fications of hydrological systems around the world with the diversion of water from natural aquatic ecosystems (2700 km3) for irrigation having well documented negative environmental effects (Richter et al., 1997; Revenga et al., 2000; WCD, 2000; MA, 2005ab; Falkenmark et al., 2007). These include salinization (20-30 million ha-Tanji and Kielen, 2004), river channel erosion, loss of biodiversity, introduction of invasive alien species, reduction of water quality, genetic isolation through habitat fragmentation, and reduced production of floodplain and other inland/coastal fisheries. Conversely, water management practices have also contributed to environmental sustainability, with the development of irrigation reducing the amount of land required for agriculture. In recent years irrigation and water storage have also been found to create new habitats for water birds in Asia, leading to population increase (Galbraith et al., 2005). Thus the coexistence of wetlands and agriculture for 10,000 years has influenced many ecological modifications (Bambaradiniya and Amerasinghe, 2004), but now the balance tends to be negative.

Improved water management can lead to more equitable water use, but this is not common.

Access to water is critical for poverty reduction with large positive impacts on agricultural productivity when combined with equitable distribution (Merrey et al., 2007). Targeted investments in water management in both rainfed and irrigated areas can effectively reduce inequity by providing more opportunities for the poor (Castillo et al., 2007). In China equity tends to increase with agricultural water management, because crops grown on irrigated land have the highest effect on lowering inequality (Huang et al., 2005). Equity in irrigation and agricultural water management are increased by equitable land distribution, secure ownership or tenancy rights, efficient input, credit, and product markets; access to information; and nondiscriminatory policies for small-scale producers and landless laborers (Smith, 2004; Hussain, 2005), but these conditions are rarely met and inequity occurs if wealthy and powerful people gain preferential access to water (Cernea, 2003). Interventions often exacerbate the existing imbalance between men and women's water ownership rights, division of labor and incomes (Ahlers, 2000; Chancellor, 2000; Boelens and Zwarteveen, 2002). The poorest farmers are often those at the end of irrigation systems because they receive less water and have the lowest certainty about the timing and amount delivered.

Improved water management can lead to efficient water use.

Better water management can result in gains in water productivity, better management of rainfed agriculture, improvements in stakeholder management of schemes and reduced evaporation. In low-yielding rainfed areas and in poorly performing irrigation systems, improved water productivity

can be achieved by more reliable and precise application of irrigation water, improved soil fertility and improved soil conservation practices. Improving water productivity- gaining more yield per unit of water-is an effective means of intensifying agricultural production and reducing environmental degradation (Molden et al., 2007b). Increased agricultural productivity can also occur when women's land and water rights are strengthened and there is gender sensitivity in the targeting of credit and input provision, training, and market linkages, especially in areas where women are the farm decision makers (Quisumbing, 1995; van Koppen, 2002). However, gains in water productivity are often overstated as much of the potential has already been met in highly productive systems; a water productivity gain by one user can be a loss to another, e.g., upstream gains in agriculture may be offset by a loss in downstream fisheries, either through increased extraction or agrochemical pollution.

Water user groups are emerging as the key social tool to meet the needs of different communities.

Access to water is critical for poverty reduction (Molden et al., 2007b). However, poor farmers often have poor access to water, as their traditional systems of water rights are overlooked by water management agencies. Smaller-scale community investments in water projects can allow better access to adequate and better quality water. One way of managing water delivery is the establishment of Water User Associations (Abernethy, 2003), but communities of water users face numerous challenges in gaining equitable and sustainable access to, and allocations of, water (Bruns and Meinzen-Dick, 2000; Meinzen-Dick and Pradhan, 2002). Social reforms to improve the equity of water allocation include providing secure water rights for users and reducing or eliminating water subsidies. Acknowledging customary laws and informal institutions can facilitate and encourage local management of water and other natural resources (CA, 2007). Clarifying water rights can ensure secure access to water for agriculture for poor women and men and other disadvantaged groups, such as the disabled (IFAD, 2006; CA, 2007) and ensure better operations and maintenance. The management of water resources can be further improved through training and capacity development. The benefits of farmer-managed irrigation schemes were confirmed in a worldwide study of 40 irrigation schemes (Tang, 1992), and a study of over 100 irrigation systems in Nepal (Lam, 1998). Management of water at the local level has to be part of an integrated process: basin, regional, national and sometimes trans-boundary (CA, 2007).

Structurally complex land use systems can enhance hydrological processes and provide some relief from water scarcity.

On a regional scale, the capacity of vegetation to trap moisture and to return it to the atmosphere by surface evaporation and transpiration affects hydrological processes and