WATER BALANCE FOR LAND DISPOSAL OF

PRETREATED WASTEWATER EFFLUENTS

Background

What IS a Water Balance?

Definition: A water balance is an assessment of the major components of a hydrologic system and includes the interactions between surface water and ground water systems. It provides a general understanding of the magnitude of recharge and discharge components.

Physical Mechanisms to Consider in a Water Balance

• When the effluent is applied to land:– It may enter the subsurface through infiltration– It may evaporate and return to the atmosphere

Where the Infiltrated Water Goes

• First, the effluent will hit the

– Where interparticle voids contain soil, moisture and air!

• Then it might enter the saturated zone, or• Return to the atmosphere by plants via

evapotranspiration

Other Water Balance Considerations

In addition to the applied effluent,

Don’t forget the rainfall!

Infiltration

Infiltration

• Definition: Infiltration is the movement of water through the soil

surface and into the soil itself.

Infiltration Rates

• Definition:The rate at which the water actually enters the soil!

• Are a function of:– Soil type– Effluent quality– Drying time between effluent applications– Etc.

Conservative Infiltration Rates *

Clay Soils

- Should not exceed 0.01 ft/day

Sandy Soils

- Should not exceed 0.03 ft/day

*Based on the infiltration surface being dried and disked/ripped at least annually.

Key Concerns

• Maintaining minimum 5 foot clearance from groundwater.

• Ensuring percolated effluent does not resurface in immediate vicinity (often due to land slope and impervious strata).

• Run-off• Groundwater mounding and/or lateral

movement of water due to impervious strata.

Evaporation

Evaporation• Definition:The transformation of water

from the liquid to the vapor state.

Source: http://www.videoweather.com/weatherquestions/What_is_evaporation.htm

How Do You Get Evaporation Data??

• Evaporation data from open water (PAN) surfaces are available from local/state authorities.

An evaporation

pan

Source: http://www.sws.uiuc.edu/atmos/statecli/Instruments/weather_instruments.htm

Calculating Site Specific Evaporation Rates

E: Design evaporation rate [in/month]

EP: PAN evaporation rate for month and location being studied [in/month]

CP: PAN coefficient to correct for excess evaporation. Usually 0.8. [-]

k: Weather correction factor

PP CEkE

Where,

Relationship Between Precipitation and PAN Evaporation

y = -0.0569x + 1.04 R2 = 0.1256

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0

Actual Precipitation [in/year]/Average Precipitation [in/year]

k=

Act

ual

Pan

Ev

apo

rati

on

[in

/yea

r]/A

ver

age

PAN

E

vap

ora

tio

n [

in/y

ear]

Source: Data points obtained from the Statewide IPM Program, Agriculture and Natural Resources, University of California (www.ipm.ucdavis.edu)

Obtaining Rainfall Data• Available from local/state/federal

authorities such as:– the Department of Water Resources– the University of California, Davis– National Weather Service

The following table shows both PAN and rainfall data obtained from UC Davis.

Example of PAN and Rainfall Data for Davis, CA

Latitude/Longitude: 38 deg 32 min N / 121 deg 46 min W Elevation: 60 ft

IPM records begin/end: June 5, 1908 /  about June, 2003 Ground cover: Bare soil

Precipitation Amount

Solar Radiation

PAN Evaporation

Max Min Max Min(F) (F) (F) (F)

January 0.13 53 37 158 49 45 0.05

February 0.13 60 40 253 54 49 0.09

March 0.10 65 43 384 61 54 0.15

April 0.03 72 45 519 70 61 0.25

May 0.02 81 50 614 80 70 0.35

June 0.01 88 55 690 88 78 0.44

July 0.00 93 56 694 94 82 0.44

September 0.01 88 53 500 85 76 0.33

October 0.03 78 48 356 74 67 0.22

November 0.08 63 41 222 59 54 0.11

December 0.09 54 36 155 50 46 0.06

Source: Statewide IPM Program, Agriculture and Natural Resources, University of California(www.ipm.ucdavis.edu)

Air Temperatures Soil TemperaturesMonth

County: Yolo

(in/day) (LY) (in)

(Calculated from daily averages)

Evapotranspiration

Source: http://wwwcimis.water.ca.gov/cimis/infoEtoOverview.jsp

What is Evapotranspiration (ET)?• A combination of two processes:

1) Evaporation – Loss of water from a vegetated field through vaporization of water from soil and plant surfaces.

