Dean,
Irrigation systems using pumps and sprinklers with pond water need to be filtered also.
I still think a system using gravity feed could be implanted so I will give you some ideas that I think will help if you go through with this..
1. A tap could be too restrictive to flow at heads it must work at in order to provide water near the pond elevation height. So if you were trying to irrigate the dam 8" below the pond water elevation height you would have 8" of head to work with at the maximum.
Toricelli found that the velocity of water exiting an orifice is a function of head. I've take the liberty of calculating the volume of water that would exit a 1/2" tap at 8" below the pond elevation height (the least restrictive scenario). The maximum volume of water would be:
Vol= Area*velocity = (PI*(.5*2.54)^2)/4 * sqrt(2*98.1*8*2.54) =79 cu cm/sec
or 1.2324 gallons/min OR 1774 gallons/day.
This is the maximum flow and so distributing the water across the dam by means of gravity would reduce what you could deliver to the dam.
2. Low head conduits are sufficient to move enough water for irrigation. To understand how much water you can move ...
go to this site. The numbers are metric but the data there shows that 3" poly of 14 ft in length will carry 1 gallon per sec under only 8" of head. To put that into perspective that is 60 gallons per minute or 3600 gallons per hour or 86400 gallons per day. So yes gravity could supply a very large quantity of water to the dam ... enough to irrigate your dam.
3. IIRC, your pond is supplied by a stream that flows almost continuously. This gives you some other options for using gravity to flood irrigate the dam.
Essentially any low head system will be flood irrigation even if the flow through any orifice of the distribution system "seems to be dripping". Low head distribution would require larger orifices of course but it is simple to calculate that the area of the distribution orifices must be a high proportion of the area of the distribution conduit. In other words, if you use 3" conduit to distribute water and the sum of the areas of the orifices is ~ the area of the conduit ... the orifices (the drip/flood holes) would carry a high proportion of what the conduit would carry and there would be "balance".
It occurs to me that you only need irrigation in dry weather when the risks of high water events are less frequent and more predictable. You may only need 2 maybe 3 flood irrigations to supply the irrigation you are seeking. Under such a scenario you could shut the main syphon and allow the maximum pond elevation to develop. A hopper in the emergency spillway would provide a reservoir at maximum head to supply distribution conduits. These could have full opening valves that could be shut off when irrigation isn't wanted. If rain is forecast, then you don't need irrigation anyway so this is good time to open the main syphon for protection of the dam and emergency spillway.
Just thinking.
