SWIMv1/SWIMv2 - soil water infiltration and movement model - simulate soil water balances

SWIMv1/SWIMv2 Categories: flow models - unsaturated zone, soil, solute transport models - unsaturated zone
 

SWIMv1/SWIMv2 Description

SWIMv1 - Soil Water Infiltration and Movement (Version 1)

Introduction to SWIMv1
SWIMFILE - Data Preparation Program
SWIMEV - The Simulation Program
SWIMPLOT - Data Presentation Program
Features of SWIMv1

Introduction to SWIMv2
Features of SWIMv2
Water Flow
Solute Flow
SWIMv2 Vegetation, Transpiration, Evaporation
SWIMv2 Management Options
SWIMv2 Inputs and Boundary Conditions
SWIMv2 Output
 

INTRODUCTION TO SWIMv1

SWIMv1 is a software package for simulating water infiltration and movement in soils. As in the real world, SWIMv1 allows addition of water to the system as precipitation and removal by runoff, drainage, evaporation from the soil surface and transpiration by vegetation.

SWIMv1 helps researchers and consultants understand the soil water balance so they can assess possible effects of such practices as tree clearing, strip mining and irrigation management.

SWIMv1 is valuable for scientists and consultants involved in land planning and land management. For example, if a development is being considered which involves tree clearing, SWIMv1 can be used to indicate salinity or surface runoff problems that could result from a change in the soil water balance associated with the removal of the trees.

SWIMv1 is an easy-to-use menu-driven system ideal for teaching students about the soil water balance. With the aid of a computer, students can simulate real-world situations and gain a feel for how the systems behave. They can investigate particular systems such as how water movement through a clay soil differs from water movement through a sandy soil, given the same rainfall conditions. This helps to provide a quick and accurate understanding of various water systems and the effects that different management practices have on the soil water balance.

SWIMv1 consists of a menu-driven suite of three programs that allow the user to simulate soil water balances using numerical solutions of the basic soil water flow equations.

The SWIMv1 simulation program obeys the basic physical law of conservation of mass while making the following assumptions:

• conditions can be treated as horizontally uniform
• flow is described by Richards' equation
• soil hydraulic properties can be described simply

While a working knowledge of soil physics and computing offer an advantage in using SWIMv1, efforts have been made to provide a user-friendly package with extensive help facilities. The three main modules comprising SWIMv1 are:

1. SWIMFILE - input data preparation program

2. SWIMEV - the simulation program

3. SWIMPLOT - output data presentation program

SWIMFILE - DATA PREPARATION PROGRAM

SWIMFILE is a menu-driven program which allows the user to edit an input data file interactively. Both the values and units of parameters can be altered, with error flagging occurring for incorrect editing.

Parameters that can be altered include those describing:

Simulation - starting time, finishing time, print interval (simulation results will be output at this interval) and water increment (simulation integration time steps are chosen so that the greatest water flux in the system, excluding transpiration, will move approximately this amount of water in one time step; a smaller increment gives better accuracy).

Vegetation characteristics - the model allows up to four vegetation types to grow together. Each type has certain characteristics that determine its water extraction pattern. A fraction of the total potential evapotranspiration (PET) is assigned to each type, with any left over being used for soil evaporation. The fraction of the potential that a type intercepts and its root length density increase sigmoidally with time. The distribution of roots with depth is assumed to be exponential, with a fixed depth constant.

Soil surface conductance - initial soil surface conductance, minimum soil surface conductance, precipitation constant and effectiveness parameter. Although SWIMv1 can cope with thin impeding layers, there is special allowance for those at the surface. A surface seal or crust can be represented as a surface conductance, and its value may decrease exponentially with cumulative rainfall energy. Alteration of these parameters will alter the surface conductance characteristics of the soil.

Surface storage - initial soil surface storage, minimum soil surface storage, precipitation constant. Runoff occurs when the surface-water depth is greater than the surface storage. To allow for a reduction of surface roughness due to rainfall, the storage decreases exponentially with precipitation energy from the given initial value towards the given minimum in exactly the same way as the surface conductance. Variable initial surface-water depth allows the simulation to begin with the surface ponding.

Runoff - runoff rate factor and runoff rate power. For a water depth that is dh above the storage, the runoff rate is equal to the given runoff rate factor times dh^P, where P is the given runoff rate power. This crudely simulates overland flow.

Soil properties - hydraulic properties together with soil depths and initial matric potential values. The hydraulic properties available include thetas (water content at field concentration), psie (air entry potential), b (minus the slope of a straight line approximating the water retention curve on a log - log plot), and ks (hydraulic conductivity at field saturation). Soil depths are those used for the simulation. Increasing the number of soil depths increases accuracy but at the expense of simulation execution speed.

Precipitation and potential evapotranspiration - cumulative precipitation and cumulative PET are given functions of time. SWIMEV uses linear interpolation to find values between those given in both cases.

SWIMFILE offers extensive help screens providing background information, examples, suggested values and assistance with general editing procedures. A data file is created on completion of this program which is then used as an input file to the simulation program, SWIMEV.

