**2. Methodological approach**

#### **2.1 Water-nitrogen interaction studies**

Studies have been conducted in the Clarion-Nicollet-Webster Soil Association in central Iowa within the Walnut Creek watershed. This 5400 ha watershed was described in Hatfield et al. (1999). Production sized fields have been used as experimental units for these studies because of the need to quantify the effects of different N rates on crop yield and water use across soil types. These production fields ranged in size from 32 to 96 ha each with a similar experimental design. The experimental design was a replicated plot design with strips of different N rates being a treatment and each treatment replicated at least three times across a field. Nitrogen rates were applied in 1997 and 1998 using a starter application at planting of 50 kg ha-1 only with the second treatment having the N starter rate and the sidedress rate determined by the Late Spring Nitrate Test (LSNT). The third treatment was the starter plus a sidedress rate to represent a non-limiting N rate of an additional 150 kg ha-1. This experimental procedure was described by Jaynes et al. (2001). In 1999 and 2000, the N application procedure was modified to further refine N application rates based on the use of the leaf chlorophyll measurements and soil test based on the results obtained from the 1997 and 1998 experiments. The rates applied were 50, 100, and 150 kg ha-1 to different soils, planting rates, and plant population densities (75,000 and 85,000 plants ha-1). Nitrogen rates were applied uniformly across each field using liquid UAN (32% N) for these studies. These applications were applied with production scale equipment to mimic producer operations.

Soil N concentrations were measured prior to spring operations, after planting, and at the end of the growing season after harvest to a soil depth of 1.5 m using a 5 cm core. Cores were subdivided into depth increments to estimate the N availability throughout the root zone. Sample position was recorded with a GPS unit to ensure accurate location of each subsequent sample. Nominal plot size for plant measurements and yield determinations was 15 x 15 m. Yields were measured on 5 m length of row for five subsamples within each plot in which no plants had been removed or measured during the growing season. Plots were replicated three times within each treatment.

Crop water use rate was measured from planting to harvest with an energy balance method (Bowen ratio in 1997 and 1998, and eddy correlation in 1999 and 2000). These units consisted of a net radiometer positioned at 3 m above the canopy, soil heat flux at 10 cm (within the row, middle of the row, and adjacent to the row), wind speed, air temperature, water vapor pressure (positioned at 0.5 and 1.5 m above the canopy), three dimensional sonic anemometer and krypton hygrometer (positioned at 1 m above the canopy), and an infrared thermometer (15 fov) positioned at 45 from nadir in a south-facing direction. All measurements were recorded at 10-second intervals and either 15 minute or 30 minute averages stored in the data acquisition unit. Data were screened to ensure proper data quality and then converted to equivalent water depth and summed for the growing season to determine crop water use. Within each soil-N combination we had a single energy balance station and this measurement technique provides a sample representative of a 50 m2 area. These instruments were positioned to provide a measurement of the representative area around the plant measurements. Missing data for a given station were treated by using the relationships among instruments within a soil type across N management practices to determine the relationships among variables. These relationships were then used to fit in

Studies have been conducted in the Clarion-Nicollet-Webster Soil Association in central Iowa within the Walnut Creek watershed. This 5400 ha watershed was described in Hatfield et al. (1999). Production sized fields have been used as experimental units for these studies because of the need to quantify the effects of different N rates on crop yield and water use across soil types. These production fields ranged in size from 32 to 96 ha each with a similar experimental design. The experimental design was a replicated plot design with strips of different N rates being a treatment and each treatment replicated at least three times across a field. Nitrogen rates were applied in 1997 and 1998 using a starter application at planting of 50 kg ha-1 only with the second treatment having the N starter rate and the sidedress rate determined by the Late Spring Nitrate Test (LSNT). The third treatment was the starter plus a sidedress rate to represent a non-limiting N rate of an additional 150 kg ha-1. This experimental procedure was described by Jaynes et al. (2001). In 1999 and 2000, the N application procedure was modified to further refine N application rates based on the use of the leaf chlorophyll measurements and soil test based on the results obtained from the 1997 and 1998 experiments. The rates applied were 50, 100, and 150 kg ha-1 to different soils, planting rates, and plant population densities (75,000 and 85,000 plants ha-1). Nitrogen rates were applied uniformly across each field using liquid UAN (32% N) for these studies. These applications were applied with production scale equipment to mimic producer operations. Soil N concentrations were measured prior to spring operations, after planting, and at the end of the growing season after harvest to a soil depth of 1.5 m using a 5 cm core. Cores were subdivided into depth increments to estimate the N availability throughout the root zone. Sample position was recorded with a GPS unit to ensure accurate location of each subsequent sample. Nominal plot size for plant measurements and yield determinations was 15 x 15 m. Yields were measured on 5 m length of row for five subsamples within each plot in which no plants had been removed or measured during the growing season. Plots

