Richard Lowrance*, Randall G. Williams, Shreeram P. Inamdar, David D. Bosch, and Joseph M. Sheridan.

Transcription

1 Evaluation of Coastal Plain Conservation Buffers using the Riparian Ecosystem Management Model (In Press, Journal of the American Water Resources Association) Richard Lowrance*, Randall G. Williams, Shreeram P. Inamdar, David D. Bosch, and Joseph M. Sheridan. 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 17 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 (4.6m) 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 (1.7 m, 35 ft) was projected to achieve at least 5% reduction of N, P, and sediment in the load cases simulated. keywords: water quality, riparian forest buffers, vegetated filter strips, nitrogen, phosphorus, nutrient loads, sediment loads, ecosystem model. * Corresponding author 1

2 INTRODUCTION Riparian ecosystem buffers are an important part of both nonpoint source pollution control and stream restoration strategies being applied by USDA and other federal and state agencies. Riparian ecosystem buffers have been adopted as a best management practice (BMP) in some state nonpoint source pollution control programs (Gilliam et al., 1997) and are one of the primary practices in the USDA Conservation Buffer Initiative. The continuous sign-up of the Conservation Reserve Program (CRP) is one of the main ways for agricultural landowners to get government assistance to install riparian ecosystem buffers. USDA has adopted a multiple zone buffer system as the standard riparian buffer for both controlling nonpoint source pollution and protecting and restoring adjacent aquatic ecosystems (Welsch, 1991; NRCS, 1995) (Figure 1). The multiple zone buffer is designed to provide Figure 1 multiple functions by having a band of permanent woody vegetation adjacent to the aquatic ecosystem (Zone 1), a second band of woody vegetation that can be managed for biomass production adjacent to Zone 1 (Zone 2); and a third band of herbaceous vegetation between the woody buffer and the source area (field), Zone 3. USDA programs will allow buffers of 1-3 zones to be applied as part of the CRP depending on the resource needs. There are minimum widths that are considered technically feasible and maximum widths that are eligible for cost share assistance and rental payments under CRP. The minimum widths for Zone 1 and Zone 2 are 4.6 m (15 ft) and 6.1 m (2 ft), respectively. Under certain conditions, USDA programs will cost share for only a Zone 1 buffer system (minimum 15 ft) or only a two zone buffer (minimum 2

3 35 ft). Based on refinements of the practice, USDA-Natural Resources Conservation Service (NRCS) has determined that a three zone buffer system will actually consist of two conservation practices, a Riparian Forest Buffer (Practice Code 391) and a Filter Strip (Practice Code 393). The Riparian Forest Buffer (RFB) can be either one or two zones. If nonpoint source pollution is not a problem a RFB (391) can be as narrow as one zone of 4.6 m. If nonpoint source pollution is a problem a two-zone buffer would be recommended and if sediment movement is a concern, an herbaceous zone 3 would be recommended and installed as a Filter Strip (393) Practice. Thus under USDA standards, a riparian ecosystem buffer could range from a minimum of a 4.6 m one zone buffer to a much wider (typically 51.8 m (17ft) maximum) buffer that has three zones in two separate conservation practices. The nonpoint source pollution control functions of many of these riparian ecosystem buffers, especially the very narrow ones, have not been well studied. Although there is more information on the function of riparian ecosystem buffers in the Coastal Plain than in most other regions (Lowrance et al., 1997) there have been no comprehensive studies of the effects of different width buffers, different vegetation, and different loadings on buffer performance. In many regions of the country there is only limited information on the role of riparian buffer ecosystems in controlling nonpoint sources of sediment and nutrients. Because of the paucity of data from many regions, the need for site specific information from numerous sites, and the ongoing application of riparian ecosystem buffers as an important part of USDA Conservation Programs, USDA and cooperating universities have developed the Riparian Ecosystem Management Model (REMM) to simulate the effects of multiple zone 3

4 riparian buffers on water, sediment, and nutrient movement from source areas to streams (Altier et al., In Press, Lowrance et al., 2). The riparian system is characterized in the model as consisting of three zones parallel to the stream corresponding to the three zone system used as a management recommendation. Although designed to simulate this three-zone system, REMM can simulate riparian conditions where one or more of the zones are absent or managed differently from the specifications. REMM is a daily time-step model. Inputs to REMM are the water, sediment, nitrogen, and phosphorus loadings in surface runoff and subsurface flow from a source area, typically a field. Outputs from REMM are the water, sediment, nitrogen, and phosphorus in surface runoff, subsurface flow, and seepage to a stream or adjacent aquatic ecosystem. Surface runoff as defined in REMM is generated by rainfall exceeding available soil water storage capacity in a zone. Subsurface flow is shallow groundwater movement within the root zone of the buffer. Seepage is flow along the soil surface below the leaf litter that is generated when subsurface flow exceeds available water storage capacity in a downslope zone. Outputs are reported as sediment size classes and nutrient species in water moving as direct surface runoff, and nutrient species moving in subsurface flow and seepage. Direct surface runoff only occurs on rain days and interacts with the litter and a soil-mixing layer in the litter. Seepage is water exfiltrating from the soil and can occur whenever there is more subsurface flow into a zone than can be stored in the zone. Seepage does not interact with the litter but can infiltrate in downslope zones. The emphasis in REMM on process simulation and the explicit representation of a multiple zone buffer system with details of water, sediment, sediment borne nutrient, and soluble nutrient movement provides insights into the functions of riparian buffers 4

