Snowmelt runoff, sediment, and phosphorus losses
under three different tillage systems
N.C. Hansen
*, S.C. Gupta, J.F. Moncrief
Department of Soil Water and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, USA
Received 29 February 2000; received in revised form 25 July 2000; accepted 31 July 2000
Abstract
In cold climates, snowmelt runoff often exceeds rainfall runoff during the year. Conservation tillage practices may be effective in reducing runoff during the cropping season but not during the snowmelt period. A plot study was conducted on a cropped hillslope to assess how tillage practices affect snowmelt runoff and the associated losses of sediment, phosphorus (P), and chemical oxygen demand (COD). Tillage systems were fall moldboard and chisel plowing with spring disking, and a ridge till system utilizing only the tillage associated with summer row cultivation. Tillage and planting were done up and down the slope. Ridge tilled plots had higher fall residue cover, retained more snow, had less surface roughness, and consequently produced more runoff than the moldboard plow treatment. The amount of runoff from chisel plowed plots was similar to runoff from ridge tilled plots despite a relatively rough surface and moderate amount of residue cover. Phosphorus losses in runoff were higher for the ridge till and chisel plow systems than for the moldboard plow system. For all tillage systems, soluble P represented a major portion (75%) of the total P loss in snowmelt runoff. Although erosive losses in snowmelt were low, the P losses were substantial and merit consideration in studies evaluating management systems impact on surface water quality in regions where snowmelt runoff is important.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Surface runoff; Phosphorus losses; Snowmelt
1. Introduction
Excessive nutrient inputs to surface waters degrade water quality through the process of accelerated eutro-phication. In many watersheds, phosphorus (P) from agricultural sources is an important pollutant (Sharp-ley et al., 1994; Lee et al., 1978). Conservation tillage is a management practice that is recommended for reducing runoff and diffuse source pollution. This practice is generally effective at reducing erosion and runoff for the critical period between planting
and canopy development. However, in cold climate regions a signi®cant fraction of annual runoff occurs in spring as a result of melting snow. Less is known about how different tillage practices impact runoff and P losses during this time. Snowmelt runoff is not as erosive as runoff caused by rain, but loss of water-soluble contaminants can be substantial (Ginting et al., 1998).
Concentration and loss of water soluble contami-nants from reduced tillage systems are often higher than from conventional tillage systems for rainfall induced runoff (McDowell and McGregor, 1984; Romkens et al., 1973; Sharpley et al., 1995). Reasons for increased soluble nutrient loss from reduced tillage systems include: (1) accumulation of nutrients at or
*Corresponding author. Tel.:1-320-589-1711;
fax:1-320-389-4870.
E-mail address: hansennc@mrs.umn.edu (N.C. Hansen).
near the soil surface due to reduced mixing of applied fertilizers or manure (Eckert and Johnson, 1985; Weil et al., 1988; Baker and La¯en, 1982), and (2) leaching of nutrients from crop residues at the soil surface (Schreiber and McDowell, 1985; Ginting et al., 1998). The importance of these factors during snow-melt runoff is not known.
The objective of this study was to evaluate the impacts of different tillage practices on runoff, sedi-ment, and P losses for the period between primary tillage in the fall and secondary tillage in the spring. Tillage treatments for this study were fall moldboard and chisel plow based systems and a ridge till system with no fall tillage.
2. Materials and methods
2.1. Runoff plot set-up
Tillage treatments for this study were implemented in the fall of 1995. The results reported in this paper are for springs of 1996 and 1997. Rectangular runoff
plots (3 m22 m) were established on a Clarion silt
loam (Typic Hapludoll, ®ne-loamy, mixed, mesic) with an 8±10% slope. The site is located in Scott County, Minnesota, and is part of the lower Minnesota River Basin. Prior to the study, the site was in a ridge
till based corn [Zea mays(L.)]-soybean [Glycine max
(L.) Merr.] rotation for 10 years. The treatments were replicated four times in an experiment having a ran-domized complete block design. Treatments were fall moldboard plow, fall chisel plow, and ridge till (no fall tillage) with all tillage done parallel to the slope. Ridge till plots were established using existing ridges. Each plot was isolated around the perimeter with vertical metal borders driven 10 cm into the soil to prevent leakage. Galvanized metal runoff collectors at the bottom of the plots channeled ¯ow to a 10.2 cm diameter polyvinyl chloride (PVC) pipe and to a tipping bucket ¯ow gauge. A data logger recorded the number and rate of tips with the use of a magnetic closure switch on each tipping bucket. Volume of runoff was calculated by the use of a calibration equation relating ¯ow rate to the rate of tipping. A composite runoff sample for each plot and each runoff event was collected by mechanically capturing a fraction of the water from each tip. Rain amounts
were measured with an electronic, tipping bucket-type rain gauge as well as with a manual rain gauge. Snow depth and density measurements were made prior to anticipated snowmelt events to determine the amount of water available for runoff. Snow depth was mea-sured with a meter stick at 20 random locations per plot. Snow density was measured by inserting a metal ring (30.5 cm diameter) through the snow pro®le, measuring the depth of snow inside the ring to calcu-late the snow volume, collecting all of the snow within the ring, determining the mass of collected snow gravimetrically, and then dividing the mass by the volume. Average snow depth and density were then used to calculate an equivalent depth of water stored on each plot.
