Bach is a water resources model, designed to be the replacement of the regional surface water modules Mozart and SIMRES in the Netherlands Hydrological Instrument (NHI). Bach is a work in progress, it is a prototype that demonstrates all essential functionalities. Further development of the prototype in a software release is planned in 2022 and 2023.

Bach is written in the Julia programming language and is built on top of the SciML: Open Source Software for Scientific Machine Learning libraries, notably ModelingToolkit.jl.

1 Status

The initial focus is on being able to reproduce the Mozart regional surface water reservoir results. Each component is defined by a set of symbolic equations, and can be connected to each other. From this a simplified system of equations is generated automatically. We use solvers with adaptive time stepping from DifferentialEquations.jl to get results.

Example timeseries of a single basin, the Hupselse Beek, with the input and output fluxes on the top, and the storage volume (the state) below.

Example bar plot of the daily waterbalance for the Hupselse Beek, comparing results of Mozart (left) and Bach (right).

2 Introduction

2.1 Water balance equations

The water balance equation for a drainage basin (Wikipedia contributors 2022) can be defined a first-order ordinary differential equation (ODE), where the change of the storage \(S\) over time is determined by the inflow fluxes minus the outflow fluxes.

\[ \frac{\mathrm{d}S}{\mathrm{d}t} = Q_{in} - Q_{out} \]

We can split out the fluxes into separate terms, such as precipitation \(P\), evapotranspiration \(ET\) and runoff \(R\). For now other fluxes are combined into \(Q_{rest}\). If define all fluxes entering our reservoir as positive, and those leaving the system to be negative, all fluxes can be summed up.

\[ \frac{\mathrm{d}S}{\mathrm{d}t} = R + P + ET + Q_{rest} \]

Such a water balance ODE can be represented in ModelingToolkit.jl as follows:

using ModelingToolkit

@variables t S(t) R(t) P(t) ET(t) Q_rest(t)  # independent and dependent variables
D = Differential(t)       # define an operator for the differentiation w.r.t. time

sys = ODESystem([D(S) ~ R + P + ET + Q_rest])

On the last line, an ODESystem is created that consists of a single equation. Before we can solve this, we need to create an ODEProblem, that provides information about the timespan of the simulation, as well as the initial conditions.

Bach can be used as a Julia package running in a Julia session or as an application initialized via TOML configuration files. Both uses are demonstrated in the demonstrations.

2.2 Time

The water balance equation can be applied on many timescales; years, weeks, days or hours. Depending on the application and available data any of these can be the best choice. In Bach, we make use of DifferentialEquations.jl and can use any of its ODE solvers. Many of these solvers make use of adaptive time stepping, which means the solver will decide how large the time steps can be depending on the state of the system.

The forcing, like precipitation, is generally provided as a time series. Bach is set up to support unevenly spaced timeseries. The solver will stop on timestamps where new forcing values are available, so they can be loaded as the new value.

Bach is essentially a continuous model, rather than daily or hourly. If you want to use hourly forcing, you only need to make sure that your forcing data contains hourly updates. The output frequency can be configured independently. To be able to write a closed water balance, we accumulate the fluxes. This way any variations in between timesteps are also included, and we can output in rather than m³s⁻¹.

2.3 Space

The water balance equation can be applied on different spatial scales. Besides modelling a single lumped watershed, it allows you to divide the area into a network of connected representative elementary watersheds (REWs) (Reggiani, Sivapalan, and Majid Hassanizadeh 1998). At this scale global water balance laws can be formulated by means of integration of point-scale conservation equations over control volumes. Such an approach makes Bach a semi-distributed model. In this document we typically use the term “basin” to refer to the REW. (In Mozart the spatial unit was called Local Surface Water (LSW)). Each basin has an associated polygon, and the set of basins is connected to each other as described by a graph, which we call the network. Below is a representation of both on the map.

Mozart Local Surface Water polygons and their drainage.

