Water Balance Estimation

In the natural environment, water is almost constantly in motion and is able to change state from liquid to a solid or a vapour under appropriate conditions. Conservation of mass requires that, within a specific area over a specific period of time, water inflows are equal to water outflows, plus or minus any change of storage within the area of interest. Put more simply, the water entering an area has to leave the area or be stored within the area. The simplest form of water balance equation is as follows:

                                P = Q + E ± ∆S

Where, P is precipitation, Q is runoff, E is evaporation and ∆S is the storage in the soil, aquifers or reservoirs.

In water balance analysis, it is often useful to divide water flows into ‘green’ and ‘blue’ water. ‘Blue’ water is the surface and groundwater that is available for irrigation urban and industrial use and environmental flows. ‘Green’ water is water that has been stored in the soil and that evaporates into the atmosphere. The source of ‘green’ water is rainfall or ‘blue’ water has been used for irrigation.

A Water Balance Analysis Can Be Used to:

• Assess the current status and trends in water resource availability in an area over a specific period of time.
• Strengthen water management decision-making, by assessing and improving the validity of visions, scenarios and strategies.

Water balance estimates are often presented as being precise. In fact, there is always uncertainly, arising from inadequate data capture networks, measurement errors and the complex spatial and temporal heterogeneity that characterises hydrological processes. Consequently, uncertainty analysis is an important part of water balance estimation as is quality control of information before used.

When the data sources are imprecise, it is often possible to omit components that do not affect changes. For example, it is possible to omit storage from an annual water balance if year-on-year storage changes (such as reservoirs) are negligible.

Some common problems that occur when water balance estimations are made include:

• Temporal and spatial boundaries are not defined.
• The quality of input data is poor.
• Double counting of water flows when water flows within an area added to water flow exiting area.
• Inappropriate extrapolation of field level information to a larger scale. Many hydrological relationships are scale dependent (e.g. runoff as a proportion of rainfall is almost always higher at smaller spatial and temporal scales).
• Intuition (often based on popular myths) is used rather than good quality information.
• The storage term(s) of the water balance is omitted.
• Political or other pressures result in unreliable estimates that have been manipulated.

Almost everyone is influenced by the water balance estimates because they are often central elements of awareness raising campaigns. Stakeholders directly involved in decision making may require more detailed information about the water available. Given that water balance analysis should be based on a needs assessment, everyone is involved in determining the outputs that are needed. The process of producing water estimates is best undertaken by experienced specialists or by staff who have undergone training and have access to specialist support.

Techniques for carrying out water balance estimation range from very simple ‘back of the envelope’ estimates to highly complex computer-based models. A sound knowledge of hydrological processes of a prerequisite of water balance estimation. It is often advisable for a project or programme to employ the services of a specialist to produce water balance estimates or, at the very last, to provide specialist advice as and when it is needed. Access to a quality-controlled information base is a good starting point for water balance estimation.

 

Step 1

Define boundaries of the area of interest and period over which water balance is to be made.

Step 2

Undertake a needs assessment of the water balance information that is required among stakeholders. This should be used as a starting point for listing and defining the water balance components that are of particular interest.

Step 3

Produce a simply schematic diagram that shows storage, inflows and outflows from the area of interests. Use the diagram as a basis for drafting a water balance equation. Check that no components are missing and that is no double counting of flows or storage.

Step 4

Identify a source(s) of quality-controlled information. Ensure that the information is relevant to the area and time period you are interested in.

Step 5

Carry out uncertainty and sensitivity analysis on this information. If there are inadequacies in the information (almost always the case), redefine the boundaries of the water balance and/or modify the water balance equation.

Step 6

Produce water balance estimates in a format that is useful for visioning, scenario building and planning with the community.

Step 7

Control these estimates regarding their quality before they are disseminated, using quality control methods described in this chapter.

Step 8

Disseminate findings to stakeholders in a form that they can understand (see also creating information material).

One tool with which you can carry out a professional water balance analysis is the WEAP (Water Evaluation And Planning) Tool – you find it here.