2) Transpiration – Water, taken into the plant through the root system, passes through pores and evaporates into the atmosphere.

Evaporation + Transpiration =

Evapotranspiration

Factors Affecting ET

• Location

• Crop Type

• Season

• Irrigation Practices

Finding ET Information• Government agencies will usually have something

called the:

• ETo is a function of:– Location– Time– Weather

(or ETo)

CIMIS ETo Reference Crop

• The California Irrigation Water Management System (CIMIS)1 uses the following reference crop:– Grass

• Closely clipped

• Actively growing

• Completely shading the soil

• Well watered

1 www.cimis.water.ca.gov

Calculating Crop Specific ETo

ETo

ETcKc

Where,

kc: Crop coefficient (ratio of ETc to ETo)

ETc: For crop of interest[in/unit time]

CIMIS Normal Year ETo Zones for California

Source: http://wwwcimis.water.ca.gov/cimis/ images/etomap.jpg

CIMIS Map

• Divides the state into 18 zones• Provides average year Etos for each zone for each

month.• Estimated standard deviation: 0.01 in/month

Other Applications• Apply crop coefficients (kc) to ETo to get crop

specific ETc.• Crop coefficients can be found at CIMIS web site.

Other ET Facts

• Largest ETcs come from irrigated pasture grasses.

• Maximum ETc roughly 0.7 time PAN evaporation rate.

• Little to no ET may occur during field preparation, harvesting, or other operations.

EXAMPLE OF DESIGN OF A LAND DISPOSAL SYSTEM

The First Things You’ll Need to Know About Designing a Land Disposal System

• Location of land disposal system: – Davis, CA (surprise, surprise)

• Area Soil Type:– Predominately clay-based

• Subsurface conditions will not limit infiltration rates.

• Design for 1-in-100 year wet season.– Estimated AWWF1-100 ~ 1.2 MGD

• ADWF = 1.0 MGD

Some data needs to be collected…

Average Water Year Data for the City of Davis

Month O N D J F M A M J J A S Total

Days of the mont 31 30 31 31 28 31 30 31 30 31 31 30 365

Average Rainfall [in] 0.90 2.40 2.79 4.03 3.72 3.01 0.96 0.56 0.18 0.03 0.03 0.30 18.91

Average PAN Evaporation [in] 6.95 3.37 1.85 1.63 2.64 4.74 7.60 10.98 13.25 13.50 12.08 9.80 88.39

Average Eto [in] 4.03 2.10 1.55 1.55 2.24 3.72 5.10 6.82 7.80 8.68 7.75 5.70 57.04

Rainfall Depth-Duration-Frequency (DDF) Information

Source: California Department of Water Resources, Division of Flood Management, Hydrology Branch

Rainfall Depth Duration FrequencyStation Station No County Lat Long. Elev. SourceOb Time Yrs Rec Slope Intercept

Davis 2 WSW A00 2294 00 Yolo 38.535 -121.775 60 CD 105

Return Period for Rainfall For Indicated Number Of Concecutive Days

1 2 3 4 5 6 8 10 15 20 30 60 W-YRRP 2 1.75 2.34 2.71 2.99 3.27 3.53 3.96 4.27 4.96 5.50 6.63 9.59 16.95RP 5 2.36 3.24 3.79 4.19 4.56 4.88 5.44 5.85 6.73 7.51 9.00 12.93 21.68

RP 10 2.75 3.85 4.51 4.97 5.41 5.73 6.37 6.82 7.77 8.72 10.43 14.90 24.37RP 25 3.23 4.60 5.41 5.95 6.46 6.77 7.47 7.96 8.98 10.14 12.10 17.17 27.40RP 50 3.57 5.16 6.07 6.65 7.22 7.51 8.26 8.77 9.81 11.13 13.26 18.75 29.45