5 not possible with models that depend on empirical mass balances. Details on REMM and the testing that has been done for Coastal Plain systems in Georgia are found in Inamdar et al., 1999a, 1999b and Lowrance et al, 2a. In this study REMM was tested for a range of 14 different buffer widths, three different buffer vegetation types, and two upland loadings. The 14 different buffers were either one-zone (hardwoods only), two-zones (hardwoods and pines), or three-zones (hardwoods, pines, and grass). The purpose of the simulation study was three-fold: 1) determine the effects of differing buffer widths ranging from a minimal Zone 1 buffer to a three zone buffer similar in size to the maximum allowable CRP buffer; 2) to determine the effects of vegetation changes as buffer width changed; and 3) to determine the effects of a higher nutrient and water loading on the suite of buffer systems. MATERIALS AND METHODS The detailed testing of REMM reported in earlier publications (Inamdar et al., 1999a, 1999b) provided the foundation for the simulations reported here. The site used for these simulations is located at the University of Georgia Gibbs Experimental Farm located approximately 5 miles west of Tifton, GA. The site, site hydrology and nutrient transport and nitrogen cycling for the site were described in a series of publications (Ambus and Lowrance, 1991; Hubbard and Lowrance 1996; Bosch et al., 1996; Lowrance et al. 2a, 2b; Sheridan et al., 1999). Water table depths at the site ranged from cm (water table at the surface) to about 2cm depending on the time of year. Selected soil and vegetation characteristics of the site are given in Table 1. Table 1 5

6 This simulation study was designed to test the types of buffers that might be installed on Coastal Plain (and other) farms as specified by USDA- NRCS practice standards Riparian Forest Buffers (practice code 391) and Filter Strips (practice code 393). The practice standards were used to develop a series of buffer scenarios that ranged from a minimum one zone buffer to a large three zone buffer by a combination of practices 391 and 393. The 14 buffer scenarios (Table 2) ranged from a 4.6 m (15 ft) one zone buffer to a 51.8 m (17 ft) three zone buffer. The Table 2 first three buffers were zone 1 buffers of 4.6, 7.6, and 1.7 m (15, 25, 35 ft) consisting of mature hardwoods. When the one zone buffer reached the minimum size for a two zone buffer (1.7 m), the buffer was changed to a two- zone buffer of 1.7, 13.7, and 16.8 m (35, 45, and 55 ft) total with mature hardwoods in Zone 1 and mature pines in Zone 2. These were buffers 4-6 (Table 2). When the total buffer width reached 16.8 m (55ft) the buffer was changed to a three zone buffer with mature hardwoods in Zone 1, mature pines in Zone 2, and perennial grass in Zone 3. These were buffers 7-14 (Table 2). For the three zone buffers, the size of the total buffer was then increased in increments to a maximum of 51.8 m (17ft) with the proportion of Zone 2:Zone 1 ranging from 1.3 to 2.34 and the width of Zone 3 remaining a constant 6.1 m (2 ft) (Table 2). The final buffer of 51.8 m was similar to the actual Gibbs Farm buffer cited above. The source area to buffer area ratio ranged from 15.4 for the 4.6 m buffer to 1.3 for the 51.8 m buffer (Table 2). The details of parameterization of REMM for this site were described in Inamdar et al., 1999a and 1999b. To summarize, data collected from the Gibbs Farm site were used for model parameters whenever possible. When data from the site were not available we used data from 6

