Surface Energy Balance, Evapotranspiration, And Surface

University of Nebraska - Lincoln
[email protected] of Nebraska - Lincoln

Biological Systems Engineering: Papers and Publications

Biological Systems Engineering

2015
Surface Energy Balance, Evapotranspiration, And Surface Coefficients During Non-Growing Season In A Maize-Soybean Cropping System
Lameck O. Odhiambo
University of Nebraska-Lincoln, [email protected]
Suat Irmak
University of Nebraska-Lincoln, [email protected]

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Odhiambo, Lameck O. and Irmak, Suat, "Surface Energy Balance, Evapotranspiration, And Surface Coefficients During Non-Growing Season In A Maize-Soybean Cropping System" (2015). Biological Systems Engineering: Papers and Publications. 447. https://digitalcommons.unl.edu/biosysengfacpub/447
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SURFACE ENERGY BALANCE, EVAPOTRANSPIRATION, AND SURFACE COEFFICIENTS DURING NON-GROWING SEASON
IN A MAIZE-SOYBEAN CROPPING SYSTEM
L. O. Odhiambo, S. Irmak

ABSTRACT. Surface energy balance components, including actual evapotranspiration (ET), were measured in a reducedtill maize-soybean field in south central Nebraska during three consecutive non-growing seasons (2006/2007, 2007/2008, and 2008/2009). The relative fractions of the energy balance components were compared across the non-growing seasons, and surface coefficients (Kc) were determined as a ratio of measured ET to estimated alfalfa (ETr) and grass (ETo) reference ET (ETref). The non-growing season following a maize crop had 25% to 35% more field surface covered with crop residue as compared to the non-growing seasons following soybean crops. Net radiation (Rn) was the dominant surface energy balance component, and its partitioning as latent heat (LE), sensible heat (H), and soil heat (G) fluxes depended on field surface and atmospheric conditions. No significant differences in magnitude, trend, and distribution of the surface energy balance components were observed between the seasons with maize or soybean surface residue cover. The cumulative ET was 196, 221, and 226 mm during the three consecutive non-growing seasons. Compared to ETref, the cumulative total measured ET was 61%, 63%, and 59% of cumulative total ETo and 43%, 46%, and 41% of cumulative total ETr during the three consecutive seasons. The type of residue on the field surface had no significant effect on the magnitude of ET. Thus, ET was primarily driven by atmospheric conditions rather than surface characteristics. The coefficient of determination (R2) for the daily ET vs. ETr data during the three consecutive non-growing seasons was only 0.23, 0.42, and 0.42, and R2 for ET vs. ETo was 0.29, 0.46, and 0.45, respectively. Daily and monthly average Kc values varied substantially from day to day and from month to month, and exhibited interannual variability as well. Thus, no single Kc value can be used as a good representation of the surface coefficient for accurate prediction of ET for part or all of the non-growing season. A good relationship was observed between monthly total measured ET vs. monthly total ETref. The R2 values for monthly total ET vs. monthly total ETref data ranged from 0.71 to 0.89 for both ETr and ETo. Using pooled data for monthly total ET vs. monthly total ETref, R2 was 0.78 for ETr and 0.80 for ETo. The slopes (S) of the best-fit line with intercept for the monthly total ET vs. monthly total ETref data were consistent for all three non-growing seasons, with S = 0.45 ±0.05 for ETr and S = 0.62 ±0.08 for ETo. The parity in R2 and S across the three non-growing seasons suggests that the same regression equation can be used to approximate non-growing season ET for field surfaces with both maize and soybean crop residue covers. Considering the extreme difficulties in measuring ET during winter in cold and windy climates with frozen and/or snow-covered conditions, the approach using a linear relationship between monthly total ET vs. monthly total ETref appears to be a good alternative to using a surface coefficient to approximate non-growing season monthly total ET. The conclusions of this research are based on the typical dormant season conditions observed at the research location and may not be generally transferable to other locations with different climatic and surface conditions. Keywords. Dormant season, Evapotranspiration, Non-growing season, Reference evapotranspiration, Surface coefficient, Surface energy balance.