The network is described as graph. Flow can be bi-directional, and the graph does not have to be acyclic.

graph LR;
    A["basin A"] --- B["basin B"];
    A --- C["basin C"];
    B --- D["basin D"];
    C --- D;

Internally a directed graph is used. The direction is defined to be the positive flow direction, and is generally set in the dominant flow direction. The basins are the nodes of the network graph. Basin states and properties such storage volume, wetted area are associated with the nodes (A, B, C, D), as are most forcing data such as precipitation, evaporation, or water demand. Basin connection properties and interbasin flows are associated with the edges (the lines between A, B, C, and D) instead.

Multiple basins may exist within the same spatial polygon, representing different aspects of the surface water system (perennial ditches, ephemeral ditches, or even surface ponding). Figure 1, Figure 2, Figure 3 show the 25.0 m rasterized primary, secondary, and tertiary surface waters as identified by BRT TOP10NL (PDOK 2022) in the Hupsel basin (as defined in the Mozart LSW’s). These systems may represented in multiple ways.

Figure 1: Hupsel: primary surface water.

Figure 2: Hupsel: secondary surface water.

Figure 3: Hupsel: tertiary surface water.

As a single basin (A) containing all surface water, discharging to its downstream basin to the west (B):

graph LR;
    A["basin A"] --> B["basin B"];

Such a system may be capable of representing discharge, but it cannot represent residence times or differences in solute concentrations: within a single basin, drop of water is mixed instantaneously. Instead, we may the group primary (P), secondary (S), and tertiary (T) surface waters. Then T may flow into S, S into P, and P discharges to the downstream basin (B.)

graph LR;
    T["basin T"] --> S["basin S"];
    S --> P["basin P"]; 
    P --> B["basin B"];

As each (sub)basin has its own volume, low throughput (high volume, low discharge, long residence time) and high throughput (low volume, high discharge, short residence time) systems can be represented in a lumped manner; of course, more detail requires more parameters.

3 Usage

3.1 Installing the compiled executable

Binaries of bach_cli can be downloaded from bach-artifacts, and are currently available for Windows only. Download and unpack the .zip file. It can be placed anywhere, however it is important that the contents of the zip file are kept together in a directory. The bach_cli executable is in the bin directory.

To check whether the installation was performed successfully, run bach_cli with no arguments in the command line will give the following message:

Usage: bach_cli 'path/to/config.toml'

3.2 Installing as a Julia package

Bach is a Julia package, that can be installed using Julia’s built in package manager, Pkg. For more information on how to use Pkg, see the Getting Started page in its documentation.

Since Bach is not yet registered in the General registry, you have to install it using the full url. Note that here suggest dev to do a development install.

pkg> dev

This will clone the git repository, put it under your home directory in .julia/dev/Bach, and add the Bach package to your project environment. Note that to receive updates, you have to pull in the latest changes yourself using git pull.

Installation may take a while since Pkg is also installing and pre-compiling the dependencies of Bach.

Once it is done, you can go back the Julia REPL and try to load Bach:

julia> using Bach

If you already have a simulation prepared, you can run it using:


Again, there will be considerable latency. For this reason, if you expect to run Bach a lot, we recommend to:

  • Use bach_cli if you only need and don’t need to do developments.
  • Adapt a workflow that keeps you session active; the first"model.toml") will take considerably more time than the second one, since more code needs to be compiled.
  • Use a system image, explained in the developer documentation, to reduce latency further
Tip: Visual Studio Code

There is a section on editors and IDEs for Julia on, scroll down to see it. We use and recommend Microsoft’s free and open source Visual Studio Code. When combined with the Julia extension it provides a powerful and interactive development experience.

Tip: Revise.jl

If you plan to make changes to the code of Bach, we recommend installing the Revise.jl package. This package allows you to modify code and use the changes without restarting Julia. Install it with add Revise from the Pkg REPL. Then create a file called .julia/config/startup.jl, and put using Revise there. This will load Revise every time you start a Julia session.

3.3 Input and output files

3.3.1 Configuration file

Bach has a single configuration file, which is written in the TOML format. It contains settings, as well as paths to other input and output files.