7 similar study sites nearby that had been part of the long-term research effort on riparian ecosystem buffers. When no local data were available, literature values were used. The zone specific parameters used for the testing of REMM were used for parameterization of the three zones in this study. Thus, parameters for zone 1 were used for any zone 1 whether it was at the edge of the field (scenarios 1-3), separated from the field by a zone 2, or separated from the field by a zone 3 plus zone 2. Although this did not represent changing hillslope conditions with changing buffer size, it allowed for more direct comparisons of the effects of the two loadings on buffers with different total sizes and different zone widths. The loading delivered to REMM was the daily volume of water and mass of sediment, nitrogen species and phosphorus species delivered in surface runoff and subsurface flow from the upland source area. For the simulations, each of the 14 buffers received a loading designated as either Normal or High (Table 3). The load numbers were based on the nature of the upland source. Normal Load (NL) was the same as the observed Gibbs Farm loading based on the Table 3 measured concentrations and volumes in surface and subsurface runoff entering the riparian buffer (Inamdar et al, 1999a, 1999b, Lowrance et al., 2a). Crops at the Gibbs Farm site were corn, corn, corn, peanuts, and peanuts for the 5 years of the study. The High Load (HL) was based on a nearby site that received liquid dairy manure from a center pivot spray application system (Vellidis et al, 1993, Lowrance et al, 1998). The annual subsurface N loading for the HL case was equal to the annual estimated loading for the dairy effluent site (Lowrance et al., 1998). The subsurface P loading and the surface runoff N and P loading for the HL case were higher than for the dairy manure application site and represented Aworst-case@ conditions for Coastal 7

8 Plain systems. The HL case was achieved by increasing all concentrations to either observed or higher than observed concentrations at the dairy effluent site and by increasing the subsurface flow into the buffer by 1%. The simulations presented here are one subset of the types of simulations possible with REMM. In the simulations reported here there were 14 buffer scenarios and two loading cases for a total of 28 simulations. Each simulation generated a very large number of output variables. In this paper, we report both the simulated loads for all scenarios and case combinations as well as the percent nutrient and sediment load reduction for each of the 28 combinations. Each load result is given as kg ha -1 yr -1 of the entire watershed (source area plus buffer) and can be compared directly to measured watershed outputs. It is important to note that the size of the overall watershed changes in these simulations: the source area remained constant at.293 ha and provided either a Normal Load or a High Load to 14 buffer scenarios. The buffers changed from 4.6 m in width to 51.8 m in width, consequently the total watershed area changed from.312 ha to.58 ha. The buffers ranged from 6.4 % of the total watershed to 39% of the total watershed. This use of REMM is different than simulating a particular hillslope so that when the size of the buffer is increased, the size of the field is decreased and the mass loadings from the field also decreased. Simulations of actual hillslopes with changing field sizes are also possible with REMM. Each loading reported and the corresponding load reduction was based on the annual mean of 5 year simulations ( ) using the same five year weather data set used in the testing reports (Inamdar et al, 1999a, 1999b, Lowrance et al, 2a). The total loads are used to 8

9 calculate a percent reduction as: ((Input-Output)/Input)*1. Results shown include the loads for all water and nutrient fluxes from all sources (surface runoff, subsurface flow and seepage) but only present the percentage reduction for total N, total P, and sediment. Results will be presented through a series of graphs showing the output of water, sediment, or nutrients from all 28 combinations of scenarios and loads. Each graph will have six different symbols representing the different combinations of NL and HL cases with one zone, two zone, or three zone buffers. Therefore in the graphs, buffers 1-3 (Table 2) were the 1 zone buffers, buffers 4-6 (Table 2) were the 2 zone buffers, and buffers 7-14 (Table 2) were the three zone buffers. Note that buffers number 3 and 4 and buffers number 6 and 7 overlap in width as the change was made from one zone to two zones and from two zones to three zones (Table 2 and all graphs). For discharge and sediment, nitrogen transport, and phosphorus transport, all results will be presented and discussed by: 1) comparing the effects of the change in buffer size/vegetation on the output and the partitioning among flow paths (water) and among flow paths and nutrient species (nutrients) and 2) comparing the effects of the change in load (Normal Vs High) on the output. Finally, simulation results of nutrient removal processes (denitrification and vegetation uptake) will be presented. Nutrient removal processes are reported as the area weighted average of all three zones for the buffer only ( kg ha -1 yr -1 of N or P ) RESULTS AND DISCUSSION Discharge and Sediment 9

10 Total discharge was affected by the change in buffer size (Figure 2a). As the buffer width increased from 4.6 to 1.7 m (15 to 35 ft) total discharge decreased, seepage decreased rapidly, and subsurface flow increased (Figures 2a-d). Seepage was very high from the 4.6m Figure 2 buffer. This was due to the large amount of subsurface flow that was moving into the small buffer from the upland. Because of the small volume of storage in the 4.6 m buffer there was a large amount of seepage produced that went directly to the stream. As buffer width increased: 1) seepage decreased because there was more volume of available subsurface storage in the soil profile of the wider buffers; 2) subsurface flow increased because more seepage water infiltrated and eventually moved laterally as subsurface flow; and 3) total discharge decreased because more of the infiltrated seepage was lost as evapotranspiration rather than as streamflow. Surface runoff was also greater for the smaller buffers because of the greater duration of saturated conditions in the smaller buffers. Total discharge and seepage changed dramatically with buffer width up to 7.6m. Beyond the 7.6 m buffer width had little effect on discharge and seepage. Subsurface flow showed a small general decrease and surface runoff a small general increase with increased buffer width. As buffer width increased, a larger proportion of the total watershed was in riparian zone and there was a larger proportion of the watershed area to generate direct surface runoff during storm events. Both the addition of pines to a hardwood buffer and of grass to a pine and hardwood buffer decreased the total water output from the riparian buffer (Figure 2a). The vegetation changes primarily affected surface runoff and subsurface flow with less effect on seepage. The changes in these flows were primarily due to higher rates of evapotranspiration for both pines 1

