It has been more than once mentioned that there are periods when a distributary is run, not full, but about three-fourths full. If that were done in the case of a distributary whose outlets were mostly moduled, the water level would probably be below the minimum limit, and the modules would not be acting as such. The outlets would not, under these circumstances, obtain their proper proportionate supplies. This difficulty can, no doubt, be got over by running the distributary full for short periods at a time instead of three-fourths full for longer periods. The people, when once they understood the case, could arrange to use the water in greater volume for two days instead of in smaller volume for three. If this arrangement comes into force it will not be necessary to design distributaries--see Chapter III, Art. 4--so as to have a good command when three-fourths full supply is run.
On nearly every distributary there are some watercourses whose command is bad, and it has been stated (Chapter II, Art. 9) that in an ordinary unmoduled distributary the sizes of the outlets in such cases should be extremely liberal. To module any such outlet would cause a lowering of the water level in the watercourse and would interfere with the irrigation. Such outlets should not be moduled. Again, there are some few outlets which are not submerged, _i.e._, there is a free fall into the watercourse. The discharge does not depend on the water level in the watercourse, and it is not affected by any enlargement or clearance of it. It depends only on the water level in the distributary. This water level, if most of the outlets are moduled, will be fairly constant. Such outlets need not be moduled, and they should not be moduled unless the other unmoduled outlets in the reach concerned are sufficiently numerous, and perhaps not even then, because moduling involves some expense.
A distributary generally has some falls which divide it into reaches. Immediately upstream of a fall the water level for a given discharge is not affected by the silting or scouring of the channel. Any outlets near to and upstream of the fall are less subject than others to variation in discharge, and are suitable for non-moduling in case a sufficient number of unmoduled outlets is not otherwise obtainable.