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Spray-distribution pattern from a prototype sludge-application vehicle under varying stand conditions
Forest Ecology and Management, 29 (1989) 213-219
213
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Short C o m m u n i c a t i o n
Spray-Distribution Pattern from a Prototype Sludge-Application Vehicle Under Varying Stand Conditions GEORGE MCFADDEN and P E T E R SCHIESS
College of Forest Resources, University of Washington, Seattle, Washington 98195 (U.S.A.) (Accepted 15 November 1988)
ABSTRACT McFadden, G. and Schiess, P., 1989. Spray-distribution pattern from a prototype sludge-application vehicle under varying stand conditions. For. Ecol. Manage., 29: 213-219. The spray-distribution pattern from a prototype sludge-application vehicle was not differentially affected by varying stand conditions during spray tests in stands of Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco). The test-stand stockings ranged from 345 to 885 trees ha-1. An optimal trail spacing of 65 m was established for the prototype vehicle. The spacing was determined by the throw distance from the vehicle and the amount of spray overlap required from adjacent trails to produce a uniform application. The tests indicated that a uniform sludge application can be achieved in a forest environment with approximately 6% of the application area consisting of access trails.
INTRODUCTION
Increasing environmental concern over the management of municipal sewage sludge has triggered research into alternative methods of waste disposal or renovation. Since 1973, the Municipality of Metropolitan Seattle (METRO) has been involved in an experimental program to study forest renovation of the sludge produced during sewage treatment. Growth response following sludge application on experimental plots has been favorable, with basal-area increases ranging up to 60% in Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) stands growing on a low-quality site (Site IV; Cole, 1982 ). Sludge application is accomplished by the use of prototype equipment that is capable of applying sludge in a forest environment. A mixing station receives the sludge from the treatment plant at approximately 22% solids, and rewaters the sludge until it contains approximately 12% solids. The rewatered sludge is loaded directly adjacent to the mix site into the application vehicle which, 0378-1127/89/$03.50
© 1989 Elsevier Science Publishers B.V.
214
GEORGEMCFADDENANDPETERSCHIESS
TABLE 1 Specifications of the rubber-tired, articulated, 4-wheel-drive sludge-application vehicle1 used in the Pack Forest spray tests Chassis: Width Wheelbase Total length Height to nozzle Gross vehicle weight
Ag-Chem Gator, Model 2004 w/modifications 3m 5m 8m 5m 22 750 kg
Spray system: Design distance
Stang 101516 monitor with Stand nozzles 50 m at 100 p.s.i, with a 2.5-cm nozzle
being designed for off-road travel, is the only piece of equipment to work directly in the forest environment. The application vehicle has a rubber-tired, articulated, 4-wheel-drive chassis with an 8300-1 tank mounted over the rear axle (Table 1 ). Sludge application is achieved through a top-mounted, directional-control spray nozzle. The spray trajectory is controlled from the cab of the vehicle. Application-vehicle access into the forest is obtained through a network of preconstructed 4-m-wide trails. The distance between the trails is based upon the distribution pattern of spray from the application vehicle. The trail spacing is as wide as possible to reduce vehicle-induced site disturbance while still providing for uniform spray distribution in the application area. A reasonable goal during timber harvest is to confine access trails to less than 15% of the area (Garland, 1983). Sludge access trail systems should also confirm to this goal if forest managers are to include sludge application in their management plans. The effect, if any, of varying stand conditions on the spray-distribution pattern from the application vehicle during under-the-canopy applications where the spray trajectory remains below the base of the live crown will determine the operational compatability of sludge application and other forest-management activities. METHODS
Site location Three young-growth stands of Douglas fir in Pack Forest, located near the town of Eatonville, Washington, 110 km south-southeast of Seattle, were used in the study. Stocking in the stands ranged from 345 to 885 trees ha-1 (Table 1The use of brand names does not constitute an endorsement by the Washington State Department of Ecology, the Municipality of Metropolitan Seattle, the University of Washington, or the authors. They are used only for the ease of description and to increase the readers' understanding.
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215
TABLE 2 S t a n d characteristics of the test areas in the Pack Forest spray tests
Trees h a - 1 (no.) M e a n diameter (cm) Relative density Basal area (m 2 h a - i ) Slope ( % ) Spray orientation
Stand I
Stand 2
Stand 3
885 25.9 9.16 49.1 0 --
600 27.9 6.94 35.1 6 Uphill
345 36.8 6.04 39.9 12 Uphill
2), giving relative densities (Curtis, 1982) of 6.04, 6.94 and 9.16 based upon mean diameters ranging from 25.9 to 36.8 cm. The stands had a uniform overstory with no understory present. Ground cover consisted of low shrubs and forbs.
