Water & Hydrology Glossary
Clear definitions for students, researchers, and professionals
226 terms found
Acid Rain
Precipitation with a pH below 5.6, caused primarily by emissions of sulfur dioxide and nitrogen oxides from fossil fuel combustion. Acid rain damages aquatic ecosystems, forests, infrastructure, and drinking water supplies.
Acoustic Doppler Current Profiler (ADCP)
A hydroacoustic instrument that measures water velocity throughout the water column by emitting acoustic pulses and analyzing the Doppler shift of signals reflected from suspended particles. ADCPs have revolutionized river discharge measurement.
Acoustic Leak Detection
Acoustic leak detection finds leaks in pressurised water pipes by listening for the noise escaping water makes and working out where it comes from.
Aggradation
The process by which a stream or river deposits sediment, raising the elevation of its bed over time. Aggradation occurs when sediment supply exceeds the transport capacity of the flow.
Allocation
Water allocation is the process of distributing available water resources among competing users and uses. It is governed by legal frameworks such as prior appropriation or riparian rights doctrines.
Alluvial Fan
A cone-shaped deposit of sediment formed where a stream emerges from a steep, confined valley onto a flatter plain. Alluvial fans are important landforms for groundwater recharge and are prone to flash flooding.
Annual Exceedance Probability
Annual exceedance probability is the chance that a flood or rainfall of a given size is equalled or exceeded in any one year.
Anomaly Detection (Water Data)
Anomaly detection finds readings or patterns in water data that differ from normal behaviour, such as sensor faults, leaks, bursts or pollution events.
Aquatic Ecosystem
A water-based ecosystem encompassing the biological communities and physical-chemical environment of freshwater, estuarine, or marine habitats. Aquatic ecosystems provide critical services including water purification, flood attenuation, and biodiversity support.
Aquifer
A body of permeable rock or unconsolidated sediment that can store and transmit significant quantities of groundwater. Aquifers serve as the primary source of well water worldwide.
Arid Climate
An arid climate is characterized by extremely low annual precipitation, typically less than 250 millimeters, and high evaporation rates. Arid regions cover about one-third of the Earth's land surface.
Base Flow
The portion of streamflow that comes from groundwater seepage and other delayed sources rather than direct surface runoff. It sustains streams during dry periods between precipitation events.
Baseflow Separation
The process of separating a stream's total discharge hydrograph into baseflow (groundwater contribution) and quickflow (surface runoff) components. Baseflow separation reveals the relative contributions of different water sources to streamflow.
Beneficial Use
Beneficial use is a legal concept requiring that water rights holders put allocated water to a recognized productive purpose, such as irrigation, municipal supply, or industrial use. Water not put to beneficial use may be subject to forfeiture.
Bioaccumulation
The gradual accumulation of a chemical substance in the tissues of a living organism over time, reaching concentrations that exceed those in the surrounding environment. Bioaccumulation of pollutants in aquatic organisms is a major water quality and human health concern.
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen consumed by microorganisms during the decomposition of organic matter in water. It is a key indicator of organic pollution in aquatic systems.
Biomonitoring
The use of living organisms to assess the ecological health and water quality of aquatic environments. Biomonitoring provides an integrated measure of environmental conditions that chemical sampling alone cannot capture.
Braided River
A river characterized by multiple interlacing channels separated by temporary islands and bars of sediment. Braided rivers typically carry high sediment loads and exhibit rapid channel changes.
Catchment
The area of land where all precipitation collects and drains into a common outlet, such as a river, lake, or ocean. Also known as a watershed or drainage basin.
Channel Migration
The lateral movement of a river channel across its floodplain over time through erosion of the outer bank and deposition on the inner bank. Channel migration creates and reworks floodplain surfaces.
Chemical Oxygen Demand
Chemical oxygen demand (COD) measures the total quantity of oxygen required to chemically oxidize all organic and inorganic matter in a water sample. It provides a broader assessment of water contamination than BOD.
Cloud Seeding
Cloud seeding is a weather modification technique that introduces nucleating agents into clouds to encourage precipitation. Silver iodide and dry ice are the most commonly used seeding materials.
Coliform Bacteria
Coliform bacteria are a group of microorganisms commonly found in the environment and in the intestines of warm-blooded animals. Their presence in water is used as an indicator of potential pathogenic contamination.
Conductivity
Conductivity is a measure of water's ability to conduct an electrical current, which is directly related to the concentration of dissolved ions. It serves as a rapid proxy for total dissolved solids in water quality monitoring.
Cone of Depression
A cone of depression is the funnel-shaped fall in water level that forms around a pumped well, deepest at the well and shallower with distance.
Confined Aquifer
A confined aquifer is a water-bearing layer held between two layers of low-permeability material, so its water is under pressure.