Crop water use rate was measured from planting to harvest with an energy balance method (Bowen ratio in 1997 and 1998, and eddy correlation in 1999 and 2000). These units consisted of a net radiometer positioned at 3 m above the canopy, soil heat flux at 10 cm (within the row, middle of the row, and adjacent to the row), wind speed, air temperature, water vapor pressure (positioned at 0.5 and 1.5 m above the canopy), three dimensional sonic anemometer and krypton hygrometer (positioned at 1 m above the canopy), and an infrared thermometer (15 fov) positioned at 45 from nadir in a south-facing direction. All measurements were recorded at 10-second intervals and either 15 minute or 30 minute averages stored in the data acquisition unit. Data were screened to ensure proper data quality and then converted to equivalent water depth and summed for the growing season to determine crop water use. Within each soil-N combination we had a single energy balance station and this measurement technique provides a sample representative of a 50 m2 area. These instruments were positioned to provide a measurement of the representative area around the plant measurements. Missing data for a given station were treated by using the relationships among instruments within a soil type across N management practices to determine the relationships among variables. These relationships were then used to fit in

**2. Methodological approach** 

**2.1 Water-nitrogen interaction studies** 

were replicated three times within each treatment.

any missing data using an approach similar to the method described by Hernandez-Ramirez (2010). Generally the length of any missing data was less than 3 hours. The amount of missing data for these experiments was less than 3% of the total data record.

Crop transpiration rates were estimated from an energy balance model that determined the soil water evaporation rate based on the leaf area index of the crop and previous precipitation amounts following the approach described by Ritchie and Burnett (1971). Soil water evaporation rates were estimated from a surface energy balance model based on crop residue cover amounts and the energy balance. Precipitation for these studies was available from a tipping bucket raingauge located at a meteorological station within 1 km of the field sites.

Crop growth and development were measured in a variety of ways. In 1997 measurements were made of yield at harvest. Beginning in 1998 and 1999, a more intensive plant regime of weekly plant measurements consisting of leaf area, phenological stage, number of leaves, dry weight, and plant height were made on 10 plants from each plot and each plot was replicated three times. In 2000, the frequency was decreased to four destructive plant samplings to represent the 6-leaf stage, 12-leaf stage, tasseling, and mid-grain fill. Leaf chlorophyll measurements were made on 30 plants in each plot with a leaf chlorophyll meter at two times per week commencing with the 6 leaf stage and continuing through late grain fill. The upper leaf was measured at the mid-leaf position until the tassel appeared then the leaf immediately above the ear position was measured. Leaf carbon and N contents were determined on dry, ground samples from same stages as the 2000 plant samples were collected. For 1999 and 2000 experiments, stalk sugar content was measured with a refractive method using sap collected from freshly cut stalks. This was done on ten plants from each plot. Grain quality parameters of protein, oil, and starch were measured on subsamples of grain collected from the hand-harvest samples. Field yields were measured with yield-monitors mounted on the producers combine. These data were registered with a GPS unit to obtain field locations.

Data analyses for these studies are based on crop yield, total seasonal water use, and N application. Water use and N use efficiency was determined by the ratio of crop yield to either seasonal crop transpiration or N application rates. Intensive plant sampling data are not described in this report but were used to understand the dynamics of plant response to changes in within season N management decisions. Likewise, the leaf chlorophyll and stalk sugar content data were used to guide decisions in the 1999 and 2000 experiments. These data sets represent a complete analysis of crop-soil-water-N interactions.