2.2. Cultural practices
Corn stalks were chopped after corn harvest each year for all tillage systems. Moldboard plowing was done on November 7, 1995, and October 24, 1996. Chisel plowing was done on November 8, 1995, and November 20, 1996. During the growing seasons, the plots were cropped to corn. Ridges were re-built with cultivation on June 26, 1995, and June 27, 1996, with a Hiniker Series I Econ-O-Till row cultivator (Hiniker,
Mankato, MN1). On the same days, the moldboard and
chisel plowed plots were row cultivated with the Hiniker row cultivator with the ridging shares in the up position leaving only 56 cm wide sweeps running at 5 cm deep. There was no application of phosphorus fertilizer during the study.
Extractable soil P (Bray and Kurtz, 1945) and organic matter content (Nelson and Sommers, 1982) were determined on 10 core composite samples taken at 0±7.5 cm and 7.5±15 cm depths from each plot on June 6, 1996, and May 22, 1997. The line transect method (La¯en et al., 1981) was used to determine percent residue cover. Cover measurements were made each spring following snowmelt in order to characterize the cover present at the time of runoff.
Surface roughness was measured in 1.0 m2areas for
each tillage system by measuring the soil surface elevation on a 10 cm grid with an electronic laser
1The company names are provided for the benefit of the readers
survey tool (TOPCON GTS-303D, TOPCON
Mid-west, Arlington Heights, IL1). The data was
interpo-lated to a grid format by kriging with the software
Surfer (version 6.04, Golden Software, Golden, CO1).
Elevation data was corrected for slope with a least squares linear regression between grid elevation and the distance along linear cross-sections parallel to the slope. Roughness was calculated as the standard deviation of the residuals between the grid elevation data and the corresponding regression line and
aver-aged over six cross-sections for each 1 m2area.
2.3. Water quality measurements
Composite runoff water samples were stored at 48C until analyzed for concentration of sediment, total phosphorus, soluble phosphorus, and chemical oxy-gen demand (COD). Total solids concentration was determined by drying duplicate 50 ml aliquots at 1058C and weighing the remaining solids. Soluble P concentration was determined colorimetrically on
®ltered samples (0.45mm, millex-ha, Millipore,
Bed-ford, MA1) by the method of Murphy and Riley
(1962). Total P was determined similarly on samples following a complete nitric/perchloric acid digestion of un®ltered samples (Olsen and Sommers, 1982). Particulate P was calculated as the difference between total P and soluble P. COD was determined by adding 2.5 ml runoff samples to commercially prepared 10 ml borosilicate glass reagent vials containing potassium dichromate and standard reaction catalysts (accu Test
COD, low range, Bioscience, Bethlehem, PA1). Vials
were heated to 1508C for 2 h and then absorbance of light was measured at 440 nm. Losses of sediment, soluble P, total P, particulate P, and COD were com-puted as the product of runoff volume and the corre-sponding concentration.
2.4. Statistical analysis
Analysis of variance was carried out using Statis-tical Analysis System software (SAS, version 6.12,
Cary, NC1). Data for runoff, sediment, P, and COD
losses were transformed to logarithmic values to control non-homogeneous variances. The means reported are geometric means. When treatment effects were signi®cant, the Duncan multiple range test was used to separate means P<0:10.
3. Results and discussion
3.1. Snow water equivalent
Because of the transient nature of snow cover, the frequency and intensity of measurements required to monitor changes in snow depth with time and space were beyond the scope of this study. Our objective in measuring snow depth was to identify whether tillage practice affected the amount of water stored in the snowpack prior to melting and runoff. Snow measure-ments were made on January 11, 1996, March 27, 1996, and March 7, 1997, in anticipation of ensuing snowmelt (Table 1). On January 11, 1996, snow water equivalent averaged 5.9 cm and there was no differ-ence due to the tillage systems. This snow partially melted on several different occasions and was com-pletely gone by March 14, 1996. The snow determina-tion on March 27, 1996 corresponds to a new accumulation of snow. On this date, the snow water equivalent was greater for the ridge tilled plots (7.0 cm) than for the moldboard (5.2 cm) or chisel plowed plots (5.1 cm). On March 7, 1997, snow water equivalent was also greater in the ridge tilled plots (9.4 cm) than in the moldboard plowed plots (5.5 cm), but neither of these measurements were different from those for the chisel plowed plots (6.3 cm). Since all plots were in close proximity to each other, it is assumed that snowfall was initially uniform. The observed differences in snow depth were due to dif-ferences in the removal of snow by wind. Furrows associated with the ridge till system protected snow from removal by wind and resulted in greater snow depth than for the other tillage systems.
