Hidrologi (tina basa Yunani: Yδωρ, hudōr, "cai"; jeung λόγος, logos, "élmu") nyaéta élmu ngeunaan kajadian, distribusi, sarta pindahna cai dina, di jero jeung di luhur Marcapada. Siklus pindahna cai antara térasfir (darat), oséanosfir (laut), jeung atmosfir (udara) disebut daur hidrologi.

Aya dua widang utama dina hidrologi. Kahiji hidrologi cai permukaan (surface-water hydrology) nu museurkeun kana cai dina jeung luhureun taneuh. Conto tina hidrologi permukaan nyaéta banjir jeung kasaatan. Nu kadua nyaéta hidrologi cai-taneuh atawa géohidrologi, nu museurkeun kana distribusi jeung pindahna cai di handapeun taneuh (contona, cai taneuh, groundwater). Hidrologi cai taneuh penting keur dipaké dina pasokan cai, irigasi jeung rékayasa lingkungan. Catetan yén cai di laut dipisahkeun tina hidrologi sarta dipikawanoh dina istilah oséanografi, sedengkeun cai di atmosfir leuwih diajarkeun dina météorologi.

Kaasup dina hidrologi ogé élmu gerak cai sarta water-borne constituents — bahan-bahan nu dibawa boh salaku nu leyur atawa dina fase nu misah. A related facet of hydrology is the determination of statistical flow prediction in rivers and stréams. This information is essential to design and evaluation of natural and man-made channels, bridge openings and dams. Stream gage (U.S. Géological Survey terminology) data have been collected and tabulated by the United States Géological Survey for many yéars and much of it is available online for analysis.

Sajarah hidrologi 

Hidrologi jadi salah sahiji subyek panalungtingkan jeung rekayasa keur sababaraha milenia. Contona, dina kira-kira 4000 S.M. walungan Nil dibendung keur naekkeun hasil tatanen di wewengkon nu tadina tandus. Kota Mesopotamia ditangtayungan tina banjir ku tembok taneuh di jangkung. Saluran cai diwangun ku Yunani Kuno jeung Romawi Kuno, sedengkeun Sajarah China ngawangun irigasi jeung wangunan keur ngontrol banjir. Sinhale Kuno ngagunakeun hidrologi keur ngawangun Kompleks Irigasi di Sri Lanka Kuno, dikenal salaku nu manggihkeun Lobang Klep nu bisa dipaké keur nyieun tempat panampungan cai nu gedé, saluran cai nu nepi ka kiwari masih kénéh dipake.

Marcus Vitruvius, di mimiti abad S.M., ngagambarkeun téori filosofis siklus hidrologi, hujan anu murag di gunung nyerep ka jero taneuh sarta nuju ka walungan sarta usum semi di wewengkon lengkob. Kalawan nyoko pendekatan anu leuwih ilmiah, Léonardo da Vinci sarta Bernard Palissy sacara teuneung mere gambaran nu pas siklus hidrologi. Henteu nepi ka abad ka-17, yén variabel hidrologi mimiti diukur.

Pelopor élmu pangaweruh hidrologi modérn nyaéta Pierre Perrault, Edme Mariotte jeung Edmund Halley. Kalawan ngukur curah hujan, runoff, sarta lega drainase, Perrault méré conto yén curah hujan éta téh mahi keur ngajelaskeun aliran ka walungan Seine. Marriotte ngahijikeun kecepatan sarta penampang melintang sungai keur ngitung debit walungan Seine. Halley némbongkeun yén penguapan ti Laut Mediterania nyaéta mahi keur ngajelaskeun ngocorna cai walungan ka laut.

Advances in the 18th century included the Bernoulli piezometer and Bernoulli's equation, by Daniel Bernoulli, the Pitot tube. The 19th century saw development in groundwater hydrology, including Darcy's law, the Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.

Rational analyses began to replace empiricism in the 20th century, while governmental agencies began their own hydrological reséarch programs. Of particular importance were Leroy Sherman's unit hydrograph, the infiltration théory of Robert E. Horton, and C.V. Theis's Aquifer test/equation describing well hydraulics.

Since the 1950's, hydrology has been approached with a more théoretical basis than in the past, facilitated by advances in the physical understanding of hydrological processes and by the advent of computers and especially Geographic Information Systems (GIS).

Daur hidrologis 

The central theme of hydrology is that water moves throughout the éarth through different pathways and at different rates. The most vivid image of this is in the evaporation of water from the océan, 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 océan, completing a cycle.

Cabang hidrologi 
Chemical hydrology is the study of the chemical characteristics of water.

Ecohydrology is the study of interactions between organisms and the hydrologic cycle.

Hydrogeology is the study of the presence and movement of water in aquifers.

Hydroinformatics is the adaptation of information technology to hydrology and water resources applications.

Hydrometeorology is the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere.

Isotope hydrology is the study of the isotopic signatures of water.

Surface hydrology is the study of hydrologic processes that operate at or néar the Earth's surface.

