Watershed Planning and Management 3(2+1)

Lesson 22 Budgeting of Water in a Watershed

A water budget is a basic tool that can be used to evaluate the occurrence and movement of water through the natural environment. Water budgets provide a foundation for evaluating its use in relationship to other important influencing conditions such as ecological systems and features and social and economic components.  The water budget process can encompass various levels of assessment, which start simple and grow more complex if there are concerns about how much water is available at any level. The higher the ‘tier’, or level, the more complex the science involved and the narrower the geographic focus. Water budgets commonly go well beyond how much water is available and where it is. It also includes a 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. This overall interdependent understanding is necessary for sound water management. Water budget studies consider the volumes of water within the various reservoirs of the hydrologic cycle and the flow paths from recharge to discharge. Water budgets need to consider this information on a variety of spatial and temporal scales.

For an understanding of the hydrology of areas with little available data, a better insight into the distribution of the physical characteristics of the catchments is provided by image processing techniques. Some of the new measurement methods (photographic systems, active radar systems etc.) could yield assessment of areal distribution or at least to some extent reliable areal totals or averages of hydrologic variable such as precipitation, evapo-transpiration and soil moisture. Some of the main hydrological application fields of remote sensing are:

1. Topography

2. Water bodies

3. Vegetation

4. Spatial rainfall patterns

5. Evaporation and soil moisture

6. Snow cover extent

7. Groundwater assessment

Geographical Information System (GIS) is often utilized in hydrological studies by coupling it with hydrological models. Two types of approaches are possible for this purpose. In the model driven approach, a model or set of models is defined and thus the spatial (GIS) input for the preparation of the input data and output maps are required. The other approach is the data driven approach. It limits the input spatial data to parameters which can be obtained from generally available maps, such as topographic maps, soil maps etc. The possibility of rapidly combining data of different types in a GIS has led to significant increase in its use in hydrological applications. It also provides the opportunities to combine different data types from different sources. One of the typical applications is use of a Digital Terrain Model (DTM) for extraction of hydrologic catchment properties such as elevation matrix, flow direction matrix, ranked elevation matrix, and flow accumulation matrix.

In a natural state an unstressed basin experiences negligible long term changes in land surface, soil moisture and groundwater storage. However, this is not always the case. Also, groundwater flows as well as impacts of human activities can result in water moving between watersheds (i.e. inter-basin flow) and may be difficult to adequately quantify.

22.2 Measurements

In simple terms a water budget for a given area 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 + SWSW_in + GW_in + ANTH_in = ET + SW_out + ANTH_out + ΔS

Where;

P = Precipitation

SW_in = Surface Water Flow in

GW_in = Ground Water Flow in

ANTH_in = Anthropogenic or Human Inputs such as Waste Discharges

ET = Evaporation and Transpiration

SW_out = Surface Water Flow out

GW_out = Ground Water Flow out

ANTH_out = Anthropogenic or Human Removals or Abstractions

ΔS = Change in Storage (Surface Water, Soil Moisture, Ground Water)

22.3 Modeling Approach

A conceptual water budget model is first developed to obtain a basic understanding of the physical flow system. An initial synthesizing of the available data can be used to gain an appreciation of the various fluxes in the watershed. This initial work may indicate, where critical data gaps exist. The use of numerical modeling can provide a more refined understanding of the flow system including both surface and groundwater. Numerical models are tools used to simplify the representation of these processes and enable quantification and evaluation of the hydrologic system at various levels – watershed, sub-watershed and site scale. Although these models can provide hard quantitative values, it is important to recognize the uncertainty in numerical modeling and use the models appropriately in making water management decisions.

The most appropriate model for water budget analysis will depend primarily on the dominant flow processes (surface water or groundwater).  If changes in the groundwater discharge will significantly affect the flow of a river, then the model used should simulate the complexities of the groundwater system. If flow in the river is most affected by surface runoff and through flow during and following storm events, then the model must be able to simulate the complexities of the surface water processes.

The three basic types of numerical models that are built and used for water budget analysis are:

1. Surface Water Models

2. Ground Water Models      

3. Conjunctive or Integrated Continuum Models

Commonly an integrated approach is used, where output from both a surface water model and a groundwater flow model is iteratively compared. Traditionally, assumptions are made about all processes in a model. The processes of greatest interest are those that are explicitly represented in the model equations. The processes considered least important are treated as lumped processes and are specified as inputs or outputs to the model. They may be spatially variable but are not explicitly derived by equations in the particular model. In a groundwater flow model the recharge is input directly and is derived from field values or output from a surface water model.

Effective Application of a Numerical Model for Water Budget Analysis Requires:

1. definition of specific objectives of the analysis at the start

2. identifying the characteristics of the hydrologic system through development of a conceptual model (review existing reports: size, spatial variations, land use variability, topography, geologic structure, etc.)

3. determination  of the scale of the problem or the level of detail that needs to be included (e.g. micro-watershed versus large river basin)

4. determination  of the appropriate time scale

5. collection or compilation of sufficient data to evaluate each process

6. suitability for linkage to GIS

7. effective calibration and validation

8. recognition and minimization of the uncertainty in the analysis

9. re-evaluation of the applicability of the analysis prior to addressing new objectives.

Secondary Considerations Include:

1. available resources (e.g. training for model application)

2. model availability, preferably from an organization that provides regular updates and technical assistance.

Keywords: Watershed, Water Budget, Numerical Models, Flow Processes.

Suggested Reading

1. Shade, P. J., & Nichols, W. D. (1996). Water Budget and the Effects of Land-Use Changes on Ground-Water Recharge, Oahu, Hawaii (No. 1412). US Government Printing Office.

2. Timlin, D., Pachepsky, Y., Walthall, C., & Loechel, S. (2001). The Use of a Water Budget Model and Yield Maps to Characterize Water Availability in a landscape. Soil and Tillage Research, 58(3), 219-231.

3. Healy, R. W. (2010). Estimating Groundwater Recharge. Cambridge University Press.