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Tile drains have been suggested as a method of measuring field-scale transport parameters, but lateral flow to the drains in the saturated zone can introduce error in the measurement of unsaturated zone parameters. Our objective was to use a two-layer transfer function model to determine field-scale unsaturated zone transport parameters from tile drain breakthrough curves measured in two 12.5 by 30.5 m plots. Plots were irrigated to steady-state tile drain flow, Cl was added as a tracer, and irrigation was continued for up to 600 h. The first layer of the transfer function model consisted of the unsaturated zone and was described by the convection-dispersion equation (CDE) probability density function (pdf). The second layer consisted of the saturated zone and was described by the gamma distribution pdf. The CDE parameters were determined from a fit of the two layer model to the drainage pdf in each tile drain. Transport parameters were similar for the two plots. Mean field-scale dispersivity ([Lambda]) for the unsaturated zone (0-20 cm) was 5.3 cm for Plot 1 and 3.4 cm for Plot 2. This was much less than [Lambda] calculated for the overall system by moment analysis ([Lambda] = 23.7 and 19.5 cm in Plot 1 and 2, respectively) suggesting that most of the dispersion of the tile drain breakthrough curves was due to lateral flow in the saturated zone. This illustrates that tile drain breakthrough curves can be used to determine field-scale unsaturated zone transport parameters provided a model is used that accounts for two-dimensional flow in the saturated zone.

Current sea-level rise partly stems from increased surface melting and meltwater runoff from the Greenland ice sheet. Multi-year snow, also known as firn, covers about 80% of the ice sheet and retains part of the surface meltwater. Since the firn cold content integrates its physical and thermal characteristics, it is a valuable tool for determining the meltwater-retention potential of firn. We use gap-filled climatological data from nine automatic weather stations in the ice-sheet accumulation area to drive a surface-energy-budget and firn model, validated against firn density and temperature observations, over the 1998–2017 period. Our results show a stable top 20 m firn cold content (CC20) at most sites. Only at the lower-elevation Dye-2 site did CC20 decrease, by 24% in 2012, before recovering to its original value by 2017. Heat conduction towards the surface is the main process feeding CC20 at all nine sites, while CC20 reduction occurs through low-cold-content fresh-snow addition at the surface during snowfall and latent-heat release when meltwater refreezes. Our simulations suggest that firn densification, while reducing pore space for meltwater retention, increases the firn cold content, enhances near-surface meltwater refreezing and potentially sets favourable conditions for ice-slab formation. [ABSTRACT FROM AUTHOR]

Our goal was to determine if anion‐exchange membranes (AEMs), which can be placed in situ in soils and act as anion exchangers, were better soil nitrate (NO3‐) assessment tools than soil extraction. During winter 1993–1994 we compared NO3‐ in the surface of tilled and non‐tilled soils that were subjected to rye cover crop and fallow winter coverage treatments. Estimates of soil solution concentrations of NO3‐ based on KC1 extracts of the AEMs were between 1 to 5 fold higher than concentrations based on soil extracts. Both methods of NO3‐ assessment documented a general decline in NO3‐ availability during the winter; however, only the AEM NO3‐ data produced statistically significant results. AEM data suggested that the combination of fall tillage and winter fallowing increased soil NO3‐ contents and led to more potential for loss of NO3‐ during the 1993–1994 winter than reduced tillage practices or planting a rye cover crop. [ABSTRACT FROM PUBLISHER]

Soil solute transport parameters are usually measured at the pedon scale, but predictions of transport at the polypedon scale are often required. Our objectives were to compare pedon- and polypedon-scale convection dispersion equation (CDE) transport parameters measured in a field leaching experiment and to compare deterministic and stochastic predictions of polypedon-scale transport. The study was conducted near Watkinsville, Ga. on a 12.5 by 30.5 m plot. Time domain reflectometry (TDR) waveguides (30 and 60 cm in length) were installed vertically at 80 locations to measure water content and impedance. The plot was irrigated and a Cl pulse was used as a tracer. We found very good agreement between the average CDE parameters measured at the pedon scale and those estimated at the polypedon scale in the 0–30 and 0–60 cm depth intervals. Although the variability in average pore velocities between pedons was large, the variability in velocities within a pedon caused by hydrodynamic dispersion was greater. As a result, a deterministic approach based on the CDE more accurately predicted the estimated polypedon scale breakthrough curve (BTC) at 30 and 60 cm depths than a stochastic approach based on the convective lognormal transfer function (CLT). This indicates that the pedon serves as a representative elementary volume (REV) for solute transport in this soil. [ABSTRACT FROM AUTHOR]

The current deficit in Greenland ice sheet mass balance is due to both a decrease in surface mass balance ( SMB ) input and an increase in ice discharge ( D ) output. While SMB processes are beginning to be well captured by observationally-constrained climate modeling, insight into D is relatively limited. We use InSAR-derived velocities, in combination with ice thickness observations, to quantify the mass flux ( F ) across a flux perimeter around the ice sheet at ∼1700 m elevation. To quantify D , we correct F for SMB , as well as changes in volume due to ice dynamics, in the area downstream of the gate. Using a 1961–1990 reference climatology SMB field from the MAR regional climate model, we quantify ice sheet mass balance within eighteen basins. We find a 2007–2011 mean D of 515 ± 57 Gt yr − 1 . We find a 2007–2011 mean total mass balance of − 262 ± 21 Gt yr − 1 , which is equal to a 0.73 mm yr −1 global sea level rise contribution. This mass loss is dominated by SMB , which accounts for 61% of mass loss in the basins where partitioning is possible. [ABSTRACT FROM AUTHOR]