An underwater curve is an equity curve rewritten as drawdown from its running peak.

An underwater curve is an equity curve rewritten as drawdown from its running peak. Instead of plotting cumulative value directly, it plots how far the current equity is below the highest value reached so far. At a new high, the underwater value is zero. After losses, the line moves below zero until the equity recovers the prior peak.

This representation makes drawdowns explicit. Depth shows the magnitude of the decline from the peak. Horizontal length shows how long the strategy remains below that peak before recovery. In a standard equity curve, both features are present but often mixed into the overall upward or downward path. In an underwater plot, they are isolated into a single view of peak to trough behavior.

The mechanism is straightforward. Let the equity series be compared with its rolling maximum. At each time point, drawdown is the current value relative to that running peak. The resulting series is zero when the strategy is at a high watermark and negative when it is below it. This is the same basic drawdown idea used in risk analysis, where maximum drawdown is defined as the largest peak to trough decline over a period. The underwater curve extends that idea from one summary statistic to a full time series of all drawdown episodes.

Because the underwater curve is a time series of drawdowns, it naturally exposes both size and duration at once. A deeper valley indicates a larger loss from peak. A wider valley indicates a longer recovery period. Multiple separate valleys indicate distinct drawdown episodes. This makes it possible to inspect not only the worst drawdown, but also the frequency and persistence of stress periods across a backtest.

For strategy evaluation, that distinction matters because two equity curves can end at similar levels while taking very different paths. One may recover quickly after shallow setbacks. Another may spend long periods below its prior high. The underwater curve makes those differences visible without requiring the reader to infer them from the slope and turning points of the equity line.

The same perspective aligns with standard risk measures used in quantitative research. Sonar Sciences defines maximum drawdown as the maximum observed loss from a peak to a trough before a new peak is attained. An underwater plot shows every such loss path, not just the maximum one. It therefore complements summary metrics by revealing the sequence, clustering, and recovery profile of drawdowns over time.

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