Field, laboratory and estimated soil-water content limits

Abstract

For the purpose of irrigation scheduling, estimates of soil-water content limits are determined using field or laboratory measurements
or empirically-based regression equations. In this study the field method involved measuring simultaneously the soil-water content (using a frequency domain reflectometer with the PR1 profile probe that relies on changes in the dielectric constant of soil), and soil-water potential (using Watermark granular matrix sensors and tensiometers) at three depths (100, 300 and 600 mm) from a 1 m2 bare plot. A retentivity relationship was developed from these measurements and the drained upper limit was estimated to be 0.355 m3·m-3 when the drainage from the pre-wetted surface was negligibly small. The lower limit, corresponding to -1 500 kPa, was estimated to be 0.316 m3·m-3. In the laboratory, soil-water content and soil matric potential were measured on undisturbed soil samples taken from the edge of the bare plot. The undisturbed soil samples were saturated and exposed to different matric potentials between -1 and -1 500 kPa. A retentivity relationship was then developed from these measurements. The laboratory method realized a drained upper limit value of 0.390 m3·m-3 at -33 kPa and a lower limit value of 0.312 m3·m-3 at -1 500 kPa. A regression equation, which uses the soil bulk density and the clay (<0.002 mm) and silt (0.002 to 0.05 mm) percentage to estimate the soil-water content at a given soil-water potential, realised a drained upper limit value of 0.295 m3·m-3 at -33 kPa and a lower limit value 0.210 m3·m-3 at -1 500 kPa. Comparisons were made between field, laboratory and regression equation methods of estimating the upper and lower soil-water content limits. The field-measured soil-water content was statistically different from the laboratory-estimated and regression equation estimates of soil-water content. This was shown from a paired t-test, where the probability levels for the laboratory and regression equation methods were 0.011 and 0.0005 respectively at the 95 % level of significance. Field method soil-water content comparisons with the laboratory method resulted in a linear regression coefficient of determination of 0.975 with a root mean square error (RMSE) of 0.064 m3·m-3. By contrast, field method comparisons with the regression equation method showed a coefficient of determination of 0.995 with an RMSE of 0.035 m3·m-3. The frequency domain reflectometry method used for monitoring soil-water content has been shown to be useful in this case of relatively homogenous soils supporting perennial crops.

Water SA Vol.32 (2) 2006: pp.155-162