Experimental Assessment of Soil Wetting Geometry under Point Source Drip Irrigation Using HYDRUS-2D Simulation

Abstract

Numerical modeling of movement of water through the soil can effectively estimate the wetting pattern below drip emitters and analyse the irrigation design parameters in arid soils. This study simulated the time-based variations of the wetting volume below the surface point source emitter using HYDRUS-2D software. The two-dimensional model was utilized to evaluate the wetting pattern in a loamy sand soil of Ghayathi, Abu Dhabi, United Arab Emirates, by considering an experiment conducted on the non-cropped soil with a discharge of 2 L h^-1 for 180 min. The irrigation experiment was performed at the surface of the soil with 15 min intervals, and surface wetted radius, surface diameter, and maximum depth were determined. The soil hydraulic parameters were found at 0-75 cm depth with 15 cm intervals. The van Genuchten-Mualem model parameters were calculated as θ r = 0.043, θ s = 0.368, α = 0.069 cm^-1, n = 1.73, and Ks = 1.56 cm h^-1 on the average of the five-layer of the soil. The simulation process involved in the 2-D model of 70 × 100 cm covered the domain by dividing it into 1,543 nodes and 3,084 triangular finite elements. The point source emitter was assumed a constant-flux boundary condition. It was observed that the wetting volume increased during the irrigation process with 12.0 - 24.0 cm of surface wetted radius and 16.0 - 74.0 cm of wetting depth. The simulated values were found to be 11.38 - 22.03 cm and 16.45 - 69.40 cm of surface wetted radius and wetting depth with the irrigation time of 180 min. Additionally, the good performance of the model was evaluated based on RMSE (1.07 cm for the wetted radius; 3.19 cm for the wetting depth), model efficiency (91.80% for the radius; 96.62% for the depth), R² (> 0.98), which slightly underestimated the experimental measurements at long irrigation durations. The accuracy of the HYDRUS‑2D model was verified by plotting a log-log regression of observed and simulated wetting dimensions for all durations. Since the slope of the line is close to one and the model efficiency is greater than 90%, there is no evidence of scale dependence. Hence, these results confirm that HYDRUS‑2D model satisfactorily describes the geometry of the wetted volume under the point source drip irrigation.