Note also DSLR may be calculated off the day of heaviest rainfall, if subsequent rainy days had only light rainfall. Another feature of the onset of a new rain spell is that the canopy is dry and can intercept its full assumed 5mm of rain. Note too that the BKDI model makes no use of wind, even though strong winds may dry out the canopy. Consider the topmost layers of soil such that their field capacity is 200 mm of available water. The index estimates how much effective rainfall is needed to saturate this depth of soil at any time.
Moisture is lost from the soil only by evaporation due to temperature effects.
The first 5 mm of rainfall is lost to the canopy. There is a problem if any of the rainfall is lost as surface run-off. DF and BKDI assume flat terrain, and thus uniform drying. DF will often vary widely with terrain in hilly country. Measure in a Stephenson screen or equivalent.