Conjunctive Use
Conjunctive use is the coordinated management of surface water and groundwater resources to maximize total water supply reliability and efficiency. It exploits the complementary characteristics of both sources.
Constructed Wetland
An engineered system designed to mimic natural wetland processes for the treatment of wastewater, stormwater, or agricultural runoff. Constructed wetlands use vegetation, soil, and microbial communities to remove pollutants.
Contaminant Plume
A contaminant plume is a body of polluted groundwater or surface water that migrates from a source of contamination, spreading through the subsurface or waterway. Its shape and extent depend on hydrogeological conditions and the nature of the contaminant.
Convective Precipitation
Convective precipitation results from the vertical movement of warm, moist air that rises, cools, and condenses. It typically produces intense, short-duration rainfall over localized areas.
Current Meter
A mechanical or electromagnetic instrument used to measure the velocity of flowing water at a specific point. Current meters were the traditional standard for stream discharge measurement before being largely replaced by ADCPs.
Cyclonic Precipitation
Cyclonic precipitation is rainfall or snowfall associated with low-pressure weather systems (cyclones) where converging air masses are forced upward. It can affect large areas and persist for extended periods.
Dam
A dam is a barrier constructed across a waterway to impound water for storage, flood control, hydropower generation, or water supply. Dams are among the most significant human modifications to the hydrologic cycle.
Darcy's Law
Darcy's Law states that groundwater flow through a porous material is proportional to the hydraulic gradient, the cross-section area and the hydraulic conductivity.
Data Assimilation
Data assimilation combines a model forecast with live observations to give a better estimate of a system's current state, such as river flow.
Data Logger
An electronic device that automatically records measurements from sensors over time at programmed intervals. Data loggers are essential components of hydrological monitoring networks, recording stage, flow, water quality, and meteorological parameters.
Delta
A landform created by the deposition of sediment where a river enters a standing body of water such as an ocean, sea, or lake. Deltas are ecologically rich, economically important, and highly vulnerable to climate change.
Denudation
The overall lowering of the Earth's land surface through the combined processes of weathering, erosion, and mass wasting. Denudation rates reflect the balance between tectonic uplift and surface processes.
Desalination
Desalination is the process of removing dissolved salts and minerals from seawater or brackish water to produce freshwater. Reverse osmosis and thermal distillation are the two primary technologies used.
Dew Point
The dew point is the temperature at which air becomes saturated with water vapor and condensation begins. It is a direct measure of atmospheric moisture content.
Digital Elevation Model
A 3D representation of terrain surface elevation data, typically stored as a raster grid. DEMs are foundational datasets for hydrological modeling, watershed delineation, and flood mapping.
Digital Twin (Water)
A digital twin is a model of a real water system kept in step with live data, used to see its current state and test decisions before acting.
Discharge
The volume of water flowing through a river cross-section per unit time. It is the fundamental measurement of streamflow, typically expressed in cubic meters per second.
Dissolved Oxygen
Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water, essential for the survival of aquatic organisms. It is one of the most important indicators of water quality and ecosystem health.
District Metered Area (DMA)
A district metered area is a section of a water network with fixed boundaries and metered inflows, used to measure and track leakage.
Drainage Basin
The entire geographical area drained by a river and its tributaries. It is defined by topographic ridges that form water divides separating adjacent basins.
Drawdown
Drawdown is how far the water level in a well or aquifer falls because of pumping, measured from the level before pumping began.
Drought
Drought is a prolonged period of abnormally low precipitation that leads to water shortages affecting agriculture, ecosystems, and human communities. It is a recurring natural hazard with significant economic impacts.
Drought Index
A drought index is a numerical indicator that quantifies drought severity based on meteorological, hydrological, or agricultural data. Common indices include the PDSI, SPI, and USDM categories.
Ecological Flow
The quantity, timing, and quality of water flows required to sustain freshwater ecosystems and the human livelihoods that depend on them. Ecological flow assessments guide water allocation decisions to balance human and environmental needs.
El Nino
El Nino is the warm phase of the El Nino-Southern Oscillation (ENSO), characterized by unusually warm sea surface temperatures in the equatorial Pacific Ocean. It influences global weather patterns, including precipitation and drought.
Electromagnetic Flow Meter
An electromagnetic flow meter, or mag meter, measures flow of a conductive liquid in a full pipe from the voltage induced as it crosses a magnetic field.
Ensemble Forecasting
Ensemble forecasting runs a model many times with slightly different inputs or settings to show the range of likely outcomes, not one answer.
Environmental Impact Assessment
A systematic process for evaluating the potential environmental effects of a proposed project or development before it proceeds. EIAs are legally required in most countries and are essential for protecting water resources from development impacts.
EPANET
EPANET is free US EPA software that models flows, pressures and water quality in pressurised drinking water pipe networks over time.
Equifinality
The idea that many different model structures or parameter sets can reproduce observed data equally well, so no single best model can be picked.
