Project Number: 5440-11210-001-08
Project Type: Trust

Start Date: Sep 01, 2012
End Date: Sep 30, 2015

Objective:
Quantify field-scale variability in soil water availability over a growing season; to develop a protocol for using surface topographic features and soil properties to predict variability in soil water availability; identify soil databases that can substitute for soil sampling and provide the information needed to implement the developed protocol.

Approach:
Three cooperating producer fields in central Nebraska will be used for the study in 2012. These fields will exhibit significant topographic variation. All fields will be under irrigated crop production and to the extent possible uniform management (e.g., fields planted at the same population of the same variety and receiving similar fertilizer and irrigation inputs across the field for each growing season). Delineation protocols will be developed and the resulting field-scale maps will be compared to processed yield maps. Apparent EC and DEM data will be collected in parallel transects across each field. The number and spacing of transects will be based on the size of the field. The ECa data will be used as input for the ESAP model and a sampling design will be generated to calibrate the model. The sample design will consist of 12 or 20 points depending on the size of the field. Soil samples will be collected to a depth of 4 feet using a hydraulic probe. Topsoil thickness will be measured and each core sectioned into 0 – 6, 6 – 12, 12 – 24, 24 – 36, and 36 – 48 inch increments. Bulk density will be determined using oven dry mass and volume of soil for each increment. Soil organic matter content will be determined by loss on ignition or estimated using soil organic carbon values. Texture will be determined using the hydrometer method. Soil water holding capacity will be calculated for each increment using bulk density, organic matter content, and texture. Soil water holding capacity will be summed for the 0 to 4 foot depth at each sample location. Soil water holding capacity and topsoil depth for the calibration sample sites will be combined with the ECa data and the resulting ESAP model will be used to estimate these soil properties at each ECa point in the field. An access tube will be inserted at the 20 points where soil samples were collected for soil property estimation. Soil water content will be measured weekly using a Dynamax soil moisture probe. These weekly measurements will be rank ordered and variation over the season calculated. In addition, ARS will soil sample three to four cooperator identified fields in western NE. Within each of these fields infiltration rate, depth of top soil, and water holding capacity will be determined at eight landscape positions.