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

The evapotranspiration (ET) estimation method by Gr ibovszki et al. (2010b) has so far been validated only at one catchment because good q uality discharge time series with the required high enough temporal resolution can probably be fou nd at only a handful of watersheds worldwide. To fill in the gap of measured data, synthetic groundw ater discharge values were produced by a 2D finite element model representing a small catchment. Geome trical and soil physical parameters of the numerical model were changed systematically and it was checked how well the model reproduced the prescribed ET time series. The tests corroborated t hat the ET-estimation method is applicable for catchments underlain by a shallow aquifer. The slop e of the riparian zone has a strong impact on the accuracy of the ET results when the slope is steep, however, the method proved to be reliable for gentle or horizontal riparian zone surfaces, which are more typical in reality. Likewise, errors sligh tly increase with the decrease of riparian zone width, and unless this width is comparable to the width of the stream (the case of a narrow riparian zone), th e ET estimates stay fairly accurate. The steepness of the valley slope had no significant effect on the r esults but the increase of the stream width (over 4 m) strongly influences the ET estimation results, so t his method can only be used for small headwater catchments. Finally, even a magnitude change in the prescribed ET rates had only a small effect on the estimation accuracy. The soil physical parameters, however, strongly influence the accuracy of the method. The model-prescribed ET values are recovere d exactly only for the sandy-loam aquifer, because only in this case was the model groundwater flow system similar to the assumed, theoretical one. For a low hydraulic conductivity aquifer (e.g. clay, silt), root water uptake creates a considera bly depressed water table under the riparian zone, ther efore the method underestimates the ET. In a sandy, coarser aquifer the flow lines never become vertica l even bellow the root zone, so the method overestimates the ET rate, thus the estimated ET va lues need to be corrected. Luckily the prescribed and estimated ET rates express a very high linear c orrelation, so the correction can be obtained by th e application of a constant, the value of which solel y depends on soil type

Hydrologic Impacts of Saltcedar Control Along a Regulated Dryland River

Tens of millions of dollars have been spent to control Tamarix (saltcedar) trees along waterways in the Southwestern United States for the purpose of increasing streamflow yet no increase in streamflow has been demonstrated. The Pecos River Ecosystem Project (PREP) served as a case study to characterize surface and groundwater interaction along the Pecos River in Texas, assess the influence of saltcedar transpiration on stream stage and water table fluctuations, and evaluate the impacts of large-scale saltcedar control on baseflows. This is the first study that has investigated the influence of saltcedar transpiration on surface and groundwater interaction and the first to provide a mechanistic explanation for the lack of measurable increase in streamflow. Neither saltcedar transpiration nor saltcedar removal influenced hydraulic gradients, streambank seepage, or stream elevations. The results of the plot scale studies indicate saltcedar transpiration along the Pecos River is lower than reported elsewhere and therefore may not yield detectable increases in baseflow. To extend the study to a much larger scale, we analyzed annual baseflows at the downstream end of 340 km river reach from 1999 (pretreatment) through 2009. Surprisingly, baseflows declined for four years after the project began despite additional acreages of saltcedar treatment each year. However, baseflow surged in 2005 and remained higher than the pretreatment year (1999) through 2009. Additional detailed analyses of reservoir release and delivery records and rainfall are needed to better understand contributions of rainfall and flow regulation to this increase. Tracer based studies to determine the relative contributions of releases and groundwater would also enable a better interpretation of the change in baseflows. We did not investigate any other reported benefits, such as restoration of native plant species, or reduced soil salinity, of saltcedar control