from an agricultural field that was fertilized with inorganic fertilizer. Field results showed the buffer reduced groundwater NO3‐N concentration moving to the stream over a five‐year period. REMM was calibrated hydrologically using daily field‐measured water table depths (WTDs), and with monthly NO3‐N concentrations in groundwater wells. Results showed simulated WTDs and NO3‐N concentrations in good agreement with measured values. The mean absolute error and Willmott's index of agreement for WTDs varied from 13‐45 cm and 0.72‐0.92, respectively, while the root mean square error and Willmott's index of agreement for NO3‐N concentrations ranged from 1.04‐5.92 mg/l and 0.1‐0.86, respectively, over the five‐year period. REMM predicted plant nitrogen (N) uptake and denitrification were within ranges reported in other riparian buffer field studies. The calibrated and validated REMM was used to simulate 33 years of buffer performance at the site. Results showed that on average the buffer reduced NO3‐N concentrations from 12 mg/l at the field edge to 0.7 mg/l at the stream edge over the simulation period, while the total N and NO3‐N load reductions from the field edge to the stream were 77 and 82%, respectively.

Estimating the optimal width of buffer strip for nonpoint source pollution control in the Three Gorges Reservoir Area, China

Following the implementation of the Three Gorges Dam Project, nonpoint source (NPS) pollution has become a serious problem in the Three Gorges Reservoir (TGR). An urgent need exists to build an ecological shelter buffer (ESB) along the TGR to improve water quality. However, to determine the optimal buffer width of the ESB for NPS pollution control is challenging because of spatial variations in topography, hydrology, slope and drainage patterns in the Three Gorges Reservoir Area (TGRA). In this study, a methodology was established for modelling the ESB using geographic information systems (GIS) technology and was applied to a small-scale ungauged watershed (Panlong basin), a typical watershed along the TGR. A grid terrain analysis, an NPS pollution model, the Soil and Water Assessment Tool (SWAT) and a riparian simulation model, Riparian Ecosystem Management Model (REMM), were combined to estimate the transport of NPS pollutants and pollutant abatement through ESB at the watershed scale. Suitable widths of the ESB were estimated for different objectives of water quality protection to reflect regional variations in physical conditions. The results demonstrated that the ESB width depends strongly on the topographic features, soils types, hydrological conditions and conservation objectives of the TGRA. The estimated ESB with 58m in the averaged width can achieve the current requirement of water quality of TGRA. Most of the areas with higher values of NPS pollutant concentrations are located in the buffer drainage areas. Accordingly, an ESB, rather than other streams, most likely offers the greatest potential to improve water quality along the reservoir, and it is necessary to install ESBs along the tributary streams for NPS pollution control and water quality protection from a watershed perspective. The resulting map of ESB can indicate specific locations best suited for ESB at watershed scales, and can be applied to field-scale planning. The methodology described in this study demonstrates its capability as a decision support tool to guide ESB building, support land-use decision making and facilitate environmental policy formulation and evaluation throughout the TGRA.

Hydrology and water quality of a field and riparian buffer adjacent to a mangrove wetland in Jobos Bay watershed, Puerto Rico

Agriculture in coastal areas of Puerto Rico is often adjacent to or near mangrove wetlands. Riparian buffers, while they may also be wetlands, can be used to protect mangrove wetlands from agricultural inputs of sediment, nutrients, and pesticides. We used simulation models and field data to estimate the water, nitrogen, and phosphorus inputs from an agricultural field and riparian buffer to a mangrove wetland in Jobos Bay watershed, Puerto Rico. We used the Agricultural Policy/Environmental eXtender (APEX) and the Riparian Ecosystem Management Model (REMM) models sequentially to simulate the hydrology and water quality of the agricultural fields and an adjacent riparian buffer, respectively. Depth to the water table surface was measured monthly at numerous sites in both field and riparian areas and were used with recording well data from outside the field to estimate daily water table depths in the field and riparian buffer and to calibrate field-scale hydrologic processes. Calibration and validation of the models were successful for the riparian buffer and in three of four field quadrants. In these areas the average simulated depth to water table for the field and the riparian buffer were within ±7% of field estimated water table depths. Over the 3-year study period, the riparian buffer represented by REMM reduced agricultural loadings to the mangrove wetland by 24% for sediment yield, and about 30% for total nitrogen and phosphorus. Simulations indicated that tropical storms and hurricanes played an important role in water and nutrient transport on this site contributing at least 63% of total sediment and nutrient loads.

