Baseflow Separation and Recession Interpretation
A 45 km² catchment has a continuous 6-month streamflow record. Total flow at the start of a dry spell is 2.4 m³/s and declines smoothly over 30 days to 0.6 m³/s. During this period there is no rain. A nearby well shows a water-table decline of 1.8 m over the same 30 days. The catchment is underlain by a fractured sandstone aquifer with a specific yield estimated at 0.05. A developer argues that the 0.6 m³/s at day 30 represents 'permanent baseflow' and can be licensed for abstraction.
1. Assess whether the recession curve supports the developer's claim of a permanent 0.6 m³/s baseflow.
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A smooth, continuous decline with no recharge is a recession curve, not a plateau. Exponential recession Q = Q0·e^(−t/k): 0.6 = 2.4·e^(−30/k) → ln(0.25) = −30/k → k ≈ 30/1.386 ≈ 21.6 days.
The flow is still falling at day 30; extrapolating, at day 60 Q ≈ 2.4·e^(−60/21.6) ≈ 0.15 m³/s, and it continues toward zero absent recharge. There is no evidence of a constant, sustainable component. The 0.6 m³/s is simply the current point on a declining curve.
Judgement: the claim is not supported. Baseflow that can be licensed must be sustained by long-term recharge, which requires at least a full annual hydrograph showing recovery after wet seasons. A 6-month record ending in a dry spell cannot establish a perennial yield.
2. Use the water-table decline and specific yield to estimate the groundwater discharge that fed the stream, and compare it with the observed streamflow decline.
Read the worked answer
Groundwater discharge estimate: volume released = area × Δh × Sy. Using the 45 km² catchment as a rough aquifer area: V = 45e6 m² × 1.8 m × 0.05 = 4.05e6 m³ over 30 days.
Mean discharge = 4.05e6 / (30 × 86400) ≈ 1.56 m³/s.
Observed streamflow decline over the same period: 2.4 − 0.6 = 1.8 m³/s average of start and end, but the decline in baseflow contribution is what the aquifer supplied. The average baseflow over the period ≈ (2.4 + 0.6)/2 = 1.5 m³/s, which is very close to the 1.56 m³/s computed from the aquifer.
Interpretation: the numbers are consistent - the stream is being fed almost entirely by draining aquifer storage, and the aquifer is being depleted. This reinforces that the flow is transient storage release, not sustainable yield. Caveat: the catchment area is only a proxy for aquifer area; if the aquifer extends beyond the catchment, the 1.56 m³/s is an over- or under-estimate depending on actual contributing area, so state this as an order-of-magnitude check.