Quantifying the local‐scale uncertainty of estimated actual evapotranspiration

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Abstract
In this study, we quantified the local‐scale uncertainty of the actual evapotranspiration estimated from a water balance approach combining two soil‐water‐content measurement techniques (time‐domain reflectometry and neutron probe sounding). The local‐scale uncertainty was quantified for 28 sampling points located within a small maize cropped field. This was done by determining progressively the expected value and uncertainty on soil‐water content, soil‐water storage, soil‐water storage variation and on bottom flux. This latter term was quantified using two different hydraulic conductivity curves (HCC), derived from a pedotransfer function (PTF) and from field measurements. Results show that uncertainty on a soil water storage estimate for the considered experimental measurements in terms of standard deviation range between 9·72 and 10·37 mm. This uncertainty is related mainly to the calibration of the soil‐water content measurement instruments. For a soil‐water storage variation estimate, the uncertainty is smaller, with values ranging between 3·48 and 5·42 mm. In this case, the main component of the total variance is the instrumental variance and the variance resulting from the technique used to integrate soil‐water‐content measurements. The expected values and uncertainties for the bottom fluxes were significantly different for the two different HCC characterization techniques in some places in the field. For estimated ET fluxes with in situ estimated HCC results show that the uncertainty ranges in terms of standard deviation between 3·82 and 5·97 mm. It should be stressed that these values can be considered as very low and that a better precision is probably very difficult to obtain as all sensitive factors contributing to the ET variance have been determined in a consistent and optimal way. Finally, it is shown that the uncertainties in estimated HCC with PTFs may result in large uncertainties in estimated ET, questioning thereby the use of PTFs in characterizing the field water balance. Copyright © 2004 John Wiley & Sons, Ltd.