Due to population growth and socio-economic development, the demand for water, energy and food is increasing, which challenges the security and sustainability of resources worldwide. Water scarcity has many negative consequences for agriculture. Drip irrigation, by transferring water to the root area of crops, reduces surface water loss and evaporation and increases the uniformity of water distribution and irrigation efficiency compared to traditional surface irrigation. One of the most important parameters in the design and implementation of drip irrigation is the accurate prediction of the soil wetting pattern dimensions around the emitters, which leads to the accurate determination of the emitters spacing and the improvement of water consumption efficiency. The use of various types of surface mulches is one of the effective methods in reducing evaporation from the soil surface, especially in arid and semi-arid areas, which can increase crop yield and water resource efficiency. Considering that the water distribution in the soil under different conditions is an effective factor in the design of a drip irrigation system, this research was conducted with the aim of evaluating the effect of plastic mulch and discharge on the soil wetting pattern dimensions and water retention.
The present study was conducted in Research Farm No. 1, Faculty of Water and Environmental Engineering, Shahid Chamran University, Ahvaz, in 2019. In order to determine the characteristics of the soil and irrigation water of the farm, which is supplied from the Karun River, the sampling was done. The texture of the field soil was silty clay loam at all depths. This research was carried out in the statistical design form of a randomized complete blocks in three replications. The main factor of testing was the application or non-application of plastic mulch on the irrigation borders and the secondary factor was the amount of discharge entering the borders. Therefore, the four treatments of this experiment were including 1) non-mulched irrigation border with a discharge of 2 liters per hour, 2) non-mulched irrigation border with a discharge of 4 liters per hour, 3) mulched irrigation border with a discharge of 2 liters per hour, and 4) mulched irrigation border with a discharge of 4 liters per hour. The soil wetting pattern dimensions, 24 hours after the end of irrigation were determined at the beginning, middle and end of the irrigation border. The soil moisture difference between the second and fourth days after irrigation was considered as a measure of soil water retention. For this purpose, soil moisture was determined after 24 and 72 hours from the end of irrigation.
Results and Discussion
According to the results, with the application of the mulch, the depth, width and wetted area of the soil increased by 13.59, 20.25 and 38.54 %, respectively, compared to the non-mulched treatments, which was significant at the probability level of 1 %. Also, the soil moisture difference between the second and fourth days after irrigation decreased from 1.96% in the non-mulched borders to 0.65% in the mulched borders. With the increase of discharge from 2 to 4 liters per hour, the average soil wetted depth decreased from 27.22 to 23.55 cm, the average soil wetted width increased from 26.72 to 31.88 cm, and the soil wetted area increased from 602.17 to 658.28 cm2, which indicates a significant effect of increasing discharge on these parameters in all cases. The increase in discharge increased soil water retention, but this effect was not significant. In the four experimental treatments, the highest and lowest wetted area and soil water retention were related to the mulched irrigation border with a discharge of 4 liters per hour and the non-mulched irrigation border with a discharge of 2 liters per hour.
Generally, the application of mulch has a greater effect on increasing the soil wetting pattern dimensions and water retention than the increase in discharge, therefore, the use of plastic mulch is recommended to save water consumption and increase the efficiency of the drip irrigation system. Considering the importance of the subject, more research in this field is suggested, including the investigation of the plastic mulch effect on the soil wetting pattern dimensions and water retention in subsurface drip irrigation and drip irrigation in sloping lands. In addition, comparing the effects of plastic mulch with natural mulch and comparing different colors of plastic mulch will be beneficial.