This could be achieved using gravity to allow the water to flow from the higher to the lower level.

This is possible in some languages but not in English. In English if something happens the same way, all the time, predictably, without variation, then there isn’t really any question of probability (‘could‘). For regular, predictable phenomena use good old present simple tense without modals:

• This is achieved using gravity to allow the water to flow from the higher to the lower level.

Only use modals for unpredictable or uncertain situations, and then think about the degree of predictability or certainty:

• This could be achieved using gravity to allow the water to flow from the higher to the lower level, but there are other, better methods.
  (= Gravity perhaps not the best method)
• In most situations this can be achieved using gravity to allow the water to flow from the higher to the lower level.
  (= Gravity usually the best method)

 Notice that could implies a more negative evaluation than can. Indonesians should think carefully about this distinction as they tend to over-use could, having been taught in school that could is more formal than can. Well, yes it is, but only in offers and requests:

• Can you pass the salt? (informal)
• Could you pass the salt, please? (formal)
• Excuse me. Would you mind passing the salt? (very formal)
• etc.