Water Budgets & River Systems

The Equation: The fundamental principle of the water budget is expressed as $P = O + E \pm S$. In this formula, $P$ represents precipitation, $O$ is total runoff (streamflow), $E$ is evapotranspiration, and $S$ represents changes in storage (soil moisture and groundwater).

Positive vs. Negative Balance: A positive water balance occurs when precipitation exceeds evapotranspiration, leading to water surplus and high river levels. Conversely, a negative water balance occurs when evapotranspiration exceeds precipitation, resulting in a water deficit and depleted stores.

Soil Moisture Cycle: This cycle involves four distinct stages: surplus (soil is saturated), utilization (plants use stored water), deficit (water is exhausted), and recharge (precipitation refills the soil pores).

Components of a Hydrograph: A storm hydrograph plots river discharge against time following a specific rainfall event. Key features include the rising limb (increase in discharge), peak discharge (maximum flow), falling limb (decrease in discharge), and lag time (delay between peak rainfall and peak discharge).

Lag Time Analysis: Short lag times indicate that water is reaching the channel quickly, often through surface runoff. Long lag times suggest that water is being delayed by processes like infiltration, throughflow, or interception by vegetation.

Baseflow vs. Stormflow: Baseflow is the normal, steady level of river flow fed by groundwater, while stormflow is the additional water added to the river as a direct result of a precipitation event.

Flashy hydrographs are characterized by steep limbs and represent systems where water reaches the river quickly. This is common in urban areas with concrete surfaces or in basins with impermeable rock like granite.

Flat hydrographs show a gentle response to rainfall. These occur in basins with high infiltration rates, dense vegetation cover, or large catchment areas that spread the water out over time.

Identify the Driver: When asked to explain a hydrograph shape, always distinguish between physical factors (geology, relief, soil) and human factors (urbanization, deforestation). This ensures a balanced and comprehensive answer.

Check the Units: Ensure you distinguish between rainfall (measured in $mm$) and discharge (measured in $m^3/s$). On a storm hydrograph, these are usually plotted on two different y-axes.

Antecedent Conditions: Always consider what happened before the storm. If the soil was already saturated from previous rain, even a small storm can produce a flashy hydrograph because infiltration capacity is zero.

Scale Matters: Remember that larger drainage basins naturally have longer lag times simply because the water has a greater distance to travel to reach the measuring point.

Throughfall vs. Throughflow: A common error is confusing these two terms. Throughfall is water dripping from leaves to the ground (above ground), while throughflow is the lateral movement of water through the soil (below ground).

Infiltration vs. Percolation: Infiltration is the entry of water into the soil surface, whereas percolation is the deeper vertical movement of water through soil layers into the underlying rock.

Discharge vs. Velocity: High velocity does not always mean high discharge. A small, fast stream may have lower discharge than a large, slow-moving river because discharge depends on the total cross-sectional area.