scholarly journals Development of physically based liquid water schemes for Greenland firn-densification models

2019 ◽  
Author(s):  
Vincent Verjans ◽  
Amber Leeson ◽  
C. Max Stevens ◽  
Michael MacFerrin ◽  
Brice Noël ◽  
...  
Keyword(s):  
Water Flow ◽  
Liquid Water ◽  
Preferential Flow ◽  
Greenland Ice Sheet ◽  
Single Domain ◽  
Richards Equation ◽  
Retention Capacity ◽  
Water Percolation ◽  
Physically Based ◽  
Dual Domain

Abstract. As surface melt is increasing on the Greenland ice sheet (GrIS), quantifying the retention capacity of the firn layer is critical to link meltwater production to meltwater runoff. Firn-densification models have so far relied on empirical approaches to account for the percolation-refreezing process, and more physically based representations of liquid water flow might therefore bring improvements to model performance. Here we implement three types of water percolation schemes into the Community Firn Model: the tipping bucket approach, the Richards Equation in a single-domain and the Richards Equation in a dual-domain, which accounts for partitioning between matrix and fast preferential flow. We investigate their impact on firn densification at four locations on the GrIS and compare model results with observations. We find that for all of the flow schemes, significant discrepancies remain with respect to observed firn density, particularly the density variability in depth, and that inter-model differences are large. The simple bucket scheme is as efficient in replicating observed density profiles as the single-domain Richards Equation. The most physically detailed dual-domain scheme does not necessarily reach best agreement with observed data. However, we find that the implementation of preferential flow does allow for more frequent ice layer formation and for deeper percolation. We also find that the firn model is more sensitive to the choice of densification scheme than to the choice of water percolation scheme. The disagreements with observations and the spread in model results demonstrate that progress towards an accurate description of water flow in firn is necessary. The numerous uncertainties surrounding firn micro- and macro-structure, its hydraulic properties, and the one dimensionality of firn models render the implementation of physically based percolation schemes difficult. An improved understanding of the parameters affecting evolution of polar firn, of the effects of the climatic forcing on the densification process and more accurate treatment of liquid water would benefit further developments.

scholarly journals Development of physically based liquid water schemes for Greenland firn-densification models

2019 ◽  
Vol 13 (7) ◽  
pp. 1819-1842 ◽  
Author(s):  
Vincent Verjans ◽  
Amber A. Leeson ◽  
C. Max Stevens ◽  
Michael MacFerrin ◽  
Brice Noël ◽  
...  
Keyword(s):  
Water Flow ◽  
Liquid Water ◽  
Preferential Flow ◽  
Greenland Ice Sheet ◽  
Single Domain ◽  
Richards Equation ◽  
Retention Capacity ◽  
Water Percolation ◽  
Physically Based ◽  
Dual Domain

Abstract. As surface melt is increasing on the Greenland Ice Sheet (GrIS), quantifying the retention capacity of the firn layer is critical to linking meltwater production to meltwater runoff. Firn-densification models have so far relied on empirical approaches to account for the percolation–refreezing process, and more physically based representations of liquid water flow might bring improvements to model performance. Here we implement three types of water percolation schemes into the Community Firn Model: the bucket approach, the Richards equation in a single domain and the Richards equation in a dual domain, which accounts for partitioning between matrix and fast preferential flow. We investigate their impact on firn densification at four locations on the GrIS and compare model results with observations. We find that for all of the flow schemes, significant discrepancies remain with respect to observed firn density, particularly the density variability in depth, and that inter-model differences are large (porosity of the upper 15 m firn varies by up to 47 %). The simple bucket scheme is as efficient in replicating observed density profiles as the single-domain Richards equation, and the most physically detailed dual-domain scheme does not necessarily reach best agreement with observed data. However, we find that the implementation of preferential flow simulates ice-layer formation more reliably and allows for deeper percolation. We also find that the firn model is more sensitive to the choice of densification scheme than to the choice of water percolation scheme. The disagreements with observations and the spread in model results demonstrate that progress towards an accurate description of water flow in firn is necessary. The numerous uncertainties about firn structure (e.g. grain size and shape, presence of ice layers) and about its hydraulic properties, as well as the one-dimensionality of firn models, render the implementation of physically based percolation schemes difficult. Additionally, the performance of firn models is still affected by the various effects affecting the densification process such as microstructural effects, wet snow metamorphism and temperature sensitivity when meltwater is present.