Table 1
Effect of tillage on the average water equivalent snow depth measured on three datesa
Measurement date Average water equivalent snow depth (cm)
Moldboard
aTillage comparisons are made within each measurement date.
Means followed by the same letter are not signi®cantly different
3.2. Residue cover
Residue cover did not differ between the two study years and is reported as the mean of both years. Residue cover was 93% for the ridge till system, 40% for the chisel plow system, and 10% for the moldboard plow system. These differences in residue cover between
tillage systems were highly signi®cant P<0:001.
3.3. Surface roughness
Soil surface elevations are illustrated in Fig. 1 for each tillage system. Surface roughness was affected by tillage treatment P<0:001. The order of
increas-ing roughness was ridge till (0.12 cm), chisel plow (0.76 cm), and moldboard plow (1.0 cm). The ridge till treatment had smooth surfaces parallel to the slope and roughness was signi®cantly less than for the chisel and moldboard plow treatments. Roughness for the fall moldboard and chisel plow treatments were not signi®cantly different from each other.
3.4. Soil extractable phosphorus and organic matter content
Soil extractable P and organic matter content were not different for the two study years and are reported as the mean of both years. Extractable P levels were very high for all tillage systems and re¯ect the history of manure application at this site. At the 0±7.5 cm
depth, P levels were affected by tillage P0:002
and were higher for the ridge till system (89 mg kgÿ1)
than either the moldboard (58 mg kgÿ1) or chisel plow
systems (64 mg kgÿ1). Extractable soil P
concentra-tions at the 7.5±15 cm depth did not differ among
tillage systems and averaged 43 mg P kgÿ1. Organic
matter content in the 0±7.5 cm depth differed with
tillage P<0:007 and was higher for ridge till
system (30 g kgÿ1) than either moldboard (22 g kgÿ1)
or chisel plow systems (23 g kgÿ1). Both P and
organic matter were concentrated in the surface layer of the ridge till treatment due to limited mixing of crop residue that accumulates at the soil surface.
3.5. Runoff
Five runoff events occurred in 1996 and three occurred in 1997 (Table 2). A runoff event on January
18, 1996, was the result of 2.5 cm of rain falling on existing snow with an average water equivalent of 5.9 cm. The remaining runoff events resulted from melting snow. Analysis of variance for runoff from individual events showed a signi®cant tillage effect on some dates but not on others (Table 2). When sums of snowmelt runoff within each year were compared,
there was a signi®cant effect of year P0:001
and tillage P0:006 and there was no year by
tillage interaction P0:514. Average runoff depth
was higher in 1996 (9.5 cm) than in 1997 (3.7 cm). Runoff from the ridge till and chisel plow plots was greater than for moldboard plow plots.
Differences in runoff volume among tillage prac-tices are partially due to the differences in the amount of water stored in the snow. One way to normalize for snow depth is to compare the percent of available water measured as runoff. The snow depth measured Fig. 1. Soil surface roughness of a 1.0 m2area for moldboard plow,
on March 27, 1999 (one day prior to runoff) provides the best comparison because (1) snow loss by wind drift was minimal, (2) no additional snow was deposited, and (3) snow was completely gone by the end of the runoff event. In other words, for this event
snow dissipation was mainly due to runoff and in®l-tration. The runoff hydrographs for March 28±April 3, 1996, along with air temperature are plotted in Fig. 2. The percent of available water that ran off was 84, 80, and 51% for the chisel plow, ridge till, and moldboard Table 2
Effect of tillage on runoff from individual snowmelt events for 1996 and 1997 and for the sum of snowmelt runoff for each yeara
Runoff date Runoff (cm) ANOVA;Pvalue: tillage
Ridge till Chisel Moldboard
Individual events
1996
January 18 2.9 1.7 0.10 0.002
February 9±10 0.83 0.71 0.10 <0.001
February 23±25 4.4 4.8 1.7 0.230
March 9±14 0.27 0.15 0.18 0.740
March 28±April 8 4.4 4.0 2.4 0.280
1997
February 17±March 2 0.24 0.89 0.26 0.440
March 8±11 1.8 2.6 1.0 0.110
March 17±27 1.5 2.2 0.55 0.370
Sums
1996 13 11 4.5 0.001 (tillage)
1997 3.5 5.7 1.8 0.006 (year)
0.514 (tillage by year)
Average 8.3 a 8.7 a 3.2 b
aGeometric means of four replications are reported. Means followed by the same letter are not signi®cantly different P<0:10.
