Evaluation of HYDROUS-2D Model for Determination of Soil Moisture Distribution under Surface and Sub-Surface Drip Irrigation of Pistachio Trees

Multidimensional nature of water flow, plant uptake, and high frequency of water application increase the complexity in modeling soil moisture dynamics from trickle irrigation. By determining soil hydraulic properties, parameters of root distribution model for pistachio trees in the field, evapotranspiration and inflow flux, soil moisture distribution was modeled using HYDRUS-2D model for surface (DI) and sub-surface drip irrigation (SDI) systems. Also, soil moisture content in the following days after irrigation was measured at different lateral and vertical distances from the tree by using Moisture Meter Profile Probe. Leaf stomatal conductance was used to test the model and parameterize water-stress response function. The h50 for pistachio tree, which represents the pressure head at which the water extraction rate is reduced by 50%, was calculated 4935 cm. HYDRUS outputs were compared with measured data in corresponding locations, and values ofME, RMSE, E and R2 statistics were obtained -0.002, 0.02, 0.7, 0.741 for DI and 0.006, 0.021, 0.761, and 0.794 for SDI respectively. The calculated transpiration by HYDRUS showed high correlation with stomatal conductance, especially in SDI. Based on plant measurements and HYDRUS results, root water uptake in SDI was significantly more than DI. Therefore, using SDI systems, by decreasing evaporation, saves more water and increases irrigation efficiency. The calculated root water uptake and measured stomatal conductance for the pistachio trees revealed that soil moisture perfectly supports plants until four days after irrigation. Thus, by decreasing irrigation interval in the field, maximum potential of drip irrigation systems can be achieved.