## 1. Introduction

[2] Riparian areas are key to understanding regional water and energy balances, particularly in semiarid regions. However, seasonal evapotranspiration (ET) in riparian areas typically have high levels of uncertainty which limit our ability to accurately estimate the groundwater portion of water budgets [*Goodrich et al*., 2000]. Riparian zones in semiarid regions often exhibit high rates of ET in spite of low-soil moisture content due to vegetation that is able to withdraw water from permanent or seasonal ground water sources. Phreatophytes, such as cottonwoods (*Populus* spp.) and willows (*Salix* spp.), in temperate, semiarid zones, are deep-rooted vegetation that fulfill a significant amount of their transpiration needs directly from the saturated zone. Since these plants are in direct contact with the saturated zone, daily changes in water table levels can be seen as a direct response to vegetation transpiration (assuming other influences are negligible or otherwise accounted for). Transpiration generally follows the diurnal solar radiation cycle causing the water table to decline throughout the day as the plants move water from the saturated zone out through the leaves. During the night, transpiration becomes negligible and the water table recovers due to net inflow from the farfield [*Loheide et al*., 2005] to replace water lost through ET.

[3] *White* [1932] presented a simple model to estimate ET from diurnal fluctuations of the water table. The method calculates the groundwater component of evapotranspiration (ET* _{g}*) from the empirical relationship

*ET*=

_{g}*S*+

_{y}δS*R*, where

*S*[−] is the specific yield,

_{y}*δS*[L/T] is the net change in water table position for 1 period (1 day), and

*R*[

*L*/

*T*] is the net recovery rate of the ground water.

*R*can be calculated over a time of day (commonly 00:00–04:00 AM) when ET is assumed to be negligible with

*R*equal to change in head [

*L*] over change in time [

*T*]. Studies have suggested improvements for the White method pertaining to the specific yield parameter [

*Meyboom*, 1967;

*Loheide et al*., 2005] and the recovery rate [

*Troxel*, 1936;

*Loheide*, 2008], but the method remains popular for calculating ET from well hydrographs.

[4] *Loheide et al*. [2005] found that the method of *White* [1932] tends to overestimate ET and they presented new guidelines for its use including a more thorough method for estimating the readily available specific yield ( ) for subdaily testing. *Gribovszki et al*. [2008] applied the Dupuit approximation for saturated flow to more accurately relate the recovery rate (*R*) to water level fluctuations and show that, for riparian zones dominated by phreatophytes, hydraulic head changes in response to transpiration can occur through the full saturated zone thickness. Much of the literature using the *White* [1932] method focuses on ways to better estimate hydraulic parameters (specific yield, primarily) but these efforts could benefit from better understanding of the role of the riparian system on daily drawdown observed in wells. *Malama and Johnson* [2010] presented an analytical model for well drawdown in response to ET which shows that drawdown magnitude will increase away from the river (even within a narrowly defined riparian corridor) in response to a spatially invariant surficial ET flux. This is due to decreasing river water contributions to ET flux with distance from the river. Failure to consider the distance of the observation well from the river can result in significant error in ET calculations from well hydrographs.

[5] Using this analytical solution, *Malama and Johnson* [2010] modeled drawdown at a range of distances from the river edge (*x*) under spatially invariant ET rates. Results from this modeling show the buffering effect the river has on drawdown in an observation well. Solving for drawdown as provides a maximum drawdown for a specified ET flux and allows for a quantitative calculation of the water contributions from aquifer storage and from the river system in diurnal fluctuations of the ground water table due to ET.

[6] In this paper, we use the model of *Malama and Johnson* [2010] with field data collected at a research well field adjacent to a controlled river in a temperate, semiarid riparian zone to (a) explain local variation in ET-induced drawdown between wells and (b) estimate the proportion of water contributed to transpiration from river leakage versus aquifer storage at different wells at different distances from the river. In this analysis, we account for local variation in ET rates due to vegetation density and compare the modeled results to observed hydrographs which show that changes in drawdown with distance from a river are recognizable, significant, and can be modeled to improve management and modeling of riparian zones.