referring to fig1 an embodiment of the irrigation system of the present invention is shown by a diagrammatic perspective illustration . in the figure , 11 designates a pump driven by an electric motor 12 which draws up irrigation liquid such as water from a reservoir 13 and delivers pressurized liquid to a pipe 14 . at a middle portion of the pipe 14 are provided a filter 15 and a control valve 16 . pipe 14 is connected with a number of branch pipes 17 made of plastic piping each having a number of very small slit openings diagrammatically designated by points 18 for continuously discharging liquid therethrough . 19 designates an air compressor driven by an electric motor 20 . the compressed air delivery port of the compressor is connected with the pipe 14 by means of a pipe 21 including a control valve 22 and a check valve 23 incorporated at a middle portion thereof . 24 designates a controller provided for controlling operation of pump 11 , the air compressor 19 and control valves 16 and 22 , in a manner as described hereinbelow . in operation , the pump 11 is generally constantly operated to supply pressurized liquid continuously to the pipe 14 and the branch pipes 17 . the pressure of the pressurized liquid may generally be of the order of 1 - 2 kg / cm 2 . the very small slit openings 18 such as the slits s shown in fig2 are designed to have a certain high throttling effect so as to provide a required distribution of irrigation as well as a required rate of irrigation on a continuous supply basis . in this normal operation , the control valve 16 is of course opened while the control valve 22 is closed and the air compressor 19 is shut down . after a period of continuous irrigating operation of the system , if the very small slit openings 18 have undergone a certain clogging , the irrigating operation of the system is temporarily stopped by shutting down the pump 11 and closing the control valve 16 . thereafter the air compressor 19 is started while the control valve 22 is opened whereby compressed air is supplied through the pipe 14 and the branch pipes 17 so as to let the compressed air be discharged through the very small slit openings 18 . the pressure of the compressed air should preferably be a little higher than that of the irrigation liquid so as to be of the order of 3 kg / cm 2 . by this discharge of compressed air through the very small slit openings 18 , substances clogging the very small slit openings are effectively removed while the slit openings are temporarily slightly magnified within the limit of the elastic deformation of the plastic material . thus the irrigation system resumes its fresh operating performance . fig2 shows some examples of evaluation of performance of the irrigation system of the present invention . a polyethylene pipe of 80 meters length and having diametric dimensions as shown in fig2 was provided with cutting slits s of 3 mm length and 200 mm spacing and used as the passage means having very small slit openings such as the branch pipes 17 in fig1 . in the diagram of fig2 point a1 shows the flow rate of water discharged by the pipe when pressurized water of the pressure of 2 kg / cm 2 and the temperature of 19 °- 20 ° c . was supplied to the pipe . after 20 days operation the flow rate of water lowered to point b1 . then , the blowing off of the cut slits was performed by supplying compressed air of the pressure of 3 kg / cm 2 to the pipe for 10 minutes . by this blowing - off operation , the flow rate of water increased to point c1 . points a2 , b2 and c2 are the results of another experiment performed with respect to a similar pipe while changing the water pressure to 1 kg / cm 2 . from these results , it will be understood that the clogging of the very small slit openings is clearly removed by the blowing - off operation by compressed air , particularly of a little higher pressure than the irrigation liquid . in this connection , the fact that the points c1 and c2 are positioned higher than the points a1 and a2 respectively is construed to be the effect of a little enlargement of the slit openings due to a plastic deformation thereof caused by a period of operation . although the invention has been shown and described with respect to a preferred embodiment thereof , it should be understood by those skilled in the art that various changes and omissions of the form and detail thereof may be made therein without departing from the scope of the invention .
an irrigation system including liquid passage means made of a plastic pipe having very small slit openings for discharging liquid therefrom , further including means for selectively supplying compressed air into the liquid passage means for occasionally blowing off the very small slit openings when they have been clogged .