Land and water management in dry climates, an unresolved dilemma

In dry areas, vegetation tends to self-organise in spatial patterns of patches or bands optimising water use; densely vegetated areas are fed by runoff coming from bare degraded areas, resulting in an overall productivity higher than the one that could be reached without patterning. Traditional farming schemes use either natural or man-made rain water harvesting settings at the hillslope scale; rocky outcrops or vegetation-cleared upslope sectors respectively provide excess water to small fields or even single fruit trees. But when the spatial scale is wider, economical or environmental conflicts emerge. Forest plantations for commercial purposes or afforestation for soil conservation or carbon sequestration in the headwaters jeopardize sufficient flows to maintain aquatic life, irrigation and water supply downstream. The encroachment of irrigated areas for food (rice), fodder (alfalfa) or fibre (cotton) may cause the deterioration of deltas (Ebro) or the desiccation of lakes or inland seas (Aral Sea). Even `soft¿ strategies such as local water harvesting schemes may have severe consequences in reducing river flows. Under a global change perspective, things are even more intricate. In the next decades, rainfall, river flows and water resources are expected to decrease in many already dry areas while several tree species will die of burn being unadapted to the new climate conditions. Reduction of forest cover in the headwaters of basins with crucial water resources may be an adaptation measure, but deforestation of wide rainy areas may decrease land evapotranspiration and tropospheric water vapour recycling from oceanic to inner continental regions. Rising concepts such as `blue¿ and `green¿ water management, Integrated Land and Water Resource Management (ILWRM) and eventually Payment for Ecosystem Services (PES) illustrate diverse ways for trying to match land and water management strategies. But the full development of these schemas is delayed by two main issues: the insufficient knowledge of the driving ...