Hydrology

Building a map of groundwater contours

Ground water is water beneath Earth's surface, often pumped for drinking water.[1] Groundwater hydrology (hydrogeology) considers quantifying groundwater flow and solute transport.[5] Problems in describing the saturated zone include the characterization of aquifers in terms of flow direction, groundwater pressure and, by inference, groundwater depth (see: aquifer test). Measurements here can be made using a piezometer. Aquifers are also described in terms of hydraulic conductivity, storativity and transmissivity. There are a number of geophysical methods[6] for characterizing aquifers. There are also problems in characterizing the vadose zone (unsaturated zone).[7]

Infiltration is the process by which water enters the soil. Some of the water is absorbed, and the rest percolates down to the water table. The infiltration capacity, the maximum rate at which the soil can absorb water, depends on several factors. The layer that is already saturated provides a resistance that is proportional to its thickness, while that plus the depth of water above the soil provides the driving force (hydraulic head). Dry soil can allow rapid infiltration by capillary action; this force diminishes as the soil becomes wet. Compaction reduces the porosity and the pore sizes. Surface cover increases capacity by retarding runoff, reducing compaction and other processes. Higher temperatures reduce viscosity, increasing infiltration.[8]^{: 250–275}

Soil moisture can be measured in various ways; by capacitance probe, time domain reflectometer or Tensiometer. Other methods include solute sampling and geophysical methods.[9]

A flood hydrograph showing stage for the Shawsheen River at Wilmington.

Hydrology considers quantifying surface water flow and solute transport, although the treatment of flows in large rivers is sometimes considered as a distinct topic of hydraulics or hydrodynamics. Surface water flow can include flow both in recognizable river channels and otherwise. Methods for measuring flow once the water has reached a river include the stream gauge (see: discharge), and tracer techniques. Other topics include chemical transport as part of surface water, sediment transport and erosion.

One of the important areas of hydrology is the interchange between rivers and aquifers. Groundwater/surface water interactions in streams and aquifers can be complex and the direction of net water flux (into surface water or into the aquifer) may vary spatially along a stream channel and over time at any particular location, depending on the relationship between stream stage and groundwater levels.

A standard NOAA rain gauge

In some considerations, hydrology is thought of as starting at the land-atmosphere boundary[10] and so it is important to have adequate knowledge of both precipitation and evaporation. Precipitation can be measured in various ways: disdrometer for precipitation characteristics at a fine time scale; radar for cloud properties, rain rate estimation, hail and snow detection; rain gauge for routine accurate measurements of rain and snowfall; satellite for rainy area identification, rain rate estimation, land-cover/land-use, and soil moisture, for example.

Evaporation is an important part of the water cycle. It is partly affected by humidity, which can be measured by a sling psychrometer. It is also affected by the presence of snow, hail, and ice and can relate to dew, mist and fog. Hydrology considers evaporation of various forms: from water surfaces; as transpiration from plant surfaces in natural and agronomic ecosystems. Direct measurement of evaporation can be obtained using Simon's evaporation pan.

Detailed studies of evaporation involve boundary layer considerations as well as momentum, heat flux, and energy budgets.

Estimates of changes in water storage around the Tigris and Euphrates Rivers, measured by NASA's GRACE satellites. The satellites measure tiny changes in gravitational acceleration, which can then be processed to reveal movement of water due to changes in its total mass.

Remote sensing of hydrologic processes can provide information on locations where in situ sensors may be unavailable or sparse. It also enables observations over large spatial extents. Many of the variables constituting the terrestrial water balance, for example surface water storage, soil moisture, precipitation, evapotranspiration, and snow and ice, are measurable using remote sensing at various spatial-temporal resolutions and accuracies.[11] Sources of remote sensing include land-based sensors, airborne sensors and satellite sensors which can capture microwave, thermal and near-infrared data or use lidar, for example.

In hydrology, studies of water quality concern organic and inorganic compounds, and both dissolved and sediment material. In addition, water quality is affected by the interaction of dissolved oxygen with organic material and various chemical transformations that may take place. Measurements of water quality may involve either in-situ methods, in which analyses take place on-site, often automatically, and laboratory-based analyses and may include microbiological analysis.

