Introduction:

When using a surface drip irrigation, the infiltrated water in the soil forms a wetted zone around an irrigation dripper. The knowledge of the soil water distribution and wetting patterns around the irrigation emitter is an important factor for designing and managing drip irrigation systems, for precise delivery of water and nutrients in the crop root zone and for matching the wetting pattern with the root growth pattern. The soil wetting pattern depends on soil hydraulic properties (soil texture, structure, hydraulic conductivity, water table) and irrigation management (Irrigation method, discharge rate, irrigation frequency, the total volume of applied water, emitter spacing and position above or below the surface). In recent years, using pulsed management in drip irrigation systems has increased because of its positive effects on the soil water distribution and consequently plant growth and development. Pulsed drip irrigation refers to irrigating for a short period of time and then waiting for another short period and repeating this on-off cycle until the entire irrigation depth is applied. In the present study, the effects of different pulsed managements with different off-time durations on the soil water distribution and wetting pattern dimensions in a clay soil was investigated, and the performance of the HYDRUS-2D model in simulating soil water distribution under pulsed drip irrigation was evaluated.

Methods:

The treatments included three pulsed drip irrigation scenarios where irrigation water was applied in two (P2), three (P3), and four (P4) cycles with two on-off time duration periods were taking either the on and off times equal (T1) or considering the off-time duration periods as three times that of the on-time duration (T2). A soil container of internal dimensions (90 cm length, 90 cm width and 120 cm depth) was used in the experiments. One sides of this container are plexi-glass sheets to monitor advancing soil wetting pattern at different times during the experiments and the other sides are metal plates. The soil texture used in this study was clay. The duration of the experiment for all treatments was three hours, so the volume of water was constant for all treatments. The gypsum blocks that were calibrated before were used to measure the soil moisture within the wetting pattern. The measurements were recorded by a reader every half an hour after the start of the experiment in each treatment. The HYDRUS-2D model were used for simulating the soil water content under different pulsed treatment. It was calibrated and validated by the soil moisture measurements using inverse modeling. In order to statistically analyze the results of applying different irrigation pulses on the soil moisture distribution and wetting pattern the t-test analysis was used at a significance level of 5%. The simulated values of soil water content by the HYDRUS-2D model were compared with the corresponding measured values using some statistical criteria including root mean square error (RMSE), mean absolute error (MAE), and model efficiency (EF).

Results:

In the present study, the effect of different pulsed management and off-time duration on soil moisture distribution in a clay soil under a drip irrigation system was investigated. The performance of HYDRUS-2D numerical model was also evaluated to simulate moisture distribution in a clay soil under pulsed drip irrigation system. The results showed that increasing the number of pulses, increased the depth and width of the soil wetting pattern by 8 and 12%. Also, increasing the off-time duration, increased soil wetting pattern dimensions (depth and width) around an irrigation dripper in a clay soil by 13.5% and 20%, respectively. Pulsed irrigation improves soil water content distribution that it does not exceed from field capacity despite the heavy soil texture. Therefore, it can be concluded that better conditions in terms of aeration and oxygenation are provided for the crop roots under pulsed drip irrigation management. Also, evaluating the simulation of soil water content distribution and soil wetting pattern dimensions with the measured ones showed that the HYDRUS-2D model has a good accuracy in estimating soil wetting pattern dimensions (R2=0.95) and simulation of soil water content distribution (RMSE=0.018) in a heavy soil under pulsed drip irrigation. Therefore, the HYDRUS-2D numerical model can be used to simulate soil wetting pattern and soil moisture distribution for the design, management and scheduling of the drip irrigation systems with pulsed management.