<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="1298" name="water chemistry" parent="437" scope="Branch of chemistry that deals with the elements and compounds in water."/><bt><term thcode="2" code="437" name="geochemistry" parent="291" scope="Study of the distribution of chemical elements and natural compounds on the earth and in the atmosphere and the chemical processes that affect the earth."/><term thcode="2" code="291" name="earth sciences" parent="1019" scope="Broad term for all science related to the study of the earth."/><term thcode="2" code="1019" name="sciences" parent="1" scope="Major educational fields, fields of study, and professional groupings within USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="703" name="marine chemistry" parent="1298" scope="Branch of chemistry that deals with the properties, composition, structure, and interaction of substances in the seas and oceans."/></nt><rt><term thcode="2" code="518" name="groundwater quality" parent="1306" scope="Fitness of subsurface water for use based on its composition and properties."/><term thcode="2" code="806" name="nutrient content (water)" parent="1304" scope="Contaminants in water that nourish organisms, especially plants. Includes nitrogen and phosphorus, either of which can lead to the harmful growth of algae and other plants when present to excess in a body of water."/><term thcode="2" code="854" name="oxygen content (water)" parent="1304" scope="Measurement of the concentration of oxygen dissolved in water, which is an important indicator of the condition of a water body."/><term thcode="2" code="1010" name="salinity" parent="1304" scope="Measure of the concentration of salts dissolved in a solution."/><term thcode="2" code="1125" name="surface water quality" parent="1306" scope="Chemical, physical, and biological characteristics of water in lakes, rivers, or streams related to its fitness for use."/><term thcode="2" code="1129" name="suspended material (water)" parent="1304" scope="Sediment or organic material carried in suspension in the water column, in contrast to material that moves on or near the bottom. May be measured directly by sampling, or indirectly by acoustic or optical backscatter or transmission."/><term thcode="2" code="1301" name="water hardness" parent="1304" scope="Indicator of the concentration of alkaline salts in water, mainly calcium and magnesium, as a measure of water quality."/><term thcode="2" code="1302" name="water pH" parent="1304" scope="The negative logarithm of the hydrogen ion concentration or activity of a water solution. Used for expressing both acidity (pH less than 7) and alkalinity (pH greater than 7), with 7 considered neutral."/><term thcode="2" code="1304" name="water properties" parent="1174" scope="Measurable physical and chemical characteristics of water from different sources."/><term thcode="2" code="1306" name="water quality" parent="1174" scope="The chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose."/><term thcode="2" code="1309" name="water sampling" parent="379" scope="Methods of collecting a representative part of a body of water for testing in controlled conditions."/><term thcode="2" code="1312" name="water temperature" parent="1304" scope="The degrees of heat of water from a given source at a specific time."/></rt></descriptor>