Irrigation, dams, and deforestation can modify evaporation patterns in a region, possibly impacting water resources in distant places. Until date, many moisture recycling research have had a regional emphasis. Van der Ent et al. present a worldwide view. They find that most of the terrestrial biosphere is affected by irrigation, deforestation, and dams, with global annual rates of water loss during the dry season of approximately 0.5% of world ice-free land surface area.
Their results suggest that future changes in land use may lead to significant shifts in local and regional hydrology. For example, if rain falls primarily during the wet season, then changing land uses could alter when and where runoff occurs, affecting stream flows and reservoir levels. Regions without sufficient precipitation during the dry season would be particularly at risk for drought-like conditions caused by land use changes.
The water losses they estimate are substantial. If all of this lost water were replaced with fresh water, it would be enough to cover the entire North American continent to a depth of about 10 meters (33 feet).
Land use changes can also influence the amount of solar radiation that reaches Earth's surface. If forests are cleared for agriculture or fuel, more direct sunlight will reach the ground, leading to greater temperatures and more rapid climate change.
Land irrigation alters the usage and distribution of water. Although drainage of such places is required to convert land to agricultural or urban development, it might result in diminished recharge to groundwater and greater floods in the developed area. Irrigation also affects the soil through changes to its moisture content and physical structure. These effects depend on many factors including the type of irrigation, the time over which it is applied, and the nature of the soil.
Irrigation can have a positive as well as negative effect on land. It allows farmers to use land that would otherwise be too dry for agriculture, while urban irrigation helps meet the needs of an increasing population. However there are costs associated with both types of irrigation: one is the cost of installing and maintaining systems such as dams and pipelines; the other is the loss of value of land used for irrigation and the impact this has on future uses of the land.
Groundwater is affected by irrigation because less rain falls when plants need water and so less flows into streams and lakes. This means that the amount of water in groundwater stores like puddles and springs decreases. If the level of groundwater drops low enough, it may be drawn down further by deep wells. The quality of groundwater is likely to be affected by the type of crop grown in an area and how this affects the flow of water through the soil.
More water vapour is present in the atmosphere when irrigation directly enhances evapotranspiration. Changes in cloud cover, precipitation, and net radiation at the ground surface may result from more atmospheric water vapor. These changes have the potential to have an impact on the climate over rather wide areas. For example, increased evaporative loss due to irrigation could lead to local drought conditions.
Irrigation also affects other components of the hydrologic cycle through soil moisture and temperature changes. Irrigated fields tend to be warmer in summer because the crops need moisture to grow and heat-dissipating leaves. This means that the soil will hold more heat during these months, leading to greater transpiration rates. Cooler temperatures in winter reduce the need for transpiration, so some of the water used for irrigation returns to the atmosphere as vapor.
Irrigation can also influence river flows by increasing the amount of water lost through runoff or draining away reservoir waters. The amount of runoff depends on many factors such as the type of crop grown, soil condition, and land use surrounding the field. If the drained area is not replaced with something else, then this loss of water can lead to further depletion of groundwater resources.
Finally, irrigation can change the distribution of rainfall by altering the flow path of clouds. Water vapor from the soil increases the concentration of clouds, making more intense rainstorms possible.
Climate change is predicted to reduce the amount of water available from surface water or groundwater in many situations, as well as increase evapotranspiration from crops and hence need for irrigation [10–16]. Changes in rainfall patterns are already affecting agriculture in many parts of the world, particularly in Africa and Asia where about 60% of the population depends on rain-fed agriculture for their survival. In addition, there is evidence that climate change will cause increased frequency of droughts and floods which would also have an impact on agriculture.
One study has estimated that if global average temperature increases by 3°C, then annual precipitation would need to decrease by around 10% for the total annual supply of water on Earth not to be affected by climate change. Another study has estimated that if the amount of water vapor in the atmosphere increases by 20%, then the amount of precipitation will need to decrease by about 15% for the total annual supply of water on Earth not to be affected by climate change. These studies indicate that if climate change causes temperatures to rise by 3°C or more or if water vapor in the atmosphere increases by 20%, then there would be significant changes to how much water is available for agriculture across the world.