Runoff, inflow, or outflow lower than expected
A large number of support questions deal with runoff, inflow, or outflow values that are lower than expected. This is usually caused by errors in data entry, or a misunderstanding of the underlying calculations.
Why am I getting so little runoff from a subcatchment?
You may be using an incorrect time span for the selected runoff method. If the span ends before the actual runoff begins, no flow will be shown. It is also possible that the span begins after the rainfall has ended. This is a common problem when switching to the Rational Method, where a much shorter time span is needed. Please see the discussion of time span for further assistance.
A second cause of low/no runoff is a low rainfall or low curve number. While the Rational Method will produce some runoff under all conditions, the SCS runoff equation dictates that there is zero runoff when the rainfall is below a certain depth. This threshold, or initial abstraction, can be especially high with low curve numbers.
Why is the inflow lower than the sum of the peaks?
A peak inflow value may seem incorrect because it is lower than the sum of the
contributing peaks. This happens when the peaks occur at different points in time, and
therefore do not add directly. This commonly occurs when a subcatchment is added to
the outflow of a reach or pond whose peak has been delayed by routing effects.
This situation will be readily apparent upon visual examination of the inflow hydrograph, which will often show two (or more) distinct peaks. The effect is accentuated with narrow peaks (short Tc), because they are likely to have less "overlap", while broad peaks will tend to overlap more and are more likely to yield a direct addition of the individual peak flows.
The effect of peak timing becomes more significant with larger watersheds. It can be used to advantage to "sequence" runoff peaks from a site, and therefore minimize the peak flow.
Why is there so little outflow from my pond?
Reduced pond outflow usually indicates that most or all of the inflow is being retained. (You can confirm this on the pond summary.) To resolve these problems:
- Check your units. If pond areas are entered in square-feet, but the units heading is set to acre-feet, the resulting pond will be MUCH larger than intended. This results in very low pond depths and therefore low outflow.
- Check your outlet elevations. Make sure each outlet device is at the correct invert elevation in relation to the stage-storage data.
- Check your outlet configuration. When using multiple outlet devices (particularly in a compound arrangement) make sure they are properly routed. You should verify the overall behavior of the outlets by examining the stage-discharge plot (not the hydrograph). If it doesn't show the basic behavior you expected, recheck the device routing.
- Check your use of primary vs. secondary outflows. While a secondary outflow can be routed on the diagram (as a dashed arrow), it will otherwise be discarded (as in the case of exfiltration).
- Make sure you define at least one stage-storage value above your highest outlet device. Since discharge is calculated only over the storage range you specify, devices above this range will have no effect. Observe the warning message that indicates that you have exceeded the highest defined stage! Review the stage-discharge curve to verify the intended discharge.