S. J. van Donk

Crop Residue Cover Effects on Evaporation, Soil Water Content, and Yield of Deficit-Irrigated Corn in West-Central Nebraska

Competition for water is becoming more intense in many parts of the U.S., including west-central Nebraska. It is believed that reduced tillage, with more crop residue on the soil surface, conserves water, but the magnitude of water conservation is not clear. A study was initiated on the effect of residue on soil water content and corn yield at North Platte, Nebraska. The experiment was conducted in 2007 and 2008 on plots planted to field corn (Zea mays L.). In 2005 and 2006, soybean was grown on these plots. There were two treatments: residue-covered soil and bare soil. Bare-soil plots were created in April 2007. The residue plots were left untreated. In April 2008, bare-soil plots were recreated on the same plots as in 2007. The experiment consisted of eight plots (two treatments with four replications each). Each plot was 12.2 m × 12.2 m. During the growing season, soil water content was measured several times in each of the plots at six depths, down to a depth of 1.68 m, using a neutron probe. The corn crop was sprinkler-irrigated but purposely water-stressed, so that any water conservation in the residue-covered plots might translate into higher yields. In 2007, mean corn yield was 12.4 Mg ha-1 in the residue-covered plots, which was significantly (p = 0.0036) greater than the 10.8 Mg ha-1 in the bare-soil plots. Other research has shown that it takes 65 to 100 mm of irrigation water to grow this extra 1.6 Mg ha-1, which may be considered water conservation due to the residue. In 2008, the residue-covered soil held approximately 60 mm more water in the top 1.83 m compared to the bare soil toward the end of the growing season. In addition, mean corn yield was 11.7 Mg ha-1 in the residue-covered plots, which was significantly (p = 0.0165) greater than the 10.6 Mg ha-1 in the bare-soil plots. It would take 30 to 65 mm of irrigation water to produce this additional 1.1 Mg ha-1 of grain yield. Thus, the total amount of water conservation due to the residue was 90 to 125 mm in 2008. Water conservation of such a magnitude will help irrigators to reduce pumping cost. With deficit irrigation, water saved by evaporation is used for transpiration and greater yield, which may have even greater economic benefits. In addition, with these kinds of water conservation, more water would be available for competing needs.

COMPARISON OF THE WEIBULL MODEL WITH MEASURED WIND SPEED DISTRIBUTIONS FOR STOCHASTIC WIND GENERATION

Wind is the principal driver of the Wind Erosion Prediction System (WEPS), which is a processbased computer model for the simulation of windblown sediment loss from a field. WEPS generates wind using a stochastic wind generator. The objectives of this study were to improve the stochastic generation of wind speed and direction and to update the wind statistics used by the generator with statistics derived from more recent, qualitycontrolled data for the 48 contiguous states of the U.S. Erosive wind power density (WPD) was chosen to evaluate how well wind is generated, since it is proportional to sediment transport by wind. It is important that WPD calculated from stochastically generated data (WPDg) closely reproduces WPD calculated from the underlying measured data (WPDm). The commonly used twoparameter Weibull model did not fit wind speed distributions well enough for application in wind erosion models. WPDg deviated more than 20% from WPDm for 168 out of the 332 stations having WPDm > 5 W m �2 . Fitting the model to the high wind speeds only, with the expectation of a better curve fit, resulted in some generated wind speeds exceeding 100 m s �1 , which is unacceptable. A more direct method uses the wind speed distributions themselves instead of the Weibull model that describes them. Wind speeds are then generated directly from the distributions using linear interpolation between data points. With this more robust direct approach, there was only one station (down from 168 stations) where WPDg deviated more than 20% from WPDm. The direct method of wind speed generation reproduces wind speeds more accurately than the Weibull model, which is important for wind erosion prediction and may be important for other applications as well.

