At about the same time, the Abbasid caliph Al-Mutawakkil (r. 232–47 / 847–61), ordered the construction of another ingenious structure on the Nile: the Nilometer. This structure was used to measure the level of seasonal flooding, which in turn enabled the agricultural authorities to predict and then calculate the quantity of seasonal harvests expected and the taxes that were due for them. The Nilometer also helped the population of the Nile Valley to plan for food shortages and rationing in periods of drought, thereby ensuring that both the wealth and the health of the nation were well managed and preserved.