SWIMEV - THE SIMULATION PROGRAM

SWIMEV solves Richards' equation numerically using recently-developed techniques that ensure efficient computation (Ross, P.J. 1990. Efficient numerical methods for infiltration using Richards' equation. Water Resources Research 26(2), 279-290). The methods ensure that mass is conserved, even when fast, approximate solutions are obtained. Although Richards' equation does not accurately describe every flow situation, it is the accepted basis of soil water flow. Within its limitations, a user can simulate infiltration, redistribution, deep drainage, simultaneous evapotranspiration by up to four types of vegetation, transient surface-water storage and runoff. Soils may be vertically inhomogeneous, but must be horizontally uniform. Macroporosity can be included if water in the macropores can move quickly to other pores.

SWIMEV accepts data input from an ASCII file created by the data preparation program, SWIMFILE, on a standard text editor.

Output from SWIMEV is written to a binary file, to the screen, and optionally to an ASCII file. The binary output file can be viewed graphically using SWIMPLOT. The FORTRAN source code for SWIMEV is printed as an appendix to the manual.

SWIMEV Data Input
Some input parameters are required by SWIMEV while others are optional. Required parameters include:

• simulation control parameters
• soil depths (and associated soil parameters)
• conductance parameters
• runoff parameters

Optional parameters include:

• number of vegetation types (and associated vegetation data)
• precipitation data
• evaporation data

Help screens can be used for guidance in selecting values for entries where there is uncertainty.

SWIMEV Data Output
Printout occurs at the times with the print interval parameter in the "Times" menu. The program adjusts its timesteps to reach each print time exactly. Output is as follows:

• Time - The current print time.
• Balance - Computational error in solving the water balance equation. This value should always be small.
• Work - The number of linear equations solved so far. The program solves the nonlinear water balance equations used to approximate Richards' equation by an iterative method that uses sets of simultaneous linear equations. Execution time is proportional to this value.
• Evaporation rates - The actual and potential evaporation rates for the soil are followed by the actual and potential transpiration rates for vegetation type 1, vegetation type 2, etc. Rates for the most recent day (or print interval, if this is less than one day). Potential rates are calculated from the given evaporation data and parameters in the "Vegetation" menus.
• ET - The cumulative actual and potential evaporation for the soil are followed by the cumulative actual and potential transpiration for vegetation type 1, vegetation type 2, etc.
• Water balance - Cumulative precipitation, runoff, infiltration, drainage and evapotranspiration are given together with current values of surface-water depth and total water in the soil profile. The balance error given above is calculated from these values plus initial values of surface and profile water.
• Water variables - Values of soil water potential, volumetric water content, soil hydraulic conductivity at that water content, and downward soil water flow rate are given at each depth.

SWIMPLOT - DATA PRESENTATION PROGRAM

SWIMPLOT pictorially displays the data from SWIMEV for each print step. VGA color displays are supported.

SWIMPLOT displays a plot of volumetric water content with depth for available and unavailable water, evaporation rates, and water balance components relative to the overall water budget.

The user has control over the display speed and the facilities to restart the display at any screen number, alter increments between screens, pause the screen and print an individual screen. Printer support is provided for Epson-FX and Hewlett Packard Laserjet II and compatible printers.

FEATURES OF SWIMv1

• Numerical solution of Richards' equation.
• Fast and accurate.
• Conservation of mass, even in fast, approximate solutions.
• Ability to handle non-uniform and layered soils.
• Ability to handle unsaturated, saturated and ponded conditions.
• Caters for transient soil surface conductance and storage.
• Calculates runoff and drainage.
• Allows simultaneous transpiration by several vegetation types.
• Extensive help screens.
• Ability to accept data in your preferred units.
• Color graphic output.

SWIMv2 - Soil Water Infiltration and Movement
(Version 2)

INTRODUCTION TO SWIMv2

SWIMv2 (Soil Water Infiltration and Movement model version 2) is a mechanistically-based model designed to address soil water and solute balance issues associated with both production and the environmental consequences of production. SWIMv2 employs fast, numerically-efficient techniques for solving Richards' equation for water flow and the convection-dispersion equation for solute transport and is suitable for personal computer applications. The model deals with a one-dimensional vertical soil profile which may be vertically inhomogeneous, but is assumed to be horizontally uniform. It can be used to simulate runoff, infiltration, redistribution, solute transport and redistribution of solutes, plant uptake and transpiration, evaporation, deep drainage and leaching. The physical system and the associated flows addressed by the model are shown schematically in Fig.1. Thermally-induced vapor flow and the temperature dependence of liquid water movement are ignored. Soil water and solute transport properties, initial conditions, and time dependent boundary conditions (e.g., precipitation, evaporative demand, solute input) need to be supplied by the user in order to run the model. SWIMv2 uses a self-adjusting time step to meet specified error criteria, and the user need only specify the maximum time step allowed. There is no real limitation to the space and time steps that can be used but they should be chosen to reflect the particular processes being studied and the questions being addressed. It is common to use one hour as the maximum time step when addressing diurnal issues, and one day for most other cropping issues that are likely to be addressed with SWIMv2. The option of both going higher (weeks, months, years) and lower (minutes, seconds) is available to the user. The SWIMv2 model conserves mass, no matter how large the space or time steps that are used in the simulation. Additional details of the SWIMv2 model and its use are included in the User Manual (Verburg et al., 1996).

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