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Week 2 Assignment: Applying Water Balance

Water balance is a very important concept in water resource management, since determining the water balance (also known as water budget) of a watershed, a surface water system such as a 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.

As a stormwater manager, you will review proposed permits or development projects to determine their potential impacts on watershed resources, water balance, and runoff.  When reviewing proposed development projects, you can use water balance to determine the impact on stormwater runoff, and/or suitable best management practices to be integrated into the designs. Make sure to use the correct units in the calculations.

For your assignment, please use this applied situation:

A proposed subdivision plans to collect stormwater 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 feet 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 the weir invert to the average elevation of soil substrate in the 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

Table 1: Data for your Water Balance Assignment calculations. 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 the 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.

What you must do:

1. Prepare a table in a spreadsheet (e.g., in Excel®) to calculate the water balance of the system estimate the water level in the proposed wetland, and complete the following tasks. (35 points possible)

In your spreadsheet, you must do the following:

1.  
   1. Begin by including the data provided in Table 1 (above) in your spreadsheet.
   2. Identify clearly in your spreadsheet which components are the inflow into the system, as well as which components are the outflow from the system.
   3. Calculate the water balance of the system to estimate the wetland water level each month, by using the equations functions of the spreadsheet. To do so, you should use the data provided, your notations of inflows and outflows.
   4. Include in the spreadsheet the net precipitation for each month. This is the difference between the monthly values of precipitation and evapotranspiration.
   5. Provide your Excel file as a deliverable.
2. Prepare a separate document and provide your recommendation regarding the permit application. In your document, you should include:
3.  
   1. Permit Recommendation Document (25 points possible). Write and submit a document with your written recommendations about whether the wetland permit request should be approved or denied, based on your background research of the site, the setting, and your calculations from the spreadsheet.  Your recommendations should include background information about the permit application, the geographic context and setting of the proposed development, including relevant factors in climate and the geographic region. Include all relevant factors, such as the purpose of an artificial wetland, how to maintain it, and whether this is an appropriate best management practice for this setting, climate, and context.
   2. Graph of water balance (10 points possible).  Create in the spreadsheet, and then integrate into your permit recommendation document, a graph (plot, etc.) that shows the various components of the water balance, including your results of the wetland water level, in one graph. Integrate this as a figure in your Permit Recommendation Document and include a descriptive title and caption, as well as refer to it in the text of your document. All axes must be labeled. Include a key (legend) for the variables you plot on your graph.
   3. Net precipitation calculation and description (5 points possible). Clearly describe in your Permit Recommendation Document your observations regarding net precipitation in your calculations each month of the year and its significance to your permit recommendations. Net precipitation is the difference between the monthly values of precipitation and evapotranspiration

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 description 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.