Water balance

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Nile basin water balance
Global distribution of water balance in the soil averaged over the years 1981-2010 from the CHELSA-BIOCLIM+ data set SWB wiki.png
Global distribution of water balance in the soil averaged over the years 1981-2010 from the CHELSA-BIOCLIM+ data set

The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. [2] [3] In hydrology, a water balance equation can be used to describe the flow of water in and out of a system. A system can be one of several hydrological or water domains, such as a column of soil, a drainage basin, an irrigation area or a city.

Contents

The water balance is also referred to as a water budget. Developing water budgets is a fundamental activity in the science of hydrology. According to the US Geological Survey: [4]

An understanding of water budgets and underlying hydrologic processes provides a foundation for effective water-resource and environmental planning and management. Observed changes in water budgets of an area over time can be used to assess the effects of climate variability and human activities on water resources. Comparison of water budgets from different areas allows the effects of factors such as geology, soils, vegetation, and land use on the hydrologic cycle to be quantified.

Equation for a basin

Water balance in a basin Water balance basin.jpg
Water balance in a basin

A general water balance equation is: [5]

P = Q + ET + ΔS

where

P is precipitation
Q is streamflow
ET is evapotranspiration
ΔS is the change in storage (in soil or the bedrock / groundwater)

This equation uses the principles of conservation of mass in a closed system, whereby any water entering a system (via precipitation), must be transferred into either evaporation, transpiration, surface runoff (eventually reaching the channel and leaving in the form of river discharge), or stored in the ground. This equation requires the system to be closed, and where it is not (for example when surface runoff contributes to a different basin), this must be taken into account.

Extensive water balances are discussed in agricultural hydrology.

A water balance can be used to help manage water supply and predict where there may be water shortages. It is also used in irrigation, runoff assessment (e.g. through the RainOff model [6] ), flood control and pollution control. Further it is used in the design of subsurface drainage systems which may be horizontal (i.e. using pipes, tile drains or ditches) or vertical (drainage by wells). [7] To estimate the drainage requirement, the use of a hydrogeological water balance and a groundwater model (e.g. SahysMod [8] ) may be instrumental.

The water balance can be illustrated using a water balance graph which plots levels of precipitation and evapotranspiration often on a monthly scale.

Several monthly water balance models had been developed for several conditions and purposes. Monthly water balance models had been studied since the 1940s. [9]

Water Balance of a System

“Making water available for its many uses and users requires tools and institutions to transform it from a natural resource to one providing services”. [10] This means that there are two types of water systems: Water Resource System (WRS) and Water Use System (WUS).

A WRS, such as a river, an aquifer or a lake, must obey water balance. For example, the volume of water that goes into an aquifer must be equal to the amount that leaves it plus its change in storage. Under various drivers, such as, climate change, population increase, and bad management, water storage of many WRS is decreasing, say per decade. This means that the volume of water in a WRS decreased after a decade, i.e., inflow was less than outflow during that time interval. [11]

In general, a WUS is a water construct of a user, such as a city, an industry, an irrigation zone, or a region, and not a geographic area. The schematic of a WUS shows the inflows and the outflows. For a WUS, change in storage is negligible (relative to its inflow) under a proper time interval, hence water balance becomes inflow equal to outflow with nine Water Path Types (WPT): [12]

A typical schematic of a Water Use System (WUS) with its fixed nine Water Path Types Figure B1.tif
A typical schematic of a Water Use System (WUS) with its fixed nine Water Path Types

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle VA+OS+PP = ET+NR+RF+RP}

Of course, instead of a river, it could be an aquifer that supplies water to a WUS as a main source. Let us briefly examine an urban water supply on an annual basis as a simplified example. It has negligible ET and PP (WUS is a piped network), has some limited amount of water from groundwater (OS), has return flow to the main source (RF) after passing through a Wastewater Treatment Plant, and RP type has various Water Path Instances (WPI), such as leakage, and water taken to irrigate green zones. Considering that the annual change in storage of an urban area is negligible, water balance equation becomes

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle VA_{riv}+OS_{gw}=NR+RF_{wwtp}+RP_{leak}+RP_{irr}}

Models

Several diagnostic measures in hydrology can be used to select and evaluate the performance of water balance models.

Applications

Types

See also

Related Research Articles

<span class="mw-page-title-main">Hydrology</span> Science of the movement, distribution, and quality of water on Earth and other planets

Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying earth or environmental science, civil or environmental engineering, and physical geography. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as environmental preservation, natural disasters, and water management.

<span class="mw-page-title-main">Groundwater</span> Water located beneath the ground surface

Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

In hydrology, discharge is the volumetric flow rate of water transported through a given cross-sectional area. It includes any suspended solids, dissolved chemicals, or biologic material in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system may equate it with outflow, contrasted with inflow.

<span class="mw-page-title-main">Infiltration (hydrology)</span> Process by which water on the ground surface enters the soil

Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meters per day but can also be measured in other units of distance over time if necessary. The infiltration capacity decreases as the soil moisture content of soils surface layers increases. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier.

The United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall–runoff–subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas. It can simulate the Rainfall- runoff, runoff, evaporation, infiltration and groundwater connection for roots, streets, grassed areas, rain gardens and ditches and pipes, for example. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. Besides, low impact development (LID) and best management practice areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators.

