Assessing drainage water quality for irrigation using the water quality index and DataFit software

In this research, the quality of drainage water was studied by using the water quality index (WQI). Water samples were taken from the Al-Dujaila River and Al-Dujaila drainage. Drainage water was diluted to different ratios with river water to decrease its salinity and increase its potential for agricultural uses. The diluted samples contained between 0 and 100% drainage water, and between 100 and 0% river water, in 10% steps – i.e., 0:10, 1:9, 2:8, 3:7, 4:6, 9:1, 8:2, 7:3, 6:4, and 5:5. After dilution of water, chemical properties, ion activities, electrical conductivity (EC), soil reaction (pH), sodium (Na+), calcium (Ca+2), magnesium (Mg+2), and total dissolved solid (TDS)) were measured in the laboratory of the University of Wasit/College of Agriculture, for computing the drainage water quality index (DWQI), sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP), and soluble magnesium percentage (Mg%). Mathematical models were generated to predict the DWQI using DataFit software, depending on the water's chemical properties, and to find the best dilution ratio, which was 9:1. Model 2 includes the DWQI with SAR, ESP that gave the best results (R2 = 99.99%, RE = 0.0007, MAE = 0.425, RMSE = 0.6, and SEE = 1.992). The diluted drainage water used in this study was not suitable for either irrigation or human use.