The definitions and associated explanations of water-related terms presented here are intended to provide the reader with a working knowledge of terms that apply to Virginia's water resources. The list is designed to assist the user in understanding and interpreting water related information that may come from sources as varied as governmental agencies, environmental groups, or the news media. While terms and definitions are fairly consistent, some terminology presented here could be defined differently to describe water resources issues in other locations.

Micro-irrigation systems can deliver water and nutrients in precise amounts and at controlled frequencies directly to the plant's root zone. With micro-irrigation systems, an extensive network of pipe is used to distribute water to emitters that discharge it in droplets, small streams, or through mini-sprayers. The major cause of failure in micro-irrigation systems is emitter plugging. Emitter plugging can severely degrade irrigation system performance and application uniformity. Because the emitters are small and can easily plug, it is important to understand the filtration and maintenance requirements of these systems and be proactive to prevent plugging. The basic components of a typical micro-irrigation system are shown in Figure 1.

A TMDL, or total maximum daily load defines the total pollutant loading a water body can receive and still meet applicable water quality standards. (Italicized terms are defined in the boxes at the bottom of each page.) A TMDL equation is developed from a study that identifies the sources of a particular pollutant in a watershed, the pollutant contribution from each source, and the pollutant reduction required to attain and maintain water quality standards. In TMDL calculations, all identified sources of the particular pollutant are quantified, including both point and nonpoint sources of pollution. Because some TMDL calculations involve assumptions and professional judgment, TMDLs also include a margin of safety to account for uncertainty. (See TMDLs [Total Maximum Daily Loads]: Terms and Definitions, Virginia Cooperative Extension publication 442-550, http://pubs.ext.vt.edu/442-550/.)

Microbial Source Tracking (MST), also commonly referred to as bacterial source tracking (BST), is a method used to determine the sources of fecal bacteria and establish whether fecal bacteria are being introduced into water bodies through human, wildlife, agricultural, or pet wastes. MST is considered to be a novel technology still in developmental stages. However, the use of MST is rapidly becoming widespread as more researchers and states are recognizing its potential.

Did you know that livestock, like humans, prefer a clean water source and are healthier and more productive when they drink clean water? Virginia producers who have restricted or eliminated livestock access to streams and farm ponds and converted to a clean, alternative water source have observed increased livestock productivity, improved water quality, and restored stream banks on their farms. As a consequence, livestock stream exclusion practices are gaining popularity across Virginia. This publication, produced through the cooperation of Virginia Cooperative Extension and the Virginia Department of Conservation and Recreation, describes the findings, experiences, and successes of individual producers who are limiting livestock stream access.

The definitions of TMDL-related terms presented here are intended to provide the reader with a working knowledge of terms that apply to Virginia's TMDL program. This is the first in a series of Virginia Cooperative Extension publications that deal specifically with TMDLs. The federal Clean Water Act requires States to develop TMDLs for streams, rivers, lakes and estuaries that do not or are not expected to meet applicable water quality standards. This glossary is designed to assist the reader in understanding and interpreting TMDL related information that may come from sources as varied as governmental agencies, environmental groups, consulting firms, or the news media.

A water-quality "impairment" exists if a body of water is unable to support its designated uses. (Italicized terms are defined in the boxes at the bottom of each page.) Virginia's water-quality standards specify that surface waters are either designated for "recreational use" (e.g., swimming, fishing, and boating) or "aquatic life use" (e.g., viable fishing populations). To support the "recreational use," the state sets numeric waterquality criteria for the maximum amount of bacteria in surface waters (Escherichia coli (E. coli)) for fresh water and enterococci for marine waters). When the concentration of bacteria exceeds the state-specified water-quality criteria, the water does not support the designated recreational use and is deemed to have a bacteria or pathogen impairment. E. coli and enterococci bacteria are found in the intestinal tracts and feces of warm-blooded animals, including humans. High counts of these bacteria indicate the presence of fecal contamination in water.

Water reuse can be defined as the use of reclaimed water for a direct beneficial purpose. The use of reclaimed water for irrigation and other purposes has been employed as a water conservation practice in Florida, California, Texas, Arizona, and other states for many years.