SOIL TEMPERATURE UNDER A DORMANT BERMUDAGRASS MULCH: SIMULATION AND MEASUREMENT

The ENergy and WATer BALance (ENWATBAL) model is a mechanistic, numerical model that simulates soil water and temperature profiles, evaporation from soil, and transpiration from crops, but it does not simulate the effects of a mulch layer. Surface vegetative mulches are becoming more common, especially in reduced -tillage systems, limiting the models applicability. Our objective was to modify ENWATBAL to enable physically based simulation of the effects of a dense mulch. As a preliminary evaluation of the model, soil temperatures simulated with the modified model were compared with those measured at Watkinsville, Georgia, in Cecil sandy loam (clayey, kaolinitic, thermic, Typic Kanhapludult) under a dense, thatchy layer of dormant bermudagrass (Cynodon dactylon, (L.) Pers.) that acted as a mulch during the simulation period. Measured daily soil temperature amplitudes at 0.04 m depth were about 2.5C during an 8-day period in December 1995. Simulated amplitudes were 12C with the original ENWATBAL model (configured for a bare soil) and 3.5C with the mulch-enhanced model. The root mean square error between hourly measured and simulated soil temperatures was 4.1C using the original ENWATBAL model and 1.1C using the mulch-enhanced model. Measured soil temperatures lagged behind those simulated, indicating that conduction may be an important process of heat transfer through the mulch. Two solution methods were tested: an iterative solution for mulch and soil surface temperatures implicit in the energy balance equations, and a linearized explicit solution of the energy balances. The latter method was 50 times faster than the iterative method without compromising accuracy; the largest linearization error was only 0.01C. The capability to simulate mulch effects increases the scope of problems where ENWATBAL is applicable.

CROP RESIDUE IN NORTH DAKOTA : MEASURED AND SIMULATED BY THE WIND EROSION PREDICTION SYSTEM

Residue cover is very important for controlling soil erosion by water and wind. Thus, the Wind Erosion Prediction System (WEPS) includes a model for the decomposition of crop residue. It simulates the fall rate of standing residue and the decomposition of standing and flat residue as a function of temperature and moisture. It also calculates residue cover from flat residue mass. Most of the data used to develop and parameterize this model have been collected in the southern U.S. We compared WEPS-simulated residue cover with that measured in south-central North Dakota for 50 two-year cropping sequences from nine crops species that were grown using no-till management. Measured data included residue mass at the time of harvest and residue cover just after seeding the next spring. Simulated residue cover significantly (P < 0.05) underestimated measured cover for 33 out of the 50 simulated cropping sequences and overestimated measured cover for five cropping sequences. Some of the differences may be explained by the fact that, for many WEPS crops, residue decomposition parameters are not based on measured field data, but on expert judgment. In addition, WEPS did not predict any stem fall for most of the crops during winter, which contradicts observations that storms flatten many residue stalks of crops such as sunflower. In addition to stem fall and residue decay by biological means, which are driven by temperature and moisture, the model needs to explicitly simulate stem fall by mechanical forces, such as wind- and snowstorms, which are important in northern climates. Furthermore, WEPS does not model the migration of unanchored residue caused by rain- or windstorms, although this does affect residue mass-to-cover ratios and susceptibility to erosion. This study will help improve the WEPS decomposition model and its parameterization, but more data on residue decay and stem fall are needed for different climates and crops to ensure the applicability of the model over a wide range of conditions.

Corn residue stocking rate affects cattle performance but not subsequent grain yield.

This study investigated effects of stocking rate on cattle performance, quality and quantity of corn residue, and impact of residue removal on grain yield for 5 yr at the University of Nebraska - Lincoln West Central Water Resources Field Laboratory near Brule, NE. Four removal treatments-1) no removal (control), 2) grazing at 2.5 animal unit month (AUM)/ha, 3) grazing at 5.0 AUM/ha, and 4) baling-were applied to a center pivot-irrigated corn field (53 ha). The field was divided into eight 6.6-ha paddocks to which replicated treatments were assigned. Samples of residue were collected in October and March (before and after residue removal) using ten 0.5-m quadrats per treatment replication. Residue was separated into 5 plant parts-stem, cob, leaf, husk, and grain-and analyzed for nutrient content. Esophageally fistulated cattle were used to measure diet quality. Cattle assigned to the 2.5 AUM/ha stocking rate treatment gained more BW ( < 0.01) and BCS ( < 0.01) than cattle assigned to the 5.0 AUM/ha treatment. Leaf contained the most ( < 0.01) CP and husk had the greatest ( < 0.01) in vitro OM disappearance (IVOMD) but the CP and IVOMD of individual plant parts did not differ ( > 0.69) between sampling dates. Amount of total residue was reduced ( < 0.05) by baling and both grazing treatments between October and March but was not different ( > 0.05) in control paddocks between sampling dates. As a proportion of the total residue, stem increased ( < 0.01) and husk decreased ( < 0.01) between October and March. Diet CP content was similar ( = 0.10) between sampling dates for the 2 grazing treatments but IVOMD was greater after grazing in the 2.5 AUM/ha grazing treatment ( = 0.04). Subsequent grain yields were not different ( = 0.16) across all 4 residue removal treatments. At the proper stocking rate, corn residue grazing results in acceptable animal performance without negatively impacting subsequent corn grain production.

