Editing Hydrology (section)

== Themes ==
{{Main|Water cycle}}
The central theme of hydrology is that water circulates throughout the [[Earth]] through different pathways and at different rates. The most vivid image of this is in the evaporation of water from the ocean, which forms clouds. These clouds drift over the land and produce rain. The rainwater flows into lakes, rivers, or aquifers. The water in lakes, rivers, and aquifers then either evaporates back to the atmosphere or eventually flows back to the ocean, completing a cycle. Water changes its state of being several times throughout this cycle.

The areas of research within hydrology concern the movement of water between its various states, or within a given state, or simply quantifying the amounts in these states in a given region. Parts of hydrology concern developing methods for directly measuring these flows or amounts of water, while others concern modeling these processes either for scientific knowledge or for making a prediction in practical applications.

=== Groundwater ===
[[File:Building a map of groundwater countours.gif|thumb|250px|right|Building a map of groundwater contours]]
Ground water is water beneath Earth's surface, often pumped for drinking water.<ref name=USGS /> Groundwater hydrology ([[hydrogeology]]) considers quantifying groundwater flow and solute transport.<ref>{{cite journal|last1=Graf|first1=T.|last2=Simmons|first2=C. T.|title=Variable-density groundwater flow and solute transport in fractured rock: Applicability of the Tang et al. [1981] analytical solution|journal=Water Resources Research|date=February 2009|volume=45|issue=2|page=W02425|doi=10.1029/2008WR007278|bibcode=2009WRR....45.2425G|s2cid=133884299 }}</ref> 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<ref name=Vereecken /> for characterizing aquifers. There are also problems in characterizing the vadose zone (unsaturated zone).<ref name=Wilson />

=== Infiltration ===
{{Main|Infiltration (hydrology)}}
Infiltration is the process by which water enters the soil. Some of the water is absorbed, and the rest [[Percolation|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 (geology)|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.<ref name="Reddy">{{cite book |last1=Reddy |first1=P. Jaya Rami |title=A Textbook of Hydrology |date=2007 |publisher=Laxmi Publ. |isbn=9788170080992 |edition=Reprint. |location=New Delhi}}</ref>{{rp|250–275}}

=== Soil moisture ===
{{main|Soil moisture}}
Soil moisture can be measured in various ways; by [[capacitance probe]], [[time domain reflectometer]] or [[Tensiometer (soil science)|tensiometer]]. Other methods include solute sampling and geophysical methods.<ref>Robinson, D. A., C. S. Campbell, J. W. Hopmans, B. K. Hornbuckle, S. B. Jones, R. Knight, F. L. Ogden, J. Selker, and O. Wendroth. (2008) "Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review."</ref>

=== Surface water flow ===
[[File:Wlmm3_hg.png|250px|thumb|A [[hydrograph|flood hydrograph]] showing [[stage (hydrology)|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 (hydrology)|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.

=== Precipitation and evaporation ===
[[File:2013-10-14 12 27 49 National Weather Service Standard Rain Gauge.JPG|thumb|upright|A standard [[NOAA]] [[rain gauge]]]]
In some considerations, hydrology is thought of as starting at the land-atmosphere boundary<ref>{{Cite book|url=https://books.google.com/books?id=uLiL5xEd680C&pg=PA113|title=Hydroecology and Ecohydrology: Past, Present and Future|last1=Wood|first1=Paul J.|last2=Hannah|first2=David M.|last3=Sadler|first3=Jonathan P.|date=28 February 2008|publisher=John Wiley & Sons|isbn=978-0-470-01018-1|language=en}}</ref> 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, snow cover or snow water equivalent for example.<ref>{{Cite journal |last1=Schilling |first1=Samuel |last2=Dietz |first2=Andreas |last3=Kuenzer |first3=Claudia |date=2024-03-20 |title=Snow Water Equivalent Monitoring—A Review of Large-Scale Remote Sensing Applications |journal=Remote Sensing |language=en |volume=16 |issue=6 |pages=1085 |doi=10.3390/rs16061085 |doi-access=free |bibcode=2024RemS...16.1085S |issn=2072-4292}}</ref>

[[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.