• Budget analyses
• Parameter estimation
• Scaling in time and space
• Data assimilation
• Quality control of data – see for example Double mass analysis

Observations of hydrologic processes are used to make predictions of the future behavior of hydrologic systems (water flow, water quality).[12] One of the major current concerns in hydrologic research is "Prediction in Ungauged Basins" (PUB), i.e. in basins where no or only very few data exist.[13]

By analyzing the statistical properties of hydrologic records, such as rainfall or river flow, hydrologists can estimate future hydrologic phenomena. When making assessments of how often relatively rare events will occur, analyses are made in terms of the return period of such events. Other quantities of interest include the average flow in a river, in a year or by season.

These estimates are important for engineers and economists so that proper risk analysis can be performed to influence investment decisions in future infrastructure and to determine the yield reliability characteristics of water supply systems. Statistical information is utilized to formulate operating rules for large dams forming part of systems which include agricultural, industrial and residential demands.

Plan view of water flow through a catchment simulated by the SHETRAN hydrological modelling system.

Hydrological models are simplified, conceptual representations of a part of the hydrologic cycle. They are primarily used for hydrological prediction and for understanding hydrological processes, within the general field of scientific modeling. Two major types of hydrological models can be distinguished:[14]

• Models based on data. These models are black box systems, using mathematical and statistical concepts to link a certain input (for instance rainfall) to the model output (for instance runoff). Commonly used techniques are regression, transfer functions, and system identification. The simplest of these models may be linear models, but it is common to deploy non-linear components to represent some general aspects of a catchment's response without going deeply into the real physical processes involved. An example of such an aspect is the well-known behavior that a catchment will respond much more quickly and strongly when it is already wet than when it is dry.
• Models based on process descriptions. These models try to represent the physical processes observed in the real world. Typically, such models contain representations of surface runoff, subsurface flow, evapotranspiration, and channel flow, but they can be far more complicated. Within this category, models can be divided into conceptual and deterministic. Conceptual models link simplified representations of the hydrological processes in an area, whereas deterministic models seek to resolve as much of the physics of a system as possible. These models can be subdivided into single-event models and continuous simulation models.

Recent research in hydrological modeling tries to have a more global approach to the understanding of the behavior of hydrologic systems to make better predictions and to face the major challenges in water resources management.

Water movement is a significant means by which other materials, such as soil, gravel, boulders or pollutants, are transported from place to place. Initial input to receiving waters may arise from a point source discharge or a line source or area source, such as surface runoff. Since the 1960s rather complex mathematical models have been developed, facilitated by the availability of high-speed computers. The most common pollutant classes analyzed are nutrients, pesticides, total dissolved solids and sediment.

• International Hydrological Programme (IHP)[15]

• International Water Management Institute (IWMI)[16]
• UN-IHE Delft Institute for Water Education[17]

• Centre for Ecology and Hydrology – UK[18]
• Centre for Water Science, Cranfield University, UK[19]
• eawag – aquatic research, ETH Zürich, Switzerland[20]
• Institute of Hydrology, Albert-Ludwigs-University of Freiburg, Germany[21]
• United States Geological Survey – Water Resources of the United States[22]
• NOAA's National Weather Service – Office of Hydrologic Development, USA[23]
• US Army Corps of Engineers Hydrologic Engineering Center, USA[24]
• Hydrologic Research Center, USA[25]
• NOAA Economics and Social Sciences, United States[26]
• University of Oklahoma Center for Natural Hazards and Disasters Research, USA[27]
• National Hydrology Research Centre, Canada[28]
• National Institute of Hydrology, India[29]

• American Institute of Hydrology (AIH)[30]
• Geological Society of America (GSA) – Hydrogeology Division[31]
• American Geophysical Union (AGU) – Hydrology Section[32]
• National Ground Water Association (NGWA)[33]
• American Water Resources Association[34]
• Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI)[35]
• International Association of Hydrological Sciences (IAHS)[36][37]
• Statistics in Hydrology Working Group (subgroup of IAHS)[38]
• German Hydrological Society (DHG: Deutsche Hydrologische Gesellschaft)[39]
• Italian Hydrological Society (SII-IHS) – http://www.sii-ihs.it
• Nordic Association for Hydrology[40]
• British Hydrological Society[41]
• Russian Geographical Society (Moscow Center) – Hydrology Commission[42]
• International Association for Environmental Hydrology[43]
• International Association of Hydrogeologists[44]
• Society of Hydrologists and Meteorologists – Nepal [45]

• Connected Waters Initiative, University of New South Wales[46] – Investigating and raising awareness of groundwater and water resource issues in Australia
• Murray Darling Basin Initiative, Department of Environment and Heritage, Australia[47]