11 and grass compared to hardwoods. The sediment input to the buffer system measured at the Gibbs Farm Site was very low compared to other agricultural areas. Total load was less than 1 kg dry sediment ha -1 yr -1. Therefore the field was not providing a level of input that would stress the sediment load reduction capacity of the buffer. Even though the sediment load was low, sediment output and the percentage reduction in sediment load were affected by changes in buffer size (Figures 3a-b). Figure 3 Most of the sediment load reduction occurred by the time the buffer was 16.8 m wide with very little increased sediment load reduction for larger buffers. Either of the 16.8 m buffers removed 9% or more of the sediment input. The sediment output was affected slightly by vegetation changes from one zone to two zones and from two zones to three zones. The HL case had a predictable effect compared to NL with greater discharge occurring for all buffer scenarios under the high load conditions (Figures 2a-d). The total runoff (HL vs NL) tend to converge in the wider buffers (Figure 2a). The HL case had larger input from subsurface flow only but caused larger outputs of all flows - surface runoff, seepage, and subsurface flow. Once water enters the riparian buffer as modeled by REMM, water can exit by either subsurface flow or seepage and can influence the amount of direct surface runoff by affecting the water table conditions. Sediment transport increased slightly in the HL case due to the increased surface runoff that occurred. Therefore, a loading case with higher subsurface flow created higher surface runoff from the buffers and this response in turn created slightly higher sediment transport. These results, in particular, demonstrate the utility of REMM and the complex interactions that are portrayed in the model. 11

12 Nitrogen Nitrogen output ranged from over 1 kg N ha -1 yr -1 for the narrow buffer with HL to less than 5 N kg ha -1 yr -1 for the widest buffers. (Figure 4a). For narrower buffers (4.6 m to 3.5 m), Figure 4 most of the output was nitrate. For wider buffers, the output was more evenly divided between nitrate and ammonium (Figures 4a-c). The nitrate output is very high for the HL case and the narrowest buffer, with most of the nitrate output moving as seepage from the 4.6 m buffer (seepage nitrate data not shown but corresponds to seepage flow shown in Figure 2d). There are not major differences in N output due to changes in vegetation types among the buffers, although the two and three zone buffers showed slight decreases in load compared to the same width of one and two zone buffers, respectively. The percent reduction of total N ranged from around 5% for the 4.6m-HL case to over 95% for the widest buffer (Figure 4d). The HL case had higher per cent load reduction for the four widest buffers (36.6m to 51.8m) because the outputs of total N were similar to the NL cases and the inputs were higher, therefore the per cent reduction was greater. For the 4.6m - NL case the total N output was about evenly divided between surface runoff, subsurface flow and seepage (Figures 5 a-d). For the HL case, the increased amount of Figure 5 seepage caused the proportions to change to about 4% subsurface flow, 4% seepage, and 2% surface runoff (Figures 5a-d). Most of the subsurface output was nitrate for the narrower buffers (Figure 5c). For all buffers, most of the ammonium output was in direct surface runoff with very little in subsurface flow or seepage (data not shown). Conversely, for all buffers the nitrate output was roughly proportional to the relative amounts of seep and subsurface flow. Seep 12

13 nitrate output decreased from 5% of total nitrate output for the 4.6m-HL buffer to 19% for the 9.2 m-hl buffer. Thus the high loadings of nitrate from the narrowest buffer HL case was due primarily to large amounts of seepage nitrate. These results are consistent with observations that nitrate that moves in seeps across the surface of mature buffers is unlikely to be removed by surface processes (Hill, 1996). A relatively constant mass of the N output was organic N for all the buffer scenarios with output ranging from 1.93 kg N ha -1 yr -1 for the 55 m- NL buffer to 5.3 N kg ha -1 yr -1 for the 4.6m -HL buffer. For the 4.6m- HL case, organic N output was only 4.9% of total N output. For the 51.8 m buffer organic N was a much higher percentage of total N (63.6 % and 82.8% for HL and NL, respectively). The Little River Watershed, an area with large buffers, typically has 6-7% of total N as organic N (Lowrance et al., 1985). Thus the results obtained from REMM were consistent with behavior of the Coastal Plain watersheds with larger buffers. Phosphorus Total P output from the widest buffer scenarios was similar to loads from watershed studies in the region (Lowrance et al., 1985). The total P output ranged from.48 kg P ha -1 yr -1 for the 55m - NL buffer to 2.69 kg P ha -1 yr -1 for the 4.6m-HL buffer (Figure 6a). Because of the low sediment input, almost all of the P that moved through the buffer system was dissolved P (Figures 6a and b). This is also characteristic of these low-gradient watersheds with extensive riparian zones (Lowrance et al, 1988). The surface runoff dissolved P output was most of the P output for all cases (Figure 6d). The surface runoff dissolved P ranged from 45% of total P for the 55m -NL buffer to 76% for the 4.6m -HL buffer. 13