# start- and endtime of the simulation
# can also be set to a date-time like 1979-05-27T07:32:00
starttime = 2019-01-01
endtime = 2021-01-01

# maintain target waterlevel in level controlled basins
add_levelcontrol = true  # default is true
# run a coupled Bach - Modflow simulation
run_modflow = false  # default is false

# all timesteps are in seconds
# do water allocation for this time horizon
update_timestep = 86400.0
# save output at this interval
output_timestep = 86400.0  # default is 86400.0
# save timesteps at this interval
# note that this is internal, and is forwarded to the DiffEq solver options
saveat = 86400.0  # default is to save every step

# location IDs that will be included in the simulation
# these location IDs are referenced in the input files below
lsw_ids = [151358, 151371, 151309]

# input files
forcing = "../data/input/6/forcing.arrow"
state = "../data/input/6/state.arrow"
static = "../data/input/6/static.arrow"
profile = "../data/input/6/profile.arrow"
network_path = "../data/input/6/network.ply"

# output file
waterbalance = "../data/output/waterbalance.arrow"

Additionally, if run_modflow = true, a Modflow specific section needs to be added to the configuration file. This part is documented in the Modflow coupling demo.

3.3.2 Network

The network, as set with network_path in the configuration file, is the path to a PLY file. This network is a directed graph, that links the different basins to each other, as described in Section 2.3.

The PLY format is a relatively simple file format that comes in binary and ASCII flavors, both of which are supported. To support viewing the network as a mesh layer in QGIS, we recommend adding a comment specifying the coordinate reference system (CRS) as documented by MDAL and shown below. In this example, only the first and last two vertices are shown, just like for the edges.

format ascii 1.0
comment crs: EPSG:28992
element vertex 8531
property float64 x
property float64 y
property float64 location
element edge 6318
property int32 vertex1
property int32 vertex2
property float32 fractions
140702.897 414305.632 10020.0
157416.633 424082.847 10037.0
47343.505 371868.984 991896.0
45651.526 372390.837 991908.0
0 11 1.0
1 25 1.0
8512 7572 1.0
8516 7677 1.0

For each vertex, the x and y coordinates are given, followed by the location ID. For each edge, the source and destination vertex are given, followed by the fraction. The vertex here refers not to the location ID, but to the zero-based index of the vertices as ordered in the file. The fraction is typically 1.0, but can be divided into smaller parts if there is a bifurcation from one basin to two downstream basins. This then represents the fraction of water flowing from the source along this edge.

The network file is allowed to be a superset of the locations IDs that you simulate (the lsw_ids configuration). A sub-network will be derived automatically.

3.3.3 Arrow files

The input and output files described below all share that they are tabular files, stored in the Apache Arrow IPC file format, also known as Feather files. This format has been chosen since it is standardized, fast, simple and flexible. It can be read and written by many different software packages and libraries in different languages. In Bach we use Arrow.jl.

Below we give details per file, in which we describe the schema of the table using a syntax like this:

column type restriction
location Int -
volume Float64 non-negative

This means that two columns are required, one named location, that contained elements of type Int, and a column named volume that contains elements of type Float64. The order of the columns does not matter, and the files are also allowed to have extra columns, which will be ignored. In some cases there may be restrictions on the values or the ordering of the rows. This is indicated under restriction.

Tables are also allowed to have rows for locations, variables or times that are not part of the simulation, these will be ignored. That makes it easy to prepare data for a larger region once, and test models on a subset of the basins.

The Arrow input files can be compressed with LZ4 or Zstd compression. Furthermore, in some of the columns, a small amount of different values are repeated many times. To reduce file sizes it may be a good idea to apply dictionary encoding to those columns.

3.3.4 State

The state table aims to capture the full state of the system, such that it can be used as an initial condition, that is potentially the outcome of an earlier simulation. Currently the only state is the volume of storage.

column type restriction
location Int -
volume Float64 non-negative

Each location specified in lsw_ids needs to be in the table.

