Is zakat obligatory on land purchased by installments, upon which a hawl (lunar year) has passed while it is in the possession of the buyer, given his lack of a firm intention for it (investment or personal construction)?

Once you purchase the land, it becomes your property, whether you pay its price in full or in installments. Zakat (definition) on land is obligatory only if you intend it for trade alone, or for trade combined with the intention of personal use (private ownership) along with the possibility of selling it for profit. However, if you did not intend it for trade at the time of purchase, then no zakat is due on it. If the land is for trade, then zakat is obligatory for the past years; its value is estimated each year, and zakat is paid on it, as zakat does not lapse with the passage of time.