Environmental quality research in the Beasley Lake watershed, 1995 to 2007: Succession from conventional to conservation practices

The Beasley Lake watershed (BLW), established for the Mississippi Delta Management Systems Evaluation Area project, represents the US Mississippi Delta region in the national Conservation Effects Assessment Project. The 915-ha (2,260-ac) BLW drains into an oxbow lake that has been monitored since 1995 when row crops were grown on 79% of the area, and the remaining area included a 25-ha (62-ac) lake and a 135-ha (330-ac) riparian forest. Currently, row crops account for 66.5% of the area with 12.4% enrolled in the Conservation Reserve Program. Cotton (<i>Gossypium hirsutum</i> L.) acreage has decreased from 63.3% to 8.9%. Historical and current research in BLW focuses on monitoring lake limnology, evaluating conservation practice effects on edge-of-field runoff, quantifying changes associated with the Conservation Reserve Program, and modeling watershed responses. Applying combinations of conservation practices can significantly reduce nonpoint source pollution. For example, converting row crops to reduced tillage and transgenic herbicide-resistant crops in BLW reduced suspended sediment (70% reduction), total phosphorus (41% reduction), and pesticide concentrations in lake water. Corresponding increases in Secchi visibility (97%) and chlorophyll <i>a</i> (a primary productivity indicator) likely contributed to improved fish productivity (e.g., fish weight increase comparing 1998 and 2004: <i>Micropterus salmoides</i> 87%, <i>Lepomis macrochirus</i>, 65%) during this period. Additional studies should quantify effects of individual practices and improve modeling tools for making better management decisions. The utilization of the Annualized Agricultural Non-Point Source (AnnAGNPS) model and the Riparian Ecosystem Management Model (REMM) together provides additional information on the effectiveness of conservation practices within the watershed by combining technology that assesses riparian buffer effectiveness in filtering nutrients at the field scale with the watershed water quality transport capabilities of the AnnAGNPS model.

Riparian Ecosystem Management Model: Sensitivity to Soil, Vegetation, and Weather Input Parameters1

Abstract: The Riparian Ecosystem Management Model (REMM) was developed by the U.S. Department of Agriculture‐Agriculture Research Service (USDA‐ARS) and its cooperators to design and evaluate the efficiency of riparian buffer ecosystems for nonpoint source pollution reduction. REMM requires numerous inputs to simulate water movement, sediment transport, and nutrient cycling in the buffer system. In order to identify critical model inputs and their uncertainties, a univariate sensitivity analysis was conducted for nine REMM output variables. The magnitude of each input parameter was changed from −50% to +50% from the baseline data in 12 intervals or, in some cases, the complete range of an input was tested. Baseline model inputs for the sensitivity analysis were taken from Gibbs Farm, Georgia, where REMM was tested using a 5‐year field dataset. Results of the sensitivity analysis indicate that REMM responses were most sensitive to weather inputs, with minimum daily temperature having the greatest impact on the nitrogen‐related outputs. For example, the 100% change (−50% to +50%) in minimum daily temperature input values yielded a 164.4% change in total nitrogen (N), a 109.3% change in total nitrate (NO3), and a 127.1% change in denitrification. REMM was most sensitive to precipitation with regard to total flow leaving the riparian vegetative buffer zone (199.8%) and sediment yield (138.2%). Deep seepage (12.2%), volumetric water content (24.8%), and pore size index (6.5%) in the buffer soil profile were the most influential inputs for the output water movement. Sediment yield was most sensitive to Manning’s coefficient (46.6%), bare soil percent (40.7%), and soil permeability (6.1%). For vegetation, specific leaf area, growing degree day coefficients, and maximum root depth influenced the nitrogen related outputs. Overall results suggest that because of the high sensitivity to weather parameters, on‐site weather data is needed for model calibration and validation. The model’s relatively low sensitivity to vegetation parameters also appears to support the use of regional vegetation datasets that would simplify model implementation without compromising results.

