We propose measuring vulnerability of selected outcome variables of concern (e.g. agricultural yield) to identified stressors (e.g. climate change) as a function of the state of the variables of concern relative to a threshold of damage, the sensitivity of the variables to the stressors, and the magnitude and frequency of the stressors to which the system is exposed. In addition, we provide a framework for assessing the extent adaptive capacity can reduce vulnerable conditions. We illustrate the utility of this approach by evaluating the vulnerability of wheat yields to climate change and market fluctuations in the Yaqui Valley, Mexico.

The vulnerability framework of the Research and Assessment Systems for Sustainability Program explicitly recognizes the coupled human-environment system and accounts for interactions in the coupling affecting the system's responses to hazards and its vulnerability. This paper illustrates the usefulness of the vulnerability framework through three case studies: the tropical southern Yucatan, the arid Yaqui Valley of northwest Mexico, and the pan-Arctic. Together, these examples illustrate the role of external forces in reshaping the systems in question and their vulnerability to environmental hazards, as well as the different capacities of stakeholders, based on their access to social and biophysical capital, to respond to the changes and hazards. The framework proves useful in directing attention to the interacting parts of the coupled system and helps identify gaps in information and understanding relevant to reducing vulnerability in the systems as a whole.

Nitrate leaching from agricultural soils can represent a substantial loss of fertilizer nitrogen (N), but a large variation in losses has been reported. We report N leaching losses under four N fertilizer treatments and two farmer's fields in the Yaqui Valley, Mexico. In these irrigated wheat systems, farmers typically apply 250 kg N as anhydrous ammonia (knifed in) or urea (broadcast), with 75% applied directly before planting and 25% at the time of the first post-planting irrigation. Over two wheat seasons, we compared typical farmer's practices to alternatives that applied less N and more closely timed fertilizer application to plant demand. Field lysimeter measurements and predictions from a water transport simulation model (called NLOSS) were used to determine the amount of N leached over the season. Approximately 5 and 2% of the applied N leached below the root zone with the typical farmer's practice in 1995-96 and 1997-98, respectively. The alternative treatments reduced N leaching losses by 60 to 95% while producing comparable economic returns to the farmers. Leaching losses from the two farmer's fields were substantially higher (about 14 and 26% of the applied N). Our results indicate that the typical farmer's practice leads to relatively high N leaching losses, and that alternative practices synchronizing fertilizer application with crop demand can substantially reduce these losses.

The overall effects of policy reforms enacted during the 1990s in Mexico on financial and economic profitability of Yaqui Valley agriculture are assessed in this study, which describes the reforms,examines how exogenous shocks affected the reform process,and documents how rural people and institutions adjusted to the changed circumstances. Virtually all of the reforms affected Yaqui Valley farmers because of the commercial character of their agriculture (relatively large, irrigated wheat farms), their close proximity to the US,and the new "openness" of Mexico's economy. By almost any standard,the reforms were both wide-ranging and successful, at least as measured in efficiency terms, yet the Yaqui Valley's rural communities face significant challenges at the start of the 21st century. The ejido communities have lost cohesiveness and even larger-scale farmers in the private sector face serious income problems. More generally, farmers have yet to find profitable new production systems, including the associated marketing institutions, which are consistent with greater reliance on world agricultural prices and diminished dependence on explicit and implicit subsidies from the government.

Nitrogen fertilization is a substantial source of nitrogen-containing trace gases that have both regional and global consequences. In the intensive wheat systems of Mexico, typical fertilization practices lead to extremely high fluxes of nitrous oxide (N2O) and nitric oxide (NO). In experiments, lower rates of nitrogen fertilizer, applied later in the crop cycle, reduced the loss of nitrogen without affecting yield and grain quality. Economic analyses projected this alternative practice to save 12 to 17 percent of after-tax profits. A knowledge-intensive approach to fertilizer management can substitute for higher levels of inputs, saving farmers money and reducing environmental costs.