MEASUREMENT AND MODELING OF HEAT TRANSFER MECHANISMS IN MULCH MATERIALS

Crop residues or mulches affect soil temperature influencing plant growth and related processes in the soil. A hot/cold plate combination was used to quantify heat transfer through several common dry test mulch materials (rubber chips, pine straw, wheat straw) and identify and quantify heat transfer mechanisms with the goal of modeling apparent thermal conductivity of the mulch. Mulch material bulk densities ranged from near 0 kg/m3 to 33 kg/m3 , mulch thickness ranged from 61 mm to 140 mm and test temperatures ranged from 20°C to 45°C. To determine the effect of thermal radiation on heat transfer, measurements were taken with the test material between both a set of low emissivity aluminum (Al) plates and a set of high emissivity black painted plates. To quantify free convection, measurements were made in a thermally unstable configuration with the hot plate on the bottom and the cold plate on top and in a thermally stable configuration with the cold plate on the bottom and the hot plate on top. In thermally unstable situations (i.e., bottom plate hot, top plate cool), free convection and conduction mechanisms best explained the heat flux. In thermally stable conditions, radiation and conduction best explained heat flux. The percentage of heat due to thermal radiation decreased as mulch thickness and density increased in both the thermal stable and unstable conditions. The percentage of heat transfer due to free convection (unstable case) and due to conduction (stable case) generally increased as mulch thickness and density increased. For a given mulch material, the thermally unstable condition results in an increased apparent thermal conductivity (k) value. The difference between the k values for stable and unstable cases tended to diminish with pine straw or wheat straw mulches compared to air. Increasing the mulch thickness (plate spacing) resulted in the most difference with low mulch densities or no mulch. Differences are probably not statistically meaningful at the high mulch densities. For pine straw the average k was 0.11 W m–1 K–1 and for wheat straw 0.08 W m–1 K–1. Models were created to develop the radiation, conduction and convection parameters for the mulches tested, with r2 values for the estimated parameter fit ranging from 0.75 to 0.99. These models could be used to estimate the apparent k of dry mulches in the field.

APPARENT THERMAL CONDUCTIVITY OF MULCH MATERIALS EXPOSED TO FORCED CONVECTION

Soil temperature controls plant growth and many related processes in the soil. A mulch or crop residue covering the soil may alter soil temperatures significantly. Available simulation models often lack experimental data for the mulch thermal conductivity and its dependence on air velocity. The apparent thermal conductivity (k) of wheat straw, pine straw, tire chips, dry sandy soil, and the thermal resistance of Bermudagrass sods were measured using a guarded hot plate at air velocities between 0 and 5 m/s. For all mulch materials, k ranged between 0.1 and 0.6 W m–1 °C–1, and increased with increasing air velocity, except for the more compact materials such as soil and, to a lesser extent, small tire chips. We found a minimum in k around 1 m/s for the thicker (> 0.1 m) layers of wheat straw and pine straw, which was tentatively attributed to interactions between the straw and the convection (free versus forced mechanism at the 1 m/s velocity). A model was created for predicting apparent thermal conductivity through mulches in thermally unstable environments. Using estimated mulch opacity parameters and fitting convection parameters, r2 values ranging from 0.72 to 0.99 were obtained. The model may be used in field situations where the soil under a mulch is warmer than the air above the mulch, which is a typical nighttime condition. The model should be tested using independently measured data.

