Ionic concentration modeling in the groundwater at Baixo Acaraú Irrigation District

Abstract

Regression equations were developed for identify the relationship between electrical conductivity (EC) and ionic concentration of Na+, Cl- and Mg2+, of the water table. Water samples were collected in ten shallow wells, monthly, from Jan/04 to Sep/05, pooled a total of 190 samples. The experimental field comprehend an Irrigated District of 8,440 ha sited in down area of Acaraú Basin, Ceará, Brazil. Analyses were done at Embrapa Agroindústria Tropical soil and water laboratory. To evaluate the regression equations accuracy, 95 samples were selected for model validation. Those samples were not used in the development of the regression analysis to avoid autocorrelation. Developed models for Cl- and Na+ ions concentration in groundwater showed values of determination coefficient (r2) and Willmott index(d) higher than 0.96. Both equations presented a high performance index (c > 0,90) with a good agreement among observed and computed values. On the other hand, Mg2+ model showed a good performance index (0.66 < c < 0.75), which should be used with carefulness to concentrations higher than.3.0 mmolc L-1.