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TECHNICAL REPORTS
Landscape and Watershed Processes

Influence of Manure Application on Surface Energy and Snow Cover

Field Experiments

^a NOAA/NESDIS/ORA, Atmospheric Research and Applications Division, 5200 Auth Rd., Rm. 601/WWB, Camp Springs, MD 20746-4304
^b Dep. of Soil Science, 1525 Observatory Drive, Univ. of Wisconsin-Madison, Madison, WI 53706-1299

* Corresponding author (Cezar.Kongoli{at}noaa.gov)

Received for publication October 23, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Application of manure to frozen and/or snow-covered soils of high-latitude, continental climate regions is associated with enhanced P losses to surface water bodies, but the practice is an essential part of most animal farming systems in these regions. Field experiments of the fates of winter-applied manure P are so difficult as to make them essentially impractical, so a mechanistic, modeling approach is required. Central to a mechanistic understanding of manure P snowmelt runoff is knowledge of snowpack disappearance (ablation) as affected by manure application. The objective of this study was to learn how solid manure applied to snow-covered fields modulates the surface energy balance and thereby snow cover ablation. Manure landspreading experiments were conducted in Arlington, WI during the winters of 1998 and 1999. Solid dairy manure was applied on top of snow at a rate of 70 Mg ha^-1 in 1998, and at 45 and 100 Mg ha^-1 in 1999. Results showed that the manure retarded melt, in proportion to the rate applied. The low-albedo manure increased absorption of shortwave radiation compared with snow, but this extra energy was lost in longwave radiation and turbulent flux of sensible and latent heat. These losses result in significant attenuation of melt peaks, retarding snowmelt. Lower snowmelt rates beneath manure may allow more infiltration of meltwater compared with bare snow. This infiltration and attenuated snowmelt runoff may partially mitigate the enhanced likelihood of P runoff from unincorporated winter-spread manure.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PHOSPHORUS RUNOFF to surface water bodies is a major environmental concern (e.g., Udoyara and Jolly, 1994; Thomann and Mueller, 1987; Pote et al., 1996). Phosphorous runoff from farm fields comes from stores in the soil that are maintained and increased by additions of processed fertilizers and manure, from dead vegetation, and directly from manure applied to the soil surface and not immediately incorporated. In the high-latitude, continental climate regions the landscape may be frozen and snow covered for four to six months of the year, during which incorporation of manure after spreading is impossible. While application of manure to frozen and/or snow covered soils is thought to cause enhanced P losses to surface water bodies (e.g., Madison et al., 1997), the practice is an essential part of most animal farming systems in these regions. The alternative to winter application is storage, but this compresses the time available for spreading (a logistical problem for producers), increases capital investment, and can lead to leakage of concentrated P from storage structures. However, the perceptions that winter applications are intolerable and that engineered solutions of storage are cost effective are so strong that publicly funded cost sharing programs are helping to construct many storage systems in the northern USA. Annual expenditures by the state agencies of Wisconsin for manure storage construction are at least $1.5 million.

	LITERATURE REVIEW ON WINTER-APPLIED MANURE

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Factors influencing P loss from nonwinter manure applications to agricultural soils have been studied (e.g., Khaleel et al., 1980; Mueller et al., 1984a,b; Wendt and Corey, 1980; Sharpley et al., 1992, 1994; Bosworth et al., 1995). In contrast, there appears to be many knowledge gaps of the fate of P in manure applied during winter. Most of the field experiments on P in runoff from various treatments of winter-applied manure predate 1980, and little new experimental data have been collected since. Winter and spring runoff experiments are tremendously challenging, and results in any given year may reflect weather influences more strongly than treatment effect.

There is great uncertainty on the relative amounts of P losses in snowmelt runoff coming from manured and unmanured fields. Minshall et al. (1969) found that total P from manured areas in southwestern Wisconsin was only one tenth of the amount of total P supplied from unmanured fields. Witzel et al. (1969) showed that P losses in spring runoff from four small watersheds, some of which had received manure application, were about the same as from plots that had received no winter manure application. Young and Mutchler (1976) found that total nutrient losses from manured plots were no greater than losses from control plots. In contrast, Klausner et al. (1976) found that manure applied on snow during active thaw periods resulted in significantly higher total soluble P loss than in control plots. Great uncertainty also exists on the effect of timing of application on P loss. For instance, Young and Mutchler (1976) found that manure applied on frozen ground significantly increased P loss as compared with manure applied on top of snow, whereas Klausner et al. (1976) found that manure applied on top of melting snow resulted in greater total soluble P loss than when the manure was applied earlier in the winter. Results are also sensitive to weather events soon after spreading. This sensitivity is exhibited in the considerable year-to-year variability in runoff and P loss from given trials, often substantially more than among treatments in a year. For instance, Hensler et al. (1970), Minshall et al. (1970), and Klausner et al. (1976) discussed what they viewed as anomalous weather closely associated with applications in these experiments.

