Assignment: Applying Water Balance
The concept of water balance is discussed in the class lectures. Water balance is a very important concept in water resource management since determining water balance (also known as water budget) of a watershed, a surface water system such as lake, or a groundwater system is essential in managing water resources.
A well formulated water balance enables planners and policy makers to make informed decisions and manage water resources sustainably. A simple water balance equation is used to determine the change in storage or water level by taking into consideration the inflows and outflows of the system. Conceptually, outflows from the systems are deducted from inflows into the system to obtain the net change in storage. In real life it is important to make sure proper units are used in the calculations.
calculate water budget of a small watershed:
P+Qin =ET+∆S+Qout,
where ET is evapotranspiration (the sum of evaporation from soils, surface-water bodies, and plants),
P is precipitation,
Q in is water flow into the watershed,
∆S is change in water storage and
Qout is water flow out of the watershed.
Applied situation: A proposed subdivision plans to collect storm water runoff and divert it into detention basins. The developer submitted an application to the County proposing to create a new wetland as part of a mitigation plan. We need to determine if there will be a sufficient supply of water to maintain the functions of the new wetland.
The site is located in the southwest of the U.S. and the weather is generally warm. The topography has a mild slope and the wetland is designed to help reduce the pollution that comes with the surface runoff by increasing the residence time and allowing the dissolved solids to settle in the wetland ecosystem and allow for phytoremediation to take place by the plants in the wetland.
The results of environmental studies have indicated that the wetland needs to maintain a minimum of 1.00 foot of water at all times in order to function as designed. In the beginning, the wetland will be filled mechanically by about 6 inches of water and that serves as the starting water level for the months to come. The values are expressed as depth in inches over baseline elevation which is approximately the average distance from weir invert to average elevation of soil substrate in wetland.
You are responsible for reviewing the permit process and as part of the process you need to determine if there will be a sufficient supply of water to maintain the functions of the wetland.
The monthly data for the components of the water balance are provided in the table below.
Data are in units of inches October November December January February March April May June July August September
Initial Fill 6.00
Groundwater Inflow 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
Runoff 4.17 1.30 0.07 1.28 0.19 0.41 0.52 0.63 0.87 1.33 2.26 6.99
Precipitation 7.16 5.23 2.26 6.17 2.88 3.87 4.06 7.78 5.37 6.48 4.58 8.50
Evapotranspiration 2.45 0.43 0.03 0.00 0.11 0.51 2.38 3.37 5.90 5.81 5.39 4.04
Groundwater Outflow 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
Outflow 0.42 5.98 2.18 7.33 2.85 3.66 2.09 4.93 0.22 1.88 1.32 11.31
Note: The values in the table are expressed as depth in inches over baseline elevation which is approximately the average distance from weir invert to average elevation of soil substrate in the wetland.
A simple conceptual representation of a water balance model for the wetland could be developed as follows by carefully examining the figure provided below.
Figure Source: Woessner, W. W. (2020)
KEY
P: Precipitation
E: Evaporation
ET: Evapotranspiration
SWI: Surface Water Inflow
SWO: Surface Water Outflow
GWI: Groundwater Inflow
GWO: Groundwater Outflow
ΔSP: The change in storage of the wetland
For this wetland system, the basic formulation is as follows:
Wetland Water Level = Starting Water Level + Inflows into the system – Outflows from the system
For the first month of the record the starting water level will be the initial fill value provided in the table. For the subsequent months, the starting water levels will be the water levels estimated for the previous month.
The inflows into the system include groundwater inflow, surface runoff and direct precipitation input into the system. Groundwater outflow from the systems is also provided as well as surface outflow through a culvert. Potential evapotranspiration from the system is also provided in the table.
Note: Water balance could be estimated either as volume or depth of ponded water level. In order to estimate the depth of the ponded water level, the volume accumulated is divided by the area. In this exercise the conversion is done and the different parameters are presented in the same unit as depth of water to simplify our exercise. In most real-life problems, precipitation and evapotranspiration are provided in units of depth and the rest of the components are obtained as flow volume per unit of time.
Questions
1. Prepare a spreadsheet in Excel® using the data provided and define the water balance equation of the system using the Equations function of Excel® and estimate the water level in the wetland using the water balance equation you have plugged into Excel®. In the Excel spreadsheet, clearly identify the inflow components into the systems and the outflow components from the systems. The table should show the data provided plus estimated values of wetland water level and net precipitation for each month. Provide excel file as a deliverable. (40 points)
2. Create a plot that shows the various components of the water cycle in one graph. (20 points)
3. Prepare a separate Word® document and provide your recommendation regarding the permit application should it be permitted or denied? (20 points)
4. Clearly describe (in the Word® document) your observations regarding net precipitation. This is the difference between the monthly values of precipitation and evapotranspiration. (10 points)
5. Include citations and references you used for this assignment. (10 points)
Deliverables (See the points for each section in the Questions section)
1. Provide the Excel® spreadsheet file in which you performed the water balance analysis. The spreadsheet should have the calculation and the plot with clear labels showing the data and the units in the axes. It should also have a legend showing the labeling of the data.
2. Provide a written response (Word® document) that clearly describes the process followed to perform the analysis, the water balance equation you used for this system, the recommendations you made regarding the permit, and the observations you have about net precipitation and the special considerations that need to be put in place in certain months of the year. The written document needs to show in clear terms how you approached this problem and should not exceed a maximum of two pages.
The discussion provided in the Word® document should include the following:
• a descriiption of the equation used in the spreadsheet.
• the final water level in the wetland as estimated using the spreadsheet formula
• a plot of the estimated water levels created in the spreadsheet.
• the observation you have regarding the difference between monthly values of precipitation and evapotranspiration.
NOTE: The written response should read like a brief technical summary that clearly articulates your recommendation and additional considerations.
