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TECHNICAL NOTES

Estimation of Soil Water Balance Components Using an Iterative Procedure

USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012. The mention of trade or manufacturer names is made for information only and does not imply an endorsement, recommendation, or exclusion by USDA-ARS
^* Corresponding author (rschwartz{at}cprl.ars.usda.gov).

Received 11 January 2007.

Quantifying the hydrologic balance at high temporal resolution is necessary to evaluate field-scale management effects on 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. Soil water contents were monitored in 2005 at 0.5-h intervals on 12 plots instrumented with time-domain reflectometry probes at 0.05, 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 soil layer at 0.24 m, changes in soil water storage were used to iteratively fit hydraulic parameters to estimate soil water fluxes into and out of the control volume. Predicted hydraulic conductivities were not significantly different (p = 0.471) from hydraulic conductivities calculated using the iterative method during three other months in 2005 and yielded drainage rates that differed by less than 0.05 mm d^–1 as compared to calculated changes in storage below the plane of zero flux. By considering the delayed response of water content measurements to precipitation inputs, cumulative infiltration and evaporation throughout a month with 103-mm precipitation could be estimated from the measured changes in soil water storage with expected uncertainties of ± 5 mm. 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 weighing lysimeter and meterological approaches.

Abbreviations: DOY, day of year • ET, evapotranspiration • ST, sweep tillage • TDR, time-domain reflectometry • UT, untilled • VGM, van Genuchten–Mualem

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