Erosion
The removal and transport of soil, rock, and sediment by water, wind, ice, or gravity. Water erosion is the dominant process shaping river valleys and is a major concern for water quality and land degradation.
Eutrophication
Eutrophication is the excessive enrichment of a water body with nutrients, especially nitrogen and phosphorus, leading to algal blooms and oxygen depletion. It is a major cause of water quality degradation in lakes, reservoirs, and coastal areas worldwide.
Evaporation Pan
An evaporation pan is a standardized open container used to measure the rate of water evaporation at a specific location. The Class A pan is the most widely used design worldwide.
Evapotranspiration
The combined process of water evaporation from soil and plant surfaces and transpiration from vegetation. It is the largest component of the water budget in most catchments.
Evapotranspiration (Remote Sensing)
The estimation of combined water loss from soil evaporation and plant transpiration using satellite-based thermal and vegetation data. Remote sensing enables spatially distributed ET estimates over large areas where ground measurements are impractical.
Fecal Coliform
Fecal coliforms are a subset of coliform bacteria that are associated with the fecal matter of warm-blooded animals. Their presence in water indicates potential contamination by sewage or animal waste.
Fish Passage
Structures and management strategies that enable fish to move past barriers such as dams, culverts, and weirs in rivers and streams. Fish passage is critical for maintaining migratory fish populations and aquatic ecosystem connectivity.
Flood Control
Flood control encompasses structural and non-structural measures designed to reduce the damaging impacts of flooding on communities, infrastructure, and the environment.
Flood Frequency
A statistical analysis that estimates the probability of a flood of a given magnitude occurring in any given year. It is used to design infrastructure and assess flood risk.
Flood Plain
A flat area of land adjacent to a river that is subject to periodic flooding. Floodplains serve as natural storage areas for excess water during high flow events.
Floodplain Mapping
The process of delineating areas adjacent to rivers and streams that are subject to inundation during flood events of specified return periods. Floodplain maps are essential for land use planning, insurance, and emergency management.
Flow Measurement
The quantification of the volume of water passing through a channel cross-section per unit time. Accurate flow measurement is the foundation of water resource management, flood forecasting, and environmental monitoring.
Fluvial Terrace
An elevated, relatively flat surface flanking a river valley, representing a former floodplain or channel level that was abandoned as the river incised to a lower elevation. Terraces record the geomorphic history of a valley.
Frontal Precipitation
Frontal precipitation occurs when warm air is lifted over cooler air along a frontal boundary between two air masses. It is the dominant precipitation mechanism in mid-latitude regions.
Gauging Station
A fixed site on a river or stream equipped with instruments to continuously monitor water level (stage) and calculate discharge. Gauging stations form the backbone of hydrological observation networks worldwide.
Geomorphic Threshold
A critical condition or tipping point that, when exceeded, triggers a significant change in landform processes or morphology. Geomorphic thresholds explain why landscapes can undergo abrupt changes in response to gradual forcing.
GRACE Satellite
The Gravity Recovery and Climate Experiment satellite mission that measures changes in Earth's gravity field to detect large-scale variations in water storage. GRACE data reveal groundwater depletion, ice sheet mass loss, and continental water balance changes.
Groundwater Recharge
The process by which water moves downward from the surface through the unsaturated zone to replenish an aquifer. It is the primary mechanism for sustaining groundwater resources.
Groundwater Telemetry
Groundwater telemetry uses sensors in wells and wireless links to send water level and quality readings automatically to a central system.
Groundwater-Dependent Ecosystem
An ecosystem that relies on groundwater for all or a significant portion of its water requirements. GDEs include springs, baseflow-fed streams, wetlands, and phreatophyte vegetation communities that access the water table.
Hardness
Water hardness is a measure of the concentration of dissolved calcium and magnesium ions in water. Hard water can cause scale buildup in pipes and reduce the effectiveness of soaps and detergents.
Heavy Metals
Heavy metals are metallic elements with high atomic weights that can be toxic to organisms even at low concentrations. Common heavy metal contaminants in water include lead, mercury, arsenic, cadmium, and chromium.
HEC-HMS
HEC-HMS is the free US Army Corps of Engineers software that turns rainfall into runoff hydrographs for whole catchments.
HEC-RAS
HEC-RAS is the free US Army Corps of Engineers software that models water levels, velocities and flooding in rivers and floodplains.
Humidity
Humidity is the amount of water vapor present in the atmosphere. Relative humidity, expressed as a percentage, is the most commonly reported measure, indicating how close the air is to saturation.
Hydraulic Conductivity
A measure of a material's ability to transmit water under a hydraulic gradient. It depends on both the properties of the porous medium and the fluid flowing through it.
Hydraulic Gradient
The change in hydraulic head per unit distance in the direction of groundwater flow. It represents the driving force that causes water to move through porous media.
