Schedule and Tasks

Tasks and timetable

Task 1, completion month 1

Install and test original version of OLAM/LEAF on supercomputer at Julich Research Center.

Task 2, completion month 2

Prepare detailed Powerpoint presentation that illustrates different types of soil/groundwater gridding structures that may be best suited for 3D groundwater modeling. Presentation will list pros and cons of the different grid structures, including their relation to the atmospheric grid system with which surface fluxes are exchanged and to the vegetation canopy. The presentation will be provided to all interested members of the vegetation, soil, and groundwater communities and will solicit comments from all regarding how different grid structures might impact modeling in each particular discipline. The purpose of this effort is to select one or more gridding structures that will be incorporated into OLAM-SOIL to make it fully 3D groundwater capable and that will address the needs of the community for vegetation modeling. At the discretion of ETH, FZJ, and ISMC, the presentations will be given orally at meetings, if desired.

Task 3, completion month 4

Complete implementation and testing of vegetation and soil/groundwater gridding structures in OLAM-SOIL. This will include extending depth of soil model to 100 m or more, following recommendations from the groundwater community. Apply model updates to OLAM-SOIL installation at Julich Research Center.

Task 4, completion month 6

Incorporate new datasets for vegetation and soil, including hydraulic properties of soil and bedrock for deep groundwater modeling, following specific recommendations by the community. Apply model updates to OLAM-SOIL installation at Julich Research Center.

Task 5, completion month 8

Develop benchmark test cases in collaboration with the community that will facilitate evaluation of ongoing upgrades to the model and help to propel acceptance of OLAM-SOIL as a community model. Install test cases at Julich Research Center.

Task 6, completion month 9

Expand documentation of OLAM and OLAM-SOIL to improve accessibility by general community. New documentation will consist of (1) detailed comments added to computer source code to supplement those already present, (2) a guide for installing, compiling, and running OLAM-SOIL, and (3) a description of the major components of OLAM-SOIL, organized by physical processes that are represented and by source code files and subroutines where such processes reside. (A document that describes in detail each of the input namelist variables that control the configuration and operation of OLAM and LEAF is already available, but will be modified to reflect OLAM-SOIL developments.)

Task 7, completion month 10

Conduct OLAM-SOIL training workshop (2 to 5 days, at the discretion of ETH, FZJ, and ISMC).

Status Report on all Tasks

The work associated with Task 2 and Task 3 was by far the most challenging and complex development under this project. In order to enable high resolution 3D groundwater modeling, OLAM-SOIL required a different soil grid structure than what is implemented in OLAM/LEAF. The LEAF model represents only vertical, but not horizontal, flow of water and energy in the soil, and therefore treats soil columns as independent of each other. Soil columns are defined based on the overlying atmospheric grid, which is generated first, and on the intersection of the topographic surface with atmospheric grid layers, which are horizontal in OLAM (Figure 1). A LEAF soil column is not permitted to extend past the confines of a single atmospheric grid cell, either horizontally or vertically. This leads to smaller and more numerous land cells on steep topographic slopes (as in the center region of the figure) and larger and fewer land cells where the land has little slope (as in the lower right of the figure). While this has certain advantages if only vertical transport needs to be represented in the soil, it leads to irregular polygon soil columns (viewed from above) that are not optimal for representing horizontal transport. Furthermore, horizontal resolution of the soil grid is not independent of the atmospheric grid. Many intended applications of OLAM-SOIL will require the soil grid to have much higher horizontal resolution than the atmosphere grid, but this is not possible in the current LEAF implementation.

Several possible methods of generating a new soil grid that is independent of the atmosphere grid were conceived in the first phase of the project. Walko evaluated and discussed these options with colleagues, including in-person meetings and visual presentations (Task 2) at ETH, FZJ, and the University of Wageningen over a two-week period in January-February 2017. One possibility that was considered but rejected was to allow the soil and vegetation gridding systems to be independent. This would have allowed the vegetation grid to interface as a hybrid between the soil and atmosphere, automatically conforming to the overlapping areas between their individual grid cells. However, it was considered important that vegetation be able to fully couple with its own soil rather than sharing that soil and its properties with vegetation (of a different type) in a neighboring area. Thus, the choice was made that vegetation and soil would share a common vertical column structure.