Measurementprocedure The measurement procedures were based upon those developed by Christiansen (1942) to test the spray-distribution pattern of sprinkler-irrigation systems. Spray collectors, which consisted of 15-cm-tall, 75-cm-diameter plastic-lined fiberglass rings, were placed in a systematic pattern in each stand. The 59.4 X 49.0-cm test area consisted of a 59.4 X 33.0-m measurement plot situated between 8.0-m buffer strips with the short axis parallel to the application trail. The measurement plot was divided in 45 subplots, each 6.6 X 6.6m square. The subplots were arranged in a 9 X 5 grid, with a row of 5 subplots fronting on the application trail. Spray collectors were placed in the center of each subplot. Each collector sampled 1.2% of the area contained in the subplot. Spray accumulation was determined by weighing the collectors before and after each application.
Application procedure To provide for replicate applications, water was used during the tests instead of sludge. The spray characteristics of water are similar to those of rewatered sludge (Anonymous, 1982), but were not quantified in the study. Three replicate tests were conducted on clear, calm days in each stand. Each test consisted of five loads of water from the application vehicle. The operator was instructed to apply the water to the entire 59.4 X 49.0-m test area. except for a 3-m buffer strip adjacent to the trail in accordance with standard sludgeapplication procedure. The buffer is designed to prevent sludge flow into the trail, and as a result, no data was recorded from the first row of collectors. The mechanical efficiency of the application vehicle was determined on the day of each test by measuring the unobstructed throw-distance; defined as the
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GEORGE MCFADDEN AND PETER SCHIESS
maximum spray distance the vehicle can achieve, the unobstructed throw-distance was calculated by positibning the application vehicle at a road junction or in an open field near the test area. The maximum throw-distance from the vehicle in a minimum of three directions was then recorded and averaged. Stand throw-distance was determined by recording the distance from the trail to the last row of collectors to accumulate any water. Slope distance was used in determining stand throw-distance, a correction factor of 1 m of additional throw distance for each 2 m of vertical drop was used to calculate slopecorrected throw-distance.
Uniformity coefficients Christensen (1942) developed the coefficient of uniformity, C,, defined by the equation:~. Cu = 10011.0- (Xd/mn)]
(1)
where d is the absolute value of the deviation of the individual observations from the mean value m, and n is the number of equally spaced observations. A statistical uniformity coefficient, C~u,proposed by Wilcox and Swailes (1947), is defined by the equation: C~u = 1 0 0 [ 1 . 0 - (a/m)]
(2)
where m is the mean of a series of equally spaced observations and a is the standard deviation of mean. Christiansen (1942) anticipated that uniformity coefficients, r~, approaching 100% were possible under ideal conditions and throw-distances less than 15 m. Longer throw-distances reduce the uniformity of application, and field values near 70% are not unusual. Wilcox and McDougald (1955) considered statistical uniformity coefficients, Csu, in the range of 75-80% as reasonable uniformity values under field conditions. RESULTS AND DISCUSSION
Throw-distance Unobstructed throw-distance varied between 35.4 and 49.5 m, (Table 3) with the minimum throw-distance occurring just prior to scheduled maintenance being performed on the application-vehicle pumping system. The maximum throw-distance was recorded following the scheduled maintenance. Water accumulated in collectors 36.1 m from the trail in stands 1 and 3, and 42.6 m from the trail in stand 2. Slope-corrected throw-distances in each test area were within 6.6 m of the corresponding unobstructed throw-distance. A supplemental throw-distance test, conducted in stand I because of the short throw distance observed prior to vehicle maintenance, gave an unobstructed throw distance of 45.9 m. The stand throw distance was 44.6 m. Throw-dis-
217
SPRAY-DISTRIBUTION PATTERN FROM A PROTOTYPE SLUDGE-APPLICATION
TABLE 3 Throw-distances (m) achieved during the Pack Forest spray tests Site
Stand 1 Stand 2 Stand 3
Throw-distance Unobstructed
Stand
Slope-corrected’
35.4 49.5 41.3
36.1 42.6 36.1
36.1 44.3 38.4
‘Assumes an additional l-m throw-distance for each 2 m of vertical drop.