Hydraulic Head
The total energy per unit weight of water at a specific point in a groundwater system, equal to the sum of elevation head and pressure head. It determines the direction of groundwater flow.
Hydraulic Model vs Digital Twin
A hydraulic model is an offline simulation used mainly for planning; a digital twin keeps a model in sync with live data to support daily operations.
Hydrograph
A graph showing the variation of discharge or water level at a point in a stream over time. It is the fundamental tool for analyzing streamflow response to precipitation.
Hydrologic Cycle
The continuous circulation of water through the Earth's atmosphere, surface, and subsurface driven by solar energy and gravity. Also called the water cycle.
Indicator Organism
An indicator organism is a microorganism whose presence in water suggests the potential presence of pathogenic organisms. Coliform bacteria and E. coli are the most commonly used indicators of water quality.
Infiltration
The process by which water on the ground surface enters the soil. It is a key process controlling the partitioning of rainfall between surface runoff and subsurface flow.
Integrated Water Resources Management
Integrated water resources management (IWRM) is a holistic approach that coordinates the development and management of water, land, and related resources to maximize economic and social welfare while maintaining ecosystem sustainability.
Interception
The process by which precipitation is caught and held by vegetation canopy or other surface cover before reaching the ground. Intercepted water eventually evaporates back to the atmosphere.
Irrigation Efficiency
Irrigation efficiency is the ratio of water beneficially used by crops to the total water delivered to the field. It measures how effectively an irrigation system converts applied water into crop production.
Isohyet
A line on a map connecting points of equal rainfall amount over a given period. Isohyetal maps are used to visualize the spatial distribution of precipitation across a region.
Isohyetal Method
The isohyetal method estimates average rainfall over an area by drawing lines of equal rainfall and weighting the rainfall between them by area.
Karst
A landscape formed by the dissolution of soluble rocks such as limestone, dolomite, and gypsum, characterized by sinkholes, caves, and underground drainage systems.
Kling-Gupta Efficiency (KGE)
A model performance score that combines correlation, bias and variability errors into one number, with 1 as a perfect fit.
Knickpoint
A sharp break or steepening in a river's longitudinal profile, often marked by a waterfall or rapids. Knickpoints migrate upstream over time and indicate active adjustment of the channel to a change in base level.
Lag Time
The time interval between the center of mass of rainfall and the peak of the resulting runoff hydrograph. It reflects how quickly a watershed responds to precipitation.
Landsat
The longest-running satellite program for Earth observation, providing continuous multispectral imagery since 1972. Landsat data are fundamental to water resource monitoring, land use change detection, and environmental assessment.
Levee
A natural or artificial embankment along a river channel that confines floodwaters and prevents inundation of adjacent land. Levees are fundamental flood control structures but can increase flood risk if they fail.
LiDAR
Light Detection and Ranging technology that uses laser pulses to create highly accurate 3D point clouds of terrain and objects. LiDAR-derived elevation data are essential for high-resolution floodplain mapping and hydraulic modeling.
LoRaWAN
LoRaWAN is a low-power wide-area radio network protocol that lets battery sensors send small amounts of data over several kilometres.
LSTM Rainfall-Runoff Model
An LSTM rainfall-runoff model is a recurrent neural network trained on past weather and flow records to predict river discharge.
Lumped and Distributed Hydrological Models
Lumped models treat a catchment as one unit with average inputs; distributed models split it into many cells, each with its own inputs and values.
Lysimeter
A device used to measure the amount of water that percolates through a soil column, enabling direct measurement of evapotranspiration and deep drainage.
Managed Aquifer Recharge
Managed aquifer recharge is the deliberate adding of water to an aquifer for later use or for environmental benefit.
Manning Equation
An empirical formula used to estimate the average velocity and discharge of open channel flow based on channel geometry, slope, and roughness. It is one of the most widely used equations in hydraulic engineering.
Maximum Contaminant Level
A maximum contaminant level (MCL) is the highest concentration of a contaminant allowed in public drinking water under the Safe Drinking Water Act. MCLs are legally enforceable standards set by the EPA.
Minimum Environmental Flow
Minimum environmental flow (or ecological flow) is the quantity, timing, and quality of water flow required to sustain freshwater ecosystems and the services they provide to people.
Model Calibration
The process of adjusting a model's parameters so its outputs match observed data, such as measured river flow or groundwater levels.
Model Predictive Control (MPC)
Model predictive control uses a model to forecast a system, picks the best control moves over a future horizon, applies the first, then repeats.
MODFLOW
MODFLOW is the USGS finite-difference computer model that simulates groundwater flow and heads in aquifers in three dimensions.
MODIS
The Moderate Resolution Imaging Spectroradiometer aboard NASA's Terra and Aqua satellites, providing daily global observations in 36 spectral bands. MODIS is widely used for large-scale water cycle monitoring including snow cover, vegetation health, and evapotranspiration.