scholarly journals Modelling liquid water transport in snow under rain-on-snow conditions – considering preferential flow

2017 ◽  
Vol 21 (3) ◽  
pp. 1741-1756 ◽  
Author(s):  
Sebastian Würzer ◽  
Nander Wever ◽  
Roman Juras ◽  
Michael Lehning ◽  
Tobias Jonas
Keyword(s):  
Water Transport ◽  
Liquid Water ◽  
Field Experiments ◽  
Preferential Flow ◽  
Richards Equation ◽  
European Alps ◽  
Wet Snow ◽  
Snow Conditions ◽  
Dual Domain ◽  
Grain Properties

Abstract. Rain on snow (ROS) has the potential to generate severe floods. Thus, precisely predicting the effect of an approaching ROS event on runoff formation is very important. Data analyses from past ROS events have shown that a snowpack experiencing ROS can either release runoff immediately or delay it considerably. This delay is a result of refreeze of liquid water and water transport, which in turn is dependent on snow grain properties but also on the presence of structures such as ice layers or capillary barriers. During sprinkling experiments, preferential flow was found to be a process that critically impacted the timing of snowpack runoff. However, current one-dimensional operational snowpack models are not capable of addressing this phenomenon. For this study, the detailed physics-based snowpack model SNOWPACK is extended with a water transport scheme accounting for preferential flow. The implemented Richards equation solver is modified using a dual-domain approach to simulate water transport under preferential flow conditions. To validate the presented approach, we used an extensive dataset of over 100 ROS events from several locations in the European Alps, comprising meteorological and snowpack measurements as well as snow lysimeter runoff data. The model was tested under a variety of initial snowpack conditions, including cold, ripe, stratified and homogeneous snow. Results show that the model accounting for preferential flow demonstrated an improved overall performance, where in particular the onset of snowpack runoff was captured better. While the improvements were ambiguous for experiments on isothermal wet snow, they were pronounced for experiments on cold snowpacks, where field experiments found preferential flow to be especially prevalent.

scholarly journals Laboratory-based observations of capillary barriers and preferential flow in layered snow

2015 ◽  
Vol 9 (6) ◽  
pp. 6627-6659 ◽  
Author(s):  
F. Avanzi ◽  
H. Hirashima ◽  
S. Yamaguchi ◽  
T. Katsushima ◽  
C. De Michele
Keyword(s):  
Grain Size ◽  
Liquid Water ◽  
Preferential Flow ◽  
Liquid Water Content ◽  
Richards Equation ◽  
Input Rate ◽  
Capillary Barriers ◽  
Physically Based ◽  
Water Transmission ◽  
Water Speed

Abstract. Several evidences are nowadays available that show how the effects of capillary gradients and preferential flow on water transmission in snow may play a more important role than expected. To observe these processes and to contribute in their characterization, we performed observations on the development of capillary barriers and preferential flow patterns in layered snow during cold laboratory experiments. We considered three different layering (all characterized by a finer-over-coarser texture in grain size) and three different water input rates. Nine samples of layered snow were sieved in a cold laboratory, and subjected to a constant supply of dyed tracer. By means of visual inspection, horizontal sectioning and liquid water content measurements, the processes of ponding and preferential flow were characterized as a function of texture and water input rate. The dynamics of each sample were replicated using the multi-layer physically-based SNOWPACK model. Results show that capillary barriers and preferential flow are relevant processes ruling the speed of liquid water in stratified snow. Ponding is associated with peaks in LWC at the boundary between the two layers equal to ~ 33–36 vol. % when the upper layer is composed by fine snow (grain size smaller than 0.5 mm). The thickness of the ponding layer at the textural boundary is between 0 and 3 cm, depending on sample stratigraphy. Heterogeneity in water transmission increases with grain size, while we do not observe any clear dependency on water input rate. The extensive comparison between observed and simulated LWC profiles by SNOWPACK (using an approximation of Richards Equation) shows high performances by the model in estimating the LWC peak over the boundary, while water speed in snow is underestimated by the chosen water transport scheme.

scholarly journals Simulating ice layer formation under the presence of preferential flow in layered snowpacks

