The water year is the best approximation of the consecutive 12 months that span the "water storage/water usage" hydrological cycle. The water year cycle is particularly obvious in the Rocky Mountains and western U.S. where snow begins to accumulate at high elevations in October and doesn't melt until the next spring and summer.

Another way to think of the Water Year is the resting/replenishing season followed by the growing, harvesting and water-consuming season. As October begins, the summer growing season comes to an end. With the coming of colder weather, evapotranspiration shuts down. In the mountains and the northern states, snows begin to fall. For much of the country and especially the northern states, the months of October through March are months where precipitation from the sky exceeds evaporation from the ground. This means that soil moisture and ground water can recharge. Runoff that reaches the rivers and streams may increase (except in cold areas where the water remains frozen). Then, when next spring comes the temperatures rise again, plants come back to life, snow melts, rivers surge. As evapotranspiration increases as plants begin to grow. By the summer months, evapotranspiration will once again exceed precipitation for most of the country. This means that soils dry out, river flow may decrease, and little or no water recharges aquifers. Drought becomes especially problematic when precipitation falls short of expectations during the spring and summer months. By next September, crops will be harvested, temperatures will again cool, and yet another water year will come to an end.