Monsoon
A monsoon is a seasonal reversal of wind patterns that brings dramatic shifts between wet and dry seasons. Monsoons are the primary source of water for billions of people in South and Southeast Asia.
Multiparameter Water Quality Sonde
A multiparameter sonde is a sealed probe with several sensors that measures water quality variables such as temperature, pH, oxygen and turbidity at once.
Muskingum Routing
A hydrologic flood routing method that predicts a river reach's outflow hydrograph from its inflow, using a storage equation with two parameters, K and x.
Nash-Sutcliffe Efficiency (NSE)
A score from minus infinity to 1 that compares a model's squared errors with the variance of the observed data; 1 is a perfect fit.
NB-IoT (Narrowband IoT)
NB-IoT is a cellular standard from 3GPP for low-power devices that send small amounts of data over existing mobile networks.
NDVI
The Normalized Difference Vegetation Index, a widely used spectral index that quantifies vegetation greenness and health from satellite imagery. NDVI is a proxy for plant biomass and is used to estimate crop water use and monitor drought impacts.
NDWI
The Normalized Difference Water Index, a spectral index designed to highlight open water features and assess vegetation water content from satellite imagery. NDWI is essential for mapping surface water extent and monitoring flood inundation.
Nephelometric Turbidity Unit
The nephelometric turbidity unit (NTU) is the standard unit for measuring turbidity in water, based on the scattering of light by suspended particles. Lower NTU values indicate clearer water.
Nitrate
Nitrate (NO3-) is a dissolved form of nitrogen that is essential for plant growth but can be a serious water contaminant at elevated concentrations. High nitrate levels in drinking water can cause methemoglobinemia (blue baby syndrome) in infants.
Non-Point Source Pollution
Non-point source (NPS) pollution comes from diffuse sources rather than a single identifiable discharge point. It is the leading cause of water quality impairment in the United States.
Non-Point Source Pollution
Water pollution originating from diffuse sources across the landscape rather than from a single identifiable discharge point. Agricultural runoff, urban stormwater, and atmospheric deposition are major non-point sources of water contamination.
Non-Revenue Water (NRW)
Non-revenue water is water put into a supply system that earns no money for the utility, through leaks, theft, meter errors or unbilled use.
Nowcasting
Nowcasting is very short-range forecasting, from minutes to a few hours ahead, mostly by moving the latest radar and satellite data forward in time.
Nutrient Loading
Nutrient loading is the total amount of nutrients, primarily nitrogen and phosphorus, entering a water body from all sources over a given period. Excessive nutrient loading drives eutrophication and algal blooms.
Orographic Precipitation
Orographic precipitation occurs when moist air is forced upward by topographic barriers such as mountains, cooling and condensing to produce rainfall or snowfall. It creates distinct wet and dry zones on opposite sides of mountain ranges.
Overland Flow
The movement of water over the land surface toward stream channels when rainfall intensity exceeds the soil's infiltration capacity or when the soil becomes fully saturated.
Oxbow Lake
A crescent-shaped lake formed when a meander bend of a river is cut off from the main channel. Oxbow lakes are important wetland habitats and indicators of past channel positions.
Palmer Drought Severity Index
The Palmer Drought Severity Index (PDSI) is a widely used meteorological drought indicator that uses temperature and precipitation data in a soil water balance model to assess drought severity on a scale from -4 to +4.
Pan Coefficient
A pan coefficient is the factor that converts evaporation measured in a pan to the evaporation expected from a lake or reference crop.
Pathogen
A pathogen is a disease-causing microorganism found in water, including bacteria, viruses, protozoa, and helminths. Waterborne pathogens are a leading cause of illness and death worldwide.
Peak Discharge
The maximum instantaneous flow rate observed during a flood event at a specific point in a stream. It is the critical parameter for flood design and risk assessment.
Percolation
The downward movement of water through soil and rock under the force of gravity, beyond the root zone toward the water table. It is distinct from infiltration, which occurs at the surface.
pH
pH is a logarithmic scale measuring the acidity or alkalinity of water, ranging from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. It is a fundamental parameter in water quality assessment.
Phosphorus
Phosphorus is a nutrient essential for biological growth that often acts as the limiting nutrient in freshwater ecosystems. Excess phosphorus in water bodies is a primary driver of eutrophication and algal blooms.
Physics-Informed Neural Network (PINN)
A physics-informed neural network is trained to fit observed data while also obeying physical equations, such as flow or mass balance laws.
Piezometer
A piezometer is a tube or sensor that measures groundwater head at one chosen depth, by being open to the ground only over a short interval.
Piezometric Surface
A piezometric surface is the imaginary level to which water would rise in wells tapping an aquifer. It is also called the potentiometric surface.
Point Source Pollution
Point source pollution is contamination discharged from a single identifiable source, such as a pipe, ditch, or outfall. It is regulated under the Clean Water Act through the NPDES permit system.
