Introduction: Leakage in water distribution networks (WDN) causes loss of water and energy resources. The amount of leakage in WDNs is influenced by pressure. Leakage prediction and optimal decision making on leakage control strategies require a deep understanding of the leakage-pressure relationship. High-density polyethylene pipes (HDPE) have been used extensively in water distribution networks in recent decades, and leakage behavior in this material is still debatable. The purpose of this study was to better understand the leakage behavior of the longitudinal slits of the high-density polyethylene pipes and the effective parameters in their elastic phase. Material and methods: To achieve the objectives of this research, a semi-industrial laboratory model was developed. Based on the preliminary studies and literature review, effective parameters were extracted and then 10 samples were prepared by considering the parameters of diameter and thickness of pipes, dimensions of slits, and temperature. Then, the samples were pressurized by a high-pressure pump and by setting the pressure, the leakage discharge of the longitudinal slits and also the temperature were measured for each pressure during the test. Using the dimensional analysis method, the results were analyzed. Results and discussion: In the longitudinal slit of HDPE pipes, the leakage flow was a function of changes in the area of the leak. Our results showed that there is a relatively linear relation between the change in the effective area of the leak position and the pressure in the elastic state. Also, the rate of changes in the area relative to pressure variations depends on the parameters such as the dimensions of slits, pipe diameter and thickness, and temperature. Among these parameters, the effect of the slits’ length and the temperature had a greater effect on the amount of leakage in longitudinal slits. Using experimental results and dimensional analysis, a relationship was developed to estimate the effective area change of the leak in the longitudinal slits in the HDPE pipes. Using this relationship, leakage can be calculated at different pressures. In the present study, the leakage exponent for longitudinal slits in the HDPE pipe was calculated in the range of 0. 44 to 1. 44 and was consistent with the proposed relationship. In the elastic state, if any of the effective parameters such as slits’ length or pressure increased, the leakage power approached 1. 5. Given the uncertainty about the size of the slits in the water distribution networks, an exact number cannot be proposed for the leakage power, but the average leakage power in our experiment was equal to one. In another experiment, the effect of temperature on leakage was investigated independently at a pressure of 5 bar and showed that an increase in temperature could increase the leakage in HDPE pipes. Also, at 40 ° C, it exited from the elastic state, and the slope of the changes increased significantly. Conclusion: The mean value of the leakage power for the current experiment was equal to one, which corresponds to the previously reported amount that was used to estimate the leakage rate of WDNs. According to this study, a relationship was used to estimate the variation in the area of the leak in terms of the shape and material of the pipe, the shape of the slits and the temperature relative to the pressure. For the first time, the temperature parameter was also investigated in relation to the variation in the area of a slit and discharge leakage. Considering the significant effect of temperature, it is suggested a new relationship to be used in the tropics to estimate the amount of leakage.