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SOIL PHYSICS

Soil Water and Shallow Groundwater Relations in an Agricultural Hillslope

^a National Soil Tilth Lab, 2110 University Blvd., Ames, IA 50011
^b Iowa Geol. Survey (Iowa DNR), 109 Trowbridge Hall, Iowa City, IA 52242

^* Corresponding author (sally.logsdon{at}ars.usda.gov).

Shallow water tables can contribute water for plant use; therefore, plant available water includes not only the water stored in the root zone, but also the water moving up from below the root zone. The purpose of this study was to quantify the amount of water moving upward to the root zone. Automated water content reflectometers were used to monitor soil water content across a landscape in Central Iowa, which had varying shallow water tables. Either manual or automated water table depths were measured. Tipping bucket raingage and eddy covariance evapotranspiration (ET) methods were used to measure rain and evapotranspiration as part of the water balance. Upward water movement ranges were determined from water balance and uncertainties for each component (rain, ET, change in soil water content). In 2006 out of 53 dry days (days that did not have any rain), 37, 43, and 46 d showed net upward flux for shoulder, backslope, and toeslope positions, shown by an uncertainty range that did not overlap zero. In 2007, 37 out of 62 dry days showed net upward flux for the toeslope position. The mean significant net upward flux for dry days was 2.6, 3.2, and 3.1 mm d^–1 for the shoulder, backslope, and toeslope positions in 2006, and 2.5 mm d^–1 for the toeslope position in 2007. Mean ET on nonrain days was 4.0 and 4.1 mm d^–1 in 2006 and 2007. Automated equipment used to develop a water balance approach provided a quantitative approach to estimate net upward soil water flux in agricultural fields.

Abbreviations: ET, evapotranspiration • CSI, Campbell Scientific Instruments