Porosity
The ratio of void space to total volume of a soil or rock material, expressed as a percentage. It indicates the maximum amount of water a material can hold.
Potential Evapotranspiration
Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired if sufficient water were available. It represents the atmospheric demand for water and is a key parameter in water balance calculations.
Precipitation
Any form of water that falls from the atmosphere to the Earth's surface, including rain, snow, sleet, and hail. It is the primary input to the hydrologic cycle.
Pressure Management
Pressure management is the control of pressure in a water network to cut leakage and bursts while keeping enough pressure for customers.
Pressure Transducer
A sensor that measures water level by detecting the pressure exerted by the water column above it. Pressure transducers are widely used for continuous water level monitoring in streams, wells, and lakes.
Prior Appropriation
Prior appropriation is a water rights doctrine used primarily in the western United States, based on the principle of 'first in time, first in right.' Senior rights holders have priority over junior users during water shortages.
Probable Maximum Precipitation
Probable maximum precipitation (PMP) is the theoretically greatest depth of precipitation that is meteorologically possible for a given area and duration. It is used in designing critical infrastructure such as dam spillways.
Pumping Test
A pumping test pumps a well at a controlled rate while water levels are recorded, to find aquifer properties and how much the well can yield.
Radar Altimetry
A satellite-based technique that measures water surface elevation by timing radar pulse reflections from the Earth's surface. Radar altimetry enables monitoring of lake levels, river stages, and reservoir storage from space.
Radar Water Level Sensor
A radar water level sensor measures water level without touching the water by timing microwave pulses sent down to the surface and reflected back.
Rain Gauge
A rain gauge is an instrument used to measure the amount of liquid precipitation over a specified time period. Standard, tipping bucket, and weighing gauges are the most common types.
Rain Harvesting
Rain harvesting (rainwater harvesting) is the collection, storage, and use of rainwater from rooftops, land surfaces, or other catchments. It is an ancient practice gaining renewed attention for water conservation and stormwater management.
Rainfall Intensity
The rate at which rain falls, expressed as depth per unit time. It is a critical factor in determining whether rainfall will infiltrate or run off the land surface.
Rainfall-Runoff Model
A model that turns rainfall and other weather inputs into a prediction of streamflow at a catchment outlet over time.
Real-Time Control (RTC)
Real-time control adjusts pumps, gates and valves automatically from live sensor readings to meet operating goals in water and sewer systems.
Recurrence Interval
The average time period between occurrences of a hydrological event of a given magnitude. A 100-year flood has a recurrence interval of 100 years, meaning a 1% chance of occurring in any year.
Remote Sensing in Hydrology
The application of satellite and airborne sensors to observe and quantify components of the water cycle across spatial scales. Remote sensing provides critical data for hydrological modeling, water management, and flood forecasting in data-scarce regions.
Remote Terminal Unit (RTU)
A remote terminal unit is a field device that reads sensors at a remote site, controls equipment there and sends the data to a central SCADA system.
Reservoir
A reservoir is a natural or artificial lake used for the storage, regulation, and controlled release of water. Reservoirs are critical components of water supply, flood control, and hydropower systems.
Return Period
The expected average time between events of a specific magnitude. Synonymous with recurrence interval, it is used to express the probability of extreme hydrological events.
Riparian Buffer
A vegetated strip of land maintained along the banks of a stream, river, or other water body that filters pollutants, stabilizes banks, and provides wildlife habitat. Riparian buffers are one of the most effective practices for protecting water quality.
Riparian Rights
Riparian rights is a water law doctrine granting landowners adjacent to a natural water body the right to make reasonable use of that water. It is the predominant system in the eastern United States and much of the world.
River Basin
The total land area that drains into a specific river system. Large river basins often cross political boundaries and require coordinated management among multiple jurisdictions.
River Morphology
The study of the physical form and structure of river channels, including their cross-sectional geometry, planform pattern, longitudinal profile, and bed features. River morphology reflects the dynamic balance between water flow and sediment transport.
Runoff
The portion of precipitation that flows over the land surface or through the shallow subsurface to reach streams and rivers. It is the primary mechanism by which water leaves a catchment.
Runoff Coefficient
The ratio of the volume of runoff to the volume of precipitation over a given area and time period. It reflects how much rainfall becomes surface runoff versus being absorbed or evaporated.
Saint-Venant Equations
A pair of equations, one for continuity and one for momentum, that describe unsteady one-dimensional flow in open channels such as rivers and sewers.
Salinity
Salinity is the total concentration of dissolved salts in water, most commonly sodium chloride. It is a key parameter for classifying water bodies and determining suitability for drinking, irrigation, and aquatic life.
Saltwater Intrusion
Saltwater intrusion is the movement of seawater or other saline water into a freshwater aquifer, usually caused by pumping.
