Modeling moisture bulb distribution on sloping lands: Numerical and regression-based approaches

Drip irrigation has been recognized as the most widely applied system in sloping lands due to high distribution uniformity and low runoff amounts. The wetted dimensions and area of moisture bulb are important factors for proper designing of drip irrigation systems, as they affect the distance between the emitters and laterals. Two physical models were constructed in the present study and the laboratory experiments were conducted using different soil samples (light, medium and heavy textures), three outflow rates (e.g., 2, 4, and 6 lit/hr) and four different land slopes (e.g., 0, 10, 20 and 30 %). The results showed that wetting pattern becomes asymmetric as the value of slope increased, so a considerable portion of the moisture front is created at the downstream part of the emitter. Then, the performance of numerical Hydrus-2D model was evaluated in simulating the moisture bulb dimensions. An additional analysis was performed through developing regression-based models for simulating the moisture bulb in both the flat and sloping lands scenarios. The obtained results showed that the root mean square error (RMSE) values of Hydrus model for simulating upstream wetting radius, downstream wetting radius and wetting depth vary between 1.18 and 2.48, 1.5-2.87 and 1.73-2.62 cm for all investigated soil types and outflow rates on slopping lands, respectively; while they were between 0.66 and 1.62, 0.98-2.3, and 0.51-0.96 cm, respectively, with applying nonlinear regression approach.