Top: cumulative ZEC balance in the Sprout pool over time. Bottom: daily inflow (shielding) and outflow (deshielding) lines. Sprout was the original shielded pool using JoinSplit operations. It has been deprecated — the declining balance shows users migrating to Sapling/Orchard.
The upper panel is how much ZEC sits in the Sprout pool; the lower is how much moved in and out each day. A balance is a stock and a flow is a rate. Reading one as the other is the mistake this chart most invites. A falling balance with healthy flow in both directions is turnover; a falling balance with no flow is abandonment. They look the same on the top panel alone.
Both sides are exact, not estimated. Value entering or leaving a shielded pool crosses the transparent turnstile and is counted by consensus, which is why these numbers can be stated precisely while nothing about the holders can be. What stays hidden is everything inside: who, how much each, and moving to whom.
Sprout is deprecated and one-directional in practice. It was the original shielded pool (2016, JoinSplits), nothing new enters, and the balance only falls as the last holders move out. Read the remaining balance as coins whose owners have not moved them -- including, almost certainly, coins whose owners no longer can.
For all pools on one axis see shielded pool balances, or net flows compared for the same question asked of direction alone.