Evapotranspiration in the Riparian Zone of the Lower Boise River with Implications for Groundwater Flow

Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite of low soil moisture content due to vegetation that is able to withdraw water from shallow aquifers. This work better defines the relationship between ET and the saturated zone by comparing the observed water table drawdown to analytically modeled drawdown in fully penetrating wells of an unconfined aquifer in response to daily ET flux. ET at the Boise Hydrogeophysical Research Site or BHRS (a riparian zone in a temperate, semi arid environment) is calculated following the approach of Batra et al. (2006) but uses site (or near site) measurements for air temperature and net radiation while relying on Landsat 5 data for quantification of vegetation. The resulting ET calculations represent a data set consisting of high resolution (30m x 30m) ET flux values obtained from minimal site measurements. Water table levels in the shallow, unconfined aquifer were monitored over the summer and fall of 2009 to record the timing and magnitude of daily fluctuations in four wells with different vegetation densities and distances from the nearby Boise River at the site. ET derived from the radiation-driven model of Batra et al. (2006) compares favorably with groundwater ET contribution rates calculated from well hydrographs (White, 1932), which further supports the finding of negligible water contribution coming from the unsaturated zone in shallow riparian aquifers in semi-arid regions (Shah et al., 2007). Using an analytical solution to ET induced drawdown (Malama & Johnson, 2010) for a semi-infinite (or river bounded) domain, daily drawdown is modeled in response to daily ET flux applied at the water table. Close agreement between observed and modeled drawdown supports the idea that ET can be estimated from well hydrographs in a wellcharacterized aquifer and conversely, if ET behavior is characterized, aquifer parameters like hydraulic conductivity and specific storage can be estimated from well hydrographs