Estimation of soil water balance components using an iterative procedure

Authors: Schwartz, Robert; Baumhardt, Roland; Howell, Terry

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2007
Publication Date: 1/23/2008
Citation: Schwartz, R.C., Baumhardt, R.L., Howell, T.A. 2008. Estimation of soil water balance components using an iterative procedure. Vadose Zone Journal. 7(1):115-123.

Interpretive Summary: Quantification of the daily hydrologic balance is necessary to evaluate field-scale management effects on infiltration and soil water storage. We developed and evaluated a procedure to estimate drainage, infiltration, and evaporation based on changes in soil water contents measured in the field. Soil water contents were monitored at 0.5 h intervals on twelve field plots. Soil hydraulic relationships were estimated using an iterative procedure during suitable periods in a given month. Once hydraulic functions were calibrated, drainage, infiltration, and evaporation could be calculated throughout the year based on measured soil water contents. The method predicted reasonable estimates of soil water drainage, infiltration, and evaporation and overcame some of the difficulties associated with traditional approaches.

Technical Abstract: Quantification of the hydrologic balance at high temporal resolution is necessary to evaluate field scale management effects on infiltration and soil water storage. Our objective was to develop and evaluate a hybrid procedure to estimate drainage, infiltration, and evaporation based on changes in plot-scale soil water storage on a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustolls). Soil water contents were monitored at 0.5 h intervals on twelve plots instrumented with time-domain reflectometery probes at .05, 0.1, 0.15, 0.2, and 0.3 m depths, and weekly using a neutron moisture meter to a depth of 2.3 m in 0.2-m increments. During periods in August 2005 when either a plane of zero flux existed or when a wetting front penetrated into an upper boundary, changes in soil water storage were used to iteratively fit hydraulic parameters to estimate soil water fluxes into and from the control volume. Predicted hydraulic conductivities were not significantly different (p=0.409) from hydraulic conductivities calculated using the iterative method during three other months in 2005 and yielded drainage rates that differed by less than 0.0049 mm d-1 as compared to calculated changes in storage below the plane of zero flux. The strategies used to partition changes in soil water storage led to reasonable estimates of infiltration and evaporation throughout a month with 103 mm precipitation. The proposed procedure permits the indirect estimation of soil water balance components useful for comparing plot-scale treatments and overcomes some of the difficulties associated with traditional approaches.