tance in the stands tested was not affected over the range of conditions tested as long as the spray trajectory remained below the base of the live crown. Field observations revealed that the spray stream lost cohesion and momentum upon contacting the live crown. Insufficient clearance below the live crown decreased throw-distance. In the Pack Forest spray tests, a clearance of 17 m at the apex of the spray trajectory was required to achieve the maximum throwdistance. Application uniformity and trail spacing The uniformity coefficients, C,, ranged from 49.4 to 60.4%, while the statistical uniformity coefficients, C,,, ranged from 40.4 to 51.8% (Table 4). There were no significant differences in spray uniformity among the stands. The spray-distribution pattern consisted of a primary zone adjacent to the trail and a secondary zone further away from the trail. The boundary between the primary and secondary zones was approximately 2/3 of the distance from the trail to maximum stand throw-distance. On average, spray accumulation was three times greater in the primary zone than in the secondary zone. There was no significant relationship between accumulation and distance from the trail in the primary zone, while in the secondary zone spray accumulation decreased as distance from the trail increased. The uniformity values in the spray tests were lower than the values associated with sprinkler applications under field conditions. The sprinkler values are based upon applications with overlap in the coverage area between adjacent sprinklers. The spray tests conducted at Pack Forest did not include overlap from adjacent application trails. The presence of a secondary application zone, where accumulations decrease as distance from the trail increases, indicated that higher uniformity values would be achieved with secondary-zone overlap. The data from the Pack Forest spray tests were overlapped mathematically to simulate an application to the test area from adjacent trails. Uniformity coefficients, C,, ranged from 66.7 to 70.9% and statistical uniformity coefficients, C,,, from 73.4 to 76.8% (Table 5) with an assumed trail-spacing of 65
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GEORGE MCFADDEN AND PETER SCHIESS
TABLE 4 Summary of results of the Pack Forest spray tests Stand 1
Stand 2
Stand 3
Total accumulation (kg) Mean accumulation (kg) Standard deviation (kg)
525.0 7.0 4.2
583.3 6.3 3.5
522.9 7.0 3.4
Uniformity coefficients: C” (%) C8”
49.4 40.4
53.2 44.1
60.4 51.8
TABLE 5 Summary results of the Pack Forest spray tests with secondary-zone overlap based upon a 65-m trail spacing
Total accumulation (kg) Mean accumulation (kg) Standard deviation (kg) Uniformity coefficients: C” (So) C,, (%)
Stand 1
Stand 2
Stand 3
1050.0 8.8 2.9
1166.6 9.0 2.6
1045.8 8.8 2.7
73.4 66.7
76.8 70.9
75.1 68.7
m. Lower uniformity values were produced for trail-spacings both wider and narrower than 65 m. The ability to produce a uniform application in a forest environment using a 65-m trail-spacing results in approximately 6% of the application area being occupied by access trials. The area occupied by sludge access trails is less than the maximum of 15% set for harvest access, and indicates that sludge application can be incorporated into forest management plans without the requirement of additional access trails.
ACKNOWLEDGEMENT
The authors express their gratitude to the Washington State Department of Ecology and the Municipality of Metropolitan Seattle for the funding provided to conduct this research.
SPRAY-DISTRIBUTION P A T T E R N F R O M A P R O T O T Y P E SLUDGE-APPLICATION
219
REFERENCES
Anonymous, 1982. Design specifications for Silvigrow application vehicle. Municipality of Metropolitan Seattle. Cole, D.W., 1982. Response of forest ecosystems to sludge and wastewater application - A case study in western Washington. In: Land Reclamation and Biomass Production with Municipal Wastewater and Sludge. Penn State University Press, 524 pp. Christiansen, J.E., 1942. Irrigation by Sprinkling. Univ. of Calif. Agric. Exp. Stn. Bull. 670, 124 pp. Curtis, R.O., 1982. A simple index of stand density for Douglas-fir. For. Sci., 28: 92-94. Garland, J., 1983. Designated skid trails minimize soil compaction. Oregon State Univ. Serv. Circ. 1110. Wilcox, J.E. and Swailes, G.E., 1947. Uniformity of water distribution of some under tree orchard sprinklers. Sci. Agric., 27: 565-583. Wilcox, J.E. and McDougald, J.M., 1955. Water distribution patterns from rotary sprinklers. Sci. Agric., 35: 217-228.