USING TEMPORALLY LIMITED WIND DATA IN THE WIND EROSION PREDICTION SYSTEM

The Wind Erosion Prediction System (WEPS) is a computer model for the simulation of windblown sediment loss from a field. The model is used to evaluate the effect of alternative cropping systems and management scenarios on wind erosion. WEPS requires hourly wind data, which for many locations are unavailable. Therefore, the objective of our research was to investigate whether wind speed and direction can be simulated adequately from temporally limited data and to determine suitable times of the day to take measurements if only a few measurements per day can be made. For three locations (La Junta, Colorado; Sidney, Nebraska; and Pendleton, Oregon), two statistical datasets were created to be used with the WEPS stochastic wind generator. The first was based on the full dataset with 24 hourly observations per day, and the second was based on a subset of four observations per day: at 0200, 0800, 1400, and 2000 hours local standard time (LT). Erosive wind power densities (WPD), calculated from both datasets, agreed well with each other. On an annual basis, the discrepancy was greatest for La Junta, with a difference of 0.8 W m -2 (6%). For the five most erosive months, the mean absolute WPD difference was less than 10% for all three locations. Prevailing wind erosion direction and WEPS-simulated soil loss also showed good agreement between the two data sets. Many other subsets of two, three, and four measurements per day performed as well or better than the 0200, 0800, 1400, 2000 LT subset. In spite of temporally limited wind data, it is possible to use WEPS to estimate wind erosion risks and the effectiveness of various conservation practices. The results of this study allow researchers to evaluate whether limited data, measured at certain times of the day, are suitable for use in WEPS. For a new station, if only a few measurements per day are going to be made, the results of this study may be used as a guide to choose the times of the day to take these measurements.

Effects of Crop Residue Removal on Soil Water Content and Yield of Deficit-Irrigated Soybean

Reduced tillage, with more crop residue remaining on the soil surface, is believed to conserve water, especially in arid and semi-arid climates. However, the magnitude of water conservation is not clear. An experiment was conducted to study the effect of crop residue removal on soil water content, soil quality, and crop yield at North Platte, Nebraska. The same field plots were planted to soybean (Glycine max) in 2009 and 2010. There were two treatments: residue-covered soil and bare soil. Residue (mostly corn residue in 2009 and mostly soybean residue in 2010) was removed every spring from the same plots using a flail chopper and subsequent hand-raking. The experiment consisted of eight, 12.2 m × 12.2 m, plots (two treatments with four replications each). Soybeans were sprinkler-irrigated, but purposely water-stressed, so that any water conservation in the residue-covered plots might translate into higher yields. After four years of residue removal, soil organic matter content and soil residual nitrate nitrogen were significantly smaller, and soil pH was significantly greater, in the bare-soil plots compared to the residue-covered plots. The residue-covered soil held approximately 90 mm more water in the top 1.83 m compared to the bare soil near the end of the 2009 growing season. In addition, mean soybean yield was 4.5 Mg ha-1 in the residue-covered plots, compared to 3.9 Mg ha-1 in the bare-soil plots. Using two crop production functions, it is estimated that between 74 and 91 mm of irrigation water would have been required to produce this extra 0.6 Mg ha-1. In 2010, mean soybean yield was 3.8 Mg ha-1 in the residue-covered plots, compared to 3.3 Mg ha-1 in the bare-soil plots. Between 64 and 79 mm of irrigation water would have been required to produce this extra 0.5 Mg ha-1. In both years, several processes may have contributed to the differences observed: (1) greater evaporation of water from the soil in the bare-soil treatment, and (2) greater transpiration by plants in the bare-soil treatment in the beginning of the growing season as a result of more vegetative growth due to higher soil temperatures in the bare-soil treatment.

DEVELOPMENT OF HOT/COLD PLATE APPARATUS FOR DETERMINING HEAT TRANSPORT MECHANISMS IN MULCH MATERIALS

To study the effects of mulches and crop residues on soil temperature, researchers have frequently used simulation models. In such models, quantification of heat transport within the mulch material is often weak and heat transport mechanisms are poorly understood. In this paper we describe an apparatus to quantify heat transport through dry mulch materials. In addition, heat transport mechanisms (conduction, thermal radiation, free and forced convection) can be identified and quantified using this apparatus. The apparatus consists of precisely controlled and monitored 0.9 m by 0.9 m hot and cold plates. The hot plate actually consists of three component plates: a test, a guard, and a bottom plate that are individually controlled (temperature) and monitored (temperature and power). The guard plate surrounds the test plate, minimizing undesired lateral heat flow. The bottom plate is positioned in parallel with the test and guard plates to insure that all wattage into the test plate moves off the top of the plate through the mulch. The correct functioning of the hot plate was verified using three reference materials with a known thermal resistance. The cold plate is based on techniques using thermoelectric devices (Peltier coolers). In addition, heat sinks and fans are used to transport heat away from the cold plate. A two-dimensional numerical simulation showed that errors caused by lateral heat flow in a sample contained between the hot and the cold plate can be neglected. The thermal conductivity of air was measured using the apparatus, yielding a value of 0.026 W m -1 C -1 , exactly matching the theoretical value, thus confirming