SAR
Synthetic Aperture Radar, an active microwave imaging technology that can observe the Earth's surface through clouds and at night. SAR is critical for flood mapping, soil moisture retrieval, and ground deformation monitoring in water resource applications.
Saturated Zone
The subsurface region below the water table where all pore spaces and fractures are completely filled with water. It is the zone from which wells draw groundwater.
SCADA (Supervisory Control and Data Acquisition)
SCADA is a control system that collects data from remote sites such as pumps, tanks and wells and lets operators watch and run them from a control room.
SCS Curve Number Method
An empirical method that estimates direct runoff depth from a storm's rainfall using one number, the curve number, set by soil and land cover.
Sediment Load
Sediment load is the total amount of solid material transported by a river or stream, including suspended sediment, bedload, and dissolved load. Excessive sediment degrades water quality and aquatic habitats.
Sediment Yield
The total amount of sediment exported from a drainage basin over a given time period, measured at the basin outlet. Sediment yield integrates all erosion, transport, and deposition processes within the watershed.
Sentinel
The European Copernicus program's constellation of Earth observation satellites providing free, open data for environmental monitoring. Sentinel-1 (SAR) and Sentinel-2 (optical) are particularly important for water resource applications.
Smart Water Meter (AMR and AMI)
A smart water meter records water use and sends readings remotely, either one way to a passing reader (AMR) or two way over a fixed network (AMI).
Snow Water Equivalent
Snow water equivalent (SWE) is the depth of water that would result from melting a given snowpack. It is a critical measurement for water supply forecasting in snow-dominated watersheds.
Soft Sensor
A soft sensor estimates a quantity that is hard or costly to measure by computing it from other, easier measurements with a model.
Soil Moisture
The water held in the pore spaces of the unsaturated soil zone between the land surface and the water table. It is a critical variable linking precipitation, runoff, and evapotranspiration.
Soil Moisture Active Passive (SMAP)
A NASA satellite mission designed to measure near-surface soil moisture content globally using microwave radiometry. SMAP data are vital for drought monitoring, flood forecasting, and agricultural water management.
Specific Yield
The volume of water that an unconfined aquifer releases from storage per unit surface area per unit decline in the water table. It represents the drainable porosity of the aquifer material.
Spectral Reflectance
The fraction of incoming electromagnetic radiation reflected by a surface at specific wavelengths. Spectral reflectance signatures of water, soil, and vegetation are the foundation of satellite-based water resource monitoring.
Stage
The height of the water surface in a stream or river above a fixed reference datum. It is the most commonly measured hydrological variable and is used to estimate discharge.
Stage-Discharge Relationship
The empirical relationship between water surface elevation (stage) and the volume of water flowing through a channel cross-section (discharge) at a specific location. This rating curve is the fundamental tool for converting continuous stage records into discharge data.
Standardized Precipitation Index
The Standardized Precipitation Index (SPI) quantifies precipitation anomalies at multiple time scales, providing a versatile measure of drought and wet conditions. It is the World Meteorological Organization's recommended drought index.
Storativity
Storativity, or the storage coefficient, is the volume of water an aquifer releases per unit area for each unit fall in head.
Storm Hydrograph
A hydrograph showing the short-term response of streamflow to a single precipitation event, including the rising limb, peak, and recession. It reveals how a watershed transforms rainfall into runoff.
Stormwater Management
Stormwater management encompasses the planning, design, and implementation of systems to control the quantity and quality of rainfall runoff from developed land. It addresses flooding, erosion, and pollution from urban surfaces.
Stream Gauge
An instrument or installation that measures the water level (stage) of a river or stream continuously. Stream gauges provide the real-time data essential for flood warning, water supply management, and hydrological research.
Streamflow
The volume of water moving through a stream channel at any given time. It integrates all upstream hydrological processes and is the most comprehensive indicator of watershed behavior.
Subsurface Flow
The lateral movement of water through soil layers above the water table toward stream channels. It is an important runoff generation mechanism in vegetated hillslopes with permeable soils.
Surrogate Model
A surrogate model is a fast, simplified model trained to copy the outputs of a slower, detailed simulation model.
Suspended Solids
Suspended solids (SS) are particles of organic and inorganic matter held in suspension in water by turbulence. Total suspended solids (TSS) is a standard measurement used in water quality monitoring.
Sustainable Yield
Sustainable yield is the amount of water that can be withdrawn from a water source (aquifer, reservoir, or basin) over the long term without causing unacceptable environmental, economic, or social consequences.
SWAT Model (Soil and Water Assessment Tool)
SWAT is a free river basin model from USDA-ARS and Texas A&M that simulates water, sediment, nutrients and crops under different land uses.
SWMM (Storm Water Management Model)
SWMM is free US EPA software that models rainfall, runoff and flow in urban storm drains, sewers and green infrastructure.
