ARS | Publication request: Soil water and shallow groundwater relations in an agricultural hillslope

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 16, 2009
Publication Date: N/A

Interpretive Summary: The amount of soil water available to a plant depends not only on how much water the soil can hold, but also the position on a hillside. This study showed that areas that had the water table within 4 1/2 feet of the soil surface had more water moving up to the root zone than did areas with deeper water table depths (up to 10 feet). This extra water in the soil root zone could be an important source of water for growing crops; however, the water could also affect movement of leached chemicals back into the root zone, water loss to the atmosphere that varies by position on the hill, and tile drainage losses. The information is important for scientists who study the water balance in agricultural fields. Ultimately, the information is needed for developing management practices that effectively use soil water in rainfed areas and for the most efficient precision farming operations.

Technical Abstract: 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) measurements measured the other components 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 non-rain days, 37, 43, and 46 had 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 non-rain days showed net upward flux for the toeslope position. The mean significant net upward flux for nonrain days was 2.6, 3.2, and 3.1 mm for the shoulder, backlsope, and toeslope positions in 2006, and 2.5 mm for the toeslope position in 2007. Mean ET on nonrain days was 4.0 and 4.1 mm 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.