In the Midwestern U.S., maize (Zea mays L.) and soybean (Glycine max (L.) Merrell) are the predominant crops mostly grown in rotation. In this region, the non-
Submitted for review in June 2014 as manuscript number 10790; approved for publication by the Natural Resources & Environmental Systems Community of ASABE in March 2015.
The mention of trade names or commercial products is for the information of the reader and does not constitute an endorsement or recommendation for use by the University of Nebraska-Lincoln or the authors.
The authors are Lameck O. Odhiambo, ASABE Member, Research Assistant Professor, and Suat Irmak, ASABE Member, Distinguished Professor, Department of Biological Systems Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska. Corresponding author: Suat Irmak, 239 L.W. Chase Hall, University of Nebraska-Lincoln, Lincoln, NE 68583-0726; phone: 402-472-4865; e-mail: [email protected]

growing season, also referred to as the dormant period, is the period during which no maize or soybean crop is planted in the fields due to cold winter temperatures. The nongrowing season occurs between the first killing frost of autumn and the last killing frost of spring (October to April). The field surface conditions during this period are characterized by the presence of plant residue, periods of frost and frozen conditions, cessation of active plant growth, and periods of snow and/or ice cover. The field surface conditions during the non-growing season have the potential to affect the magnitude of individual components of the surface energy balance, including actual evapotranspiration (ET). The total amount of incoming solar radiation during this period of the year (October to April) is substantially reduced due to the large solar zenith angles and

Transactions of the ASABE

Vol. 58(3): 667-684 © 2015 American Society of Agricultural and Biological Engineers ISSN 2151-0032 DOI 10.13031/trans.58.10790

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short day lengths, which can impact the surface energy balance and residue cover interactions. Horton et al. (1996) provide a comprehensive review of how crop residue on the soil surface affects components of the surface energy bal-
ance. The shortwave reflectivity (α) of light-colored residue is considerably greater than that of a dark-colored soil surface, thereby reducing the amount of solar radiation that becomes available at the soil surface for ET [i.e., latent heat (LE)], sensible heat (H), or soil heat (G) fluxes (Aase and Tanaka, 1991; Hares and Novak, 1992; Bristow, 1988; Horton et al., 1994; Bussiere and Cellier, 1994, Sauer et al., 1997, 1998a). Residue on the soil surface also blocks the incoming radiation that would otherwise reach the soil and affects vapor transfer and loss of heat by conduction, convection, and evaporation (Horton et al., 1994; Steiner, 1994). Sauer et al. (1998b) measured all surface energy balance components of a maize residue-covered field during the non-growing season on snow-free days. They found that on overcast days with a dry surface 42% to 75% of the available energy was consumed by LE, and on continuous sunny days with a dry surface less than 21% of the available energy was partitioned into LE. Sauer et al. (1998a) found that on wet surfaces during snow-free periods, less energy was partitioned into LE on sunny days than on overcast days (<19% on sunny days vs. >38% on overcast
days). Snow cover with its high α also reduces the amount of solar radiation that becomes available at the soil surface for LE or G. Sauer (1998a) measured the surface energy balance of a maize residue-covered field during melting of the snow cover. They found that the net radiation and snowmelt/storage terms dominated the energy balance during the snowmelt period, and peak LE and G fluxes were below 100 W m-2.
During the non-growing season, ET occurs predominantly by evaporation from exposed soil surfaces, intercepted water by crop residue, and from snow and/or ice cover. Total non-growing season ET can be relatively small as compared to the yearly water balance, but knowledge of the ET processes during the dormant period is necessary for developing strategies for conserving water in the soil and root zone, and for estimating the effectiveness of nongrowing season precipitation in recharging the soil water storage for the subsequent growing season. Furthermore, quantifying ET losses during non-growing season and determining how ET impacts surface runoff and groundwater recharge which is essential for modeling the hydrologic water balance and also for better understanding of the transport process of agricultural chemicals that may occur during the non-growing season. Lewan (1993) measured ET from bare soil and cover crop surfaces during winter in southwestern Sweden and found no difference in ET between bare soil and cover crop surfaces. Lewan (1993) also found that total ET during the non-growing season was 75% of winter precipitation. Hatfield et al. (1996) measured ET over three fields with different crop covers during winter in central Iowa using a Bowen ratio system. They found that daily ET ranged from less than 1 mm d-1 to over 3 mm d-1, and the evaporative fraction ranged from 40% to 90%. Prueger et al. (1998) conducted a three-year study in central