3.3.5 Static

The static table provides data that is fixed per location. The local_surface_water_type can be either V for free draining basins or P for level controlled basins.

column type restriction
location Int -
target_level Float64 -
target_volume Float64 -
local_surface_water_type Char ‘V’ or ‘P’

Each location specified in lsw_ids needs to be in the table.


Of target_level and target_volume, currently only target_volume is used, and only for level controlled basins. This variable should be moved to the forcing table to allow dynamic targets.

3.3.6 Profile

The profile table defines the physical dimensions of the storage reservoir of each basin. Depending on the basin type, the columns are used differently. For free draining basins, the level is not used in the calculation, but may be used to convert calculated volumes to levels for output. For level controlled basins, the discharge is not used.

column type unit restriction
location Int - sorted
volume Float64 \(m^3\) sorted per location and start at 0
area Float64 \(m^2\) sorted per location and non-negative
discharge Float64 \(m^3 s^{-1}\) sorted per location and non-negative
level Float64 \(m\) sorted per location

The level is in meters above a datum that is the same for the entire model. An example of the first 5 rows of such a table is given below. The first 4 rows define the profile of location 10020. The number of rows can vary per location. Using a very large number of rows may impact performance.

location volume area discharge level
10020 0.0 1.36404e5 0.0 -0.105
10020 24726.2 1.36404e5 0.0 0.095
10020 49452.5 1.36404e5 0.00942702 0.295
10020 2.49735e6 1.36404e5 0.942702 20.095
10037 0.0 50663.3 0.0 2.129

Each location specified in lsw_ids needs to be in the table at least once.

3.3.7 Forcing

All dynamic data can be added to the forcing table.

column type restriction
time DateTime sorted
variable String -
location Int -
value Float64 depending on variable

Here is an example of the first and last two rows of a forcing table:

time variable location value
2019-01-01T00:00:00 precipitation 150006 2.89352e-10
2019-01-01T00:00:00 precipitation 150016 2.89352e-10
2020-12-31T00:00:00 evaporation 151690 1.15741e-9
2020-12-31T00:00:00 evaporation 151707 1.15741e-9

The forcing table is a narrow table. When the simulation arrives at a time given in the forcing data, it will update all the values given for that time. These values then stay constant until the next update, there is no interpolation in between. This format give maximum flexibility to update different variables at different times.

3.3.8 Water balance

The water balance table outputs all water balance terms at a regular output frequency, as defined by output_timestep in the configuration file. The units are \(m^3\), which is the total volume of flow into the basin since the previous output timestep. The inital timestep of the simulation, with all values set to zero, is also written to the file. Since flow out of the basin is given a negative value, and the storage change is included, you can sum all components to verify the closure of the water balance.

column type restriction
time DateTime sorted
variable String -
location Int -
value Float64 non-negative volume (\(m^3\))

The water balance table has the same schema as the forcing table. Since the output frequency is constant, and equal for all variables, you can pivot this table from narrow to wide data to get the different water balance components as columns. Having them as a narrow table can make it easier to process, since for a given model you may not know the water balance components ahead of time.

3.4 Example input files

From this link you can download an existing schematization for the Netherlands that was used for testing purposes during development. It is provided here as an example to help people get started. Based on the description of the input files above, you can also generate your own schematization using your tools of choice. We have used DataFrames.jl and Arrow.jl to prepare the tables, but one can equally use for instance Python’s pandas.


PDOK. 2022. “Dataset: Basisregistratie Topografie (BRT) TOPNL.”
Reggiani, Paolo, Murugesu Sivapalan, and S. Majid Hassanizadeh. 1998. “A Unifying Framework for Watershed Thermodynamics: Balance Equations for Mass, Momentum, Energy and Entropy, and the Second Law of Thermodynamics.” Advances in Water Resources 22 (4): 367–98.
Wikipedia contributors. 2022. “Drainage Basin — Wikipedia, the Free Encyclopedia.”