simple solution: wear a HR monitor. If you know your running HR's from before, try to match that in the pool for the same amount of time. BUT.....because you are being supported by the water, and something about the way the blood flows back to the heart from the hydrostatic pressure (dont' quote me on that), HR's will generally be lower in the pool for the same "effort." So you might need to go longer at a slightly lower HR in the pool to burn the same calories (If you tried to match your running HR's exactly, it might be too taxing/too hard to do).
Or, if you don't know your running HR's from before, estimate an avg of 165 (that's kinda high, but if you want to be on the safe side, estimate high). If you ran 10 miles a day before, for lets say 70 minutes at 165, then pool run and or bike for 80 minutes a day at 150. THAT will burn the same calories.