Iowa to evaluate the partitioning of available energy to ET during the non-growing season by measuring micrometeorological parameters used for estimating surface energy balance components. They found that energy partitioning at the surface over rye, oats, and bare soil during the nongrowing period is driven by climate, snow, residue cover, and available energy. They also observed that seasonal ET totals from mid-October through late February ranged from 118 to 205 mm for the three-year study. In a more recent study, Hay and Irmak (2009) evaluated non-growing season evaporative losses in relation to available energy and precipitation of a maize residue-covered subsurface dripirrigated field as part of the Nebraska Water and Energy Flux Measurement, Modeling, and Research Network (NEBFLUX; Irmak, 2010). They measured the evaporative losses using a Bowen ratio energy balance system (BREBS) on an hourly basis and averaged over 24 h for three consecutive non-growing periods. They found that ET was about 50% of the available energy for wet seasons and about 41% of the available energy for dry seasons. Seasonal cumulative ET ranged from 133 to 167 mm and exceeded precipitation by 21% during the dry season. The ratios of ET to precipitation were 0.85, 1.21, and 0.41 during the three consecutive years. ET was approximately 50% of ETo and 36% of ETr in both the first and second year, whereas ET was 32% of ETo and 23% of ETr in the third year. Overall, measured ET during the dormant season was generally most strongly correlated with radiation terms, particularly Rn, surface albedo, incoming shortwave radiation, and outgoing longwave radiation. Suyker and Verma (2009) evaluated the contributions of non-growing season ET to annual ET totals. They found that non-growing season ET ranged from 100 to 172 mm and contributed 16% to 28% of the annual ET in irrigated/rainfed maize and 24% to 26% in irrigated/rainfed soybean. They found that the amount of crop residue on the soil surface explained 71% of the variability in non-growing season ET totals.
A major drawback in determining ET during the nongrowing season is the lack of robust surface coefficients (Kc), which could be used to predict ET from calculated reference (potential) evapotranspiration (ETref) (i.e., ET = Kc × ETref). Because estimating ET during the non-growing season is characterized by non-crop surface conditions, the coefficient might be more appropriately referred to as a “surface” coefficient rather than “crop” coefficient; however, the usage and application would be the same. Crop coefficients (Kc) developed for the growing season cannot be used to estimate ET during the non-growing season due to the field surface conditions, which are significantly different from the field surface during the growing season. Very few studies have been conducted to determine Kc values for the non-growing season. Wright (1991, 1993) conducted a series of wintertime measurements of ET using the dual precision weighing lysimeter systems at Kimberly, Idaho. The lysimeter measurement surfaces included clipped fescue grass and bare soil conditions of disked wheat stubble, disked alfalfa, disked bare soil, dormant alfalfa, and winter wheat. Wright (1991) found that the Kc for an alfalfareference surface based on the ASCE standardized Penman-Monteith equation (ASCE, 2005) rarely reached 1.0

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during winter. However, the mean Kc approached or exceeded 0.80 for periods having nearly continuous distributions of precipitation. Hay and Irmak (2009) measured average surface coefficients over the three seasons as 0.44 and 0.33 for grass- and alfalfa-reference surfaces, respectively. Using geometric mean Kc values to calculate ET using a Kc·ETref approach over the entire non-growing season yielded adequate predictions, with overall root mean square deviations of 0.64 and 0.67 mm d-1 for ETo and ETr, respectively. Estimates of ET using a dual crop coefficient approach were good on a seasonal basis but performed less well on a daily basis.
While the aforementioned studies provide important information on surface energy balance, ET, and surface coefficients during the non-growing season in agricultural fields, there is a need for more research on fields with different surface conditions due to the great diversity in cropping patterns, climatic and soil conditions, and management practices. The objective of this research was to measure and compare the surface energy balance components and evapotranspiration in a ridge-tilled maize-soybean rotation field during three consecutive non-growing seasons, and also to determine how the magnitudes of individual energy balance components and ET change during the period. Non-growing season Kc values were determined from the relationships between the measured ET and estimated ETref.
MATERIALS AND METHODS
STUDY SITE Field measurements for this research were conducted on
a 14.5 ha subsurface drip irrigation (SDI) field located at the University of Nebraska-Lincoln/Institute of Agriculture and Natural Resources, South Central Agricultural Laboratory near Clay Center, Nebraska (40° 34′ N, 98° 8′ W, 552 m above mean sea level). The climate of the area is sub-humid with warm and dry summers and very cold and windy winters with average temperatures usually below 0°C. The warmest month of the year is usually July with an average maximum temperature of 30°C, while the coldest month of the year is January with an average minimum air temperature of -10°C. The annual average precipitation is about 700 mm. Rainfall is not evenly distributed throughout the year. The wettest month of the year is usually May with an average rainfall of 120 mm. The soil in the research field is classified as Hastings silt loam, which is well drained soil with a 0.5% slope. The particle size distribution is 15% sand, 62.5% silt, and 20% clay with 2.5% organic matter content. The soil field capacity (θfc) is 0.34 m3 m-3, the permanent wilting point (θwp) is 0.14 m3 m-3, and the saturation point (θsat) is 0.51 m3 m-3. In 2007, the field was planted with soybean after a maize crop the previous year; in 2008, the field was again planted with soybean. The field was under ridge tillage with crop residue evenly spread on the soil surface after harvest (Irmak, 2010).
ESTIMATING SURFACE RESIDUE COVER Several methods are accepted for estimating the amount

