Soil and irrigation heterogeneity effects on drainage amount and concentration in lysimeters: A numerical study

Water and solute fluxes measured from lysimeters located in the field can be used to estimate evapotranspiration, for irrigation scheduling and in solute leaching management. System-imposed heterogeneities are expected to affect the variability of the measured fluxes, and therefore the uncertainty of data obtained using lysimeters. In this study, local heterogeneities in soil hydraulic conductivity and dripper discharge rate were studied and their effect on drainage amount and concentration assessed. Three-dimensional simulations were performed with HYDRUS (2D/3D) with 100 simulations per treatment. The effect of three levels of soil and irrigation heterogeneities was studied for lysimeters of two different sizes (1 m² and 0.5 m²). Additionally, three leaching fraction levels and water uptake reduction due to solute stress were evaluated. Coefficient of variations of the drainage amount and solute concentrations were evaluated for the different scenarios. Irrigation heterogeneity caused higher variability in drainage amount while soil heterogeneity caused higher variability in drainage concentration. The larger the lysimeter, or the higher the leaching fraction, the lower the variability for both drainage concentration and amount. Combined soil and irrigation heterogeneities produced no synergistic effect, suggesting that the variability measured in lysimeters was governed by the factor that caused the highest variability. When water uptake reduction due to salinity was considered, the same trends were observed. The results from this study can help to decide if to use either drainage concentration or amount values, for saline water irrigation management using lysimeters, according to the soil or irrigation heterogeneity levels.