My father, may Allah (swt) be pleased with him, wrote in his letter to me : "If you finish part of your wudu and the water runs out before you complete it, then when you bring water to continue, complete your wudu if the parts you previously washed are still wet. But if they have dried, start your wudu over. If some parts of your wudu dry before you complete it, but water is still available to you, simply wash the remaining parts, whether the earlier parts have dried or not."