Soil wetting dynamics under subsurface drip irrigation: Influence of soil texture, bulk density, and emitter discharge

Abstract

Subsurface drip irrigation (SDI) is an efficient irrigation method widely used in arid and semi-arid regions; however, its effectiveness largely depends on soil physical properties and the resulting soil water distribution. This study investigated soil wetting pattern dynamics under SDI and developed empirical models to predict wetted soil geometry as influenced by emitter discharge, installation depth, irrigation time, soil texture, and bulk density. Laboratory experiments were conducted using two representative soils from the Kurdistan Region of Iraq (Semel—silty clay and Zakho—clay loam). Emitters with discharge rates of 2 and 4 L h⁻¹ were installed at depths of 12.5, 25, and 37.5 cm, and equal water volumes (8 L) were applied to monitor the advancement of the wetting front. Nonlinear regression models were developed to estimate maximum horizontal wetted diameter (H) and average vertical wetted depth (V) as functions of emitter discharge (q), irrigation time (t), clay content (c), and bulk density (ρb). The models showed strong predictive performance, with coefficients of determination (R²) ranging from 0.87 to 0.98. Model validation indicated acceptable accuracy, with root mean square error (RMSE) values of 1.676–5.033 cm, mean absolute error (MAE) of 4.531–10.745 cm, and mean absolute percentage error (MAPE) of 5.468–10.613%. The index of agreement (d) ranged from 0.968 to 0.995, while mean bias error (MBE) values were close to zero, indicating minimal systematic error. The results demonstrated that wetting front expansion was primarily governed by irrigation time, whereas emitter discharge had a comparatively smaller influence, particularly at shallow depths. Increased clay content reduced both horizontal and vertical wetting dimensions, while the effect of bulk density varied with emitter depth. Lower discharge applied over longer durations enhanced lateral water movement, whereas higher discharge promoted vertical flow and deep percolation. The developed models provide practical tools for predicting soil water distribution and can support the optimization of emitter spacing, irrigation scheduling, and water-use efficiency in SDI systems under semi-arid conditions.