2016 ◽  
Vol 10 (6) ◽  
pp. 2731-2744 ◽  
Author(s):  
Nander Wever ◽  
Sebastian Würzer ◽  
Charles Fierz ◽  
Michael Lehning
Keyword(s):  
Snow Cover ◽  
Water Flow ◽  
Liquid Water ◽  
Preferential Flow ◽  
Layer Formation ◽  
Flow Paths ◽  
One Dimensional ◽  
Preferential Flow Paths ◽  
Matrix Flow ◽  
Dual Domain

Abstract. For physics-based snow cover models, simulating the formation of dense ice layers inside the snowpack has been a long-time challenge. Their formation is considered to be tightly coupled to the presence of preferential flow, which is assumed to happen through flow fingering. Recent laboratory experiments and modelling techniques of liquid water flow in snow have advanced the understanding of conditions under which preferential flow paths or flow fingers form. We propose a modelling approach in the one-dimensional, multilayer snow cover model SNOWPACK for preferential flow that is based on a dual domain approach. The pore space is divided into a part that represents matrix flow and a part that represents preferential flow. Richards' equation is then solved for both domains and only water in matrix flow is subjected to phase changes. We found that preferential flow paths arriving at a layer transition in the snowpack may lead to ponding conditions, which we used to trigger a water flow from the preferential flow domain to the matrix domain. Subsequent refreezing then can form dense layers in the snowpack that regularly exceed 700 kg m−3. A comparison of simulated density profiles with biweekly snow profiles made at the Weissfluhjoch measurement site at 2536 m altitude in the Eastern Swiss Alps for 16 snow seasons showed that several ice layers that were observed in the field could be reproduced. However, many profiles remain challenging to simulate. The prediction of the early snowpack runoff also improved under the consideration of preferential flow. Our study suggests that a dual domain approach is able to describe the net effect of preferential flow on ice layer formation and liquid water flow in snow in one-dimensional, detailed, physics-based snowpack models, without the need for a full multidimensional model.

scholarly journals Modeling liquid water transport in snow under rain-on-snow conditions – considering preferential flow

2016 ◽  
Author(s):  
Sebastian Würzer ◽  
Nander Wever ◽  
Roman Juras ◽  
Michael Lehning ◽  
Tobias Jonas
Keyword(s):  
Water Transport ◽  
Liquid Water ◽  
Field Experiments ◽  
Preferential Flow ◽  
Richards Equation ◽  
European Alps ◽  
Wet Snow ◽  
Snow Conditions ◽  
Dual Domain ◽  
Grain Properties

Abstract. Rain-on-snow (ROS) has the potential to generate severe floods. Thus, precisely predicting the effect of an approaching ROS event on runoff formation is very important. Data analyses from past ROS events have shown that a snowpack experiencing ROS can either release runoff immediately or delay it considerably. This delay is a result of refreeze of liquid water and water transport mechanisms in the snowpack. Water percolation is depending on snow grain properties but also on the presence of structures such as ice layers or capillary barriers. During sprinkling experiments, preferential flow was found to be a process that critically impacted the timing of snowpack runoff. However, current one-dimensional snowpack models are not capable of addressing this phenomenon correctly. For this study, the detailed physics-based snowpack model SNOWPACK is extended with a water transport scheme accounting for preferential flow. The implemented Richards' Equation solver is modified using a dual-domain approach to simulate water transport under preferential flow conditions. To validate the presented approach, we used an extensive dataset of over 100 ROS events from several locations in the European Alps, comprising meteorological and snowpack measurements as well as snow lysimeter runoff data. The model was tested under a variety of initial snowpack conditions, including cold, ripe, stratified and homogeneous snow. Results show that the model accounting for preferential flow (PF) demonstrated an improved overall and in particular more balanced performance. While the improvements were small for experiments on isothermal wet snow, they were pronounced for experiments on cold snowpacks, where field experiments found preferential flow to be especially prevalent.

scholarly journals Simulating ice layer formation under the presence of preferential flow in layered snowpacks

2016 ◽  
Author(s):  
Nander Wever ◽  
Sebastian Würzer ◽  
Charles Fierz ◽  
Michael Lehning
Keyword(s):  
Snow Cover ◽  
Water Flow ◽  
Liquid Water ◽  
Preferential Flow ◽  
Swiss Alps ◽  
Layer Formation ◽  
Flow Paths ◽  
One Dimensional ◽  
Preferential Flow Paths ◽  
Dual Domain