Surface runoff contribution of nitrogen during storm events in a forested watershed

We examined total Kjeldahl nitrogen (TKN) loading to a small forested stream during storm events. We hypothesized that upper soil and litter layers in riparian area are primary source of higher TKN concentrations during storm. A storm water sampling program was carried out to gather requisite flow and water quality data to calibrate and validate water and nutrient components of the Riparian Ecosystem Management Model for TKN. Water quality and storm flow data collected from January 2000 to December 2003 were used to simulate the hydrology and nitrogen transport over a second-order watershed within the Fort Benning Military Installation, Georgia. Intensive sampling conducted from October 2002 to May 2003 provided the necessary data to characterize the rising limb, peak, and recession limb of six major storm events. Simulated runoff and storm TKN loads were compared with their corresponding observed or calculated values. Hydrology and nitrogen data collected from February 21, 2003 to December 31, 2003 were used for the model validation. The hydrology component of the model showed a Nash-Sutcliffe efficiency of 87% for the validation period. The average absolute difference between simulated and calculated TKN loads was 25%. Even though the monthly water budget indicated the dominance of subsurface flow, TKN contribution from direct runoff was significantly greater than that from subsurface flow. On an average, 73% of the observed total TKN load at the watershed outlet was contributed by surface runoff during storm events. The results suggested that the surface runoff during the storm events washed off the nitrogen from the forest floor and transported to the stream.

GIS-Based Integration of SWAT and REMM for Estimating Water Quality Benefits of Riparian Buffers in Agricultural Watersheds

The Soil and Water Assessment Tool (SWAT) is a process-based, watershed-scale model with a hydrologic response unit (HRU) as the basic computation element, which makes it difficult to accurately represent riparian buffers using their physical parameters (e.g., vegetation structure). On the other hand, the field-scale Riparian Ecosystem Management Model (REMM) provides the opportunity to consider details of hydrologic processes within a riparian buffer zone. However, the runoff and its associated constituents from the upland area that is hydraulically connected to the riparian buffer zone must be provided as inputs into REMM. The rationale proposed here is that the integration of SWAT and REMM would improve the assessment of riparian buffers, which is vital to watershed management but which has not been described in the literature. The objective of this study was to develop a GIS interface that integrated SWAT and REMM for estimating water quality benefits of riparian buffers in agricultural watersheds. For modeling purposes, the interface subdivided a watershed into a number of sub-basins, each of which was further subdivided into drainage areas of isolated impoundments (e.g., wetlands), concentrated flow, and riparian buffers using available GIS data. As a result, riparian buffers received runoff and associated pollutants from corresponding contribution areas to mimic actual field conditions. The interface facilitated transferring the SWAT outputs into REMM and computing the site characteristic parameters (e.g., length and width) of the riparian buffers. The outputs from subsequent REMM runs were in turn taken as inputs into SWAT for channel routing and further simulation. The interface was used to assess water quality benefits of riparian buffers in the Lower Canagagigue Creek watershed located in southern Ontario, Canada. The results indicated that the existing riparian buffer system achieved a 27.9% abatement in sediment and a 37.4% reduction in total phosphorus. The model runs demonstrated that the GIS interface was easy to use and could serve as a protocol for integrating models with distinctly different spatial scales.

QUANTIFYING THE SENSITIVITY OF THE RIPARIAN ECOSYSTEM MANAGEMENT MODEL (REMM) TO CHANGES IN CLIMATE AND BUFFER CHARACTERISTICS COMMON TO CONSERVATION PRACTICES

