"The big issue"
Water scarcity is not having enough water to meet the country's population's demands.
Q2 - Main reasons: Pollution, human population expansion, city expansion. Q3 - Southeastern Parts of Spain are drying to the point that they are resembling an african climate. Q4 - Probably, as Spain is not done drying yet.
Can be measured by several factors such as
Amount of salt
Water quality may be a concern for Spain as in s some parts of Spain the water is drinkable while in others it isn't. It is still rather a small problem as Spain can still make bottled water but this problem may grow bigger and bigger.