Telemetry
The automated transmission of measurement data from remote field stations to a central data management system. Telemetry systems enable near-real-time access to streamflow, water quality, and weather data from remote locations.
Theis Equation
The Theis equation gives drawdown at any distance and time around a well pumping a confined aquifer at a constant rate.
Thermal Infrared
Electromagnetic radiation emitted by all objects based on their temperature, detectable by satellite sensors in the 8-14 micrometer wavelength range. Thermal infrared imagery is essential for mapping land and water surface temperatures.
Thermal Pollution
Thermal pollution is the degradation of water quality caused by artificial changes in water temperature, most commonly from the discharge of heated water from power plants and industrial facilities.
Thiessen Polygon Method
The Thiessen polygon method estimates average rainfall by giving each rain gauge a weight equal to the area nearest to it.
Tipping Bucket Rain Gauge
A tipping bucket rain gauge records rainfall by counting the tips of a small see-saw bucket that empties each time it fills with a fixed depth of rain.
Total Dissolved Solids
Total dissolved solids (TDS) is a measure of the combined content of all dissolved inorganic and organic substances in water, including minerals, salts, and metals. It is a general indicator of water quality.
Total Organic Carbon
Total organic carbon (TOC) measures the total amount of carbon bound in organic compounds in a water sample. It indicates the level of organic contamination and the potential for disinfection byproduct formation.
Transmissivity
Transmissivity is the rate at which an aquifer passes water through its full saturated thickness. It equals hydraulic conductivity times thickness.
Turbidity
Turbidity is a measure of the cloudiness or haziness of water caused by suspended particles. It is a key indicator of water quality and treatment effectiveness.
Ultrasonic Flow Meter
An ultrasonic flow meter measures how fast water moves in a pipe or channel using sound pulses, then multiplies velocity by area to give flow.
Unconfined Aquifer
An unconfined aquifer is a water-bearing layer whose upper limit is the water table, open to the atmosphere through the soil above it.
Unit Hydrograph
A theoretical hydrograph representing the direct runoff response of a watershed to one unit of effective rainfall applied uniformly over a specified duration. It is a fundamental tool in rainfall-runoff modeling.
Vadose Zone
The vadose zone is the ground between the land surface and the water table, where pores hold both air and water. It is also called the unsaturated zone.
Virtual Water
Virtual water is the total volume of freshwater used to produce a commodity, good, or service. The concept reveals the hidden water embedded in traded products and helps explain global water resource transfers.
Water Balance
The water balance is an accounting of all water inputs, outputs, and storage changes within a defined system such as a watershed, lake, or aquifer. It is the foundation of hydrology and water resources management.
Water Budget
A water budget is a quantitative accounting of water inflows, outflows, and changes in storage within a defined system over a specific time period. It is essential for water resources planning and management.
Water Footprint
A water footprint is the total volume of freshwater used directly and indirectly by an individual, community, business, or nation. It includes blue water (surface/groundwater), green water (rainwater), and grey water (pollution assimilation).
Water Pricing
Water pricing is the practice of charging users for water supply and sanitation services to recover costs, allocate resources efficiently, and promote conservation. Pricing structures range from flat fees to volumetric and tiered rates.
Water Quality Index
A water quality index (WQI) is a numerical score that integrates multiple water quality parameters into a single value to provide a simplified, overall assessment of water quality conditions.
Water Reuse
The practice of treating and reusing wastewater for beneficial purposes such as irrigation, industrial cooling, groundwater recharge, and in some cases, potable water supply. Water reuse is an increasingly critical strategy for water-scarce regions.
Water Stress
Water stress occurs when the demand for water exceeds the available supply during a certain period, or when poor quality restricts water use. It affects over two billion people worldwide.
Water Table
The water table is the top of the saturated zone in an unconfined aquifer, where water pressure equals atmospheric pressure.
Water Treatment
Water treatment encompasses the processes used to make water suitable for its intended use, including drinking, industrial, irrigation, or environmental discharge. It involves physical, chemical, and biological methods to remove contaminants.
Watershed Management
Watershed management is the integrated planning and implementation of practices to protect and improve the water resources, land, and ecosystems within a watershed boundary.
Weir
A low dam or barrier constructed across a channel with a precisely shaped crest over which water flows, allowing discharge to be calculated from the measured water depth upstream. Weirs are primary flow measurement structures for small to medium streams.
Wetland
An area where water covers the soil or is present at or near the surface for all or part of the year, supporting hydrophytic vegetation and hydric soils. Wetlands provide critical ecosystem services including flood storage, water filtration, and biodiversity habitat.
Wetland Delineation
The field-based process of identifying and mapping wetland boundaries using three diagnostic criteria: hydric soils, hydrophytic vegetation, and wetland hydrology. Wetland delineation is required for regulatory compliance under the Clean Water Act.
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