Lesson 21 Water Budget

21.1 What is Water Budget?

A water budget is a measure of the amount of water entering and the amount of water leaving a system. It is a way to evaluate all the sources of supply and the corresponding discharges with respect to a basin or aquifer. It is a basic tool that can be used to evaluate the occurrence and movement of water through the natural environment. To maintain a balance in the ecological system, one must account for the incoming (source of water) and outgoing (water losses in the system) water resources. A water budget may be used to manage development of water resources within a region, and to ensure a sustainable supply of water over time.

Water budget commonly provides knowledge about how much water is available, where is it available with detailed understanding of the flow dynamics. These flow dynamics include the origin and movement of groundwater and surface water as well as the interaction between the two systems. Water budget studies consider the volumes of water within the various reservoirs of the hydrologic cycle and the flow paths from recharge to discharge. The water budget takes into account the water cycle, evapo-transpiration, groundwater and surface water supplies, and inter basin (import and export) transfers of the water. Thus, a water budget is important to understand since it provides crucial information regarding the carrying capacity of the land with regard to water resources. Figure 1 shows the conceptual model of water budget.

21.2 Components of Water Budget

The most basic equation for water budgets is based on the hydrologic cycle, where water moves from the atmosphere to the Earth’s surface to various destinations, and finally back to the atmosphere:

P = I + ET + R                                            (1)

Where; P = precipitation, I = infiltration, ET = evapo-transpiration, R= runoff

Fig. 21.1

Fig. 21.1. A Conceptual Water Budget.

(Source: http://water.usgs.gov/watercensus/water-budgets.html)

21.2.1 Precipitation

Precipitation is the sole input to the water budget under natural conditions. Precipitation comes in various forms.

21.2.2 Infiltration/Recharge

Infiltration of water into the land surface is a critical component of a water budget. Without infiltration, all water would either evaporate from the land surface or runoff to surface waters. The term “infiltration” means that the water moves downward from the land surface into the soil.  Some infiltration water moves toward streams just below the land surface. This water is not used for potable water supplies, generally, but is invaluable for ecosystem health. Other infiltrated water penetrates more deeply in to soil layers and can recharge aquifers. Under natural conditions, water from aquifers is transmitted to surface waters after periods ranging from years to millennia. Infiltration is highly dependent on the amount, intensity and season of precipitation. A lower percentage of precipitation infiltrates to become ground water during periods, where soils are very dry (e.g., summer), brief storms, high-intensity storms, and when the ground is frozen.

21.2.3 Evapo-Transpiration

Evapo-transpiration is a combination of two terms evaporation and transpiration. Evaporation involves the transformation of surface water to atmospheric water. Transpiration occurs when water in plants moves from shallow soils to the root system to the leaves, transporting nutrients and energy, and then evaporates from the leaves. The two terms are usually considered together in water budget calculations. Both evaporation and transpiration tend to be higher during periods of hot weather, low humidity and high wind. Of course, nearly all transpiration occurs during the growing season in case of agricultural crops and round the years for natural vegetation like forest.

21.2.4 Runoff

Runoff occurs when precipitation falls onto the land surface and moves toward surface waters. Runoff is affected by a wide variety of factors, such as:

  1. soil type and depth

  2. the presence or absence of vegetation

  3. the presence or absence of impervious surfaces such as pavement or buildings

  4. general topography

  5. the extent to which water is trapped in puddles and never gets to major surface waters,

  6. the intensity of the precipitation

  7. the form of the precipitation (such as snow or rain), and

  8. the amount of moisture in the soil just before the precipitation.

In simple terms a water budget for a given area (watershed or basin) can be looked at as water inputs, outputs and changes in storage. The inputs into the area of investigation (precipitation, groundwater or surface water inflows, anthropogenic inputs such as waste effluent) must be equal to the outputs (evapo-transpiration, water supply removals or abstractions, surface or groundwater outflows) as well as any changes in storage within the area of interest.

In the simplest form this can be expressed as:

Inputs = Outputs + Change in storage

P + SWSWin + GWin + ANTHin = ET + SWout + ANTHout + ΔS                   (2)    

Where; P = precipitation, SWin = surface water flow in, GWin = groundwater flow, ANTHin = anthropogenic or human inputs such as waste discharges, ET = evaporation and transpiration (evapo-transpiration), SWout = surface water flow out, GWout = groundwater flow out, ANTHout = anthropogenic or human removals or abstractions, ΔS = change in storage (surface water, soil moisture, groundwater)

More detail is incorporated into the water budget to account for additional physical aspects. Essentially, three compartments are considered in the water budget determination i.e.  the ground surface; the unsaturated zone and the saturated zone.