213
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Short C o m m u n i c a t i o n
Spray-Distribution Pattern from a Prototype Sludge-Application Vehicle Under Varying Stand Conditions GEORGE MCFADDEN and P E T E R SCHIESS
College of Forest Resources, University of Washington, Seattle, Washington 98195 (U.S.A.) (Accepted 15 November 1988)
ABSTRACT McFadden, G. and Schiess, P., 1989. Spray-distribution pattern from a prototype sludge-application vehicle under varying stand conditions. For. Ecol. Manage., 29: 213-219. The spray-distribution pattern from a prototype sludge-application vehicle was not differentially affected by varying stand conditions during spray tests in stands of Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco). The test-stand stockings ranged from 345 to 885 trees ha-1. An optimal trail spacing of 65 m was established for the prototype vehicle. The spacing was determined by the throw distance from the vehicle and the amount of spray overlap required from adjacent trails to produce a uniform application. The tests indicated that a uniform sludge application can be achieved in a forest environment with approximately 6% of the application area consisting of access trails.
INTRODUCTION
Increasing environmental concern over the management of municipal sewage sludge has triggered research into alternative methods of waste disposal or renovation. Since 1973, the Municipality of Metropolitan Seattle (METRO) has been involved in an experimental program to study forest renovation of the sludge produced during sewage treatment. Growth response following sludge application on experimental plots has been favorable, with basal-area increases ranging up to 60% in Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) stands growing on a low-quality site (Site IV; Cole, 1982 ). Sludge application is accomplished by the use of prototype equipment that is capable of applying sludge in a forest environment. A mixing station receives the sludge from the treatment plant at approximately 22% solids, and rewaters the sludge until it contains approximately 12% solids. The rewatered sludge is loaded directly adjacent to the mix site into the application vehicle which, 0378-1127/89/$03.50
© 1989 Elsevier Science Publishers B.V.
214
GEORGEMCFADDENANDPETERSCHIESS
TABLE 1 Specifications of the rubber-tired, articulated, 4-wheel-drive sludge-application vehicle1 used in the Pack Forest spray tests Chassis: Width Wheelbase Total length Height to nozzle Gross vehicle weight
Ag-Chem Gator, Model 2004 w/modifications 3m 5m 8m 5m 22 750 kg
Spray system: Design distance
Stang 101516 monitor with Stand nozzles 50 m at 100 p.s.i, with a 2.5-cm nozzle
being designed for off-road travel, is the only piece of equipment to work directly in the forest environment. The application vehicle has a rubber-tired, articulated, 4-wheel-drive chassis with an 8300-1 tank mounted over the rear axle (Table 1 ). Sludge application is achieved through a top-mounted, directional-control spray nozzle. The spray trajectory is controlled from the cab of the vehicle. Application-vehicle access into the forest is obtained through a network of preconstructed 4-m-wide trails. The distance between the trails is based upon the distribution pattern of spray from the application vehicle. The trail spacing is as wide as possible to reduce vehicle-induced site disturbance while still providing for uniform spray distribution in the application area. A reasonable goal during timber harvest is to confine access trails to less than 15% of the area (Garland, 1983). Sludge access trail systems should also confirm to this goal if forest managers are to include sludge application in their management plans. The effect, if any, of varying stand conditions on the spray-distribution pattern from the application vehicle during under-the-canopy applications where the spray trajectory remains below the base of the live crown will determine the operational compatability of sludge application and other forest-management activities. METHODS
Site location Three young-growth stands of Douglas fir in Pack Forest, located near the town of Eatonville, Washington, 110 km south-southeast of Seattle, were used in the study. Stocking in the stands ranged from 345 to 885 trees ha-1 (Table 1The use of brand names does not constitute an endorsement by the Washington State Department of Ecology, the Municipality of Metropolitan Seattle, the University of Washington, or the authors. They are used only for the ease of description and to increase the readers' understanding.
SPRAY-DISTRIBUTION PATTERN FROM A PROTOTYPE SLUDGE-APPLICATION
215
TABLE 2 S t a n d characteristics of the test areas in the Pack Forest spray tests
Trees h a - 1 (no.) M e a n diameter (cm) Relative density Basal area (m 2 h a - i ) Slope ( % ) Spray orientation
Stand I
Stand 2
Stand 3
885 25.9 9.16 49.1 0 --
600 27.9 6.94 35.1 6 Uphill
345 36.8 6.04 39.9 12 Uphill
2), giving relative densities (Curtis, 1982) of 6.04, 6.94 and 9.16 based upon mean diameters ranging from 25.9 to 36.8 cm. The stands had a uniform overstory with no understory present. Ground cover consisted of low shrubs and forbs.
