Before leaving for Indonesia, I had a chance to spend at day at CSIRO in Perth/Floreat with Kevin Petrone and Richard Silberstein. They both work on a range of topics, but perhaps most in the news is their work related to this now famous chart of Perth water supply inflow. The data and chart are from the water suppliers and Kevin and Richard's group's work is in understanding what's going on. Each bar is a year's flow and the higher bars are the wetter years. The average flow for the last 5 years has been about 1/6th what it was from 1911-1974.
You might think of it this way. You have a bank account and there's income and expenses. When your balance gets low, the bank starts charging you fees. These fees keep knocking you below the minimum balance threshold. So you get more fees and keep falling behind. It's possible to break the cycle, but it takes a pay rise or maybe hitting the lottery.
Similarly, in a few years rainfall was low so the water table dropped. That makes it harder to get water from the landscape to the river. So then there's less runoff and the soils around the rivers dry out. Then the water table drops further and even if there's more rain, there's not as much of a connection between the landscape and the river.