plow systems, respectively. Thus, moldboard plowing was more effective in limiting snowmelt runoff than ridge tilling or chisel plowing, mainly due to surface storage. Runoff from the ridge till and chisel plow treatments suggests that neither roughness nor residue cover measurements are good indicators of how these practices affect snowmelt runoff when tillage is par-allel to the slope.
3.6. Sediment, phosphorus, and COD losses
Snowmelt runoff is not expected to cause substan-tial interrill erosion because snow normally melts gradually and because soil detachment is limited
when the soil is frozen. Average annual sediment loss
(Fig. 3) differed with tillage practice P0:016and
year P0:009but there was no signi®cant tillage
by year interaction P0:826. Sediment loss
aver-aged 0.26 and 0.11 Mg haÿ1 for 1996 and 1997,
respectively. Average sediment loss was lower for the moldboard plow system than for the chisel plow system. This difference was mainly due to differences in runoff volume between these tillage systems. Sedi-ment losses were not different between the moldboard plow and ridge till systems. Higher runoff with the ridge till treatment was offset by a lower sediment concentration than with the moldboard plow treat-ment. Lower sediment concentrations for the ridge
till system were due to the high degree of residue cover.
The moldboard plow treatment resulted in less soluble, particulate, and total phosphorus loss than either the ridge till or chisel plow treatments (Fig. 3). For all tillage systems, soluble P loss was the dominant form of P loss in snowmelt runoff, averaging 75% of the total P loss. Relative differences in P loss among tillage systems followed a trend similar to that observed for runoff. However, the relative magnitude of particulate P loss did not follow the trend observed for loss of total solids. This indicates that solids in runoff from plots with different tillage systems dif-fered in P concentration. The ratio of particulate P loss
(g haÿ1) to sediment loss (kg haÿ1) represents the
average P concentration of the solids in runoff
(g kgÿ1). Average concentration of P in runoff solids
followed the order: ridge till (2.2 g kgÿ1), chisel plow
(1.1 g kgÿ1), and moldboard plow (0.82 g kgÿ1). This
difference in P concentration of solids in runoff is due to the accumulation of P at the soil surface of the ridge till and, to a lesser extent, the chisel plow systems.
The magnitude of all P losses, especially soluble P, in this study were large compared to other plot studies that evaluated annual loss of P in runoff (Ginting et al., 1998). Snowmelt runoff can be an important source of P to receiving waters. Tillage practices with no fall tillage are more susceptible to losses of soluble P in snowmelt runoff due to a smooth soil surface, the accumulation of P at the soil surface, and the leaching of P from crop residue.
Average annual losses of COD were affected by
tillage P0:001 and year P<0:001 and there
was no tillage by year interaction P0:647. Losses
were higher with both the chisel plow and the ridge till systems than with the moldboard plow system. As with P losses, the difference in COD losses among tillage systems were mainly due to differences in runoff volume.
4. Conclusions
When compared to a moldboard plow based system, snowmelt runoff was higher for the ridge till system with no fall tillage. This difference was due in part to an increased snow depth in the furrows of the ridge till system, but also to the lack of roughness parallel to the
direction of runoff. Despite differences in snowmelt runoff, sediment loss was not different between the ridge till and moldboard plow systems. Surface crop residue with the ridge till system did not limit runoff but did reduce the concentration of sediments in run-off. For the chisel plow system, surface roughness and residue cover were intermediate to the other tillage systems. However, both runoff and sediment losses were greater than for the moldboard plow system and similar to the ridge till system.
Soluble P, particulate P, and total P losses were lower for the moldboard plow system than for the ridge till and chisel plow systems. Soluble P domi-nated total P loss for all tillage systems, averaging 75% of total P loss. Due to an accumulation of P at the soil surface, suspended sediments lost in runoff from the ridge till system had a higher P concentration than sediment from the moldboard plow system. Runoff volume and the P concentration at the soil surface had a more important effect on P loss in snowmelt runoff than the reduction of particulate matter in runoff.
Conservation tillage practices implemented to reduce erosion and the movement of P to surface waters during spring and summer months can result in an increase in runoff during the snowmelt period. This runoff can be an important source of P, with losses mainly as soluble P. The relative importance of runoff during this time period compared to summer months deserves consideration when recommending tillage practices for cold climate regions.
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