Thus the amount of consumptive use is low. By contrast, agricultural irrigation accounts for more consumptive use than any other sector. Irrigation is important because it increases the area of land that can be productively farmed, raises crop yields, stabilizes productivity, facilitates crop diversity, enhances farm income and employment, helps alleviate poverty, and contributes to regional development. But after irrigation, typical return flows range from virtually nil to about half the water withdrawn. In recent years, given the depletion of groundwater reserves and competition from cities, the use of water for irrigation has leveled off and even decreased in regions of the United States and elsewhere.
Water consumed in agricultural irrigation is greater than that used in any other sector.
The hydrologic cycle is the central concept of the science of hydrology. As energy from the sun causes water to evaporate from both land and ocean surfaces, it also produces differential heating and movement of air masses. Water vapor is transported with the air masses and, under the right conditions, converted to precipitation. From the perspective of a global water budget, precipitation on land surfaces provides more than adequate volumes of water for human and environmental needs. But the availability of water at the place and time of need is often hampered by the uneven distribution of precipitation, along with regional variations in surface water and groundwater resources.