FE Exam – Water Budget

For the FE exam, visualizing different inflow and outflow parameters that contribute to the change in storage of a small watershed, lake, river, stream or any other water body is the most important step we need to understand before applying a water budget analysis.

The goal of this post is to help us look visualize these inflow and outflow values – allowing us to have a pictorial view before diving into practice problems.

What Does Water Budget Mean?

A hydrologic balance, or water budget, is the basic concept of the conservation of mass. In this case, the mass will simply be water.

The basic concept behind a water budget is that the rate change of water stored in a region or space is balanced out by the rate of water that flows into and out of the region. Hydrologic engineers use this concept to arrive at useful climate change predictions, water-resource sustainability guidelines, water management recommendations, and environmental planning.    

Because water can neither be created nor destroyed, a water budget requires that within a region in space that:

Water inflow – Water outflow = Change in stored water

To account for the inflow and outflow values, we can define the water budget in more complete terms by using the Surface Water System Hydrologic Budget as described on page. 290 in FE Handbook 10.1.

\begin{equation}
P+Q_{\text {in }}-Q_{\text {out }}+Q_{g}-E_{s}-T_{s}-I=\Delta S_{s}
\end{equation}
\begin{equation}
\begin{aligned}
&P=\text { precipitation } \\
&Q_{\mathrm{in}}=\text { surface water flow into the system } \\
&Q_{\mathrm{out}}=\text { surface water flow out of the system } \\
&Q_{\mathrm{g}}=\text { groundwater flow into the stream } \\
&E_{s}=\text { surface evaporation } \\
&T_{s}=\text { transpiration } \\
&I=\text { Infiltration } \\
&\Delta S_{s}=\text { change in water storage of surface water system }
\end{aligned}
\end{equation}

FE Exam Prep Course

Are you looking to learn more about the water budget with practical applications you can apply on your FE exam? Our FE exam prep course includes plenty of examples covering the water budget with full step by step solutions covering not only the calculation aspect but also the concepts.