Measurementprocedure The measurement procedures were based upon those developed by Christiansen (1942) to test the spray-distribution pattern of sprinkler-irrigation systems. Spray collectors, which consisted of 15-cm-tall, 75-cm-diameter plastic-lined fiberglass rings, were placed in a systematic pattern in each stand. The 59.4 X 49.0-cm test area consisted of a 59.4 X 33.0-m measurement plot situated between 8.0-m buffer strips with the short axis parallel to the application trail. The measurement plot was divided in 45 subplots, each 6.6 X 6.6m square. The subplots were arranged in a 9 X 5 grid, with a row of 5 subplots fronting on the application trail. Spray collectors were placed in the center of each subplot. Each collector sampled 1.2% of the area contained in the subplot. Spray accumulation was determined by weighing the collectors before and after each application.
Application procedure To provide for replicate applications, water was used during the tests instead of sludge. The spray characteristics of water are similar to those of rewatered sludge (Anonymous, 1982), but were not quantified in the study. Three replicate tests were conducted on clear, calm days in each stand. Each test consisted of five loads of water from the application vehicle. The operator was instructed to apply the water to the entire 59.4 X 49.0-m test area. except for a 3-m buffer strip adjacent to the trail in accordance with standard sludgeapplication procedure. The buffer is designed to prevent sludge flow into the trail, and as a result, no data was recorded from the first row of collectors. The mechanical efficiency of the application vehicle was determined on the day of each test by measuring the unobstructed throw-distance; defined as the
216
GEORGE MCFADDEN AND PETER SCHIESS
maximum spray distance the vehicle can achieve, the unobstructed throw-distance was calculated by positibning the application vehicle at a road junction or in an open field near the test area. The maximum throw-distance from the vehicle in a minimum of three directions was then recorded and averaged. Stand throw-distance was determined by recording the distance from the trail to the last row of collectors to accumulate any water. Slope distance was used in determining stand throw-distance, a correction factor of 1 m of additional throw distance for each 2 m of vertical drop was used to calculate slopecorrected throw-distance.
Uniformity coefficients Christensen (1942) developed the coefficient of uniformity, C,, defined by the equation:~. Cu = 10011.0- (Xd/mn)]
(1)
where d is the absolute value of the deviation of the individual observations from the mean value m, and n is the number of equally spaced observations. A statistical uniformity coefficient, C~u,proposed by Wilcox and Swailes (1947), is defined by the equation: C~u = 1 0 0 [ 1 . 0 - (a/m)]
(2)
where m is the mean of a series of equally spaced observations and a is the standard deviation of mean. Christiansen (1942) anticipated that uniformity coefficients, r~, approaching 100% were possible under ideal conditions and throw-distances less than 15 m. Longer throw-distances reduce the uniformity of application, and field values near 70% are not unusual. Wilcox and McDougald (1955) considered statistical uniformity coefficients, Csu, in the range of 75-80% as reasonable uniformity values under field conditions. RESULTS AND DISCUSSION
Throw-distance Unobstructed throw-distance varied between 35.4 and 49.5 m, (Table 3) with the minimum throw-distance occurring just prior to scheduled maintenance being performed on the application-vehicle pumping system. The maximum throw-distance was recorded following the scheduled maintenance. Water accumulated in collectors 36.1 m from the trail in stands 1 and 3, and 42.6 m from the trail in stand 2. Slope-corrected throw-distances in each test area were within 6.6 m of the corresponding unobstructed throw-distance. A supplemental throw-distance test, conducted in stand I because of the short throw distance observed prior to vehicle maintenance, gave an unobstructed throw distance of 45.9 m. The stand throw distance was 44.6 m. Throw-dis-
217
SPRAY-DISTRIBUTION PATTERN FROM A PROTOTYPE SLUDGE-APPLICATION
TABLE 3 Throw-distances (m) achieved during the Pack Forest spray tests Site
Stand 1 Stand 2 Stand 3
Throw-distance Unobstructed
Stand
Slope-corrected’
35.4 49.5 41.3
36.1 42.6 36.1
36.1 44.3 38.4
‘Assumes an additional l-m throw-distance for each 2 m of vertical drop.