of crop residue on the soil surface (Morrison et al., 1993). They include the line-transect method, photo-comparison method, remote sensing methods, and calculation methods based on measured grain yield. Generally, researchers have observed that the amount of crop residue can be estimated from measured grain yield. For example linear relationships between the amount of crop residue and grain yield have been reported for small grains (McCool et al., 2006) and corn (Linden et al., 2000), such that grain yield has been used to predict residue yield (Johnson et al., 2006). In this research, the percentage of the field surface covered with crop residue was estimated using a procedure based on the relationship between measured crop yield, crop residues produced, and residue decay over winter (Odhiambo and Irmak, 2012). Wortmann et al. (2008) approximated that 1 ton of residue (at 10% moisture) is produced with 1.02 ton of maize grain yield and 0.82 ton of soybean. The amount of crop residue left on the soil surface after winter weathering was determined from tables of typical percent residue remaining after winter weathering developed by Shelton et al. (2000). The residue remaining on the soil surface during the 2006/2007 non-growing season was from a maize crop harvested in October 2006. The yield of the 2006 maize crop was 11.6 ton ha-1, and the amount of residue produced at harvest was estimated at 11.4 ton ha-1. About 90% of maize residue remains after winter weathering (Shelton et al., 2000). The residue remaining on the surface during the 2007/2008 and 2008/2009 non-growing seasons was from soybean crops that were harvested in October 2007 and October 2008. The yield of the 2007 soybean crop was 4.7 ton ha-1 (Irmak et al., 2014), and the amount of residue produced at harvest was estimated at 5.7 ton ha-1. The yield of the 2008 soybean crop was 4.9 ton ha-1 (Irmak et al., 2014), and the amount of residue produced at harvest was estimated at 6.0 ton ha-1. About 75% of soybean residues remain after winter weathering (Shelton et al., 2000). The fraction of the soil surface covered with crop residue (Cr) was estimated as a function of the mass of residue (Gregory, 1982), which is expressed as:

Cr = 1− exp(− AmM )

(1)

where M is total residue mass (ton ha-1), and Am is an empirical parameter that converts mass to an equivalent area and varies with residue characteristics and randomness of distribution. Reported values of Am for maize and soybean are 0.32 and 0.20, respectively (Gregory, 1982).

SURFACE ENERGY BALANCE COMPONENTS The energy balance components at the field surface were
measured using a BREBS (Radiation and Energy Balance Systems, REBS, Inc., Bellevue, Wash.), which was installed inside the field. The BREBS has been used extensively and successfully to determine ET above various vegetation surfaces, yielding ET values that compare well with data from other techniques (Lafleur and Rouse, 1990; McGinn and King 1990; Ham et al. 1991; Kjelgaard et al. 1994; Verma et al. 1976; Bausch and Bernard 1992; Irmak 2010). The energy balance equation is written as:

Rn = LE + H + G + Q

(2)

