OUTFLOW COMPONENTS

Evapotranspiration (EVAP)

Evapotranspiration is the result of surface water and soil evaporation and transpiration from the plants within the depressional basin.

Since the precise period when the seven day ponding

or fourteen day saturation will change from year to year, it cannot be assumed that the evaporation or evapotranspiration rate will the the maximum rate during the growing season.

Some Sources of EVAP Data

NRCS Field Office Technical Guide (FOTG) or in the Irrigation Field Guide.

The FAO Modified Blainey Criddle method (FAO Paper 24)

Annual Free Water Surface Evaporation map in the NOAA Tech. Report Nation Weather Service (NWS) 33, “Evaporation Atlas for the Contiguous 48 United States”

Conversion of Mean Annual Evaporation

To facilitate conversion of the average annual evaporation rate to monthly rates, a percentage-by-month can be generated using NOAA Technical Report NWS 34, “Mean Monthly, Seasonal and Annual Pan Evaporation for theUnited States,: dated December 1982.

Steps for EVAP Chart

1. The first column of the chart are the months

2. Obtain the monthly percentage

3. Compute the average monthly average EVAP

4. Compute the average daily EVAP rate for each

month.

5 a) compute the EVAP for the number of days for

the critical duration

b) Sum the EVAP for the total number of months and divide this number by the number of months used.

Example 6Calculating Average EVAP

For the site used in Example 1, A site located at the southeast corner of Morton County, Kansas

Step 1. the Average Annual Evaportation is:

67 inches

Calculating Average EVAP continued

Step 2. The WETS Table shows the 50% chance, 28 degree killing frost free period to be 4/9 thru 10/26. The monthly percentages for Kansas are as follows:

Month Percent

April 9

May 12

June 14

July 15

August 14

Sept. 10

October 7

Calculating Average EVAP continued

Steps 3, 4 & 5: Figure for 7-Day Ponding (critical period = 7 days)

Month Daily Ave. 7-Day Ave.

Month % (INCHES) (INCHES) (INCHES)

April 9 6.03 0.20 1.41

May 12 8.04 0.26 1.81

June 14 9.38 0.31 2.19

July 15 10.05 0.32 2.69

August 14 9.38 0.30 2.12

Sept. 10 6.70 0.22 1.56

Oct. 7 4.69 0.15 1.10

Total 54.27 1.76 12.88

Daily Average: 0.25 in. 7-Day Average EVAP: 1.84 in.

Soil WATER (SW)

The soil moisture is the total water that the soil can hold when saturated, without any water standing above the soil surface.

Available Water Holding Capacity

AWC can be used for the total soil moisture, unless one believes that the soil is at wilting point (15 bar pressure) at the time that the critical period begins. » If this is the case then additional water

would need to be added to go from 1/10 bar to soil saturation.

Example 7 Soil Water Holding Capacity

HORIZON DEPTH (in/in) (in)

A 0 - 8 0.21 1.68

Bt 8 - 12 0.12 0.48

Total 2.16

» If the 10 percent additional water is added

SW = AWC x 1.10 = 2.16 x 1.1 = 2.38 in.

Deep Percolation

Any water movement starting at the ground surface and moving down below the root zone is classified as deep percolation

Example 8 -- Deep Percolation

PERMi = perm. rate (in/hr) x 24 hr/da x (i) days

i = critical duration for deep perc.

for i = 7 days

PERM7 = perm. rate (in/hr) x 24 hr/da x 7 days perm. rate = 0.002 in/hr

= 0.002 in/hr x 24 hr/da x 7 days = 0.34 in.