Conservation practices, such as buffers, are often installed to mitigate the effects of nutrients and sediment runofffrom agricultural practices. The Riparian Ecosystem Management Model (REMM) was developed as process-based modelto evaluate the fate of nutrients and sediment through a riparian buffer up to the edge of a stream. A one-at-a-time sensitivityanalysis was performed on REMM to evaluate the effects that changing herbaceous buffer scenarios have on N, P, andsediment in surface and ground water. Vegetation characteristics such as rooting depth, LAI, and plant height, along with somephysical buffer characteristics were varied within their typical range and compared to a base case scenario. Modeloutputs were not sensitive to plant height or LAI, but moderately sensitive to changes in SLA. Model outputs were onlysensitive to rooting depth as roots became shallower in the soil profile. Sediment yield and dissolved nitrate in surface waterwere the most sensitive to changes in Mannings n, while other soil physical characteristics such as surface roughness, surfacecondition, and % bare soil had little to no effect on model outcomes. Dissolved surface nitrate, organic P, and dissolvedsubsurface nitrate were all moderately sensitive to changes in saturated hydraulic conductivity and the slope of the buffer.Results indicate that within the model, many vegetation characteristics do not directly play a role in the physical transportof nutrients and sediment in surface and subsurface water; therefore, utilizing REMM to evaluate effects of specificherbaceous plant types may have limited value unless specific leaf area or rooting depth are considered. It would be possibleto model and perhaps achieve specific load reductions by modifying slope and other physical characteristics or by consideringforest versus grass buffers.

RIPARIAN ECOSYSTEM MANAGEMENT MODEL: HYDROLOGY PERFORMANCE AND SENSITIVITY IN THE NORTH CAROLINA MIDDLE COASTAL PLAIN

A riparian buffer installed along streams is one alternative that can be used to reduce the delivery of nitrogen,phosphorus, and sediment to the stream. The Riparian Ecosystem Management Model (REMM) has been developed tosimulate surface and subsurface riparian buffer hydrology, sediment transport, litter and sediment interactions, vegetationgrowth, and soil carbon, nitrogen, and phosphorus dynamics. For this model to accurately simulate transport and fate ofnutrients in the shallow groundwater beneath a riparian buffer, the subsurface hydrology component must be verified. Twoyears of field data were utilized to evaluate and test the sensitivity of the hydrology component of REMM in the North CarolinaMiddle Coastal Plain. Daily simulated water table depth was compared to observed water table depths across a 15 m widebuffer. The simulated water table depth was sensitive to hydrologic parameters such as groundwater inputs from the uplandinto zone 3, stream depth, and buffer slope. Average absolute errors between simulated and observed water table depth werefound to be 0.35 to 0.36 m, while relative errors ranged from 0.12 to 0.15 m. Simulated evapotranspiration (ET) was higherin zone 3 compared to zones 1 and 2, although all three zones were parameterized alike. Flow into the buffer from the streamis not simulated by REMM, although it occurred frequently during data collection. Estimates of ET should be improved forherbaceous and grass vegetation types to improve water table depth predictions. Also, for conditions where the streamcontributes flow to the groundwater, an additional component in REMM is necessary.

EVALUATION OF COASTAL PLAIN CONSERVATION BUFFERS USING THE RIPARIAN ECOSYSTEM MANAGEMENT MODEL1

ABSTRACT: Riparian buffers are increasingly important as watershed management tools and are cost‐shared by programs such as Conservation Reserve that are part of the USDA Conservation Buffer Initiative. Riparian buffers as narrow as 4.6m (15ft) are eligible for cost‐share by USDA. The Riparian Ecosystem Management Model (REMM) provides a tool to judge water quality improvement by buffers and to set design criteria for nutrient and sediment load reduction. REMM was used for a Coastal Plain site to simulate 14 different buffers ranging from 4.6 m to 51.8 m (15 to 170 ft) with three different types of vegetation (hardwood trees, pine trees, and perennial grass) with two water and nutrient loads. The load cases were low sediment/low nutrient‐typical of a well managed agricultural field and low sediment/high nutrient‐typical of liquid manure application to perennial forage crops. Simulations showed that the minimum width buffer (4.6 m) was inadequate for control of nutrients under either load case. The minimum width buffer that is eligible for cost share assistance on a field with known water quality problems (10.7 m, 35 ft) was projected to achieve at least 50 percent reduction of N, P, and sediment in the load cases simulated.