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where Rn is net radiation (positive downward), LE is latent heat flux (positive upward), H is sensible heat flux (positive upward), G is soil heat flux (positive downward), and Q is the residual of the closure of the energy balance. The energy fluxes are expressed in W m-2. In the following sections, each term in the energy balance equation is described, showing the measurement equipment and parameters that were measured or calculated.
Net Radiation at Field Surface Net radiation (Rn) is the difference between incoming to-
tal hemispherical radiation and outgoing total hemispherical radiation. Rn was measured directly using a Q*7.1 net radiometer, manufactured by Radiation and Energy Balance Systems (REBS). Frequent care and maintenance were used to ensure that the radiometer was level and that the transparent polyethylene shields on the net radiometer did not become less translucent due to UV radiation as a result of aging or scouring from wind-driven particles.
Soil Heat Flux and Soil Temperature Soil heat flux (G) is the heat transferred from the surface
downward via conduction to warm the subsurface. A temperature gradient must exist between the surface and the subsurface for heat transfer to occur. G was measured using three REBS HFT-3.1 heat flux plates installed in the soil below the net radiometer at a depth of 0.05 to 0.06 m below the soil surface. In close proximity to each heat flux plate, soil thermocouple probes (REBS STP-1) were installed 0.05 to 0.06 m below the soil surface to measure the temporal change in temperature of the soil layer above the HFT-3. The G measurements were adjusted for soil temperature and moisture as measured by three REBS SMP1 R soil moisture probes installed in the same location as the soil temperature sensors and soil heat flux plates (Irmak, 2010).
Sensible and Latent Heat Fluxes Sensible heat flux (H) above a field surface is the heat en-
ergy transferred between the surface and air when there is a temperature gradient between the surface and the air above. Latent heat flux (LE) at the field surface is the quantity of heat absorbed or released by water undergoing a change of state, such as ice changing to water or water to vapor (evaporation), at constant air temperature and pressure. Both H and LE were determined from measured Rn, G, and air temperature (T) and specific humidity (q) gradients at two levels. T and q were measured using two platinum resistance thermometers and monolithic capacitive humidity sensors (REBS models THP04015 and THP04016, respectively) with resolutions of 0.0055°C for temperature and 0.033% for relative humidity. The BREBS used an automatic exchange mechanism that physically exchanged the air temperature (T) and relative humidity (q) sensors at two heights above the canopy every 15 min. The lower exchanger sensors level was raised to a height of 2 m above the field surface, and the distance between the upper and lower exchanger sensors level was kept at 1 m throughout the non-growing season. Using the classical equations of the turbulent diffusion of heat and water, and assuming that the transfer coefficients of heat and water vapor are equal, Bowen (1926) and Tanner (1960) showed that:

β = cp ΔT

(3)

λ Δq

where β is Bowen ratio, cp is specific heat of air for constant pressure, λ is latent heat of vaporization of water, q is
specific humidity, ΔT is the gradient of air temperature at
the two levels of measurement, and Δq is the gradient of specific humidity at the two levels of measurement. H and LE are then estimated from:

H = 1+ββ ( Rn − G) (4)

LE = 1+1β ( Rn + G) (5)
The BREBS and other datasets used in this research are part of the Nebraska Water and Energy Flux Measurement, Modeling, and Research Network (NEBFLUX; Irmak, 2010) that operates eleven BREBS and eddy covariance systems over various vegetation surfaces. Detailed description of the microclimate measurements, including LE, H, G, Rn, and other microclimatic variables (e, T, q, wind speed (u), α, and soil temperature) are presented in Irmak (2010).

REFERENCE (POTENTIAL) EVAPOTRANSPIRATION DURING NON-GROWING SEASON
The reference surface during the non-growing season in winter is characterized by a vegetative cover (grass or alfalfa) that changes from active growth to dormant or dead vegetative cover, snow cover, and freezing soil conditions. As the winter period nears the end, snowmelt increases the soil moisture, the soil warms up, and the vegetative cover begins to become active again. This reference surface condition during the non-growing season bears a stark contrast with the standardized hypothetical reference for calculating ETref, which consists of a surface of green, well-watered grass (or alfalfa) of uniform height, actively growing and completely shading the ground. While it is recognized that ETref equations do not represent measurable quantities of ET from reference surfaces during most of the non-growing season, the calculated ETref may be useful as an evaporative index and was used in this research to calculate non-growing season surface coefficients. The weather data needed for calculating ETref were collected at a weather station about 500 m from the research field. The weather station was maintained on natural grass cover without irrigation but somewhat meeting the reference condition criteria. The ASCE Committee on Evapotranspiration in Irrigation and Hydrology recommended that two crops be adopted as approximations for ETref (ASCE, 2005). The symbols and definitions given are: ETo = ETref for a short crop having an approximate height of 0.12 m (similar to grass), and ETr = ETref for a tall crop having an approximate height of 0.50 m (similar to alfalfa). Grassreference (potential) evapotranspiration (ETo) and alfalfareference evapotranspiration (ETr) were calculated using the Penman-Monteith equation (Monteith, 1965) with a fixed canopy resistance (ASCE, 2005):

