Water in the critical zone: soil, water and life from profile to planet

Earth is unique in the combination of abundant liquid water, plate tectonics and life, providing the broad context within which the critical zone exists, as the surface skin of the land. Global differences in the availability of water provide a major control on the balance of processes operating in the soil, allowing the development of environments as diverse as those dominated by organic soils, by salty deserts or by deeply weathered lateritic profiles. Within the critical zone, despite the importance of water, the complexity of its relationships with the soil material continue to provide many fundamental barriers to our improved understanding, at the scales of pore, hillslope and landscape. Water is also a vital resource for the survival of increasing human populations. Intensive agriculture first developed in semi-arid areas where the availability of solar energy could be combined with irrigation water from more humid areas, minimising the problems of weed control with primitive tillage techniques. Today the challenge to feed the world requires improved, and perhaps novel, ways to optimise the combination of solar energy and water at a sustainable economic and environmental cost.