Irrigators are increasingly challenged to maintain or even increase production using less water, sometimes of poorer quality, and often from unconventional sources. This paper describes the main features of a newly developed software-based Decision Support System (DSS), with which the fitness for use (FFU) of water for irrigation (IrrigWQ) can be assessed. The assessment considers site-specific factors, several non-traditional water constituents, and the risk of negative effects. The water balance components of a cropping system and the redistribution of solutes within a soil profile are assessed with a simplified soil water balance and chemistry model. User-friendly, colour-coded output highlights the expected effects of water constituents on soil quality, crop yield and quality, and irrigation infrastructure. Because IrrigWQ uses mainly internationally accepted cause–effect relationships to assess the effect of water quality constituents, it is expected to find universal acceptance and application among users. IrrigWQ also caters for calculating so-called Water Quality Requirements (WQRs). WQRs indicate the threshold levels of water quality constituents for irrigation at specified levels of acceptability or risk. WQRs assist water resource managers in setting site-specific maximum threshold levels of water quality constituents that can be tolerated in a water source before impacting negatively on successful irrigation.

User-friendly Windows software, CLIMGEN-UP, was developed based on the CLIMGEN weather data generator. CLIMGEN-UP calculates and stores monthly statistical parameters, based on historic data of daily rainfall, minimum and maximum temperature. It can then generate and store long-series of daily rainfall, minimum and maximum temperature, based on the monthly statistical parameters. Historic and generated data can be displayed graphically. CLIMGEN-UP is written in Delphi v. 5.0 and makes use of a Paradox database. The program is available for use with Windows 98/ME/2000 and NT on an IBM-PC or compatible computer, and is supplied as executable code on 3.5″ disk. Possible applications are in hydrology, environmental and ecosystem management as well as agricultural risk assessment where long-term simulations of system behaviour are required.

Distribution of water and energy is non-uniform in widely spaced, micro-irrigated, hedgerow crops. For accurate water use predictions, this two-dimensional variation in the energy and water balance must be adequately accounted for. To this end, a user-friendly, two-dimensional, mechanistic soil water balance model (SWB-2D), has been developed. Energy is partitioned at the surface depending on solar orientation, row direction and canopy size, shape and leaf area density. Water is assumed to be distributed uniformly at the surface in the case of rainfall, whilst micro-irrigation only wets a limited portion of the field. Crop water uptake is calculated as a function of evaporative demand, soil water potential and root density. Evaporation is also calculated as being either limited by available energy or by water supply. Water is redistributed in the soil in two dimensions with a finite difference solution to the Richards’ equation. A field trial was set up to test the 2-D soil water balance model in a citrus orchard at Syferkuil (Pietersburg, South Africa). Model predictions generally compared well to actual soil water content measured with time domain reflectometry probes. Scenario modelling and analyses were carried out by varying some input parameters (row orientation, canopy width, wetted diameter and fraction of roots in the wetted volume of soil) and observing variations in the output of the soil water balance. The model holds potential for improving irrigation scheduling and efficiency through increased understanding and accuracy in estimating soil water reserves, since it accounts for the differing conditions in the under-tree irrigated strip and inter-row rainfed areas.

User-friendly Windows software (ChemEq) was developed for calculation of solution chemical equilibrium and precipitation- dissolution of lime and gypsum. ChemEq calculates ionic concentrations and activities, pH, solid phase gypsum and lime, sodium adsorption ratio and electrical conductivity from initial concentrations of ions, gypsum and lime. The program is written in Delphi v. 5.0 and is available for use with Windows 98/ME/2000 and NT on an IBM-PC or compatible computer. The main applications are in the calculation of chemical equilibrium of CaSO4 solutions.

The sustainability of irrigation with gypsiferous mine water and different irrigation management practices was evaluated using a milti-disciplinary approach, where crop response was investigated along with the impact on soil and groundwater resources. Field trials carried out at two mines (Landau and Kleinkopje Colliery, Mpumalanga Province, South Africa) indicated that a wide range of species can be cropped for commercial purposes under irrigation with this water. Chemical analyses of groundwater underlying irrigated areas indicated that contamination of groundwater did not occur after three years. The results of a glasshouse trial indicated that the inclusion of NH4 + for N-fertilization in a NO3 −-NH4 + ratio of 2:1 to 1:1 is advantageous to root and top growth of wheat. According to simulations run with the Soil Water Balance (SWB) model and the CLIMGEN weather data generator, soil chemical and physical properties will not be irreparably damaged after thirty years of irrigation. Perennial pastures, irrigated at high frequency, provided the highest net farm income and water utilization.

. The most common approach for the estimation of crop water requirements is to pair a crop factor with the evaporation from a reference surface. In this study, a user-friendly computer tool was developed to facilitate the calculation of daily FAO (Food and Agricultural Organization of the United Nations, Rome, Italy) Penman-Monteith reference crop evaporation (ET 0 ), and to estimate errors that can arise if solar radiation, wind and vapour pressure data are not available. The ET 0 calculator imports comma, tab or space-delimited daily weather data files in any user-specified format. It displays graphically and processes statistically, ET 0 values calculated from full and incomplete weather data sets. The program is written in Delphi with a Paradox database and includes a comprehensive, context-sensitive help file. Sensitivity analyses were carried out for three locations as examples. The error in predicting ET 0 using estimated weather parameters was reduced by using 5-day averages of ET 0 rather than daily values. Although some error is incurred by estimating weather parameters, this is somewhat compensated for by the absence of any error that may have been associated with the measurements.

The use of gypsiferous mine water for irrigation of agricultural crops is a promising technology that could solve problems related to both shortage of irrigation water and disposal of effluent mine drainage. The long-term effect of irrigation with lime-treated acid mine drainage on soil properties and catchment salt load was investigated. The soil water-salt balance-crop growth model (SWB) and the CLIMGEN weather data generator were used to simulate 30 years of irrigation with gypsiferous mine water for different irrigation management scenarios, followed by 20 years of dry land summer cropping, to determine if the problem of salt disposal was merely being postponed. Generated weather input data were for Bethal (South Africa), soil input data for a Plinthic Ferralsol (FAO-UNESCO) and crop input data for a rotation of pearl miller (Pennisetum glaucum cv. SA Standard) and oats (Avena sativa L. cv. Overberg). The soil appeared to act as an effective salt sink, with large quantities of calcium sulphate (340-404 Mg ha(-1)) being precipitated in 30 years, and with negligible amounts of remobilization thereafter. Dissolved salts in the soil solution increased during winter when rainfall is negligible. The highest concentrations occurred deeper in the profile, which is desirable because rooting is less dense there and plant growth is thus less affected. Due to large amounts of gypsum precipitation near the surface, the ions in solution were mainly Mg(2+) and SO(4)(2-). Simulated root weighed, saturation electrical conductivities indicated that summer cropping should present no problems, but for certain winter crops a leaching fraction should be applied to obtain maximum yields. Drainage water quality was variable, depending heavily on rainfall. Peak salt levels were around 9.6 g L(-1). Depending on irrigation strategy, between 418 and 636 Mg ha(-1) of salts was leached over 30 years. Once irrigation ceased, very little leaching occurred. Annual irrigation depended on rainfall, but averaged at about 1100 mm per annum. About 350 mm of percolation occurred, resulting in net utilization of around 750 mm annually. About 40% of the salts added through irrigation could be immobilized in the soil profile. It is concluded that year round, high frequency irrigation, with a leaching fraction in winter, should be an effective and economical means of utilizing large quantities of gypsiferous water without causing irreparable damage to soil resources. (C) 1999 Elsevier Science B.V. All rights reserved.