Laboratory investigation of the relationship between trench dimensions and infiltration capacity in unsaturated environment

Introduction
The urgent and increasing human need for water resources around the world, especially in arid and semi-arid regions, has focused more attention on researching new methods of storing and reusing groundwater and surface water (Karim, 2018). In order to design a suitable artificial recharge method, sufficient information about the distribution of water flow in the soil is required. On the other hand, researching water flow distribution in porous media without modeling field conditions is time-consuming and costly. The purpose of this research is to investigate the effect of trench dimensions on infiltration capacity in unsaturated environment and to introduce artificial recharge method suitable for desert areas

 Materials and Methods
In order to achieve the goals of this research, the physical model was built in the hydraulic laboratory of Birjand University. In the conducted tests, the depth and width of the trench (separately) were variable and other fixed factors were considered. Considering that the purpose of this research is to investigate the effect of trench dimensions on the amount of infiltration capacity in an unsaturated environment. Therefore, in the first stage, water with a flow rate of 2.2 liters per minute entered the trench with a depth of 10 cm, a width of 8 cm and a length of 80 cm (the length of the trench was constant in all the experiments) and the volume of water exiting from the measurement model became. In the next step, keeping the depth of the trench constant, the width of the trench was 0.5, 0.75, 1.25, and 1.5 times, and the volume of water coming out of the model was measured for different widths. Also, considering the width of the trench as fixed, the depth of the trench was changed with the mentioned ratios. After the water reached the stagnation level, the output water volume was measured at 5-minute intervals for 60 minutes for all the inlet flow rates of the model. Then, after 60 minutes, the output water volume (V_out) from the model was measured for another 30 minutes. Then the inlet flow was stopped and the output flow rate from the model was measured at 5 minute intervals for another 120 minutes.

Results and Discussion
The amount of   is directly affected by the changes in depth and width of the trench. In equal volumes, in cases where the ratio of depth to width is greater, the value of  is greater.In Figure 1, the trend of changes in the volume of the trench with the trend of changes in the flow output from the model according to the changes in the depth of the trench and also the changes in the width of the trench are examined. The results have shown that the slope of changes in the volume of the trench is not proportional to the slope of the output flow and shows a higher value. The slope of changes in trench volume is 20%, while the slope of changes in output flow is 2.8%, and the slope of changes in output flow is 0.8%. This shows that depth changes are more effective than trench width changes in increasing the amount of infiltration.
Also, the effects of the infiltration level on the output flow rate from the tank were investigated. Investigations show that the trend of changes in the trench level changes with a greater slope than the trend of the output flow rate from the model. The slope of the trench surface change trend was calculated as 14.29% for the trench depth change, while the slope of the model output flow rate change for the trench depth change is 2.8%. also . The slope of the trench surface changes was calculated as 5.7% per trench width change, while the slope of the model output flow rate changes was 0.8% per trench width changes.

Conclusions
The results showed that in the conditions where the width of the trench was considered constant and its depth was considered variable, compared to the conditions where the depth of the trench was considered constant and its width was considered variable; The changes in the output current from the model are greater. For example, in the case where the width of the trench was considered fixed, for every 1.5 times the depth of the trench, the output water volume increased by 7%. 1.5 doubling of the width of the trench, the volume of the outflow water increased by 1.3 percent. Also, the results showed that the trend of changes in the volume of the trench and the infiltration level of the trench is more than the trend of the changes in the flow rate of the model. The trend of changes in trench volume is 7 times and 25 times the ratio of changes in flow rate output from the model per change in depth of the trench and the trend of changes in flow rate output from the model per change in width of the trench, respectively. The trend of changes in the infiltration level of the trench was calculated by the change of the depth of the trench, the ratio of the trend of the output flow rate changes was calculated 5 times from the model. Also, the change trend of the infiltration level of the trench for the change of the trench width, the ratio of the change trend of the output flow rate was obtained from the model 7 times