A field investigation of phreatophyte‐induced fluctuations in the water table

Abstract

[1] Hydrographs from shallow wells in vegetated riparian zones frequently display a distinctive pattern of diurnal water table fluctuations produced by variations in plant water use. A multisite investigation assessed the major controls on these fluctuations and the ecohydrologic insights that can be gleaned from them. Spatial and temporal variations in the amplitude of the fluctuations are primarily a function of variations in (1) the meteorological drivers of plant water use, (2) vegetation density, type, and vitality, and (3) the specific yield of sediments in the vicinity of the water table. Past hydrologic conditions experienced by the riparian zone vegetation, either in previous years or earlier within the same growing season, are also an important control. Diurnal water table fluctuations can be considered a diagnostic indicator of groundwater consumption by phreatophytes at most sites, so the information embedded within these fluctuations should be more widely exploited in ecohydrologic studies.

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…Likewise, the principle drivers of water use by vegetation in GDEs were aquifer attributes (S y , regional groundwater flow), meteorology (solar radiation, vapour pressure deficit), environmental stress, and vegetation attributes (LAI, species composition) (Cleverly et al, 1997;Perkins and Sophocleous, 1999;Dahm et al, 2002;Cleverly et al, 2006;Butler et al, 2007;Lautz, 2008;Abudu et al, 2010). In general, these controls are observed in the wider literature on the controls of vegetation water use Whitley et al, 2009).…”

“…In some circumstances groundwater uptake by vegetation can be seen as a diel fluctuation in the depth-to-groundwater , as first identified in groundwater hydrographs by White (1932). These daily fluctuations in depth-to-groundwater cease when the water table falls below the rooting zone (Butler et al, 2007) or when vegetation is dormant (Lautz, 2008;Martinet et al, 2009;Miller et al, 2010). However, changes in the density of water with temperature can cause expansion and contraction of an aquifer (Post and von Asmuth, 2013), leading to the erroneous conclusion that the vegetation is accessing groundwater.…”

“…This approach has been successfully applied in the Okavango Delta in Botswana (Bauer et al, 2004), an upland grassland catchment in central Argentina (Engel et al, 2005), an oak/grassland site on the Great Hungarian Plain of eastern Hungary (Nosetto et al, 2007), the Sopron Hills of western Hungary (Gribovszki et al, 2008), the Gobi desert of northwestern China (Wang et al, 2014), and various sites in the USA (Butler et al, 2007;Lautz et al, 2008;Martinet et al, 2009). The White method tends to over-estimate ET g (Loheide et al, 2005;Martinet et al, 2009).…”

“…4 the oscillating curve represents the cycle of groundwater drawdown arising from evapotranspiration (ET) during the day followed by a "rebound" of the water table when ET returns to zero at night. The dashed straight line (with slope = r) provides an estimate of the recovery rate, which is how fast the water table rises in the absence of groundwater use (Butler et al, 2007). After accounting for recovery, the daily drawdown of the water table is scaled by the effective specific yield (S y ), or the volume of water (per unit surface area of an unconfined aquifer) released from the soil pores with a given change in depth-to-groundwater (White, 1932):…”

An analytic solution for groundwater uptake by phreatophytes spanning spatial scales from plant to field to regional

“…Their presence is readily observed in diurnal water table fluctuations [5], whereby hydraulic lift from groundwater influences not only the distribution and abundance of the species which use it but also the entire hydrologic balance of the community [6]. Evapotranspiration from phreatophytes has been estimated to represent most of the 20-50% of total long-term water budget depletions in large river systems that are ascribed to natural vegetation [7], and survival of some species in arid systems has been shown to depend completely on a plant's ability to tap water from permanent water tables [8].…”