Entrapped air in soils beneath the water table is one of the key factors controlling the hydraulic behavior under conditions of ponded infiltration, in perched waters, and in unconfined aquifers. The term quasi-saturated soils defines the soils with entrapped air, and the term quasi-saturated hydraulic conductivity defines the relationship between the hydraulic conductivity and entrapped air content. This paper focuses on an investigation of how entrapped air, along with other factors, affects the three-stage temporal behavior of the quasi-saturated hydraulic conductivity of soils. During the first stage the quasi-saturated hydraulic conductivity of soils decreases by as much as 5-8 times, presumably because mobile entrapped air blocks the largest pores. During the second stage, as the mobile entrapped air is discharged from the core, the quasi-saturated hydraulic conductivity of the soils slowly increases. When the mobile air is removed, the remaining immobile entrapped air is discharged as a dissolved phase, and the. quasisaturated hydraulic conductivity increases rapidly by about 1-2 orders of magnitude, essentially reaching the value of the saturated hydraulic conductivity. During the third stage the hydraulic conductivity is decreased to minimum values. The effects of sealing at the soil surface and microbiological activities are assumed to be major factors in the final decrease of the hydraulic conductivity. This three-stage temporal behavior of percolation in loam soils is repeatable. A new power law and an exponential relationship are proposed to describe the quasi-saturated hydraulic conductivity of loams as a function of the entrapped air content. in developing (1) a theory of two-phase flow [Morel-Seytoux, 1973], (2) methods for field investigations and monitoring [Stephens et al., 1984], (3) methods for managing groundwater systems for many practical applications, such as irrigation and drainage systems [Powers, 1934; Aver3:anov, 1950; Luthin, 1957], injection of water in the vadose zone [Stephens and Neuman, 1982a; Stephens et al., 1984], secondary water recovery [Moridis and Reddell, 1991a, b], remediation of waste site soils using air barriers [Wilson and Clarke, 1994], artificial recharge of groundwater [Todd, 1980], and (4) designing waste Copyright 1995 by the American Geophysical Union. Paper number 95WR01654. 0043-1397/95/95WR-01654505.00 water treatment plants using filtration and aeration [Sekoulov, 1982; Amirtharajah, 1993]. Air from the atmosphere can migrate down to depths of several meters [Massmann and Farrier, 1992] or as much as 100 m [Montazer et al., 1988] as the atmospheric pressure fluctuates. In the case of soil contamination by volatile organic compounds (VOC), air in the vadose zon e can become mixed with VOC [Cho et al., 1993]. If part of this air has been immobilized as entrapped gas, it can be a long-term source of VOC in the soils. The VOC contained in this immobile air phase is not easily removed during air extraction. Consequently, the effectiveness of remediation activit...

Abstract. Plant functional traits determine vegetation responses to environmental variation, but variation in trait values is large, even within a single site. Likewise, uncertainty in how these traits map to Earth system feedbacks is large. We use a vegetation demographic model (VDM), the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), to explore parameter sensitivity of model predictions, and comparison to observations, at a tropical forest site: Barro Colorado Island in Panama. We define a single 12-dimensional distribution of plant trait variation, derived primarily from observations in Panama, and define plant functional types (PFTs) as random draws from this distribution. We compare several model ensembles, where individual ensemble members vary only in the plant traits that define PFTs, and separate ensembles differ from each other based on either model structural assumptions or non-trait, ecosystem-level parameters, which include (a) the number of competing PFTs present in any simulation and (b) parameters that govern disturbance and height-based light competition. While single-PFT simulations are roughly consistent with observations of productivity at Barro Colorado Island, increasing the number of competing PFTs strongly shifts model predictions towards higher productivity and biomass forests. Different ecosystem variables show greater sensitivity than others to the number of competing PFTs, with the predictions that are most dominated by large trees, such as biomass, being the most sensitive. Changing disturbance and height-sorting parameters, i.e., the rules of competitive trait filtering, shifts regimes of dominance or coexistence between early- and late-successional PFTs in the model. Increases to the extent or severity of disturbance, or to the degree of determinism in height-based light competition, all act to shift the community towards early-successional PFTs. In turn, these shifts in competitive outcomes alter predictions of ecosystem states and fluxes, with more early-successional-dominated forests having lower biomass. It is thus crucial to differentiate between plant traits, which are under competitive pressure in VDMs, from those model parameters that are not and to better understand the relationships between these two types of model parameters to quantify sources of uncertainty in VDMs.

Extreme weather, fires, and land use and climate change are significantly reshaping interactions within watersheds throughout the world. Although hydrological-biogeochemical interactions within watersheds can impact many services valued by society, uncertainty associated with predicting hydrologydriven biogeochemical watershed dynamics remains high. With an aim to reduce this uncertainty, an approximately 300-km 2 mountainous headwater observatory has been developed at the East River, CO, watershed of the Upper Colorado River Basin. The site is being used as a testbed for the Department of Energy supported Watershed Function Project and collaborative efforts. Building on insights gained from research at the "sister" Rifle, CO, site, coordinated studies are underway at the East River site to gain a predictive understanding of how the mountainous watershed retains and releases water, nutrients, carbon, and metals. In particular, the project is exploring how early snowmelt, drought, and other disturbances influence hydrological-biogeochemical watershed dynamics at seasonal to decadal timescales. A system-of-systems perspective and a scale-adaptive simulation approach, involving the combined use of archetypal watershed subsystem "intensive sites" are being tested at the site to inform aggregated watershed predictions of downgradient exports. Complementing intensive site hydrological, geochemical, geophysical, microbiological, geological, and vegetation datasets are long-term, distributed measurement stations and specialized experimental and observational campaigns. Several recent research advances provide insights about the intensive sites as well as aggregated watershed behavior. The East River "community testbed" is currently hosting scientists from more than 30 institutions to advance mountainous watershed methods and understanding.

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Flood plains play a potentially important role in the global carbon cycle. The accumulation of organic matter in flood plains often induces the formation of chemically reduced groundwater and sediments along riverbanks. In this study, our objective is to evaluate the cumulative impact of such reduced zones, water table fluctuations, and temperature gradients on subsurface carbon fluxes in a flood plain at Rifle, Colorado located along the Colorado River. 2-D coupled variably-saturated, non-isothermal flow and biogeochemical reactive transport modeling was applied to improve our understanding of the abiotic and microbially mediated reactions controlling carbon dynamics at the Rifle site. Model simulations considering only abiotic reactions (thus ignoring microbial reactions) underestimated CO 2 partial pressures observed in the unsaturated zone and severely underestimated inorganic (and overestimated organic) carbon fluxes to the river compared to simulations with biotic pathways. Both model simulations and field observations highlighted the need to include microbial contributions from chemolithoautotrophic processes (e.g., Fe ?2 and S-2 oxidation) to match locally-observed high CO 2 concentrations above reduced zones. Observed seasonal variations in CO 2 concentrations in the unsaturated zone could not be reproduced without incorporating temperature gradients in the simulations. Incorporating temperature fluctuations resulted in an increase in the annual groundwater carbon fluxes to the river by 170 % to 3.3 g m-2 d-1 , while including water table variations resulted in an overall decrease in the simulated fluxes. We conclude that spatial microbial and redox zonation as well as temporal fluctuations of temperature and water table depth contribute significantly to subsurface carbon fluxes in flood plains and need to be represented appropriately in model simulations. Keywords Flood plain Á Reduced zones Á Subsurface carbon dynamics Á Temporal variability Á Biogeochemical processes