(8b)
IL is the total amount of water input to the lake by precipitation, runoff, etc. The value of Q L is the total water output from surface and underground runoff. The value ofE L is the amount of lake evaporation. The value of δ 18OI, δ 18OQ andδ 18Oev are the stable isotopic ratios of oxygen in the inflow runoff, the runoff out of the lake, and lake evaporation. Assuming that the oxygen stable isotope ratio of runoff out of the lake is equal to the lake water, usingQ L=I L-E L and δ 18OQ =δ 18OL to change the formula 8, we arrive at formula 9.