We have quantified the sensitivity of modeled climate to hydrologic behavior of land (parameterized by the water-holding capacity of soil). Findings were shown to be relevant to several problems of current interest in climate dynamics (Milly and Dunne, 1994, abstract, paper). The study produced quantitative estimates of the effect of water-resource development on global temperature and precipitation; provided evidence supporting the concept of ecological optimality of plant rooting depth, and provided estimates of the role that roots play in the climatic impact of tropical deforestation.