Part of Ongoing Effect
If you play Flood and then I play Drought, the game tries to apply both affinity changing effects in layer 5. Since you played Flood first, the game first gives the affected sites a minimum of one water affinity (W). Then, when my Drought ability is applied, the affected sites have no water affinity. Therefore, the Drought 'wins.' If Flood had been played second, then the Flood effect would have been applied second, and thus would 'win.' Remember that if an activated or triggered ability creates an ongoing effect, the effect applies based on when the ability activated or triggered, not when the original source entered the realm. For example, Floodplain's activated ability would 'win' against a Drought even if the Floodplain entered the realm before Drought, as long as the Floodplain's activated ability was used after Drought.