SAWCal: A user-friendly program for calculating soil available water quantities and physical quality indices

Abstract

Conventional plant available water (PAW), least limiting water range (LLWR), integral water capacity (IWC) and integral energy (EI) are different approaches to quantify the soil water availability for plants. The PAW, the soil water between field capacity (FC) and permanent wilting point (PWP), is based merely on the soil matric potential. The LLWR in addition considers oxygen deficit and high soil penetration resistance limitations for plant growth excluding some of the restrictions of the PAW concept. The IWC approach adapts better with natural phenomena since it considers that the impacts of soil physical limitations on water uptake by plants are gradual and not in a cut-off form. Nevertheless, its calculation is laborious and time-consuming. The EI quantifies the energy required by plants to remove a unit amount of water from the soil over the available water range. As more attention is given to new soil available water (SAW) approaches nowadays, developing an algorithm and computer graphical user interface (GUI) for calculation of these indices (especially the IWC) would help to popularize the approaches’ application. Here we report the development of a software called SAWCal (Soil Available Water Calalculator) for calculating PAW, LLWR, IWC, and EI and related quantities. It also calculates the soil physical quality index (S), the slope of the water retention curve at its inflection point, proposed by Dexter (2004a,b,c). This software only needs the parameters of soil water retention and soil penetration resistance models. The C#, being an evolution of C++, as an object-oriented programming (OOP) was employed to develop the program. The parameters of single-porosity van Genuchten (1980) and dual-porosity Durner (1994) models are used to characterize the water retention and hydraulic conductivity functions for the soils with uni- and bi-modal pore size distributions, respectively. For establishing a continuous function of soil penetration resistance vs. matric suction, SAWCal uses a modified van Genuchten (1980) model or a power model depending on the user’s choice. When needed, the integration is performed numerically. SAWCal is a help-assisted user-friendly GUI to guide the user step-by-step through the procedures. It is also possible to export the program outputs in ASCII format compatible with MS Office and text-editing programs. The software is provided as a free supplementary material to this paper.