Despite the uncertainties described above, most data show significantly lower runoff volumes from manured areas as compared with nonmanured ones (Table 1) . This observation has important implications for P loss. Assuming that this decreased runoff from manured areas occurs because of greater infiltration, this should lead to reduced peak water flows, reduced particle transport, and greater opportunity for sorption of solution P. Studies have shown that manure applied to the soil surface improves soil physical properties, resulting in higher infiltration (e.g., Ginting et al., 1998a,b). Manure can also increase infiltration by absorbing rainfall impact or slowing down runoff water passing through the manure layer (e.g., Young and Holt, 1978). Young and Mutchler (1976) and Young and Holt (1978) noted the efficiency of manure in retarding runoff and soil loss, attributing this retardation mainly to the manure serving as a mulch. These findings refer to cases when manure is applied directly to the soil. However, some of the winter landspreading trials reporting lower runoff in Table 1 involved manure spread on top of snow. When manure is applied on top of snow, it mainly interacts with the snow underneath, and the significance and mechanisms of such interaction in controlling snowmelt have not been studied. Here, (micro)meteorology is important. For example, manure albedo is much lower than that of snow, resulting in higher absorption of incoming shortwave radiation. Similarly, manure thermal properties may influence partitioning of surface energy and the energy available for melt of the snow underneath. Braun (1990) treated snow–manure interactions in a physically based fashion, but his investigations were with liquid manure, and his concepts and findings cannot be extrapolated to solid manure. Manure containing bedding and of relatively low water content might be expected to alter the surface energy balance differently than water liquid manure.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Field Experiments
The field study was conducted in 1998 and 1999 at the Arlington Agricultural Research Station, located 25 km north of Madison, WI. Manure used in all experiments was from a dairy barn bedded with chopped corn (Zea mays L.) stalks and scraped daily. In 1998, the site was flat, previously plowed land. About 70 Mg ha^-1 wet weight was applied on top of snow with conventional manure handling equipment. Spreading started on day of year 30 (hereafter day) on top of about 20 cm of snow on the ground, and continued for 5 d, resulting in a plot 100 x 150 m in size. Average thickness of applied manure was about 3.5 cm. Although there were spots where snow was visible, coverage was essentially complete.

In 1999, two sites were selected: one on a south-facing slope, and the other on a northwest-facing slope. Both sites were on a 5% slope previously planted to alfalfa (Medicago sativa L.), and had accumulated equal snow amounts prior to manure spreading. Manure was applied on four smaller-sized plots (20 by 7 m) at rates of 45 and 100 Mg ha^-1. Application was on Day 15 on top of about 30 cm of snow on the ground. Hereafter we refer to the 100 Mg ha^-1 rate as heavy, where complete coverage was obtained with an average manure thickness of 4 cm, and the 45 Mg ha^-1 rate as patchy, with manure coverage of about 60% and thickness varying from 0 to 5 cm. A rough estimate of wet bulk density of the applied manure was obtained from the application rate and average thickness. In 1998 a manure application rate of 70 Mg ha^-1 and average thickness of 3.5 cm corresponded to a bulk density of 200 kg m^-3, and a rate of 100 Mg ha^-1 and 5 cm thickness in 1999 corresponded to a bulk density of 250 kg m^-3. As dry bulk density of dairy cow manure is about 100 kg m^-3 (Bohnoff, 1985), manure moisture was about 50% on a wet weight basis for 1998 (100 kg m^-3 of water) and 60% in 1999 (150 kg m^-3 of water).

