Generally cell sap of root hair cells contain more ion concentration compared to water present out side.But to understand carefully cell sap have low water concentration compared to water outside root hair.We know water rushes from high to low concentration.Hence water outside root hair enters into root hair.

Water-Epidermis of root-root cortex-Endodermis of root-Xylem of root.

Cellsap=High ion concentration+low water concentration.
Soilwater=Low ion concentration+high water concentration.
1 5 1
If the cell sap of root hair cells contains high concentration of ions ,water will not pass into the vacuole of root hair by osmosis . therefore water flows out of the cell instead of entering into the xylem vessels.