Nutrient cycles link agricultural systems to their societies and surroundings; inputs of nitrogen and phosphorus in particular are essential for high crop yields, but downstream and downwind losses of these same nutrients diminish environmental quality and human well-being. Agricultural nutrient balances differ substantially with economic development, from inputs that are inadequate to maintain soil fertility in parts of many developing countries, particularly those of sub-Saharan Africa, to excessive and environmentally damaging surpluses in many developed and rapidly growing economies. National and/or regional policies contribute to patterns of nutrient use and their environmental consequences in all of these situations. Solutions to the nutrient challenges that face global agriculture can be informed by analyses of trajectories of change within, as well as across, agricultural systems.

This paper is part 2 of a two-part study evaluating the climatic effect of one of the nation's most rapidly expanding metropolitan complexes, the Greater Phoenix, Arizona, region.

Part 1, using a set of sensitivity experiments, estimated the potential impact of observed landscape evolution, since the dawn of the Landsat satellite era on the near surface climate, with a primary focus on the alteration of the surface radiation and energy budgets and through use of high-resolution, 2km grid spacing, Regional Atmospheric Modeling System (RAMS) simulations with circa 1973, circa 1992, and circa 2001 landscape data sets.

In this paper, part 2, we address the role of the previously discussed surface budget changes and subsequent repartitioning of energy on the mesoscale dynamics and thermodynamics of the region, the effect on convective rainfall, and their association with the large-scale North American Monsoon System (NAMS). Our results show that contrasts in surface heating resulting from landscape change are responsible for the development of preferentially located mesoscale circulations, on most days, which were stronger for the 2001 compared to the 1973 landscape, due to increased planetary boundary layer (PBL) heating via enhanced turbulent heat flux.

The effect of these stronger circulations was to warm and dry the lower part of the PBL and moisten the upper part of the PBL for the 2001 relative to the 1973 landscape. The precise physical pathway(s) whereby precipitation enhancement is initiated with evolving landscape, since the early 1970s, reveals a complicated interplay among scales (from the turbulent to the synoptic scale) that warrants future research. Precipitation recycling, however, was found to be an important driver in the overall sustenance of rainfall enhancement.

Although this study was not designed to investigate other radiative forcing factors such as greenhouse gas emissions and aerosols, the results of our sensitivity experiments do suggest that regional land use change is an important element of climate change in semiarid environments characterized by large urban areas with scarce water resources.

This paper is part 1 of a two-part study that evaluates the climatic effects of recent landscape change for one of the nation's most rapidly expanding metropolitan complexes, the Greater Phoenix, Arizona, region. The region's landscape evolution over an approximate 30-year period since the early 1970s is documented on the basis of analyses of Landsat images and land use/land cover (LULC) data sets derived from aerial photography (1973) and Landsat (1992 and 2001). High-resoultion, Regional Atmospheric Modeling System (RAMS), simulations (2-km grid spacing) are used in conjunction with consistently defined land cover data sets and associated biophysical parameters for the circa 1973, circa 1992, and circa 2001 time periods to quantify the impacts of intensive land use changes on the July surface temperatures and the surface radiation and energy budgets for the Greater Phoenix region.

The main findings are as follows: since the early 1970s the region's landscape has been altered by a significant increase in urban/suburban land area, primarily at the expense of decreasing plots of irrigated agriculture and secondarily by the conversion of seminatural shrubland. Mean regional temperatures for the circa 2001 landscape were 0.12C warmer than the circe 1973 landscape, with maximum temperature differences, located over regions of greatest urbanization, in excess of 1C. The significant reduction in irrigated agriculture, for the circa 2001 relative to the circa 1973 landscape, resulted in dew point temperature decreases in excess of 1C. The effect of distinct land use conversion themes (e.g., conversion from irrigated agriculture to urban land) was also examined to evaluate how the most important conversion themes have each contributed to the region's change climate.

The two urbanization themes studied (from an initial landscape of irrigated agriculture and seminatural shrubland) have the greatest positive effect on near-surface temperature, increasing maximum daily temperatures by 1C. Overall, sensible heat flux differences between the circa 2001 and circa 1973 landscapes result in a 1 Wm^-2 increase in domain-wide sensible heating, and a similar order of magnitude decrease in latent heating, highlighting the importance of surface repartitioning in establishing near-surface temperature trends. In part 2 of this study, we address the role of the surface budget changes on the mesoscale dynamics/thermodynamics, in context of the large-scale environment.

Managing food production systems on a sustainable basis is one of the most critical challenges for the future of humanity, for the obvious reason that people cannot survive without food. Ecosystem health is both a “means” and an “ends” to resilient crop and animal production. Being fundamentally dependent on the world’s atmosphere, soils, freshwater and genetic resources, these systems are among the most essential ecosystem services on the planet. They are also the largest global consumers of land and water, the greatest threats to biodiversity through habitat change and invasive species, significant sources of air and water pollution in many locations, and major determinants of biogeochemical change from local to global scales (Vitousek et al. 1997, Matson et al 1997, Naylor 2000, Smil 2000). The inherent interplay between human welfare, food production, and the state of the world’s natural resources underscores the need to manage these systems for resilience—to anticipate change and shape it in ways that lead to the long-run health of human populations, ecosystems, and environmental quality.