Measurements and Instrumentation
Field measurements included surface temperature, albedo, longwave and shortwave radiation, manure thermal conductivity, snow depth and water equivalent, and heat fluxes at the soil–snow and manure–snow interfaces. Snow albedo and radiation components were measured by a Kipp and Zonen (Delft, the Netherlands) Type CNR1 net radiometer. A single instrument was moved among treatments; manure albedo was measured relative infrequently (once every two to three days) since it changed more slowly than that of snow. Surface temperature of manured and control plots was measured daily as the average of three readings between 1100 and 1500 h local standard time with an Everest (Fullerton, CA) Model 120 infrared thermometer. Manure thermal conductivity was measured with a conductivity probe similar to the Decagon (Pullman, WA) Thermolink TL1. Reported values are the average of four measurements taken on the dates shown on Table 2 . Conductive heat fluxes were measured hourly with HFT3 flux plates (REBS, Seattle, WA). Reported values are the average of four measurements at each interface, except when some plates were displaced from the interface during melting and exposed to the sky. Snow depth was taken every day at about 1300 h by a snow stake as the average of four depth readings. Snow water equivalent (SWE) was taken daily as the average of three measurements on unmanured plots, and once every two to three days on manured ones. In 1998, SWE measurements were taken only on the unmanured plot, and in 1999 on both manured and unmanured plots. To measure SWE, snow cores were taken at three measurement sites on each plot with a steel tube specifically made for this purpose. The tube had walls 1 mm thick, and was 5 cm in diameter and 90 cm in height, with a serrated cutting end. The tube was pushed and rotated into the snow gently to avoid compaction. After the end contacted the soil, a flat plastic plate was slid over the end to hold the snow in the tube. The snow was emptied to a plastic bag and sealed carefully. The depth of the snow was immediately measured in the hole created by the core removal. Snow water equivalent and density were determined gravimetrically.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The winter of 1998 was warmer than normal, with average monthly air temperatures for January, February, March, and April higher than the long-term normal for 1960–1990. In 1999, the average air temperature for February was much higher than normal (Table 3) . These higher temperatures were associated with relatively high amounts of precipitation in the form of rain (Table 4) . For example, more rain than snow fell in February in 1998 and in January and February in 1999. For both years, total amount of rain for the period January–March was nearly double that of snow. In 1998, all snow fell prior to application of manure on Day 30 (Table 4). Subsequent precipitation events on Days 41 and 47 were in the form of rain, so manure remained on top of snow. In 1999, however, a recorded amount of 5 mm of snow water equivalent fell on Day 24, depositing some 5 cm of snow on manure (Table 4). The new snow covered the manure completely, but disappeared in a few days, leaving the manure exposed again. The manure remained on top of snow for the rest of the experiment.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Measured snow depth and standard deviation in the control and manured plot in 1998.

View larger version (40K): [in this window] [in a new window]   Fig. 2. Measured snow depth and standard deviation in the control, patchy, and heavy plots in 1999 in (A) the south-facing site and (B) the northwest-facing site.

View larger version (31K): [in this window] [in a new window]   Fig. 3. Measured equivalent water depth of snowpack and standard deviation in the control, patchy, and heavy plots during major melting of the snow in 1999 in (A) the south-facing site and (B) the northwest-facing site. Water equivalent depths clearly resembled snow depth changes.

View larger version (31K): [in this window] [in a new window]   Fig. 4. Measured snow albedo in the control and manured plot in 1998, and in the control, patchy, and heavy plots in 1999 in the south-facing and northwest-facing sites.

View larger version (29K): [in this window] [in a new window]   Fig. 5. Surface radiometric temperature measured midday in the control and manured plot in 1998 and in the control, patchy, and heavy plots in 1999 in the south-facing and northwest-facing sites.

Assuming the snow is beneath the manure and fluxes in manure and snow are positive downward, the net energy available for melt, M_sp, is:

[1]

where G_m and G_sl are conductive fluxes in W m^-2 at the manure–snow and soil–snow interfaces, respectively. Assuming a manure–snow interface temperature of 0°C for melting snow, the energy balance of the manure layer is:

[2]

where c_m is the volumetric heat capacity of manure in J m^-3 °C^-1, d_m is the thickness of the manure in m, T_m is the manure surface temperature in °C (T_m/2 represents the manure temperature averaged over the manure layer), R_nm is the net radiation at the manure surface in W m^-2, U_m is the sum of sensible and the latent heat fluxes in W m^-2, and t is the time in s.

The net radiation is:

[3]

where R_snm and R_lnm are the net shortwave radiation and longwave radiation of the manure surface in W m^-2, respectively. Assuming G_sl in Eq. [1] is insignificant during spring melt events and energy transport through the manure by conduction, the energy available for melt in Eq. [1] can be expressed as:

[4]

where k_m is the manure thermal conductivity in W m^-1 K^-1.

Combining Eq. [2] through [4], we obtain the expression in terms of net shortwave radiation R_snm:

[5]

Note that all the terms on the right-hand side of Eq. [5] are a function of surface manure temperature T_m, whereas R_snm is only a function of manure albedo and the incoming solar radiation. For a given R_snm, partitioning of the energy between M_sp and the rest will depend mainly on manure properties, mainly k_m and d_m. For instance, a low k_m value and thick manure lead to reduced M_sp (Eq. [4] and [5]).