Abstract. For physics based snow cover models, simulating the formation of dense ice layers inside the snowpack has been a long time challenge. Their formation is considered to be tightly coupled to the presence of preferential flow, which is assumed to happen through flow fingering. Recent laboratory experiments and modelling techniques of liquid water flow in snow have advanced the understanding of conditions under which preferential flow paths or flow fingers form. We propose a modelling approach in the one-dimensional, multi-layer snow cover model SNOWPACK for preferential flow that is based on a dual-domain approach. The pore space is divided into a part that represents matrix flow and a part that represents preferential flow. Richards equation is then solved for both domains. We found that preferential flow paths arriving at a layer transition in the snowpack may lead to ponding conditions. Subsequent refreezing then can form dense layers in the snowpack, that regularly exceed 700 kg m−3. A comparison of simulated density profiles with bi-weekly snow profiles made at the Weissfluhjoch measurement site at 2536 m altitude in the Eastern Swiss Alps for 16 snow seasons showed that several ice layers that were observed in the field could be reproduced. However many profiles remain challenging to simulate. The prediction of the early snowpack runoff also improved under the consideration of preferential flow. Our study suggests that a dual domain approach is able to describe the net effect of preferential flow on ice layer formation and liquid water flow in snow in one-dimensional, detailed, physics based snowpack models, without the need for a full multi-dimensional model.

The Daisy agroecological system model: A physically based model for preferential water flow and solute transport in drained agricultural fields

2021 ◽  
Author(s):  
Efstathios Diamantopoulos ◽  
Maja Holbak ◽  
Per Abrahamsen
Keyword(s):  
Solute Transport ◽  
Water Flow ◽  
Preferential Flow ◽  
Column Experiment ◽  
Soil Matrix ◽  
Physically Based Model ◽  
Physically Based ◽  
Rapid Transport ◽  
Preferential Water

<p>Preferential water flow and solute transport in agricultural systems affects not only the quality of groundwater but also the quality of surface waters like streams and lakes. This is due to the rapid transport of agrochemicals, immediately after application, through subsurface drainpipes and surface water. Experimental evidence attributes this to the occurrence of continuously connected pathways, connecting the soil surface directly with the drainpipes. We developed a physically-based model describing preferential flow and transport in biopores and implemented it in the agroecological model Daisy. The model simulates the often observed rapid transport of chemicals from   the upper soil layers to the drainpipes or to deeper layers of the soil matrix. Based on field investigations, biopores with specific characteristics can be parameterized as classes with different vertical and horizontal distributions. The model was tested against experimental data from a column experiment with an artificial biopore and showed good results in simulating preferential flow dynamics. We illustrate the performance of the new approach, by conducting five simulations assuming a two-dimensional simulation domain with different biopore parametrizations, from none to several different classes. The simulation results agreed with experimental observations reported in the literature, indicating rapid transport from the soil to the drainpipes. Furthermore, the different biopore parametrizations resulted in distinctly different leaching patterns, raising the expectation that biopore properties could be estimated or constrained based on observed leaching data and direct measurements.</p>

scholarly journals Development and Testing of a Frozen Soil Parameterization for Cold Region Studies

2007 ◽  
Vol 8 (4) ◽  
pp. 690-701 ◽  
Author(s):  
Xia Zhang ◽  
Shu Fen Sun ◽  
Yongkang Xue
Keyword(s):  
Phase Change ◽  
Water Flow ◽  
Liquid Water ◽  
Frozen Soil ◽  
Picard Iteration ◽  
Richards Equation ◽  
Cold Region ◽  
Field Station ◽  
Mixed Form ◽  
Ice Content

Abstract Proper simulation of soil freezing and thawing processes is an important issue in cold region climate studies. This paper reports on a frozen soil parameterization scheme for cold region studies that includes water flow and heat transfer in soil with water phase change. The mixed-form Richards’ equation is adopted to describe soil water flow affected by thermal processes in frozen soil. In addition, both liquid water and ice content have been taken into account in the frozen soil hydrologic and thermal property parameterization. To solve the complex nonlinear equation set and to ensure water conservation during simulation of complex phase change processes, efficient computational procedures have been designed and a new modified Picard iteration scheme is extended to solve the mixed-form Richards’ equation with phase change. The frozen soil model was evaluated using observational data from the field station at Rosemount, Minnesota, and the Tibet D66 site. The results show that the model is capable of providing good simulations of the evolution of temperature and liquid water content in frozen soil. Comparisons of simulation results from sensitivity studies indicate that there is a maximum difference of about 50 W m−2 in sensible and ground heat fluxes with and without the inclusion of the effect of ice content on matric potential and that using the exponential relationship between hydraulic conductivity and ice content produces realistic results.