USING GLEAMS AND REMM TO ESTIMATE NUTRIENT MOVEMENT FROM A SPRAY FIELD AND THROUGH A RIPARIAN FOREST

With the increased number of large animal production facilities in eastern North Carolina, nutrient accumulation is becoming a problem in surface waters and groundwater. To protect these water sources, management practices to reduce nutrient movement or accumulation are being evaluated using computer models. The computer models, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) and a version of Riparian Ecosystem Management Model (REMM), were used to estimate nitrogen and phosphorus transport of nutrients through a riparian buffer zone from an agricultural field that received swine lagoon effluent. The models simulated annual application rates of effluent equivalent to 500 and 1000 kg N/ha. The GLEAMS model provided the weather data and nutrient concentrations in the soil, sediment, and leachate for input into REMM. Assuming a 1000 kg N/ha loading rate, GLEAMS monthly average NO3 –N leachate concentrations were within 14% of the observed data, and REMM–simulated NO3 –N leachate concentration was within 5% of the observed data. Both models provided an adequate estimation of nitrogen transport through the system. GLEAMS simulations of PO4 –P leachate followed the general trend of observed data. However, there was no apparent response in simulated PO4 –P leachate concentrations for the two loading rates (95 and 190 kg P/ha), indicating a problem in the phosphorus calculations in the model. The REMM–simulated PO4 –P leachate was greater than observed concentrations and was affected by the inputs obtained from GLEAMS. The pre–release version of REMM provided good estimates of the nutrient transport, and with a few improvements, official releases of REMM have the potential to provide better estimates of nutrient movement through the riparian buffer zone.

GIS-BASED COUPLING OF GLEAMS AND REMM HYDROLOGY: II. FIELD TEST RESULTS

Agricultural fields border riparian areas in many locations. These riparian areas provide important filtering capabilities for agricultural chemicals and sediment leaving upland agricultural areas. Several models have been developed for upland agricultural areas and for riparian zones. The need to integrate such models through a flexible structure which represents the field conditions is highly desirable. Since water is the primary mechanism for pollutant transport, the system must be capable of managing both surface and subsurface water movement. The hydrology components of the upland agriculturally oriented water quality model, GLEAMS, and the riparian ecosystem management model, REMM, were coupled through a GIS to allow evaluation of agricultural and forest management impacts on water movement for typical field situations in the southeastern Coastal Plain. Programs and subroutines were developed to allow delineation of field areas, identification and maintenance of data based on the field map, and incorporation of data to the input data format for each model. The model system was capable of representing the shallow groundwater levels from forest management practices of clear-cut, thinned, and mature forests in field comparisons with measured values from the Gibbs Research Farm near Tifton, Georgia, for 27 months of comparison data between 1992 and 1994. Simulated monthly average shallow groundwater levels were within 0.14, 0.08, and 0.25 m of measured groundwater levels for the clear-cut, thinned, and mature forest treatments, respectively. Correlation results for the same treatments indicated at least an r 2 of 0.78 for all simulated versus measured monthly shallow ground water results. The model system did not respond as well in representing field conditions on total monthly runoff. Average monthly differences in total simulated runoff were 0.73, 1.19, and 0.48 cm lower (highly significantly) than measured results for the clear-cut, thinned, and mature forest management treatments from the Gibbs Farm site, respectively. No correlation was indicated between simulated and measured total runoff. The model system did, however, indicate the runoff trends expected due to changes in forest management. As the number of trees and canopy increased, runoff decreased. The model system has the potential of providing a cost effective method of incorporating multiple model characteristics into management practice evaluations.

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 pineforest 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 thehydrologic input to REMM. Uncalibrated simulation results for a five-year period were compared to measured values forthe same time period. The overall error in zone 2 and zone 1 mean water table depths was about 0.07 m, althoughabsolute errors were higher. The water table dynamics simulated by REMM were similar to observed although lags wereobserved in the response of the simulated water table to large rainfall events. Mixed results were obtained from observedversus simulated surface runoff comparisons, primarily due to large variability in observed runoff depths along theriparian transect. Simulated surface runoff depths for zone 3 were within one standard deviation for four out of the fiveyears. For zone 2, surface runoff depths could only be simulated within one standard deviation for two out of the fiveyears. Simulated seasonal total depths of surface runoff did not always agree with observed values but usually followedboth similar temporal and spatial patterns. Annual hydrologic budgets produced total streamflow comparable to thoseestimated for the riparian buffer site. These results provide an adequate basis for subsequent testing of other REMMmodel components including water quality and nutrient cycling.