<span class="mw-page-title-main">HBV hydrology model</span>

The HBV hydrology model, or Hydrologiska Byråns Vattenbalansavdelning model, is a computer simulation used to analyze river discharge and water pollution. Developed originally for use in Scandinavia, this hydrological transport model has also been applied in a large number of catchments on most continents.

<span class="mw-page-title-main">Groundwater recharge</span> Groundwater that recharges an aquifer

Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface. Groundwater recharge also encompasses water moving away from the water table farther into the saturated zone. Recharge occurs both naturally and through anthropogenic processes, where rainwater and or reclaimed water is routed to the subsurface.

Geographic information systems (GISs) have become a useful and important tool in the field of hydrology to study and manage Earth's water resources. Climate change and greater demands on water resources require a more knowledgeable disposition of arguably one of our most vital resources. Because water in its occurrence varies spatially and temporally throughout the hydrologic cycle, its study using GIS is especially practical. Whereas previous GIS systems were mostly static in their geospatial representation of hydrologic features, GIS platforms are becoming increasingly dynamic, narrowing the gap between historical data and current hydrologic reality.

Groundwater models are computer models of groundwater flow systems, and are used by hydrologists and hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.

<span class="mw-page-title-main">Soil salinity control</span> Controlling the problem of soil salinity

Soil salinity control refers to controlling the process and progress of soil salinity to prevent soil degradation by salination and reclamation of already salty (saline) soils. Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration.

<span class="mw-page-title-main">Surface water</span> Water located on top of land forming terrestrial bodies of water

Surface water is water located on top of land, forming terrestrial waterbodies, and may also be referred to as blue water, opposed to the seawater and waterbodies like the ocean.

<span class="mw-page-title-main">SahysMod</span>

SahysMod is a computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge in irrigated agricultural lands, using different hydrogeologic and aquifer conditions, varying water management options, including the use of ground water for irrigation, and several crop rotation schedules, whereby the spatial variations are accounted for through a network of polygons.

<span class="mw-page-title-main">Runoff model (reservoir)</span> Type of water motion

A runoff models or rainfall-runoff model describes how rainfall is converted into runoff in a drainage basin. More precisely, it produces a surface runoff hydrograph in response to a rainfall event, represented by and input as a hyetograph. Rainfall-runoff models need to be calibrated before they can be used.

<span class="mw-page-title-main">SaltMod</span>

SaltMod is computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge (hydrology) in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules. The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.

<span class="mw-page-title-main">Hydrological model</span>

A hydrologic model is a simplification of a real-world system that aids in understanding, predicting, and managing water resources. Both the flow and quality of water are commonly studied using hydrologic models.

<span class="mw-page-title-main">Agricultural hydrology</span>

Agricultural hydrology is the study of water balance components intervening in agricultural water management, especially in irrigation and drainage.

GSSHA is a two-dimensional, physically based watershed model developed by the Engineer Research and Development Center of the United States Army Corps of Engineers. It simulates surface water and groundwater hydrology, erosion and sediment transport. The GSSHA model is used for hydraulic engineering and research, and is on the Federal Emergency Management Agency (FEMA) list of hydrologic models accepted for use in the national flood insurance program for flood hydrograph estimation. Input is best prepared by the Watershed Modeling System interface, which effectively links the model with geographic information systems (GIS).

The following outline is provided as an overview of and topical guide to hydrology:

<span class="mw-page-title-main">Catchment hydrology</span> Hydrology of drainage basins

Catchment hydrology is the study of hydrology in drainage basins. Catchments are areas of land where runoff collects to a specific zone. This movement is caused by water moving from areas of high energy to low energy due to the influence of gravity. Catchments often do not last for long periods of time as the water evaporates, drains into the soil, or is consumed by animals.

DPHM-RS is a semi-distributed hydrologic model developed at University of Alberta, Canada.

References

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  2. Sutcliffe, J.V. (2004). Hydrology: A Question of Balance. Intl Assn of Hydrological Sciences (IAHS).
  3. Viessman, W.; Lewis, G.L. (1996). Introduction to Hydrology (Fourth ed.). HarperCollins College Publishers.
  4. Healy, R.W.; Winter, T.C.; LaBaugh, J.W.; Franke, O.L (2007). Water Budgets: Foundations for Effective Water-Resources and Environmental Management. U.S. Geological Survey. p. 90.
  5. "Water Balance". 31 January 2016.
  6. "RainOff, surface runoff model" . Retrieved 2010-07-28.
  7. "Free articles and software on drainage of waterlogged land under irrigation" . Retrieved 2010-07-28.
  8. "Sahysmod groundwater model" . Retrieved 2010-07-28.
  9. Xu, C.-Y.; V.P. Singh (1998). "A Review on Monthly Water Balance Models for Water Resources Investigations". Water Resources Management. 12 (1): 31–50. doi:10.1023/A:1007916816469. S2CID   153801907.
  10. High Level Panel on Water. "Bellagio principles on valuing water" (PDF). United Nations, Sustainable Development Goals (SDG). Retrieved 24 February 2021.
  11. National Geographic. "8 mighty rivers run dry from overuse". National Geographic. Retrieved 25 Feb 2021.
  12. Haie, Naim (2020). Transparent Water Management Theory: Sefficiency in Sequity. Springer.