Outflow Equation

Outflow = (EVAPn + SW + PERMi ) x Pa

where n = number of days evap

i = critical duration for infiltration

Pa = depression bottom surface area

Example 9 Outflow Calculation

Using the site used in Example 1, located at the southeast corner of Morton County, Kansas, with a depression bottom surface area of 1.0 acres

Outflow = (EVAP7 + SW + PERM7) x Pa

= (1.84 + 2.38 + 0.34)in. x 1.0 ac = 4.56 ac-in

Example of aWater Balance

Water Balance

For hydrology component of Wetland Delineation to meet Inundation:

Inflow - Outflow>=0.00 ac-in (for specific period)

Inflow = (RO / Ps ) + (Rd x Pa )

Outflow = (EVAPn + SW + PERMi ) x Pa

Example 10 -- Water Balance

Inflow = (5.00 / 1.13) + (2.95 x 1.0) = 7.37 ac-in

Outflow = (1.84 + 2.16+ 0.34)in. x 1.0 ac. = 4.34 ac-in

w/ 30% additional to AWC

= (1.84 + 2.38+ 0.34)in. x 1.0 ac. = 4.56 ac-in

Inflow - Outflow = 7.37 - 4.34 = + 3.03 ac-in

w/ 30% additional to AWC

Inflow - Outflow = 7.37 - 4.56 = + 2.81 ac-in

Maximum Depression Surface Area

Pa-max = DA x (50% chance RO) = _____ Acres

(EVAPn+SW+PERMi - Rd ) x Ps

Pa-max = Maximum Acres allowed in Wetland with

existing inputs

Example 11 - Maximum Depression SurfaceArea

Pa-max = ( 25 x 0.20 ) = 6.9 Acres

(1.84+ 2.38+0.34 - 2.95 ) x 1.13

Minimum Drainage Area (AD)

AD = [(EVAPn + SW + PERMi - Rd) x Ps RO

Example 12 Minimum Drainage Area

Usng Example 10 and asuming 10% addition to AWC

AD = (1.84 + 2.38 + 0.34 - 2.95) x 1.13

0.20

= 9.1 acres/wetland acre

Example 13 -- Kimball Co.

The site is a playa depression Given: RO = 0.08 ac-in/ac RCN = 81 DA = 34 acres Rd = 2.26 in. EVAP = 27 in/year ( make chart below) PERM = 0.003 in/hr SW = 2.36 in.

Minimum Ponding ...

Given the above informatiion, what is the

minimum ponding depth required to offset losses

for seven (7) days?

Min. water depth = Total Outflow (losses)

Outflow = (EVAP7 + SW + PERM7 ) x Ps

= (0.87+2.36+0.50) x 1.13 = ____

in.

Example 13 -- EVAP

MONTH % OF Annual Monthly Et 7-days Et

BY MONTH INCHES INCHES APRIL 6 ______ ______ MAY 13 ______ ______ JUNE 19 ______ ______ JULY 25 ______ ______ AUGUST 21 ______ ______ SEPTEMBER 13 ______ ______ OCTOBER 3 ______ ______ TOTAL ______ AVERAGE Evapotranspiration (EVAP7)

______(total/7mo.)

Minimum Ponding ...

Given the above informatiion, what is the

minimum ponding depth required to

offset losses for seven (7) days?

Min. water depth = Total Outflow (losses)

Outflow = (EVAP7 + SW + PERM7 ) x

Ps

= (0.87+2.36+0.50) x 1.13

= ____ in.

Example 13 Minimum Ponding Depth

Given the above informatiion, if surveying the depression to find the wetland surface area, what is the minimum ponding depth required to offset losses for seven (7) days?

Min. water depth = Total Outflow (losses)

Outflow = (EVAP7 + SW + PERM7 ) x Ps

= (0.87+2.36+0.50) x 1.13 = ____ in.

Example 13 -- Problem

What is the maximum playa depression near level bottom surface area that will meet the wetland criteria of 7 days ponding and minimum drainage area per acre of wetland??

Use a depression shape factor Ps of 1.13

Example 13 -- Max. Wetland Area

Pa-max = DA x (50% chance RO) = ____ ac

(EVAP7+SW+PERM7 -Rd)xPs

Pa-max = 34 x 0.08 = ____ ac

(0.87+2.36+0.50 -2.26)x1.13

Example 13 -- Minimum AD

AD = [(EVAP7 + SW + PERM7 - Rd) x Ps

RO

AD= (0.87+2.36+0.50 - 2.26) x 1.13 = ____ ac

0.08

Questions and Review

Enduntil next time