Alterations in climate due to increasing atmospheric concentrations of carbon dioxide and other trace gases must be viewed with concern, if not alarm, because of the risk of adverse impacts on the availability and quality of water resources. Hydrologic changes may lead to alterations in pressure on food resources in many regions of the world. This paper explores the mechanisms by which food security - defined here as the ability of a region to produce or otherwise provide food without threatening the well-being of either the population or the existing political systems - could be affected by climate-induced changes in water resources. Shifts in the timing, magnitude, or location of major hydrologic phenomena, such as runoff, soil moisture, or monsoons, are discussed, together with the important vulnerabilities of agricultural productivity to changes in water availability. Some of the concomitant political and socioeconomic implications are also presented, with suggestions for ways to enhance the reliability of food resources dependent on uncertain water supplies.

At first glance, it appears that the region is well endowed with water resources. While accounting for only about 16 percent of the world's land surface, it receives 22 percent of its precipitation and produces 28 percent of its renewable water resources. However, the region is home to 53 percent of the world's population and its IRWR [internal renewable water resources] are only about half of the world's average. By and large, most countries in Asia (except for Singapore, Maldives and Republic of Korea) are still not considered water-stressed. However, even in countries with high level of per capita water availability, large segments of the population are beset with water scarcity as a result of the large variations in the spatial and temporal distributions of rainfall and stream flow. About 84 percent of the total water withdrawal of the region is for agriculture. Irrigation accounts for the bulk of water use in agriculture due to the large irrigation base and the fact that approximately half of the irrigated area is devoted to flooded rice production. Although still relatively small in most Asian countries, the industrial, municipal and environmental demands for water continue to increase at a rate twice that of population growth. The proportion of water available for agriculture is projected to decline to 62 percent worldwide and 73 percent in developing countries by 2020. Hence, the opportunities for expanding the irrigation base are limited. If food security is to be maintained, ways of increasing water productivity must be found. A host of interrelated issues must be addressed in the short-and-medium terms if a water crisis in the region is to be averted. Among others, these include: changing demand patterns, impact of climate change and watershed modifications on dependable water supply, low water productivity in agriculture,in adequate investments in water supply systems to meet future demands, a shift toward decentralized and privatized water service and market mechanisms, the need to reduce non-revenue water, sustaining and expanding the irrigation base, recycling wastewater for agriculture, access to water of the rural and urban poor, groundwater mining, the need to strengthen international collaboration for managing shared water resources, protection of freshwater ecosystems, and fostering a favorable environment for a comprehensive water resources policy and program frameworks. The specific issues in irrigation that are more pressing were discussed. Suggestions on how to address these issues were presented. Two very significant changes in irrigation development policy and strategy in many developing countries of Asia were discussed. First is the shift from large, gravity irrigation systems towards small privatized farmer-controlled irrigation systems such as shallow tubewells, low-lift pumps, farm reservoirs and diversion dams. Second is the dramatic shift from the virtual monopoly of the central role of national government towards greater participation of other players such as WUAs [water users association], NGOs [non governmental organizations], and local government units. The public sector activities are now focused on improving the management of water at the main system level and adopting new policies aimed at creating incentives for WUAs or cooperatives to take over the O and M [operation and management] of the rest of the irrigation facilities.

The usual vulnerability assessment is often sectoral and hazard-specific. With the nexus approach on water, energy and food (WEF), it is recognized that these three sectors have interactions and synergies and tradeoffs in their activities. Security has five dimensions, namely: availability, accessibility, affordability, accessibility, quality and sustainability. This study involved developing and implementing an integrated vulnerability assessment (IVA) methodology and framework of WEF security nexus applied to a watershed. The framework considered the watershed with three sub-systems of ecological, energy and food interacting with water as the common element. The same concept of vulnerability assessment was used for IVA as a function of exposure, sensitivity and adaptive capacity. IVA was operationalized by identifying variables or parameters pertaining to relationships among WEF and inclusion of sectoral variables related to the various dimensions of security. Based on the study, IVA of WEF nexus is a more holistic approach in assessing vulnerability. IVA account for the relationships among the sectors, in contrast to the sectoral approach. Using the combined climate risks due to different hazards (intense typhoons, erratic rainfall, severe drought, and temperature rise) gives a broader coverage unlike the hazard-specific approach. Parameters used were applicable for IVA of the watershed area. Additional relevant variables can be included if data are available.