14 The percent reduction for total P ranged from 62% for the 4.6m-HL buffer to 9% for the 51.8m-HL buffer. As with N, the percent retention was higher for the wider buffers and the HL case because outputs were similar and inputs were higher. Denitrification and Vegetation Uptake Denitrification and vegetation uptake of N and P are mechanisms for nutrient removal in buffers simulated with REMM (Figures 7a-c). Denitrification rate showed a large response to changes in buffer size, especially for the HL case. The highest denitrification was simulated in Figure 7 Figure 7 the 1.7 to 16.8m two-zone buffers. These buffers had much of the water infiltrating and had very wet conditions for much of the year. The denitrification rate decreased with increasing buffer size in the three zone buffers. This response indicated that total denitrification from the system was at a maximum (probably limited by nitrate availability) and that the per ha rate declined as the size of the buffer increased. Both the NL and HL case showed an increase in denitrification rate in the first three buffers (slight for the NL case). This was related to the large amount of seepage produced by the two narrowest buffers (4.6 m and 7.6 m). Although these two narrow buffers remained wet, thus causing the seepage, they could not receive much of the N input and it was removed in seepage (Figure 5d). The 4.6m and 7.6 m buffers were not able to retain the incoming water and nitrogen and thus produced a large amount of seepage and seepage N movement. The decrease in seepage nitrate output from the 4.6 to 1.7 m buffer is mirrored by the increase in denitrification (Figures 5d and 7a). Nitrate that was not removed by seepage from the 1.7 m buffer went into the soil and was available for either denitrification or plant uptake. Denitrification showed a very large response to the HL case compared to the NL case 14

15 (Figure 7a). Except for the 4.6 m buffer, the HL denitrification was at least double the NL denitrification. The maximum denitrification of about 14 kg N ha -1 yr -1 was similar to rates measured at the dairy wetland riparian forest buffer restoration (Lowrance et al., 1995). The response to denitrification was consistent with observations that if more nitrogen is added to and retained in these high-water table, carbon rich soils that much of it will be denitrified. In contrast to the HL cases, denitrification rates for the NL cases varied only slightly for the range of buffers simulated. The denitrification rates for the NL cases were similar to those measured at the Gibbs Farm site (Ambus and Lowrance, 1991; Inamdar et al., 1999b). For the eight three-zone buffers, denitrification declined on a per ha basis as the size of the buffer increased (Figure 7a) but total denitrification losses of N from the buffer increased. Vegetation uptake of N and P was higher in the narrow buffers than in the wider buffers for both the HL and NL cases (Figures 7b and c). This is a direct effect of the vegetation type. The vegetation uptake of both N and P, while largely unaffected by the change in buffer size, was mainly affected by the change in buffer vegetation, as expected. Because of a number of parameters in REMM, primarily the maximum N and P concentration for growth, the uptake of both nutrients was higher for the one zone buffer (all hardwoods) than for the other buffers. Adding either pines or grass to the buffer decreased the per ha vegetation uptake compared to hardwoods. For both the HL and NL case, the narrow buffers had higher per ha nutrient uptake than the wide buffers. The vegetation uptake was largely affected by the type of vegetation in the buffer while denitrification was primarily affected by wetness and nitrogen availability. The N and P uptake response of the vegetation to the HL case was less than the 15

