Influence of landscape position on soil water flux in a cropped field

Automated soil moisture equipment can be used to compare landscape position effect on soil water fluxes; unfortunately, site-specific calibrations are often needed for the newer soil moisture probes. The purpose of this study was to demonstrate functional calibration of water content reflectometers (CS616), and to demonstrate use of soil water data to examine soil water fluxes as affected by landscape position. Soil water flux depends only on change in soil water content, so confidence in the data is increased compared with absolute water content. The CS616 period data can be converted to the square root of the apparent permittivity, which is linearly related to soil water content. For soils with high charge clays, the CS616 data are positively related to soil temperature as well as to soil water content. Each site - depth combination was first corrected for temperature, then calibrated assuming a slope of 0.09. Finally we determined diurnal hourly soil water flux, daily soil water loss for rainfree periods, and soil water increase due to rainfall events. These fluxes were compared among seven sites instrumented to 0.6 m depth. The evapotranspiration (ET) rate started increasing at 6 a.m., but soil water loss (WL) rate did not increase until 8 a.m., probably because dew evaporated. The WL was less that ET because of dew evaporation, and transpiration occurred from soil deeper than 0.6 m. For some intense rains, soil water gain (WG) was greater than total rain at some toeslope and depressional sites, probably due to runon from upslope. Usually WG after a rain was less than the total rain because the wetting front penetrated more deeply than 0.6 m, and some water was intercepted by canopy, ran off, or drained out nearby tiles.