=== Remote sensing ===
[[File:NASA's_GRACE_Sees_Major_Water_Losses_in_Middle_East.jpg|thumb|upright=1.3|Estimates of changes in water storage around the [[Tigris]] and [[Euphrates]] Rivers, measured by NASA's [[GRACE (satellite)|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.]]
{{Main|Remote sensing}}
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.<ref>{{cite journal|last1=Tang|first1=Q.|last2=Gao|first2=H.|last3=Lu|first3=H.|author4-link=Dennis P. Lettenmaier|last4=Lettenmaier|first4=D. P.|title=Remote sensing: hydrology|journal=Progress in Physical Geography|date=6 October 2009|volume=33|issue=4|pages=490–509|doi=10.1177/0309133309346650|bibcode=2009PrPG...33..490T |s2cid=140643598}}</ref> Sources of remote sensing include land-based sensors, airborne sensors and [[Earth observation satellite|satellite sensors]] which can capture [[Passive microwave sensor|microwave]], [[near-infrared|thermal and near-infrared]] data or use [[lidar]], for example.

=== Water quality ===
{{Main|Water quality}}

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 [[Bacteriological water analysis|microbiological analysis]].

=== Integrating measurement and modelling ===
* Budget analyses
*[[Parameter estimation]]
* Scaling in time and space
* [[Data assimilation]]
* Quality control of data – see for example [[Double mass analysis]]

=== Prediction ===
Observations of hydrologic processes are used to make [[predictions]] of the future behavior of hydrologic systems (water flow, water quality).<ref>{{Cite journal|last1=Archibald|first1=J.A.|last2=Buchanan|first2=B.P.|last3=Fuka|first3=D.R.|last4=Georgakakos|first4=C.B.|last5=Lyon|first5=S.W.|last6=Walter|first6=M.T.|date=July 2014|title=A simple, regionally parameterized model for predicting nonpoint source areas in the northeastern US|journal=Journal of Hydrology: Regional Studies|language=en|volume=1|pages=74–91|doi=10.1016/j.ejrh.2014.06.003|bibcode=2014JHyRS...1...74A |doi-access=free}}</ref> 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.<ref>{{Cite journal|last1=Beck|first1=Hylke E.|last2=Pan|first2=Ming|last3=Lin|first3=Peirong|last4=Seibert|first4=Jan|last5=Dijk|first5=Albert I. J. M.|author6-link=Eric Franklin Wood|last6=Wood|first6=Eric F.|date=2020-09-16|title=Global Fully Distributed Parameter Regionalization Based on Observed Streamflow From 4,229 Headwater Catchments|journal=Journal of Geophysical Research: Atmospheres|language=en|volume=125|issue=17|doi=10.1029/2019JD031485|bibcode=2020JGRD..12531485B |issn=2169-897X|doi-access=free|hdl=1885/317576|hdl-access=free}}</ref>

=== Statistical hydrology ===
The aims of Statistical hydrology is to provide appropriate statistical methods for analyzing and modeling various parts of the hydrological cycle.<ref>{{Citation |last=Loftis |first=Jim C. |title=Analysis of Water Quality Random Variables |date=2019-04-30 |url=http://dx.doi.org/10.1061/9780784415177.ch10 |work=Statistical Analysis of Hydrologic Variables |pages=381–405 |access-date=2023-05-19 |place=Reston, VA |publisher=American Society of Civil Engineers|doi=10.1061/9780784415177.ch10 |isbn=9780784415177 |s2cid=182417172 }}</ref> 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 (business)|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.

=== Modeling ===
[[Image:shetran plan view dunsop.jpg|thumb|Plan view of water flow through a [[catchment]] simulated by the [[SHETRAN]] hydrological modelling system]]
{{main|Hydrological modeling}}
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:<ref>Jajarmizadeh et al. (2012), Journal of Environmental Science and Technology, 5(5), p.249-261.</ref>
* Models based on data. These models are [[Black box (systems)|black box]] systems, using mathematical and statistical concepts to link a certain input (for instance rainfall) to the model output (for instance [[Surface runoff|runoff]]). Commonly used techniques are [[Regression analysis|regression]], [[transfer function]]s, 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 [[open channel flow|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 [[behavioral modeling in hydrology|behavior of hydrologic systems]] to make better predictions and to face the major challenges in water resources management.

=== Transport ===
{{Main|Hydrologic transport model}}
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 (pollution)|point source]] discharge or a [[line source]] or [[Area source (pollution)|area source]], such as [[surface runoff]]. Since the 1960s rather complex [[mathematical model]]s have been developed, facilitated by the availability of high-speed computers. The most common pollutant classes analyzed are [[nutrient]]s, [[pesticide]]s, [[total dissolved solids]] and [[sediment]].