scholarly journals A 2D model for simulating heterogeneous mass and energy fluxes through melting snowpacks

2016 ◽  
Author(s):  
Nicolas R. Leroux ◽  
John W. Pomeroy
Keyword(s):  
Water Flow ◽  
Liquid Water ◽  
Preferential Flow ◽  
Initial Conditions ◽  
Surface Fluxes ◽  
Porous Media Flow ◽  
Accurate Estimation ◽  
Flow Paths ◽  
Preferential Flow Paths ◽  
The Impact

Abstract. Accurate estimation of the water flux through melting snowpacks is of primary importance for runoff prediction. Lateral flows and preferential flow pathways in porous media flow have proven critical for improving soil and groundwater flow models, but though many physically-based layered snowmelt models have been developed, only 1D matrix flow over level ground is currently accounted for in snow models. Snowmelt models that include these processes may improve snowmelt discharge timing and contributing area calculations in hydrological models. A two-dimensional snow model (SMPP – Snowmelt Model with Preferential flow Paths) is presented that simulates heat and water flows through both snowpack matrix and preferential flow paths, as well as snowmelt and refreezing of meltwater. The model assumes thermodynamic equilibrium between solid and liquid phases and uses the latest improvements made in snow science to estimate snow hydraulic and thermal properties. A finite volume method is applied to solve for the 2D heat and water equations. The use of a water entry pressure for dry snow combined with consideration of the impact of heterogeneities in surface fluxes and internal snow properties – density, grain size and layer thickness – allowed calculation of the formation of preferential flow paths in the snowpack. The simulation of water flow through preferential flow paths resulted in liquid water reaching the base of the snowpack earlier than for a homogeneous wetting front. Moreover, the preferential flow paths in the model increased the exchange of energy between the snow surface and the internal snowpack, resulting in faster warming of the snowpack. A sensitivity analysis, conducted on the snow internal properties showed that initial conditions such as density and temperature, should be carefully measured in the field to accurately estimate liquid water percolating through the snowpack. Furthermore, two empirical coefficients used in the water flow equation were showed to greatly impact model outputs. This heterogeneous flow model is an important tool to help understand snowmelt flow processes in complex and level terrains and to demonstrate how uncertainty in snowmelt-derived runoff calculations might be reduced.

scholarly journals Observations of capillary barriers and preferential flow in layered snow during cold laboratory experiments

2016 ◽  
Vol 10 (5) ◽  
pp. 2013-2026 ◽  
Author(s):  
Francesco Avanzi ◽  
Hiroyuki Hirashima ◽  
Satoru Yamaguchi ◽  
Takafumi Katsushima ◽  
Carlo De Michele
Keyword(s):  
Grain Size ◽  
Spatial Variability ◽  
Liquid Water ◽  
Laboratory Experiments ◽  
Preferential Flow ◽  
Liquid Water Content ◽  
Water Infiltration ◽  
Richards Equation ◽  
Capillary Barrier ◽  
Capillary Barriers

Abstract. Data of liquid water flow around a capillary barrier in snow are still limited. To gain insight into this process, we carried out observations of dyed water infiltration in layered snow at 0 °C during cold laboratory experiments. We considered three different finer-over-coarser textures and three different water input rates. By means of visual inspection, horizontal sectioning, and measurements of liquid water content (LWC), capillary barriers and associated preferential flow were characterized. The flow dynamics of each sample were also simulated solving the Richards equation within the 1-D multi-layer physically based snow cover model SNOWPACK. Results revealed that capillary barriers and preferential flow are relevant processes ruling the speed of water infiltration in stratified snow. Both are marked by a high degree of spatial variability at centimeter scale and complex 3-D patterns. During unsteady percolation of water, observed peaks in bulk volumetric LWC at the interface reached  ∼ 33–36 vol % when the upper layer was composed by fine snow (grain size smaller than 0.5 mm). However, LWC might locally be greater due to the observed heterogeneity in the process. Spatial variability in water transmission increases with grain size, whereas we did not observe a systematic dependency on water input rate for samples containing fine snow. The comparison between observed and simulated LWC profiles revealed that the implementation of the Richards equation reproduces the existence of a capillary barrier for all observed cases and yields a good agreement with observed peaks in LWC at the interface between layers.

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