Manure application significantly altered the surface energy balance. Representative days are shown for the manured area in 1998 (Fig. 6) and the heavy plot in the south-facing site in 1999 (Fig. 7) . The heavy plot in the northwest-facing site showed a similar response as the south-facing site. During these measurements manure was on top of snow, so net shortwave and longwave radiation (Fig. 6B and 7B) refer to the manure surface. Heating of the manure surface during daytime caused outgoing longwave radiation to increase, making the net longwave more negative. Net radiation (Fig. 6C and 7C) follows the expected diurnal cycle. Net energy available for melt of the snow beneath the manure was estimated from the measured fluxes at the manure–snow and snow–soil interfaces (Fig. 6A and 7A). As shown, the net energy available for melt (M_sp in Eq. [5]) is only a small fraction of the net radiation, reaching about 50 W m^-2 in 1998, and 80 W m^-2 in 1999 (Fig. 6A and 7A). As G_sl is negligible (Fig. 6A and 7A), M_sp is practically equal to the heat flux at the manure–snow interface. The large, unaccounted-for energy represents the heating of the manure {(C_mD_m/2)dT_m/dt in Eq. [5]} and the loss in turbulent sensible and latent heat flux (U_m in Eq. [5]). An indication of such a partitioning was, for instance, the high surface manure temperature relative to that of the snow surface found on manured plots, especially during warm, sunny days (e.g., Fig. 5), and slower melting of the snow observed in all of our manure treatments. This slow-down of melting was not as pronounced in partially manured plots as in the heavily manured ones. In partially manured plots, however, the spreading pattern was important. We observed that melt retardation was more pronounced in snow beneath the largest (15 cm in diameter) patches. A likely explanation of this phenomenon is that small manure patches have a high ratio of perimeter to area, compared with big patches. Radiant and sensible heat from the perimeter edges melts snow adjacent and beneath, so snow beneath small patches disappears more rapidly than from beneath larger patches.

View larger version (31K): [in this window] [in a new window]   Fig. 6. Measured surface fluxes in the manured plot in 1998. (A) Measured conductive flux at the manure–snow and soil–snow interface. (B) Measured net long-wave and solar radiation. (C) Measured net radiation and energy available to the snow beneath the manure.

View larger version (33K): [in this window] [in a new window]   Fig. 7. Measured surface fluxes in the heavy manured plot in 1999. (A) Measured conductive flux at the manure–snow and soil–snow interface. (B) Measured net long-wave and solar radiation. (C) Measured net radiation and energy available to the snow beneath the manure.

The surface energy balance concept adequately explained the effects of manure on snow cover ablation in our experiments. Braun (1990) used energy balance to explain changes to snow cover caused by the landspreading of liquid manure, but considered only a reduction of albedo by the manure, which increased melt rates by increasing the net shortwave radiation flux to the snowpack. This phenomenon is well documented (e.g., De Quervain, 1948; Slaughter, 1969; Megahan et al., 1970; Dunne and Black, 1971; Nicholson, 1976; Warren, 1984; Conway et al., 1996). Studies conducted on albedo-reducing materials other than manure applied to snow-covered fields such as dust, soot, and volcanic ash of certain thickness, suggest that this can significantly accelerate melt. Megahan et al. (1970) measured net radiation variations in plots treated with various albedo-reducing materials and found a significant treatment effect. New snow tended to decrease treatment effects, but net radiation was still significantly greater on treated snow as compared with untreated snow, even with up to 4.1 cm of new snow on treated surfaces. Similar effects were reported by Conway et al. (1996), who found that albedo reductions from dust accumulations increased melt as much as 50% compared with untreated areas.

Our field experiments showed that the manure retarded snowmelt, a phenomenon not possible in the Braun's (1990) model. Melt retardation has also been commonly observed in snow-covered fields overlain by thick accumulations of surface dust (Drake, 1980; Ostrem, 1959). Ostrem (1959) found that the switch from advancing of melt rates to retardation occurred at a dust thickness of 0.5 cm, a phenomenon explained by Drake's (1980) simplified, energy balance model. Drake's (1980) work showed that changes caused to the surface albedo and temperature by albedo-reducing materials on top of snow were both important to predicting whether snowmelt is retarded or advanced.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Manure landspreading experiments conducted in Arlington, WI during the winters of 1998 and 1999 showed that solid dairy manure containing cornstalks applied on top of snow retarded melt, in proportion to the manure applied. This retardation was caused by manure acting as an insulator. Although manure albedo was much lower than that of snow, a significant portion of the energy absorbed as shortwave radiation was lost in longwave radiation and turbulent fluxes of sensible and latent heat, leading to reduced energy available for melt of the snow beneath the manure. Thus, manure decoupled the energy regime of the snow underneath from that of the overlying air and radiation environment. Although melt retardation has been noted in previous winter manure studies, it was not until now adequately explained. Delayed melting of snow beneath manure favors infiltration of meltwater, and greater infiltration presumably creates opportunities for sorption of solution P on soil, diminishing the amount lost in runoff.

	ACKNOWLEDGMENTS

 
This research was supported by USDA-Hatch funds through project WIS0395, administered by the University of Wisconsin-Madison College of Agricultural and Life Sciences.

	REFERENCES

TOP ABSTRACT INTRODUCTION LITERATURE REVIEW ON WINTER... MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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