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Daily water balance ​

A daily water balance approach allows a closer inspection of the behaviour of a disposal system subjected to short-term rain events which are otherwise averaged out in monthly water balances. You can thus use the 12-day model to refine the monthly model. There are many approaches to this, but you might consider the following steps:

  1. Complete a monthly water balance in Table 5, using adopted rain and other appropriate inputs, and continue to change inputs until the system dimensions just manage to avoid wastewater runoff in any month.

  2. Choose from Table 5 (or its graph) a month which you wish to study in more detail. For example, you could choose the month in which effluent depth is greatest (and the system most sensitive to inputs). Make a note of the effluent depth and the retained monthly rainfall.

  3. Click Daily balance, and enter the same effluent depth in the box provided. If you used a crop factor to calculate ET, check that your value is appropriate. Check too that the slope factor is suitable.

  4. Enter historical or artificial rainfall figures for one or more of the twelve daily boxes.

  5. Complete either the temperature or pan evaporation inputs for all of the twelve days.

  6. Inspect the table (there is no graph) for overflow. Experiment with different daily rainfalls and ET's. Keep notes of the sensitivity of the system. If you are sizing a system which should not overflow, you should return to the inputs to Table 5 or Table 4, and alter the wetted area, and inspect the daily water balance again.

  7. Once satisfied with the inputs to and results of the daily water balance, you may revisit your original climatic data, and for the month in question, alter your adopted rainfall. Go to Table 4 Trench has calculated a new wetted area for the month. Use this new value in Table 5. If you have selected suitable inputs, you have now refined your system dimensions to cope with the short term rainfall you modelled on a 12-day basis.