Answer: high rates of evaporation
Salinity is the measure of amount of salt dissolved in the water. The area with a plenty of run off will have very low salinity as the run off will take away the surface particles might be containing salt in it. The area with high rates of precipitation will have very low salinity this is because of the fact that the rainfall will take away the salts present on the surface to other location because of water erosion.
The area with high rates of evaporation will have highest salinity because the water or other liquid components gets removed from the surface leaving behind all the solid sediments which may also include salts.