16 denitrification response with very little response in the P uptake (Figures 7b and c). This is generally an indication that the vegetation is not limited by P for the NL case and thus there was not a large response for the HL case. The exception was the HL case for the one zone buffers (4.6 m to 1.7 m). For these hardwood buffers, there was an increase in the N uptake rate as the buffer size increased. Because luxury uptake is possible in REMM, this response may have been due to luxury uptake of N by the woody hardwood vegetation or it may have been due to other soil/vegetation interactions. The lack of response of P uptake was in contrast to the increased P transport for the HL case. Phosphorus retention in riparian systems is generally not as efficient as N retention because of there is not a gaseous loss of P (as with N loss by denitrification). The REMM simulations showed that P removal was as efficient as N removal but that the P removal did not enhance vegetation uptake. Thus P removal was apparently due to soil reactions of P. Future simulations using REMM will examine details of the factors that control P retention in these systems. Use of REMM Simulations for Watershed Management Decision-Making These REMM simulations of mature Coastal Plain buffers provide a tool for judging the adequacy of nutrient load reductions for the types of buffers installed under the USDA Conservation Buffer Initiative. Depending on the level of load reduction desired and the level of load expected, one can recommend the proper size buffer. For instance, in watersheds with impaired water quality due to nutrients the watershed wide nutrient load reduction is typically in the 3-4% range (Chesapeake Bay Program, 1991; Gilliam et al, 1997) potentially necessitating field scale load reductions of 4-6%. Thus, for Coastal Plain environments, these load 16

17 reduction estimates indicate that the minimal buffer (4.6m) was not adequate for N load reduction for either the NL or HL case. This was partly due to the large amount of seepage generated in this buffer due to the consistently wet conditions. To achieve N load reduction of at least 5% for both cases required at least a 1.7m (35 ft) buffer (Figure 4d). This is the minimum width of a two- zone riparian forest buffer (Practice Code 391) typically required for USDA cost-share programs such as CRP when nutrient load reduction is needed. This buffer would also provide for adequate control of sediment and P for the given loadings. Therefore, the USDA minimum riparian forest buffer for controlling excess nutrients appeared to be adequate for mature buffers in the Southeastern Coastal Plain for relatively high N and P loading and low sediment loading. It should be noted that this conclusion is only valid for mature riparian forest buffer systems in the southeastern Coastal Plain under the stated load conditions (or similar or lower load conditions). Although results from field studies (Vellidis et al, 1993; Lowrance et al, 2b) showed that both restored riparian forest buffers and buffers with a clear-cut of a mature Zone 2 have nutrient removal capability, these simulations only apply to mature riparian forest buffers and to adjacent grass filter strips. As with any simulations of complex systems using a complex model, there were numerous factors affecting the outcome. For use of REMM in this particular set of simulations, the important task was to evaluate the types of buffers being installed under the USDA Conservation Buffer Initiative. In this case, we simulated mature buffers in Coastal Plain conditions because these are the conditions for which REMM has been tested. Once the model is tested for other regions, similar types of analyses can be done for these regions. Although REMM 17

18 can be used with re-established vegetation and does account for growth of vegetation, we recommend that the first applications of REMM in a region be to more stable buffers that have well-established woody or herbaceous vegetation. 18

19 LITERATURE CITED Ambus, P. and R. Lowrance Denitrification in two riparian soils. Soil Science Society of America Journal 55: Bosch, D.D., J.M. Sheridan, and R.R. Lowrance Hydraulic gradients and flow rates of a shallow coastal plain aquifer in a forested riparian buffer. Trans. ASAE 39(3): Chesapeake Bay Program Baywide nutrient reduction strategy -199 Progress Report. CBP, Report No. 2. Annapolis, MD. Gilliam, J.W., D.L. Osmond, and R.O. Evans Selected agricultural best management practices to control nitrogen in the Neuse River Basin. North Carlina Ag. Res. Serv. Tech. Bull. 311, North Carolina State University, NC. Hill, A.R Nitrate removal in stream riparian zones. Journal of Environmental Quality 25: Hubbard, R.K. and R. Lowrance Solute transport and filtering through a riparian forest. Transactions of the ASAE 39(2): Inamdar, S.P., J.M. Sheridan, R.G. Williams, D.D. Bosch, R.R. Lowrance, L.S. Altier, and D.L. Thomas. 1999a. Riparian Ecosystem Management Model (REMM): I. Testing of the Hydrologic Component for a Coastal Plain Riparian System. Transactions of the ASAE 42(6): Inamdar, S.P., R.R. Lowrance, L.S. Altier, R.G. Williams, and R.K. Hubbard. 1999b. Riparian Ecosystem Management Model (REMM): II. Testing of the Water Quality and Nutrient Cycling Component for a Coastal Plain Riparian System. Transactions of the ASAE 19

