Watershed modeling is at the heart of modern hydrology, supplying rich information that is vital to addressing resource planning, environmental, and social problems. Even in light of this important role, many books relegate the subject to a single chapter while books devoted to modeling focus only on a specific area of application. Recognizing the

Published by the American Geophysical Union as part of the Water Science and Application Series, Volume 6. During the past four decades, computer-based mathematical models of watershed hydrology have been widely used for a variety of applications including hydrologic forecasting, hydrologic design, and water resources management. These models are based on general mathematical descriptions of the watershed processes that transform natural forcing (e.g., rainfall over the landscape) into response (e.g., runoff in the rivers). The user of a watershed hydrology model must specify the model parameters before the model is able to properly simulate the watershed behavior.

Comprehensive account of some of the most popular models of large watershed hydrology ~~ of interest to all hydrologic modelers and model users and a welcome and timely edition to any modeling library

John Wesley Powell, U.S. scientist and geographer, put it best when he said that a watershed is: ...that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community. Watersheds come in all shapes and sizes. They cross sectorial boundaries (e.g. county, state/province, and country). No matter where you are, you are in a watershed! World-wide, watersheds supply drinking water, provide r- reation and respite, and sustain life. Watersheds are rich in natural capital, producing goods (agriculture and fisheries products) and services (industry and technology) for broad geographic areas. In many countries, at the base of watersheds where tributaries empty into large water-bodies (e.g. estuaries, seas, oceans) are centers of society and are typically densely populated areas. These areas serve as concentrated centers of the socio-economic system. They also are centers of domestic and international trade, tourism, and c- merce as well as the center of governments (capitals) where local, regional and national legislatures are located. As we all live in a watershed, our individual actions can directly affect it. The cumulative effects of all the individual actions of everyone within a watershed may be, and often are devastating to the quality of water resources and affect the health of living things including humans. Therefore, watershed systems are highly subject to threat to human security and peace.

Systems Analysis and Modeling in Food and Agriculture is a component of Encyclopedia of Food and Agricultural Sciences, Engineering and Technology Resources in the global Encyclopedia of Life Support Systems (EOLSS), which is an integrated compendium of twenty one Encyclopedias. Systems analysis and modeling is being used increasingly in understanding and solving problems in food and agriculture. The purpose of systems analysis is to support decisions by emphasizing the interactions of processes and components within a system. Frequently investigated systems level questions in agriculture and food are relevant to the 6 E's: Environment, Energy, Ecology, Economics, Education, and Efficiency. The theme on Systems Analysis and Modeling in Food and Agriculture with contributions from distinguished experts in the field provides information on key topics related to food and agricultural system. The coverage include an overview of food system; system level aspects related to energy, environment, and social/policy issues; knowledge bases and decision support; computer models for crops, food processing, water resources, and agricultural meteorology; collection and analysis methods for data from field experiments; use of models and information systems. This volume is aimed at the following a wide spectrum of audiences from the merely curious to those seeking in-depth knowledge: University and College students Educators, Professional practitioners, Research personnel and Policy analysts, managers, and decision makers and NGOs.