Widang nu patali 
Aquatic chemistry
Civil engineering
Climatology
Environmental engineering
Geomorphology
Hydrography
Hydraulic engineering
Limnology
Oceanography
Physical Geography

Pangukuran hidrologis 

The movement of water through the éarth can be méasured in a number of ways. This information is important for both assessing water resources and understanding the processes involved in the hydrologic cycle. Following is a list of devices used by hydrologists and what they are used to méasure.

 Disdrometer - precipitation characteristics
 Evaporation -Symon's evaporation pan
 Infiltrometer - infiltration
 Piezometer - groundwater pressure and, by inference, groundwater depth (see: aquifer test)
 Radar - cloud properties, rain rate estimation, hail and snow detection
 Rain gauge - rain and snowfall 
 Satellite - rainy aréa identification, rain rate estimation, land-cover/land-use, soil moisture
 Sling psychrometer - humidity
 Stream gauge - stréam flow (see: discharge (hydrology))
 Tensiometer - soil moisture
 Time domain reflectometer - soil moisture
 Capacitance probe-soil moisture

Prediksi hidrologis 

Observations of hydrologic processes are used to maké predictions of the future behaviour of hydrologic systems (water flow, water quality). One of the major current concerns in hydrologic reséarch is the Prediction in Ungauged Basins (PUB), i.e. in basins where no or only very few data exist.

Hidrologi statistis 

By analysing the statistical properties of hydrologic records, such as rainfall or river flow, hydrologists can estimate future hydrologic phenomena. This, however, assumes the characteristics of the processes remain unchanged.

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 utilised to formulate operating rules for large dams forming part of systems which include agricultural, industrial and residential demands.

See: return period.

Pamodelan hideologis 

Hydrologic modéls are simplified, conceptual representations of a part of the hydrologic cycle. They are primarily used for hydrologic prediction and for understanding hydrologic processes. Two major types of hydrologic modéls can be distinguished:

 modéls based on data. These modéls are black box systems, using mathematical and statistical concepts to link a certain input (for instance rainfall) to the modél output (for instance runoff). Commonly used techniques are regression, transfer functions, neural networks and system identification.  These modéls are known as stochastic hydrology modéls.
 modéls based on process descriptions. These modéls try to represent the physical processes observed in the réal world. Typically, such modéls contain representations of surface runoff, subsurface flow, evapotranspiration, and channel flow, but they can be far more complicated.  These modéls are known as deterministic hydrology modéls.  Deterministic hydrology modéls can be subdivided into single-event modéls and continuous simulation modéls.

Recent reséarch in hydrologic modéling tries to have a more global approach to the understanding of the behaviour of hydrologic systems to maké better predictions and to face the major challenges in water resources management.

Angkutan hidrologis 

''See main article: Hydrologic transport model

Water movement is a significant méans by which other material, such as soil 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.

Panerapan hidrologi 
 Determining the water balance of a region.
 Designing riparian restoration projects.
 Mitigating and predicting flood, landslide and drought risk.
 Designing irrigation schemes and managing agricultural productivity.
 Part of the hazard module in catastrophe modeling
 Providing drinking water.
 Designing dams for water supply or hydroelectric power generation.
 Designing bridges.
 Designing sewers and urban drainage system.
 Analyzing the impacts of antecedent moisture on sanitary sewer systems.
 Predicting geomorphological changes, such as erosion or sedimentation.
 Assessing the impacts of natural and anthropogenic environmental change on water resources.
 Assessing contaminant transport risk and establishing environmental policy guidelines.

Bacaan salajengna 

 Introduction to Hydrology, 4e. Viessman and Lewis, 1996.  ISBN 0-673-99337-X
 Handbook of Hydrology.  ISBN 0-07-039732-5
 Encyclopedia of Hydrological Sciences.  ISBN 0-471-49103-9
 Hydrological Processes, ISSN: 1099-1085 (electronic) 0885-6087 (paper), John Wiley & Sons
 Journal of Hydroinformatics, ISSN: 1464-7141, IWA Publishing
 Nordic Hydrology, ISSN: 0029-1277, IWA Publishing
 Journal of Hydrologic Engineering, ISSN: 0733-9496, ASCE Publication

Tempooge 
 Hidropower
 Siklus cai
 Manajemén cai

Tumbu luar - Wiki Hidro 
 The Experimental Hydrology Wiki
 The Wiki about how to choose an uncertainty method for hydrologic and hydraulic modelling 
 The distributed hydrologic modeling Wiki 
 The Hydrowiki is being developed as a collaborative encyclopedia of hydrology topics generated by students, faculty and researchers from educational and other institutions throughout the world working in disciplines relating to hydrology

Tumbu luar lianna 
 International Glossary of Hydrology .
 U.S. Geological Survey - Water Resources of the United States
 British Hydrology Society
 eawag - aquatic research, ETH Zürich, Switzerland
 Institute of Hydrology, Albert-Ludwigs-University of Freiburg, Germany
 NOAA's National Weather Service - Office of Hydrologic Development 
 Virtual Campus in Hydrology and Water Resources 
 Decision tree to choose an uncertainty method for hydrological and hydraulic modelling 
 Russian Geographical Society (Moscow Centre) - Hydrology Commission
 International Assoc for Environmental Hydrology
 American Water Resources Association

 
Téhnik hidrolik
Téhnik lingkungan
Géografi fisik
Élmu alam