Conjunctive use of groundwater with surface-water still receives relatively little systematic consideration -- this despite being (in terms of practical water resources management) one of the most effective ways of improving drought water-supply security for climate-change adaptation, through utilising the large natural groundwater storage of most aquifers to buffer the high flow variability and drought propensity of many surface watercourses. This paper will provide an overview of present, mostly spontaneous, conjunctive use for both irrigated agriculture and urban water-supply in the developing world, and highlight the great potential of 'planned conjunctive management'. It is primarily, though not exclusively, relevant to alluvial plains, which often have important rivers and major aquifers in close juxtaposition, in the presence of major demand centres such as large-scale irrigation systems and important urban conurbations. The core of the paper will be a detailed assessment of those hydrological settings and water-supply challenges in which a conjunctive vision of groundwater and surface water is critical for effective water-resource management and the efficient use of available aquifer storage to improve water-supply security : * Irrigated alluvial plains with shallow water-table and tendency for soil water-logging and salinisation with serious negative impacts on crop production * situations where modifications to surface-water irrigation infrastructure and practices may inadvertently result in serious reduction of groundwater recharge, and thus availability of groundwater storage during drought * 'consumptive groundwater development' in middle basin locations which can severely impact downstream river baseflow and aquatic ecosystems. The assessment will be largely based on a synthesis of extensive field evaluations of conjunctive groundwater and surface-water use, and associated water-resource management challenges, gained by the World Bank GW-MATe Programme during 2001-11, and this will be supplemented by some additional data. In terms of important irrigated agriculture areas it will draw on experience from both Asia and Latin America -- and make particular reference to the Indo-Gangetic Plain (Pakistan & Indian Punjab and Uttar Pradesh), Northern China and some Andean valleys and outwash plains (in Peru & Argentina). The potential for planned conjunctive management also applies to urban water-supply -- and examples of encouraging progress in this regard come from Lima-Peru and Bangkok-Thailand, whilst other cities like Lucknow-India and Recife-Brasil are only just beginning to confront the challenge. Integrated numerical modelling of irrigation-canal flows, groundwater use and aquifer response, soil-water status and crop water-use is a great aid to evaluating the benefits of planned conjunctive management (by varying the spatial and temporal use of groundwater and radical rescheduling of surface-water distribution and allocation). Agricultural production can be notably increased (through improvements in cropping intensity and water productivity) without compromising groundwater-use sustainability. But the impediments to promoting more rational and better planned conjunctive use in established irrigation-canal command areas can be significant : * socio-political dominance of 'head-water farmers' in irrigation canal-commands, who refuse to reduce surface-water intakes so as to allow a greater proportion of available flow to reach tail-end users * inadequate understanding of conjunctive use and the potential role of groundwater by irrigation engineers, since it is poorly taught in academic engineering centres * split responsibility for surface-water and groundwater development and/or management, with organisations and agencies which tend to 'mirror' historical irrigation water-supply realities and perpetuate the status quo * inadequate water resource and water-supply charging systems with a large cost differential (as felt by users) between groundwater and surface water for irrigation because of the tradition of providing canal-water at highly subsidised cost. These impediments must be overcome by a long-term campaign of education about the risks of soil water-logging and salinisation, and the collateral benefits of groundwater pumping--with the introduction of revised water allocation criteria incorporating incentives for balanced groundwater use. The administration of water-use can be delegated (within a sustainable and transparent framework) to appropriate irrigation water-user associations in which groundwater-only users are also federated. However, the institutional dimension of conjunctive management is significantly more complex than where surface water or groundwater alone is the predominant water-supply source. In many alluvial regions, the authority and capacity for water-resources management are mainly retained in surface water-oriented agencies, because of the historical relationship with the development of irrigated agriculture (from impounding reservoirs or river intakes and major irrigation canals). This has led to little interest in complementary and conjunctive groundwater management. Reform of this situation will be essential--such as strengthening the groundwater-resource management function and/ or creation of an overarching and authoritative 'apex agency.