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 Ground water is water beneath Earth's surface, often pumped for drinking water. 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
Soil moisture Soil moisture can be measured in various ways; by
capacitance probe,
time domain reflectometer or
tensiometer. Other methods include solute sampling and geophysical methods.
Surface water flow 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.
Precipitation and evaporation rain gauge In some considerations, hydrology is thought of as starting at the land-atmosphere boundary 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.
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 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. 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.
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
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). 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.
Statistical hydrology The aims of Statistical hydrology is to provide appropriate statistical methods for analyzing and modeling various parts of the hydrological cycle. 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.
Modeling 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: • 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.
Transport 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. == Organizations ==