Precipitation falls onto the ground surface and then can either:

  1. evapo-transpirate back to the atmosphere

  2. runoff from the surface to surface water bodies (e.g. streams, lakes and wetlands)

  3. move downward to the unsaturated zone, or

  4. be removed for human water supply purposes

In turn, water that moves to the unsaturated zone can either:

  1. evapo-transpirate back to the atmosphere;

  2. move laterally as interflow to discharge to local surface water bodies; or

  3. move downward to the saturated zone

Similarly, water that moves to the saturated zone can:

  1. evapo-transpirate back to the atmosphere (e.g. via plants whose roots extend to near the water table)

  2. move in the groundwater system and eventually discharge into a surface water body; or

  3. be removed for human water supply purposes

Fig. 21.2

Fig. 21.2. The Water Budget Components.

(Source: http://www.conservation-ontario.on.ca/watershed_management/reports/IWM_WaterBudgetOverview_Final_Jun2.pdf)

Figure 21.2 illustrates that evapo-transpiration can occur from any of the three compartments (ground surface, unsaturated and saturated zones). This figure also shows anthropogenic inputs and/or abstractions. These are both related to human intervention in the water cycle. Inputs would occur in an instance, where water external to a watershed was being brought into and disposed of within the watershed, thereby increasing the water volume in the watershed. Supplies or abstractions would occur, where water was being withdrawn from either a surface water body or the groundwater system and was being removed from the watershed. It is important to note that these human interventions are often difficult to account for in a water budget owing to the fact that a certain portion of the withdrawn water is likely re-circulated back within the same watershed (e.g. through irrigation or through leakage from municipal infrastructure, etc.). Figure 21.2 also shows inputs into the three compartments (i.e. surface water inputs, interflow inputs and groundwater inputs). Water budgets are generally carried out on a watershed or sub-watershed scale and the surface water inputs and interflow inputs tend to be negligible.

Mathematically, the water budget can be expressed as follows:

P = RO + AET + I + D + A ± ΔI ± Δs ± Δg                                                        (3)                

Where; P = precipitation, RO = surface runoff, AET = actual evapo-transpiration, I = interflow, D = groundwater discharge, A = anthropogenic inputs (septic systems) and/or supplies/abstractions, ΔI = change in land surface storage, Δs = change in soil moisture storage, Δg = change in groundwater storage

Following the above water budgeting equation:

Stream Flow Discharge (SFD) = I + D + RO                                               (4)

Infiltration =  P – AET – RO – ΔS – ΔI                                                      (5)

Aquifer Recharge = P – AET – RO – ΔS – ΔI – I                                                 (6)

Over long periods of time in an unstressed, natural state basin (no groundwater pumping or other anthropogenic influences), the natural inputs will balance the natural outputs so that the change in storage will be zero. Soil moisture storage may vary considerably on a daily basis but the net change (Δs) over an annual cycle will be negligible compared to other water budget components. Similarly, groundwater storage and land surface storage may fluctuate on a monthly or annual basis, but Δg and ΔI will approach zero (steady state) over an extended period of time provided other water budget components remain essentially constant. If Δs, ΔI and Δg equal zero, then substitution of equation (6) into equation (3) reveals that

Aquifer recharge, R = D + A                                                                       (7)

Substitution of equation (3) into (2) gives us

Stream flow discharge, SFD = P – AET – A                                                         (8)

If groundwater pumping is small, (i.e., A~0), then annual recharge can be equated to groundwater discharge and stream flow discharge will be the difference between precipitation and actual evapo-transpiration.

R = D                                                                                                                   (9)

The preceding quantification assumes the groundwater divides would have to correspond to a large degree to the surface water divides in a 3-dimensional sense and this depends on the size of the study area and the nature of the groundwater flow system. It is important to understand the relationship between the groundwater and surface water divides. Where these divides are not coincidental, groundwater inputs within the surface watershed may not be reflected in the groundwater discharge within the surface watershed.

21.3 Factors affecting Water Budget of a Watershed

The natural "water budget" accounts for all the water entering the watershed, how it travels through the watershed or is stored, and all the water leaving the watershed. The natural factors affecting water budget of a watershed are as below:

  1. rainfall and snowfall

  2. temperature and wind

  3. vegetation

  4. soil properties (surface and subsurface)

  5. stream and channel characteristics (vegetated or clear channels)

In addition to these natural factors, human activities and alterations of the watershed greatly affect the water budget. These factors include:

  1. How much water is pumped out of the ground or the river and where that water ends up ?

  2. How much water is stored in reservoirs and later pumped out ?

  3. How much water is "imported" from other watersheds ?

  4. How much water is "exported" to other watersheds or lost from the watershed (via water pipes, sewers, or through increased evaporation) ?

Keywords: Water Budget, Precipitation, Infiltration, Evapo-Transpiration, Runoff.


Suggested Readings

  • Carter, D. B. (1973). Applications of the Water Budget in Physical Geography: An annotated bibliography. CW Thornthwaite Associates.

  • Landsberg, H. E. (1983). Variations in the Global Water Budget. Boundary-Layer Meteorology, 27(4), 413-413.

  • Lin, S., & Gregg, R. (1988). Water Budget Analysis: Water Conservation Area 1. Water Resources Division, Resource Planning Department, South Florida Water Management District.

Last modified: Friday, 7 February 2014, 5:18 AM