Comparative hydrology across AmeriFlux sites: The variable roles of climate, vegetation, and groundwater

Abstract

[1] Watersheds can be characterized as complex space-time filters that transform incoming fluxes of energy, water, and nutrients into variable output signals. The behavior of these filters is driven by climate, geomorphology, and ecology and, accordingly, varies from site to site. We investigated this variation by exploring the behavior of evapotranspiration signals from 14 different AmeriFlux sites. Evapotranspiration is driven by water and energetic forcing and is mediated by ecology and internal redistribution of water and energy. As such, it integrates biological and physical controls, making it an ideal signature to target when investigating watershed filtering. We adopted a paradigmatic approach (referred to as the null model) that couples the Penman-Monteith equation to a soil moisture model and explored the deviations between the predictions of the null model and the observed AmeriFlux data across the sites in order to identify the controls on these deviations and their commonalities and differences across the sites. The null model reproduced evapotranspiration fluxes reasonably well for arid, shallow-rooted systems but overestimated the effects of water limitation and could not reproduce seasonal variation in evapotranspiration at other sites. Accounting for plant access to groundwater (or deep soil moisture) reserves and for the effects of soil temperature on limiting evapotranspiration resolved these discrepancies and greatly improved prediction of evapotranspiration at multiple time scales. The results indicate that site-specific hydrology and climatic factors pose important controls on biosphere-hydrosphere interactions and suggest that plant-water table interactions and early season phenological controls need to be incorporated into even simple models to reproduce the seasonality in evapotranspiration.

Developing predictive insight into changing water systems: use-inspired hydrologic science for the Anthropocene

“…Strategically, many existing observation networks could be leveraged to generate valuable data sets for interdisciplinary hydrology (see Table 2). For instance, the Fluxnet data sets provide unrivalled, high resolution information about evapo-transpiration dynamics, but insufficient soil moisture observations to allow water budget closure (Thompson et al, 2011b). Adding these soil moisture observations would increase the hydrologic value of these sites at a relatively small cost, compared to the costs of the eddy-covariance instrumentation already present.…”

“…Comparative hydrology supersedes case study or watershed-specific approaches to hydrologic research, and has the potential to separate sitespecific phenomena from generalizable trends (Falkenmark and Chapman, 1989;McDonnell et al, 2007). With information about hydrological behavior in a large number of watersheds, robust testing of hypotheses generated from comprehensive investigations of a small number of sites (Thompson et al, 2011b) can be generalized via the statistical power of a large data set. Alternatively, statistical and top-down modeling approaches (Sivapalan et al, 2003) applied to many watersheds can be used to identify and explore emergent patterns (Voepel et al, 2011), which can then become targets of hypothesis formation and mechanistic inquiry.…”

“…In recent years, comparative studies have also addressed the classification of vegetation biomes via water balance (Stephenson, 1990(Stephenson, , 2003, hydro-climatic controls on NDVI and transpiration (Thompson et al, 2011b;Voepel et al, 2011), controls on hydrologic regimes Olden et al, 2011;Sawicz et al, 2011), the relationship between available energy and soil development (Pelletier et al, 2013;Rasmussen, 2012;Rasmussen et al, 2011), the implications of different management regimes (Huntjens et al, 2011;Borowski et al, 2008) on water resources, and -in an initial sense -emergent patterns in human-hydrologic interactions (Srinivasan et al, 2012). Figure 5 illustrates three different relationships that emerge from 430 watersheds when considering patterns of hydro-climate (Budyko's curve), geomorphology (Abrahams curve) and vegetation cover (aridity -NDVI curve).…”

Soil moisture controls on patterns of grass green-up in Inner Mongolia: an index based approach

“…Thompson et al (2011) and Ye et al (2012) showed that not accounting for vegetation phenology can lead to poor predictions of both seasonal evaporation and runoff variability in the northeastern USA, and that a temperature correction that mimics the phenology can help to improve the predictions. More globally, Cayan et al (2001) showed that in areas of the world with significant deciduous forests, changes in transpiration associated with phenology can contribute to significant changes in seasonal runoff.…”

Species‐specific transpiration responses to intermediate disturbance in a northern hardwood forest

“…These processes, and their interactions with changes to canopy structure and species composition could also accumulate to affect total transpiration over longer (daily-seasonal-annual) time scales by affecting water use efficiency and the effective degree of daily stomata-closing stresses the trees experience post disturbance. By analyzing and understanding these fast daily dynamics rather than longer-term periods (monthly) or periods of drought stress, it may be possible to improve modeling of transpiration at the subdaily time step [Thompson et al, 2011] as well as to show more realistic coupling between hydrodynamic phenomena and gross primary productivity in land surface models [Fatichi and Ivanov, 2014].…”