RainOff :   a conceptual hydrologic rainfall-runoff (surface drainage) simulation model with a nonlinear reservoir
Totally free download of software

The RainOff computer program, or simulation model, calculates runoff from rainfall on watersheds (hydrologic catchment areas) whose extension is limited by the condition that the rainfall can be assumed evenly distributed over the area. The program can be freely downloaded, it is free shareware in the public domain.
      The rainfall is converted into recharge by routing through a retention reservoir from which water escapes depending on predetermined characteristcs as storage capacity, initial storage and maximum rate of escape (representing evaporation or subsurface drainage). This first reservoir gives the recharge (rainfall excess) as overflow.
      The recharge is routed through a second, non-linear, reservoir with a reaction factor whose value depends on the storage level. Thus, the conversion (transformation) of rain into runoff (surface drainage) and flooding is produced.
      Modelling with RainOff gives the opportunity of predicting the hydrology of the watershed (catchmnent) and of flood forecasting as well as simulating discharge and water table height in subsurface drainage systems. For the latter case, the model program offers a calculator to compose the reservoir reaction factor or response function from the characteristics of the system.

The reaction (response) factor can be determined from recorded rainfall-runoff data. The program uses various reservoir functions and selects the optimal function. Alternatively, it is possible to introduce the reservoir function, together with rainfall data, and the program will calculate the runoff.
      More details are given in the program itself.
On 12 December 2010 an extra graph was provided for the recorded rainfall-runoff statistics and trends, thanks to a suggestion by Mr. Juan Victoria.
      On 27 August 2012 the program was updated to include a quadratic reservoir function and on 1 december 2012 a Help function and an observed-calculated graph was added.
      On 3 January 2018 the program was given more versatility by dividing the runoff data into a high and a low range with different reaction factors.

The program starts giving clicking on RainOffT.Exe. More information is given in the program itself.

A description of the procedure and equations used in RainOff is found in this paper on the reservoir model.
A lecture note ("Data Analysis") on drainage research with examples of RainOff applications is found on the articles page.
An example of the application of the Curve Number Method for the design of a surface drainage system for sugar cane plantations in a humid tropical coastal area can be found in a chapter ("Agricultural Drainage Criteria"), also on the articles page.
An article on rainfall-runoff relations of a small valley in Sierra Leone using RainOff with a non-linear reservoir has appeared in the International Journal of Environmental Science, see this website or find it in this place
The data used in the Sierra Leone articles were found in this report of A.Huizing
      A comparison of results of the drainage software EnDrain and RainOff for steady and non-steady state drainage flow and water level can be consulted at ResearchGate or on this web site
      The application of RainOffT is also demonstrated with the rainfall-runoff data of the "Herbornseelbach" in Hesse, Germany. Read the report using this link.
      The use of RainOff for the evaluation of the hydrologic effects of agricultural subsurface drainage systems can be inspected at this validation page and at application.

Experiences: For improvement, I am interested to learn about your experiences with RainOff. For this there is a contact form.


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1 - The model can also be used for hydrograph analysis.
2 - The original model (RainOff) used the periods without recharge to detect the "signature" of the catchment area (watershed). If in the record such periods are missing, the version (RainOffT), that detects the characteristics by numerical optimization, can be used. Though it needs more calculation time, its versatility caused the original model to be replaced by the latter.
3 - Also a version named RainOffQ can be obtained on request. It does the same as RainOffT, but it uses a quadratic function for the reaction factor instead of linear. It uses still more time, but in some cases the accuracy may be higher.
4 - The latest version, DrainCalc offers the possibility to determine the drainage reaction factors from both the measured drain discharge and depth of the water table. Once the reaction factors are known, the model can be used to simulate drain discharge and depth of the water table under varying hydrological conditions. Alternatively, the reaction factors can be found from the parameters of the subsurface drainage system.

introduction to rainfall-
             runoff software Introduction screen of RainOff program
The hydrologic RainOff simulation program converts
rainfall into runoff (surface drainage) depending on
the characteristics of the watershed or catchment area.
The software uses a reservoir model with inflow
creating storage of water and outflow.
The model can also be applied to surface drainage.
rainfall-runoff simulation Example of RainOff application
The software model gives output graphics.
In this program there is an example of a fairly close
fit of the observed and simulated discharge (runoff)
of the stream in a small valley, as was calculated by
this program from the rainfall
simulated drain discharge Example of results of the RainOff model simulating drain discharge
The drainage system is to be characterised by a response function (or reaction factor)
alpha = A * runoff + B,
as can be seen using the option "Predict discharge or runoff from rainfall given the alpha function" in the input tabsheet. For the calculation of A and B from the characteristics of the system, a calculator is built in.

In this example the discharge of a subsurface drainage system is calculated for regular inputs of irrigation water using a reaction factor alpha=0.1 (A=0, B=0.1). For more details see the analysis article in section 4.2: Transient recharge-discharge relations