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0.408Δ ( Rn − G ) + γ Cn (es − ea )u2

ETref =

T + 273

(6)

Δ + γ (1 + Cd u2 )

where Δ is the slope of the saturation vapor pressure at mean air temperature curve (kPa °C-1), Rn and G are the net radiation and soil heat flux density (MJ m-2 d-1 for daily data or MJ m-2 h-1 for hourly data), γ is the psychrometric constant (kPa °C-1), T is daily or hourly mean temperature (°C), u2 is the mean wind speed at 2 m height (m s-1), and es−ea is the vapor pressure deficit (kPa). The coefficients in the numerator (Cn) and the denominator (Cd) are given specific values depending on the calculation time step and the reference crop. The output units from equation 4 are in mm d-1 for the daily time step and in mm h-1 for the hourly time step. For the daily data, Rn is input in MJ m-2 d-1 and G is assumed to be zero. For the hourly calculations, G is assumed equal to
10% of Rn when Rn ≥ 0, and G is assumed equal to 50% of Rn when Rn< 0. The coefficients used in the ETref equation were Cn = 900 and Cd = 0.34 for ETo and Cn = 1600 and Cd = 0.38 for ETr. In this research, ETref was calculated based on an hourly time step and then summed to daily values.

CALCULATION OF SURFACE COEFFICIENTS
DURING NON-GROWING SEASON Crop or surface coefficients (Kc) are generally defined as
empirical ratios of ET to ETref (Wright, 1981):

Kc = ET / ETref

(7)

where Kc is a dimensionless surface coefficient for a particular field surface and soil moisture condition, ET is actual evapotranspiration from the field surface, and ETref is reference evapotranspiration. ET during the non-growing season is predominantly in the form of evaporation from the field surface and includes evaporation from soil profile and residue surfaces, and sublimation from ice and snow surfaces. Kc calculated using ETref = ETr and ETref = ETo are denoted by Kcr and Kco, respectively, and are considered to represent surface coefficients. The dual Kc method (Wright, 1981) divides the Kc value into a “basal” crop coefficient (Kcb) representing crop transpiration plus evaporation from the soil surface by diffusive evaporation, and an “evaporation” coefficient (Ke) representing evaporation from the soil surface. This method would not bring significant improvement to the estimated Kc values for a non-growing season with no crops on the field, as Kcb tends to be relatively small (≤0.1) and approaches zero when there is snow cover on the surface or when the soil surface is frozen. Furthermore, the dual Kc method requires daily calculation time steps, which is suitable for day-to-day irrigation management but not for water balance studies during the non-growing season.
RESULTS AND DISCUSSION
FIELD SURFACE CONDITIONS The research field had virtually no active plant or weed
growth during the non-growing seasons. As an example, figure 1 shows the changes in field surface conditions over

(a)

(b)

(c)

(d)

Figure 1. Change in field surface conditions during the 2007/2008 non-growing season (a) immediately after crop harvest, (b) during snow cover, (c) during snow melt, and (d) toward the end of the non-growing season.

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Table 1. Estimated amount of crop residue remaining on the soil

surface at the beginning and end of the non-growing seasons and the

estimated fraction of soil surface covered with residue.

NonGrowing Season

Crop

Yield (ton ha-1)

Residue at Start of Season[a]

M

Cr

Residue at End of Season[a]

M

Cr

2006/2007 Maize 11.6 11.4 0.97

10.3 0.96

2007/2008 Soybean 4.7

5.7 0.68

4.3 0.58

2008/2009 Soybean 4.9

6.0 0.70

4.5 0.59

[a] M = total mass of crop residue on soil surface (ton ha-1), and

Cr = fraction of soil surface covered with crop residue (%).

the non-growing season in 2007/2008 following harvest of the soybean crop that was planted after maize. Even though the field was devoid of weeds, it had significant amounts of surface residue that underwent very little change over the winter. Table 1 shows the estimated amounts of crop residue remaining on the soil surface at the beginning and end of the non-growing seasons and the estimated fraction of soil surface covered with residue for the entire research period. The soil surface during the 2006/2007 non-growing season was 97% covered with maize residue immediately after harvest. Maize residue is less fragile and is little affected by winter weathering. The percentage of the soil surface covered by maize residue was reduced by winter weathering by only 1% (i.e., to 96%). Since a soybean crop results in less residue than maize; during the 2007/2008 and 2008/2009 non-growing seasons an estimated 68% and 70%, respectively, of the soil surface was covered with soybean residue. Soybean residue is fragile, and the percentage of the soil surface covered in 2007/2008 and 2008/2009 was reduced to 58% and 59%, respectively, by winter weathering. These results show that the soil surface

