How can we determine supplemental irrigation needed to reduce soil salinity knowing ETP of a crop and the annual precipitation?
A very simplistic view, assuming no runoff, is that the crop will use all the water up to its potential evapotrainspiration. So if you know the precipitation, then you need to add enough water to exceed the potential ET. The excess water will infiltrate below the root zone, carrying the salt with it. The amount of excess needed will depend on the soil type and how fast you want the salt to be leached out.
But you also need to consider the seasonality of the crop vs the rainfall. If you have annual crops and winter rainfall, that can leach the salt built up over the summer. In addition, preferential flow in the soil can result in less efficient salt removal.
If you are interested in a particular area, the farm services or agricultural extension agencies may have guidance.
Here is some supplemental material for the good answer above. It won’t let me comment, http://www.fao.org/docrep/x5871e/x5871e04.htm