Hydrology is the study of the movement, distribution, and quality of water on Earth. It encompasses both the hydrologic cycle and water resources. The discipline of hydrology includes the fields of hydrometeorology, surface hydrology, hydrogeology, drainage basin management, and water quality.

Hydrologists can be found working in earth or environmental science, physical geography, geology, or civil and environmental engineering. They may be engaged in activities such as hydraulic modeling, flood mapping, catchment flood management plans, shoreline management plans, estuarine strategies, coastal protection, and flood alleviation.

Hydrological Models

Hydrologic models are simplified, conceptual representations of a part of the hydrologic, or water, cycle. They are primarily used for hydrologic prediction and for understanding hydrologic processes.

Two major types of hydrologic models can be distinguished:

  1. Stochastic Models. These models are black box systems, based on data and 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, neural networks and system identification. These models are known as stochastic hydrology models.
  2. Process-Based Models. 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. These models are known as deterministic hydrology models. Deterministic hydrology models can be subdivided into single-event models and continuous simulation models.

Recent research in hydrologic modelling has taken a more global approach to the understanding of the behaviour of hydrologic systems in an attempt to make better predictions and to address the major challenges in water resources management.

Hydrological models may also be classified based on the particular aspect of the hydrological cycle which they address. Two of these more specialized models are:

    1. Groundwater models. These are computer models of groundwater flow systems, and are used by hydrogeologists.


      Image from Wikipedia: Hydrological Modelling – see link.

      Groundwater models are employed to simulate and predict aquifer conditions. Groundwater models need:

      • hydrological inputs – rainfall, evapotranspiration and surface runoff, which determine the recharge; these inputs may vary both from time to time and from place to place
      • operational inputs – inputs which concern human interferences with the water management like irrigation, drainage, pumping from wells, watertable control, and the operation of retention or infiltration basins; these inputs may also vary in time and space
      • external conditions – initial and boundary conditions. Boundary conditions can be related to levels of the water table, artesian pressures, and hydraulic head along the boundaries of the model on the one hand (the head conditions), or to groundwater inflows and outflows along the boundaries of the model on the other hand (the flow conditions). The may also include quality aspects of the water like salinity. The initial conditions refer to initial values of elements that may increase or decrease in the course of the time within the model. The initial and boundary conditions may vary from place to place. The boundary conditions may be kept either constant or be made variable in time.
      • hydraulic parameters – topography, thicknesses of soil layers and their horizontal/vertical hydraulic conductivity (permeability to water), aquifer transmissivity and resistance, aquifer porosity and storage coefficient, as well as the capillarity of the unsaturated zone.

      Groundwater models may also have chemical components like water salinity, soil salinity and other quality indicators of water and soil, for which inputs may be needed.

  1. Surface water models. These are computer models used to understand surface water systems and potential changes due to natural or anthropogenic influences.


    Image from Wikipedia: Hydrological Modelling – see link.
    • Runoff models are mathematical models describing the rainfall – runoff relations of a rainfall catchment area, drainage basin or watershed. More precisely, they produces a surface runoff hydrograph as a response to a rainfall hydrograph input. In other words, these models calculate the conversion of rainfall into runoff.
    • Hydrological transport models are mathematical models used to simulate river or stream flow and calculate water quality parameters.

GIS and Hydrology

Geographic Information Systems (GIS) have become particularly useful and important tools in hydrology for the scientific study and management of water resources (see link). Since water is constantly in motion, its occurrence varies spatially and temporally throughout the hydrologic cycle. Thus, the study of water using GIS is especially practical.

A watershed is a spatial area, and the occurrence of water throughout its space varies by time. Hydrologists make use of a hydrologic budget (a balance between inputs, outputs, and storage) when they study a watershed. In the hydrologic budget, inputs come from precipitation, surface flows, and groundwater flows. Outputs leave as evapotranspiration, infiltration, surface runoff, and surface/groundwater flows. All of these quantities, including storage, can be measured or estimated, and their characteristics can be graphically displayed in GIS and studied.

As a subset of hydrology, hydrogeology is concerned with the occurrence, distribution, and movement of groundwater. Moreover, hydrogeology is concerned with the manner in which groundwater is stored and its availability for use. The characteristics of groundwater can readily be input into GIS for further study and management of the water resources.

Hydrological Modelling Software

Ocean Ecology has used a variety of programs for hydrological modelling. Several examples are:

    1. Hydrologic Engineering Center’s River Analysis System [HEC-RAS]. HEC-RAS is a computer program that models the hydraulics of water flow through natural rivers and other channels. The program is one-dimensional, meaning that there is no direct modeling of the hydraulic effect of cross section shape changes, bends, and other two- and three-dimensional aspects of flow. The program was developed by the US Department of Defense, Army Corps of Engineers in order to manage the rivers, harbors, and other public works. This model is an example of a process-based, surface water model. Specifically, it is a hydrological transport model.


    1. Arc Hydro Tools and Data Model. Arc Hydro is a data model and toolset for integrating geospatial and temporal water resource information that can be run within ESRI’s ArcGIS geographic information system. Thus, Arc Hydro supports hydrologic and hydraulic analysis within a GIS application. Although implemented in a commercial GIS environment, the data model and toolset are in the public domain and available free of charge. Arc Hydro is used to define the watersheds, stream networks, channels, structures, measurement stations, and land surface properties that cover the study region. This geographic description is then combined with time series information on water resources measurements for the region, resulting in a “data model” that describes the complex water landscape in a simplified way. By linking hydrologic simulation models to the data model, a “hydrologic information system” is created.


Arc Hydro.

Arc Hydro is a surface water model.

    1. Arc Hydro Groundwater Toolbar. The Arc Hydro Groundwater Toolbar is a set of public domain tools for ArcGIS (ArcScene) which perform the following tasks.


      Data Preparation

      • Creates trend or constant surfaces to represent head, thickness, porosity etc.
      • Creates well feature dataset storing well locations, pumping rate and transmissivity for computing drawdown.
      • Creates particle feature dataset storing particle locations for particle tracking. A group of particles are automatically created around a pumping well.
      • Reverses (flip) the direction of the DarcyFlow output direction raster for backward tracking.



      • Computes drawdown for a single or multiple pumping wells.
      • Creates residual, direction and magnitude using the DarcyFlow method.
      • Performs particle tracking for a single or multiple particles using ParticleTrack method. In the case of multiple particles, the paths are appended in a single feature dataset.
      • Creates concentration distribution of a pollutant using PorousPuff method.
      • Determines capture zone of a pumping well.


      Arc Hydro Groundwater Tools.

Arc Hydo Groundwater is a process-based groundwater model.

    1. MODFLOW, PMWIN, and MODFLOW Toolbox. MODFLOW is the U.S. Geological Survey’s modular finite-difference flow model, which is a computer code that solves the groundwater flow equation. The program is used by hydrogeologists to simulate the flow of groundwater through aquifers. MODFLOW ia a process-based groundwater model. PMWIN (Processing Modflow for Windows) is a non-commercial GUI (graphical user interface) for MODFLOW processing and visualization.


The PMPATH component of PMWIN – see link.

To help integrate the Arc Hydro data model with MODFLOW, a set of geoprocessing tools, the MODFLOW Toolbox, were developed to link the data model structure to the model. The toolbox includes a set of tools to create the features needed for storing input and outputs of a MODFLOW model and to help users view the modeling results within ArcGIS.

MODFLOWTools as seen in ArcGIS
– see link.