The new OLAM-SOIL grid that was chosen to meet all design criteria can be generated independently of the atmosphere grid (e.g., at much higher resolution), and it has the same basic hexagonal structure as the atmosphere grid (Figure 2). The local mesh refinement algorithms developed for the atmosphere grid can also be applied (independently) to the soil grid, which enables arbitrarily high horizontal resolution in any desired locations. Cell-to-cell horizontal communication, required in OLAM-SOIL for modeling groundwater transport, is also enabled using the same form of grid connectivity information as used for the OLAM atmosphere grid.

Because land cells in OLAM-SOIL can interface with multiple cells of the atmosphere grid, land-atmosphere coupling is more complicated than in LEAF. Atmosphere properties must be locally averaged to land cell locations, where turbulent and radiative fluxes are computed, and the fluxes must then be mapped back to the atmosphere cells.

Development and testing of the new OLAM-SOIL grid structure and its flux exchange coupling with the atmosphere grid is complete (Task 3), including programming of parallel communication steps. The final implementation is actually a hybrid of the original and new soil grid structures. The model user may specify geographic regions (e.g., the continent of Europe) where 3D groundwater modeling is to be activated, and those where it will not be done (e.g., all other continents). OLAM-SOIL then sets up the new grid structure in the groundwater modeling area, normally at higher resolution than the atmosphere grid, and uses the original LEAF soil grid elsewhere.

While not specifically listed among the project tasks, it was understood that in addition to implementation of the new OLAM-SOIL grid structure, full implementation of 3D groundwater transport would be done as well. This was a relatively straightforward task because Richard’s Equation was already solved in LEAF (in the vertical) using the van Genuchten parameters, and the new OLAM-SOIL grid provided the infrastructure for horizontal cell-to-cell interaction just as it is implemented in the atmosphere grid of OLAM.

An expansion of Task 3 that was not fully anticipated resulted from discussions of 3D groundwater modeling during Walko’s visit to FZJ. At issue was the fact that groundwater is exchanged between soil or rock material and lakes. Furthermore, Walko was made aware that the ParFlow model of groundwater flow treats deep and shallow lakes by different methods, the former represented as adjacent to soil areas and the latter existing on top of the soil. It was
thereby decided to develop the same representation in OLAM-SOIL. To that end, development of the new OLAM-SOIL grid included both types of lakes and the corresponding pathways of water and energy transport between lakes and soil. Mass and energy are conserved in these transports. Other processes represented in the lake model are thus far relatively simple, but will be improved in a future project

One other development that was only partly anticipated was that OLAM’s unique numerical method for solving Richard’s equation would require modification when applied to the high pressures encountered deep below the water table or in some confined aquifers. The near incompressibility of water poses a challenge for modeling saturated groundwater flow in that it leads to a stiff set of governing equations. The widely used ParFlow groundwater model employs a 3D implicit numerical solver to obtain stable solutions to this system, but that method is highly complex and difficult to implement efficiently on massively parallel computers. OLAM/LEAF avoids this problem by effectively increasing the specific storage of individual grid cells. A grid cell is permitted to become oversaturated, i.e., to contain slightly more water than the specified porosity. Whereas in nature, this would cause pressure head (confinement pressure) to become extremely high, the rate of pressure increase with water content in OLAM-SOIL is specified to be much less. Increasing the elasticity of the system in this way allows horizontal transport to be fully explicit in the computational system and therefore easily solved on parallel computers. The drawback is that porosity is not strictly imposed as an upper limit for water content. The oversaturation was always acceptably small in LEAF applications where the soil model extended to a depth of only a few meters and pressure head was therefore moderate, but with OLAM-SOIL extending to much greater depths, high pressure head will sometimes be encountered. In order to correct this, a relaxation term has been implemented that gradually increases the pressure head over time when oversaturation occurs. This has the effect of forcing excess water out of a grid cell until oversaturation is reduced to near zero. If confinement pressure is positive and subsaturation occurs, the confinement pressure is gradually reduced until saturation returns or confinement pressure drops to zero. The characteristic time scale of the relaxation term is set to approximately 1 hour, but the numerical solution is highly insensitive to the relaxation time for all situations that will be encountered in modeling applications.

