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crop growth and, therefore, yield. This is because the path- way for transpiration water losses in plants is the same one that allows for plant uptake of carbon dioxide, which is the raw material for photosynthesis. Both exchange processes occur through pores called stomates on the leaf surface. When soil water is not limiting, which is usually the case under irrigated conditions, stomates are fully open. When this condition exists, both transpiration and photosynthesis are occurring at maximum rates allowed by current condi- tions both internal and external to the plant. If soil water becomes limiting, stomates begin to close, limiting both transpiration water losses and photosynthesis. A key ingredient of irrigation water management is the ability to estimate the magnitude of ET losses for any given set of conditions. The most important factors that have to be accounted for are: 1.) the local weather conditions; and 2.) the cropping system for which estimates are needed (type of crop, planting date, etc.). Local weather conditions are important, because ET is driven by weather factors that determine the drying power of the air. A branch of sci- ence known as agricultural meteorology has provided good insight into the variables that drive evaporation of water from soil and crop surfaces. We can accurately predict ET losses in a given area from measurements of four local weather variables; solar radiation, temperature, humidity, and wind. To be useful, these measurements have to be made under a standardized set of conditions. By conven- tion, the variables are measured using instrumentation of specific design located within large areas devoted to stands of irrigated grass or alfalfa. The data from these measure- ments are then used in specially calibrated equations that accurately predict the daily rate of ET for these standard- ized conditions. The values obtained from this process provide standardized measurements of ET that are referred to as reference ET. The term, reference, refers to stan- dardization of the entire process including type of crop used under the weather- monitoring instrumentation, the weather variables measured, and the calculations performed. When all these factors are accounted for, the ET of the reference crop, which is designated as reference ET, can be estimated with great accuracy. In most cases, reference ET values are generated on a daily basis. The specific calculations used are from a set of calculations known as combination equa- tions. The common name of Penman is often used to refer to these equations. Crop /Soil ET Coefficients -- Reference ET values ap- ply to a specific reference crop grown under a set of local weather conditions. To be useful for other crops within the area in which the reference values were obtained, reference ET values have to be adapted to fit these other crops. This is accomplished by adjusting the reference ET values by use of a crop coefficient. As a general rule, crop coeffi- cients must account for factors such as type of crop, stage of development, and all other aspects of culture that might contribute to variability in the extent and distribution of canopy cover. Variation in available soil water also can be used to adjust crop coefficients for any period of the grow- ing season. Locally adapted crop coefficients are available for most kinds of crops that are likely to be grown in a given area. These coefficients provide daily adjustments to the reference ET values generated each day throughout the growing season. In practice, the coefficient is used as a multiplier, such that the actual daily ET for a given crop on a specific day of the season is the product of the reference ET obtained for that date times the crop coefficient for that same date. The procedures described here are for use under conditions where soil moisture is not limiting. If moisture does become limiting, an additional adjustment factor, called the soil coefficient, can be applied in addition to the crop coefficient. Concentrated Animal Feeding Operations EPA has developed the "Producers Guide" to help concentrated animal feeding operation (CAFO) owners and operators understand how to comply with federal CAFO regulations. The guide is intended for large or medium facilities, or those which have been designated by a state permitting author- ity as a CAFO. The guide provides background and other information on: CAFO regulations; determining what facilities are affected; how to apply for a National Pollution Discharge Elimination System ( NPDES) permit; require- ments that would be contained in a permit; the compliance assurance process; and contact information for each state's NPDES program. The guide will soon be available from EPA regional offices, and is now available online at Western States Water, November 28, 2003 S