tance in the stands tested was not affected over the range of conditions tested as long as the spray trajectory remained below the base of the live crown. Field observations revealed that the spray stream lost cohesion and momentum upon contacting the live crown. Insufficient clearance below the live crown decreased throw-distance. In the Pack Forest spray tests, a clearance of 17 m at the apex of the spray trajectory was required to achieve the maximum throwdistance. Application uniformity and trail spacing The uniformity coefficients, C,, ranged from 49.4 to 60.4%, while the statistical uniformity coefficients, C,,, ranged from 40.4 to 51.8% (Table 4). There were no significant differences in spray uniformity among the stands. The spray-distribution pattern consisted of a primary zone adjacent to the trail and a secondary zone further away from the trail. The boundary between the primary and secondary zones was approximately 2/3 of the distance from the trail to maximum stand throw-distance. On average, spray accumulation was three times greater in the primary zone than in the secondary zone. There was no significant relationship between accumulation and distance from the trail in the primary zone, while in the secondary zone spray accumulation decreased as distance from the trail increased. The uniformity values in the spray tests were lower than the values associated with sprinkler applications under field conditions. The sprinkler values are based upon applications with overlap in the coverage area between adjacent sprinklers. The spray tests conducted at Pack Forest did not include overlap from adjacent application trails. The presence of a secondary application zone, where accumulations decrease as distance from the trail increases, indicated that higher uniformity values would be achieved with secondary-zone overlap. The data from the Pack Forest spray tests were overlapped mathematically to simulate an application to the test area from adjacent trails. Uniformity coefficients, C,, ranged from 66.7 to 70.9% and statistical uniformity coefficients, C,,, from 73.4 to 76.8% (Table 5) with an assumed trail-spacing of 65
218
GEORGE MCFADDEN AND PETER SCHIESS
TABLE 4 Summary of results of the Pack Forest spray tests Stand 1
Stand 2
Stand 3
Total accumulation (kg) Mean accumulation (kg) Standard deviation (kg)
525.0 7.0 4.2
583.3 6.3 3.5
522.9 7.0 3.4
Uniformity coefficients: C” (%) C8”
49.4 40.4
53.2 44.1
60.4 51.8
TABLE 5 Summary results of the Pack Forest spray tests with secondary-zone overlap based upon a 65-m trail spacing
Total accumulation (kg) Mean accumulation (kg) Standard deviation (kg) Uniformity coefficients: C” (So) C,, (%)
Stand 1
Stand 2
Stand 3
1050.0 8.8 2.9
1166.6 9.0 2.6
1045.8 8.8 2.7
73.4 66.7
76.8 70.9
75.1 68.7
m. Lower uniformity values were produced for trail-spacings both wider and narrower than 65 m. The ability to produce a uniform application in a forest environment using a 65-m trail-spacing results in approximately 6% of the application area being occupied by access trials. The area occupied by sludge access trails is less than the maximum of 15% set for harvest access, and indicates that sludge application can be incorporated into forest management plans without the requirement of additional access trails.
ACKNOWLEDGEMENT
The authors express their gratitude to the Washington State Department of Ecology and the Municipality of Metropolitan Seattle for the funding provided to conduct this research.
SPRAY-DISTRIBUTION P A T T E R N F R O M A P R O T O T Y P E SLUDGE-APPLICATION
219
REFERENCES
Anonymous, 1982. Design specifications for Silvigrow application vehicle. Municipality of Metropolitan Seattle. Cole, D.W., 1982. Response of forest ecosystems to sludge and wastewater application - A case study in western Washington. In: Land Reclamation and Biomass Production with Municipal Wastewater and Sludge. Penn State University Press, 524 pp. Christiansen, J.E., 1942. Irrigation by Sprinkling. Univ. of Calif. Agric. Exp. Stn. Bull. 670, 124 pp. Curtis, R.O., 1982. A simple index of stand density for Douglas-fir. For. Sci., 28: 92-94. Garland, J., 1983. Designated skid trails minimize soil compaction. Oregon State Univ. Serv. Circ. 1110. Wilcox, J.E. and Swailes, G.E., 1947. Uniformity of water distribution of some under tree orchard sprinklers. Sci. Agric., 27: 565-583. Wilcox, J.E. and McDougald, J.M., 1955. Water distribution patterns from rotary sprinklers. Sci. Agric., 35: 217-228.