Task 1 was carried out during Walko’s visit to FZJ. OLAM was installed on the supercomputer and successfully executed in a test simulation. Although other proposed tasks called for their respective code developments to be installed on the supercomputer as well, there has so far been no need, and instead the final version of OLAM-SOIL will be installed and tested upon the completion of this project.

Task 4 is in progress but close to completion. It calls for replacing the use of soil textural classes and associated hydraulic pedotransfer functions (PTFs) in LEAF with SoilGrids datasets from ISRIC that contain soil compositional and chemical properties and new PTFs that directly use that information. The principal work under this task has been to enable the new datasets to be input and interpolated onto the OLAM-SOIL grid, to implement the two PTF models that have been chosen so far (Vereecken; de Boer), and to adapt memory access to the new data throughout the model wherever the soil parameters are used. In addition, it was necessary to implement formulas for soil thermal conductivity and heat capacity that are based on SoilGrids data rather than on soil textural class, and the methods described by de Vries (1963) and Farouki (1981) and currently used in the Tethys-Chloris (Fatichi 2017) and CLM (Oleson, 2013) models were adopted for OLAM-SOIL. One further aspect of Task 4 that was developed through discussions with Dani Or, Simone Fatichi, and Harry Vereecken during this project was to implement a representation of soil structure and its impact on hydraulic conductivity in soils containing significant vegetation and under conditions of saturation or near saturation. It was determined that observed GPP would be used to define vegetation abundance according to a formulation provided by Fatichi, and the influences of soil mineral content and soil wetness would be based on recent research by Vereecken, Or, and others. Most of this implementation has been completed. The original statement of Task 4 included adoption of new vegetation data, but it was later mutually agreed that this first year of the project should instead focus solely on the soil, its properties, and on groundwater modeling.

Task 5 is to develop and run test cases that will examine the performance of OLAM-SOIL and explore the sensitivity of the global climate system to details of the soil model. One such test was run during this project using a preliminary version of OLAM-SOIL that could switch between activation and de-activation of the soil structure effect and examine the resulting differences in climate variables. Climatological averages over a 5-year simulation period revealed an impact on atmospheric variables, lending support to the argument that soil structure is an important factor to represent in soil models. However, it is necessary to augment the tests to include an ensemble of comparative climate simulations in order to increase the statistical sample size. These simulations will be performed once all current developmental tasks for OLAM-SOIL have been completed. Another set of tests that specifically examine groundwater flow have been described in Kollet et al. (2016). Some of these tests will be performed with OLAM-SOIL in order to verify that the model produces the expected results.

Task 6 is to develop various forms of documentation of OLAM and OLAM-SOIL to augment what has previously been developed. This task has been ongoing at every stage of the project so far in that all new coding has been intermixed with abundant in-code documentation. In addition, the OLAM User’s Guide, a PDF document that describes how to set up and run a model simulation, has continually been updated to reflect new control variables that a model user must set to operate the new SOIL component. Both these activities will continue apace with the remaining model development until completion of the project. Other documents are under development that provide an introduction to OLAM and OLAM-SOIL, describe their unique features and capabilities, how to install and run them, and how to generate visual (graphical) output and analyses. Some of these documents have been provided to a new OLAM webpage that is hosted at ETH, and more documents will be added upon completion.

Task 7 is to conduct a training workshop on OLAM-SOIL. This has been arranged to be held on 9 December 2017 in New Orleans, Louisiana, as part of the AGU Fall Meeting. Robert Walko will be the primary presenter, with assistance from Simone Fatichi and Stefan Kollet. Documentation developed under Task 6, as well as that existing previously, will be provided for the workshop.

 

References

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