ERROR ANALYSIS OF AN EVAPORATION METHOD FOR DETERMINING HYDRODYNAMIC PROPERTIES IN UNSATURATED SOIL

Solving soil unsaturated flow problems requires knowledge of the water retention theta(h) and unsaturated hydraulic conductility K(theta) re lationships. The purpose of this study was to investigate more thoroug hly the properties and accuracy of an evaporation method described by Wind (1969) for determining theta(h) and K(theta) from laboratory core s. Evaporation from a vertical column of soil was first simulated usin g numerical solution of Richards equation for a given set of soil hydr aulic properties. The simulated data were then used to evaluate the ab ility of Wind's method to provide estimations of the retention and uns aturated conductility curves when measurement errors were taken into a ccount. The main sources of error were (i) errors due to the position of the tensiometers in the sample, (ii) errors due to the calibration of the transducers used for the pressure head measurements, and (iii) errors due to layering in the soil column. The estimated water retenti on curves were sensitive only to soil layering. On the other hand, sma ll uncertainties in the position of the tensiometers (1 or 2 mm), and in the calibration curve of the transducers for the pressure head meas urements (1-5%) had a great influence on the estimated hydraulic condu ctivity curves. A correction procedure was proposed and was satisfacto ry when errors of position in the tensiometers were taken into account . Results also showed that temperature corrections related to viscosit y of liquid water were targe. Finally, this method gives poor estimate s of hydraulic conductivities when raw tensiometric data are corrupted with small errors in their position or calibration.