5.1 Water Budgets
Water budgets can be applied at multiple scales to assess groundwater-surface water exchange for streams, lakes and wetlands. They account for all water sources, sinks and changes in storage within a defined boundary and time frame (e.g., Figure 1; Figure 38; Figure 49). Healy and others (2007) prepared an excellent public-domain publication on water budgets for resource and environmental management that addresses formulation of and methods used to quantify components of water budgets at various scales. Earlier work by Winter (1981) addressed the challenges of computing groundwater components of lake budgets. He cautioned against setting groundwater components as the budget unknowns because this approach causes budget uncertainties and errors to be incorporated in the computed (residual) groundwater exchange values. Winter (1981) stressed the importance of independently quantifying components of the water budget.
At an individual feature scale (reach of a stream, lake, and wetland) a water budget boundary and time interval are required when developing a water budget. The budget requires quantification of the effluent and influent exchange of groundwater with the defined budget boundaries (e.g., a watershed; shoreline of a surface-water feature). For example, the groundwater inflow for a lake setting (Figure 38) is defined by Equation 1 when using a lake water budget with the lake shoreline as the boundary. Each component requires quantification and an estimate of error (e.g., Healy et al., 2007).
|
Pin + Swin + GWin = GWout + SWout + Eout + ETout − S
|
(1) |
where:
| Pin | = | precipitation falling within the lake domain (L3/T) |
| SWin | = | surface-water flow into the lake (L3/T) |
| GWin | = | groundwater discharge to the lake (L3/T) |
| GWout | = | lake water leakage to groundwater (may include the flow of water to phreatophytes located near the shoreline) (L3/T) |
| SWout | = | surface-water flow out of lake (L3/T) |
| Eout | = | direct evaporation from the lake water surface (L3/T) |
| ETout | = | loss of water from plants within the lake by evapotranspiration (L3/T) |
| S | = | volumetric net change in storage of lake water, an increase in volume is positive and a decrease is negative (L3/T) |
Standard hydrology texts (e.g., Hornberger et al., 1998; Dingman, 1994; Watson and Burnett, 1993) and hydrogeology texts (e.g., Freeze and Cherry, 1979; Domenico and Schwartz, 2000; Fetter, 2001; Schwartz and Zhang, 2003; Fritts, 2012; Weight, 2019) discuss techniques to quantify each component of a water budget. Box 3 presents the derivation of a water budget for Mirror Lake in New Hampshire (Healy et al., 2007) and addresses estimation of error associated with the budget components. Though water budgets are valuable, they should be used in conjunction with other methodologies when finer resolution of site-specific exchange is needed.