The question asks to identify what the flow-mass curve graphically represents. Let's analyze the options to understand the concept of a flow-mass curve in hydrology.

Understanding the Flow-Mass Curve

A flow-mass curve is a fundamental tool used in hydrology, particularly for water resource management. It visually represents the accumulation of water flow (discharge) over a specific period. The curve is constructed by plotting the cumulative discharge against time, typically in chronological order. This means that at any point on the curve, the vertical value indicates the total volume of water that has passed a particular point up to that specific time.

Components of the Flow-Mass Curve

• Cumulative Discharge/Volume: This is represented on the vertical (y-axis). It's the sum of discharges measured over successive time intervals. For instance, if daily discharges are $Q_1, Q_2, Q_3, \dots$, the cumulative discharge at day $n$ would be $Q_{cum, n} = Q_1 + Q_2 + \dots + Q_n$.
• Time: This is represented on the horizontal (x-axis). The time axis follows a chronological sequence, showing the progression of flow measurements over days, months, or years.

Therefore, the flow-mass curve provides a visual summary of the total water volume available over time, ordered chronologically. This is crucial for tasks like estimating water availability for reservoirs or planning water supply systems.

Analyzing Other Options

Let's look at why the other options are not the correct representation of a flow-mass curve:

• Option 1: cumulative discharge and time - While this is partially correct, it misses the explicit mention of volume and the chronological aspect which are key components often implied or directly stated in the definition. Option 3 is more precise.
• Option 2: discharge and percentage probability of flow being equaled or exceeded - This describes a flow-duration curve, which shows the frequency distribution of flow magnitudes, not the cumulative volume over time.
• Option 4: discharge and time in chronological order - This describes a hydrograph, which shows the instantaneous discharge rate at different times, rather than the cumulative volume.

Based on the analysis, the most accurate description for a flow-mass curve is its representation of cumulative discharge (volume) and time in chronological order.