after maize during the 2006/2007 non-growing season had as much as 25% to 35% more field surface covered by crop residue as compared to 2007/2008 and 2008/2009 nongrowing seasons, which followed soybean.
METEOROLOGICAL PARAMETERS The meteorological parameters observed in this research
were air temperature (T), solar radiation (Rs), wind speed (WS), vapor pressure deficit (VPD, estimated), precipitation, snow cover depth, and soil temperature measured at 0.06 m depth. The monthly means of T, Rs, WS, and VPD were compared with the long-term (1983-2009) monthly means as presented in figure 2. The long-term monthly mean T fell from about 10.9°C at the beginning of October, reached a minimum value of -3.6°C in January, and then increased to about 9.1°C by the end of April (early spring). Similarly, the long-term mean monthly Rs fell from about 11.3 MJ m2 d-1 at the beginning of October to a minimum value of 6.7 MJ m2 d-1 in December and then increased to 17.7 MJ m2 d-1 at the end of April. The WS (fig. 2c) is adjusted for the measurement height of 2 m. The long-term monthly mean WS increased from 3.3 m s-1 at the beginning of October to 4.6 m s-1 at the end of April. The monthly means of WS for 2008/2009 were higher than the long-term averages.
The VPD (fig. 2d) is defined as the difference between the ambient (actual) vapor pressure and the saturation vapor pressure of the water present in the atmosphere at a given temperature. Because VPD has a nearly straight-line relationship with the rate of evapotranspiration, it is a strong measure of the evaporative demand of the atmosphere

Figure 2. Weather data during the research period vs. long-term trends: (a) average monthly air temperature (T), (b) monthly total incoming solar radiation (Rs), (c) average monthly wind speed (WS) adjusted for measurement height of 2 m, and (d) average monthly vapor pressure deficit (VPD).

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above the field surface. The 2006/2007 and 2007/2008 nongrowing seasons exhibited a lower atmospheric evaporative demand than the 26-year averages for December, January, and February, whereas the atmospheric evaporative demand for the 2008/2009 non-growing season was higher than the long-term normal for January, February, and March. Past studies have indicated that one unit change in VPD can result is as much as 10% to 30% change in the estimated reference (potential) ET (Saxton, 1975; Sadler and Evans, 1989; Yoder et al., 2005; Irmak et al., 2006). In the present research, the largest difference in VPD between the 2006/2007, 2007/2008, and 2008/2009 non-growing seasons and the long-term averages was less than 0.25 kPa; hence, the seasonal differences in VPD could not have contributed substantially to the differences in ET. The T, Rs, WS, and VPD in each of the three dormant periods were similar in trend and magnitude to the long-term averages, indicating that all three periods studied were representative of the typical non-growing season weather conditions expected at the research location.
Figure 3 shows the precipitation distribution as well as cumulative precipitation during the non-growing seasons. In 2006/2007, very little precipitation occurred early in the season, and most of the precipitation occurred in the second half of the season, with peaks in mid-February and late April. Total cumulative precipitation during the 2006/2007

non-growing season was 418 mm. In 2007/2008, significant amounts of precipitation occurred early October, early to mid-December, mid- to late February, and late March through April. Total cumulative precipitation during the 2007/2008 non-growing season was 497 mm. The 2008/2009 non-growing season was different in that it started with high amounts of precipitation, followed by a dry or minimal precipitation period, but frequent amounts of precipitation until late March and late April when considerable amounts of precipitation occurred. The 2008/2009 non-growing season was relatively dry as compared to the non-growing seasons in 2006/2007 and 2007/2008. Total cumulative precipitation during the 2008/2009 nongrowing season was 360 mm.
The snow depths presented in figure 4 are estimated snow cover depths at Clay Center, Nebraska, reported by the local National Weather Service (NWS) office. The data were compiled by NWS using reports from local law enforcement, volunteer spotters, and cooperative observers. The 2006/2007 non-growing season had 23 days with snow cover on the field surface, concentrated between midJanuary and mid-February. The 2007/2008 non-growing season had 55 days with snow cover on the field surface, concentrated between early December and early February. The 2008/2009 non-growing season had 24 days with snow cover on the field surface, concentrated between mid-

Figure 3. Daily precipitation amounts and distribution during the non-growing seasons in (a) 2006/2007, (b) 2007/2008, and (c) 2008/2009, and (d) cumulative precipitation during the three non-growing seasons.

