Numerical Validation of a Diurnal Streamflow-Pattern- Based Evapotranspiration Estimation Method

The evapotranspiration (ET) estimation method by Gribovszki et al. (2010b) has so farbeen validated only at one catchment because good quality discharge time series with the requiredhigh enough temporal resolution can probably be found at only a handful of watersheds worldwide. Tofill in the gap of measured data, synthetic groundwater discharge values were produced by a 2D finiteelement model representing a small catchment. Geometrical and soil physical parameters of thenumerical model were changed systematically and it was checked how well the model reproduced theprescribed ET time series. The tests corroborated that the ET-estimation method is applicable forcatchments underlain by a shallow aquifer. The slope of the riparian zone has a strong impact on theaccuracy of the ET results when the slope is steep, however, the method proved to be reliable forgentle or horizontal riparian zone surfaces, which are more typical in reality. Likewise, errors slightlyincrease with the decrease of riparian zone width, and unless this width is comparable to the width ofthe stream (the case of a narrow riparian zone), the ET estimates stay fairly accurate. The steepness ofthe valley slope had no significant effect on the results but the increase of the stream width (over 4m)strongly influences the ET estimation results, so this method can only be used for small headwatercatchments. Finally, even a magnitude change in the prescribed ET rates had only a small effect on theestimation accuracy. The soil physical parameters, however, strongly influence the accuracy of themethod. The model-prescribed ET values are recovered exactly only for the sandy-loam aquifer,because only in this case was the model groundwater flow system similar to the assumed, theoreticalone. For a low hydraulic conductivity aquifer (e.g. clay, silt), root water uptake creates a considerablydepressed water table under the riparian zone, therefore the method underestimates the ET. In a sandy,coarser aquifer the flow lines never become vertical even bellow the root zone, so the methodoverestimates the ET rate, thus the estimated ET values need to be corrected. Luckily the prescribedand estimated ET rates express a very high linear correlation, so the correction can be obtained by theapplication of a constant, the value of which solely depends on soil type.