Irrigation Scheduling of Walnut Seedlings Using HYDRUS-1D and Taguchi Optimization Approach

Water management of seedling cultivation in nurseries necessitates proper irrigation scheduling, especially in arid and semiarid regions that cope with water shortage. To this end, triggered irrigation scheduling of walnut seedlings in a field nursery was performed using the HYDRUS-1D model and Taguchi optimization approach. Soil moisture content variation and canopy cover (CC) development were monitored in the study field by a profile probe (PR2 sensor) and aerial-view images, respectively. Cross-validation technique was performed in order to calibrate and validate the HYDRUS-1D model. Two sets of combinations comprised of four and five factors with five levels for each were designed to represent 256 and 3,125 different scenarios, respectively, for optimizing triggered irrigation using the Taguchi method. An objective function equal to the sum of irrigation efficiency (IE) and normalized transpiration (NT) was considered to maximize using the Taguchi optimization process. Results of the cross-validation technique showed a reasonable agreement between measured and simulated data [root-mean-square error $(\mathrm{RMSE})=0.025{\mathrm{m}}^3{\mathrm{m}}^{-3}$, mean absolute error $(\mathrm{MAE})=0.018{\mathrm{m}}^3{\mathrm{m}}^{-3}$, and Nash-Sutcliff efficiency $(\mathrm{NSE})=0.79$ in both calibration and validation stages on average]. Running 25 combinations for each set resulted in the objective function values of 1.50 ($\mathrm{IE}=0.52$ and $\mathrm{NT}=0.98$) and 1.80 ($\mathrm{IE}=0.92$ and $\mathrm{NT}=0.88$) for the first and second data sets, respectively. The best scenario led to 74% less water consumption compared to actual irrigation practice. The results showed that the combination of the Taguchi optimization approach and HYDRUS-1D model is successfully applicable and provides a promising method for optimizing the triggered irrigation scheduling of walnut seedlings.