20 42(6): Lowrance, R. R., R.A. Leonard, L.E. Asmussen, and R.L.Todd Nutrient budgets for agricultural watersheds in the southeastern Coastal Plain. Ecology 66: Lowrance, R. and R.A. Leonard Streamflow nutrient dynamics in coastal plain watersheds. Journal of Environmental Quality 17: Lowrance, R., G. Vellidis, and R.K. Hubbard Denitrification in a restored riparian forest wetland. Journal of Environmental Quality 24: Lowrance, R., L.S. Altier, J.D. Newbold, R.R. Schnabel, P.M. Groffman, J.M. Denver, D.L. Correll, J.W. Gilliam, J.L. Robinson, R.B. Brinsfield, K.W. Staver, W. Lucas, and A.H. Todd Water quality functions of riparian forest buffers in the Chesapeake Bay Watershed. Environmental Management 21: Lowrance, R., Johnson, Jr, J.C. Newton, G.L. and Williams, R.G Denitrification from soils of a year-round forage production system fertilized with liquid dairy manure. Journal of Environmental Quality 27: Lowrance, R. L.S. Altier, R.G. Williams, S.P. Inamdar, D.D. Bosch, R.K.Hubbard, and D.L. Thomas. 2a. The Riparian Ecosystem Management Model. Journal of Soil and Water Conservation 55: Lowrance, R., R.K. Hubbard, and R.G. Williams. 2b. Effects of a managed three-zone riparian buffer system on shallow groundwater quality in the southeastern coastal plain. Journal of Soil and Water Conservation. 55: NRCS, Riparian Forest Buffer, 392. USDA-Natural Resources Conservation Service, 2

21 Watershed Science Institute, Seattle, Wash. Sheridan, J.M., R.R. Lowrance, and D. D. Bosch Management effects on runoff and sediment transport in riparian forest buffers. Transactions of the ASAE 42(1): Vellidis, G., R. Lowrance, M.C. Smith, and R.K. Hubbard Methods to assess the water quality impact of a restored riparian wetland. Journal of Soil and Water Conservation 48: Vellidis, G. R.K. Hubbard, J.G. Davis, R. Lowrance, R.G. Williams. J.C.Johnson, Jr., and G.L. Newton Nutrient concentrations in the soil solution and shallow groundwater of a liquid dairy manure land application site. Transactions of the ASAE 39(4) Welsch, D.J Riparian Forest Buffers. USDA-FS Pub. No. NA-PR USDA-FS, Radnor, PA. 21

22 LIST OF TABLES Table 1. Riparian buffer site characteristics derived from the Gibbs Farm data and used in the buffer scenarios for this study. Table 2. Riparian buffer zone width and vegetation combinations used in REMM simulation studies. Table 3. Normal (NL) and high (HL) load cases for the riparian buffers simulated in this study. 22

23 TABLE 1. Riparian buffer site characteristics derived from the Gibbs Farm data and used in the buffer scenarios for this study. Parameter Zone 1 Zone 2 Zone 3 Soil series Alapaha loamy sand Alapaha loamy sand Tifton loamy sand (loamy, siliceous, thermic Arenic Plinthic Paleaquult) (loamy, siliceous, thermic Arenic Plinthic Paleaquult) (fine-loamy siliceous, thermic Plinthic Kandiudult) Vegetation Mature Hardwoods Mature Pine Perennial Grass Surface Slope (%) Area (ha) Total Soil Thickness (m) Permeability (cm hr -1 )* 13, 11,.5 13, 11,.5 13, 11,.5 Porosity (cm cm -1 )*.35,.38,.4.35,.38,.4.35,.38,.4 Field Capacity (cm cm -1 ) Wilting Point (cm cm -1 ) Vegetation Leaf Area Index *for soil layers 1, 2, and 3, respectively 23

24 TABLE 2. Riparian buffer zone width and vegetation combinations used in REMM simulation studies. Buffer Number Total Buffer Width (m (ft)) Source:buffer Area ratio Zone Width (m (ft)) Zone 1 (hardwood) Zone 2 (pine) Zone 3 (grass) (15) (15) (25) (25) (35) (35) (35) (15) 6.1 (2) (45) (17.6) 8.4 (27.4) (55) (2.2) 1.6 (34.8) (55) (15) 6.1 (2) 6.1 (2) (65) (17.6) 8.4 (27.4) 6.1 (2) (8) (21.5) 11.7 (38.5) 6.1 (2) (1) (26.7) 16.2 (53.3) 6.1 (2) (12) (31.9) 2.8 (68.1) 6.1 (2) (14) (37.1) 25.3 (82.9) 6.1 (2) (16) (42.3) 29.8 (97.7) 6.1 (2) 24

25 (17) (45) 32. (15) 6.1 (2) 25

26 TABLE 3. Normal (NL) and High (HL) Riparian Buffer Load Scenarios. Annual Water or Nutrient Load Normal (NL) High (HL) Total field runoff (mm) Surface runoff (mm) Subsurface flow (mm) Total Nitrogen (kg/ha) Total Nitrate (kg/ha) Total Ammonium (kg/ha) Surface Nitrogen (kg/ha) Surface Nitrate (kg/ha) Subsurface Ammonium (kg/ha) Subsurface Nitrogen (kg/ha) Subsurface Nitrate (kg/ha) Subsurface Ammonium (kg/ha) Total Phosphorus (kg/ha) Total Dissolved Phosphorus (kg/ha) Surface Phosphorus (kg/ha) Surface Dissolved Phosphorus (kg/ha) Subsurface Dissolved Phosphorus (kg/ha)

