During periods of water stress, a common and understandable question is: “Hasn’t it just rained?”

Rainfall matters but in Knysna’s water system, rainfall totals on their own are not a reliable indicator of recovery.

In this short explainer I set out why increases in river flow and dam storage, rather than rainfall alone, determine whether the system is stabilising or still under pressure.

Rainfall is a potential input

Rainfall is best understood as a potential input into the water system. Whether that input improves supply depends on several factors:

• How much rain falls
• How quickly it falls
• Where it falls within the catchment areas
• How dry the soils are beforehand
• How much is lost to evaporation, runoff, or groundwater recharge

Only the portion of rainfall that results in sustained river flow or stored water meaningfully improves water security.

Why river flow matters more than rainfall totals

Knysna’s water supply depends heavily on abstraction from the Knysna and Gouna rivers. When rainfall is sufficient, it produces baseflow, that is steady, ongoing river flow that can be reliably abstracted.

However, after dry periods, rainfall often produces only a short-lived runoff pulse:

• River levels rise briefly
• Flow peaks for hours or a day
• The pulse then passes through the system
• Baseflow remains weak or collapses back to near-zero

In these cases, rainfall may be visible on SAWS rain charts and home gauges but does not restore sustainable abstraction conditions.

This is why the Department of Water & Sanitation's river gauges are so important: They show whether rainfall has translated into usable, ongoing water, not just a brief runoff.

Why storage dam levels are the decisive indicator

For Knysna’s off-channel storage dams (Akkerkloof and Glebe), recovery depends on active pumping from rivers, not direct rainfall.

This means:

• Rain falling directly onto the dam surface has only a minor effect
• Storage increases only when river flow is strong enough to support pumping
• If river flow is constrained, dam levels will stabilise or decline even after rain

As a result, dam surface extent and stored volume provide a clear, visual record of whether the system is rebuilding or drawing down reserves.

Shallow systems amplify uncertainty

When dams and rivers are at very low levels:

• Small changes in water depth can look dramatic
• Sediment, algae, and lighting conditions can alter satellite appearance
• Rainfall that would normally help may be absorbed before reaching channels

This is why “it looks wetter” can be misleading without supporting river flow or dam storage data.

What counts as meaningful improvement?

From a system perspective, rainfall only becomes meaningful when it produces one or more of the following:

• Sustained increases in river flow above baseflow thresholds
• A visible and persistent expansion of dam water extent
• Measurable increases in stored volume over multiple days

If rainfall does not move these indicators, the system remains under stress regardless of how much rain was recorded.

How I treat rainfall data

In the satellite and river monitoring log:

• Rainfall is not treated as recovery by default
• It is referenced only to explain changes (or lack of change) in river flow or storage
• The primary indicators remain measured river flow and dam behaviour

This approach avoids false reassurance during short rainfall events and helps to focus attention on the indicators that actually govern the towns water security.

In summary

Until rainfall produces sustained river flow and measurable increases in stored water, Knysna’s water system remains operational but vulnerable.

Monitoring river gauges and storage behaviour provides a clearer, earlier signal of risk than rainfall totals alone.