WETTED AREA CALCULATIONS: The "Large" and "No wetted area possible" messages The wetted area required for disposal changes each month if ET-Rr changes. Conservatively, therefore, the maximum wetted area would be selected, to avoid effluent overflow in all months. However, this neglects the storage capability of the trench or bed, which, if adequate, will prevent effluent overflow. This is the basis of Table 5. If Trench showed a 'Large' wetted area in Table 4, it means your combination of evapotranspiration retained rainfall and LTAR produced a negative or zero Disposal Rate i.e. (ET - Rr + LTAR) <=0. Since Wetted Area = Effluent Volume divided by Disposal Rate, a zero Disposal Rate (i.e. dividing by zero) results in an infinite wetted area, and a negative Disposal Rate produces a negative wetted area. Trench assumes you can't have either, and produces a 'Large' message. A 'Large' message may also occur because: (a) you entered wrong numbers in the climate table and/or in the Adopted LTAR entry box, or (b) your Adopted LTAR is small (or zero, when, for example, you are modelling lined disposal facilities), and/or you are modelling a high rainfall area with low evaporation rates. An example is clayey soils in cold wet places. A negative Disposal Rate means that, for the month in question, you have a water excess ie the two trench outputs (evapotranspiration and absorption losses) are less than the retained rain. This is before any effluent is added. This is acceptable for a month or several months provided there is a positive net annual Disposal Rate. Conversely, if the net annual rate is negative or zero, in- ground effluent disposal cannot work. Trench then shows the 'No wetted area possible' message.