27 LIST OF FIGURES Figure 1. Diagram of a three zone riparian ecosystem buffer. Figure 2. a) annual total runoff, b) annual surface runoff, c) annual subsurface runoff, and d) annual seepage runoff from the 14 buffer scenarios under HL and NL. Figure 3. a) annual sediment output, and b) annual percentage sediment load reduction for the 14 buffer scenarios under HL and NL. Figure 4. a) annual total N output, b) annual nitrate output, c) annual ammonium output, and d) annual percentage total N load reduction for the 14 buffer scenarios under HL and NL. Figure 5. a) annual surface total N output, b) annual subsurface total N output, c) annual subsurface nitrate output, and d) annual seepage total N output from the 14 buffer scenarios under HL and NL. Figure 6. a) annual total P output, b) annual dissolved P output, c) annual surface dissolved P output, and d) annual percentage total P load reduction for the 14 buffer scenarios under HL and NL. Figure 7. a) annual denitrification, b) annual N uptake by plants, and c) annual P uptake by plants from the 14 buffer scenarios under HL and NL. 27

28 Figure 1. Diagram of a three zone riparian ecosystem buffer. 28

29 4 a) Total Runoff 4 b) Surface Runoff Total Runoff (mm/yr) Subsurface Runoff (mm/yr) Buffer W idth (m) c) Subsurface Runoff Buffer W idth (m) Surface Runoff (mm/yr) Seep Runoff (mm/yr) Buffer W idth (m) d) Seep Runoff 2 3 Buffer W idth (m) Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 2. a) annual total runoff, b) annual surface runoff, c) annual subsurface runoff, and d) annual seepage runoff from the 14 buffer scenarios under HL and NL. 29

30 Sediment Out (kg/ha/yr) a) Sediment Out 2 3 Buffer Width (m) 4 5 Sediment Load Reduction (%) b) Sediment Load Reduction Buffer Width (m) 4 5 Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 3. a) annual sediment output, and b) annual percentage sediment load reduction for the 14 buffer scenarios under HL and NL. 3

31 Total N Out (kg/ha/yr) Total Ammonium Out (kg/ha/yr) a) Total N Out Buffer W idth (m) c) Total Ammonium Out Buffer W idth (m) Total Nitrate Out (kg/ha/yr) Total N Load Reduction (%) b) Total Nitrate Out Buffer W idth (m) d) Total N Load Reduction (%) Buffer W idth (m) 5 5 Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 4. a) annual total N output, b) annual nitrate output, c) annual ammonium output, and d) annual percentage total N load reduction for the 14 buffer scenarios under HL and NL. 31

32 Total Surface N Out (kg/ha/yr) Subsurface Nitrate Out (kg/ha/yr) a) Total Surface N Out Buffer Width (m) c) Subsurface Nitrate Out Buffer Width (m) Total Subsurface N Out (kg/ha/yr) Total Seep N Out (kg/ha/yr) b) Total Subsurface N Out Buffer W idth (m) d) Total Seep N Out Buffer W idth (m) Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 5. a) annual surface total N output, b) annual subsurface total N output, c) annual subsurface nitrate output, and d) annual seepage total N output from the 14 buffer scenarios under HL and NL. 32

33 Total P Out (kg/ha/yr) Surface Diss. P Out (kg/ha/yr) a) Total P Out Buffer W idth (m) c) Surface Dissolved P Out Buffer W idth (m) Total Dissolved P Out (kg/ha/yr) Total P Load Reduction (%) b) Total Dissolved P Out Buffer W idth (m) d) Total P Load Reduction Buffer W idth (m) Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 6. a) annual total P output, b) annual dissolved P output, c) annual surface dissolved P output, and d) annual percentage total P load reduction from the 14 buffer scenarios under HL and NL. 33

34 Denitrification (kg/ha/yr) a) Denitrification 2 3 Buffer Width (m) 4 5 Vegetation N Uptake (kg/ha/yr) b) Vegetation N Uptake Buffer Width (m) 4 5 Vegetation P Uptake (kg/ha/yr) c) Vegetation P Uptake 2 3 Buffer Width (m) 4 5 Normal - 1 Zone High - 1 Zone Normal - 2 Zone High - 2 Zone Normal - 3 Zone High - 3 Zone Figure 7. a) annual denitrification, b) annual N uptake by plants, and c) annual P uptake by plants from the 14 buffer scenarios under HL and NL. 34

35 35