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Figure 4. Snow cover depth on the experimental field surface and soil temperature variation during the non-growing seasons in (a) 2006/2007, (b) 2007/2008, and (c) 2008/2009, and (d) monthly average soil temperature compared between the three non-growing seasons.

December and mid-February. Figure 4d shows that the soil temperature measured at 0.06 m below the soil surface gradually decreased from 8°C or 13°C in October to below 0°C in December and January and then gradually increased to about 8°C or 10°C in April. There was only a slight difference in interannual soil temperature changes between the three consecutive non-growing seasons.
SURFACE ENERGY BALANCE COMPONENTS Figure 5 shows daily variation of Rn, LE, H, and G dur-
ing the three consecutive non-growing seasons. Gaps in the data are days when the measuring equipment malfunctioned due to ice formation on the instrumentation and/or other issues, or days when maintenance was being performed on the BREBS. The period from October to late December is a transition from summer to winter when day length becomes shorter and the total incoming solar radiation gradually decreases. The shortening of day length runs from the summer solstice (longest day of the year), which occurs on June 21 or 22, to the winter solstice (shortest day of the year), which occurs on December 21 or 22. The period from late December to mid-March is usually associated with freezing temperatures and snowfall at the research location. The period from mid-March to April is a transition from winter cold to summer, during which the day length and total incoming solar radiation gradually increase. The transition from winter to summer runs from the

vernal equinox (day and night equal in length), which occurs on March 20 or 21, to the summer solstice.
Rn is a positive value when incoming shortwave radiation exceeds outgoing radiation, allowing the field surface to absorb energy. When the outgoing radiation is greater than the incoming radiation, Rn becomes negative. Daily values of Rn during the non-growing season ranged from 20.5 to 168.8 W m-2 with a mean of 48.5 W m-2 in 2006/2007, from -31.1 to 175.7 W m-2 with a mean of 47.4 W m-2 in 2007/2008, and from 28.2 to 162.6 W m-2 with a mean of 54.3 W m-2 in 2008/2009. The 2006/2007 non-growing season had six days with negative Rn values in January 2007, while the 2007/2008 non-growing season had 44 days with negative Rn values occurring in early December 2007, late January 2008, and mid-February 2008. The 2008/2009 non-growing season had 13 days with negative Rn, which occurred in late December and midFebruary. Rn is distributed (partitioned) as LE, H, and G components, depending on the field surface and atmospheric conditions, and their interactions. When evaporation is taking place from the field surface, there is a positive LE flux. A positive LE flux is upward (away from the field surface), indicating that the surface is losing energy to the air above. Sometimes there is also condensation of water vapor present in the atmosphere to a liquid form on the field surface. During the condensation process, the LE flux is negative, indicating that it is converted to H flux, which

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TRANSACTIONS OF THE ASABE

Figure 5. Surface energy balance components measured in the experimental field during the non-growing seasons: (a) 2006/2007, (b) 2007/2008, and (c) 2008/2009. Rn = net radiation, LE = latent heat flux (actual evapotranspiration), H = sensible heat flux, and G = ground heat flux. Gaps in the data are days when the measuring equipment malfunctioned or when maintenance was being performed.

causes an increase in the temperature of the air. Daily values of LE during the non-growing season ranged from -19.4 to 106.2 W m-2 with a mean of 24.9 W m-2 in 2006/2007, from -4.8 to 107.3 W m-2 with a mean of 28.9 W m-2 in 2007/2008, and from 0.6 to 151.8 W m-2 with a mean of 31.6 W m-2 in 2008/2009. The 2006/2007 non-growing season had seven days with negative LE values occurring in late January 2007, the 2007/2008 non-growing season had 29 days with negative LE values occurring between early December 2007 and mid-February 2008, and the 2008/2009

non-growing season had no days with negative LE values. Heat is initially transferred into the air by conduction as
air molecules collide with those of the field surface. As the air warms, it circulates upward via convection. When the surface is warmer than the air above, heat is transferred upward into the air as a positive H flux. The transfer of heat increases the air temperature but cools the surface. If the air is warmer than the surface, heat is transferred from the air to the surface, creating a negative H flux. If heat is transferred out of the air, the temperature of the air decreases

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