RP 100 3.91 5.71 6.73 7.35 7.97 8.22 9.01 9.54 10.60 12.06 14.36 20.23 31.35RP 200 4.24 6.25 7.37 8.03 8.70 8.92 9.75 10.28 11.35 12.96 15.42 21.65 33.15RP 500 4.66 6.95 8.21 8.92 9.66 9.83 10.69 11.23 12.30 14.11 16.78 23.44 35.40

RP 1000 4.98 7.49 8.85 9.59 10.38 10.50 11.39 11.93 12.99 14.95 17.77 24.75 37.02RP 10000 6.03 9.25 10.96 11.79 12.75 12.69 13.65 14.19 15.18 17.63 20.93 28.90 42.06

Step 1 – Development of Monthly Disposal Potential

• Some calculations need to be done:

]/[072.21001 monthinfallnAverageRaifallRain

2.072 = Value for RP 100 for 30 days (see previous slide) / Average value (found in Appendix O) = 14.36 in / 17.31 in

Average Rainfall: From Davis average water year data table.

Where,

Monthly 100 RP Rainfall Events:

1 in 100 Year Rainfall Event Pond Evaporation

]/[1001 monthinPANkCnEvaporatioPond P

Where,

Cp = 0.8 (typical value)

k = 0.922 (from Precipitation vs. PAN Evaporation chart, k = -0.0569[Average Precipitation] + 1.04)

PAN Evaporation: From water year data from the City of Davis

Monthly Infiltration

]/ [# ]/[ inf monthdaysofkmonthinonInfiltrati 12[in/ft]

Where,

kinf = 0.01 feet/day (from equation)

Losses or Gains from Pond and Disposal Area

PondsfromGainNetorlossNet

AreaDisposalfromGainNetorlossNet

onInfiltratipirationEvapotransfallRain

onInfiltratinEvaporatioPondfallRain

Ponds:

Disposal Area:

Voila! A Disposal Potential Table

Month O N D J F M A M J J A S Total

Raifall1-100 [in] 1.86 4.97 5.78 8.35 7.72 6.23 1.99 1.16 0.37 0.06 0.06 0.62 39.2

Pond Evaporation 1-100 [in] -5.13 -2.49 -1.36 -1.20 -1.95 -3.50 -5.61 -8.10 -9.77 -9.96 -8.91 -7.23 -65.2

Evapotranspiration1-100 [in] -4.03 -2.10 -1.55 -1.55 -2.24 -3.72 -5.10 -6.82 -7.80 -8.68 -7.75 -5.70 -57.0

Infiltration [in] -3.72 -3.60 -3.72 -3.72 -3.36 -3.72 -3.60 -3.72 -3.60 -3.72 -3.72 -3.60 -43.8

Net Loss (-), Gain(+) from ponds

area a [in]-6.98 -1.11 0.70 3.43 2.41 -0.99 -7.22 -10.66 -13.00 -13.61 -12.57 -10.21 -69.8

Net Loss (-), Gain(+) from disposal

area b [in]-5.89 -0.73 0.51 3.08 2.12 -1.21 -6.71 -9.38 -11.03 -12.34 -11.41 -8.68 -61.7

Step 2 – Analysis of Table of Disposal Potential

DISPOSAL

• Disposal area used from March through October

• Disposal potential of 61 in/year under 1-in-100 year conditions.

Step 2 (Cont.) – Analysis of Table of Disposal Potential

STORAGE/RUN-OFF

• Disposal area gains water from Dec. through Feb.– Must store rainwater or let it run off.– Run-off approach normally taken when there is

a large disposal area.• Requires that disposal area not receive effluent for

one month prior to run-off (for this case, November is reserved for drying out period).

Step 2 (Cont.) – Analysis of Table of Disposal Potential

STORAGE/RUN-OFF (CONT.)

• All effluent stored from November through February.

• Likely effluent storage in latter half of October.

• Likely effluent storage in March.

• Limited storage necessary in early April (based on monthly pond gain/loss)

Step 2 (Cont.) – Analysis of Table of Disposal Potential

• 1-in-100 year storage pond disposal:• –69.8 in/year

Step 3 – Estimation of Annual Flow, Storage Volume and Pond

Area

Annual Flow

Calculate Approximate Annual Flow:

7 months: Number of disposal months (April – October)

1.2 MGD: Flow during wet months (AWWF)

5 months: Number of storage months (November – March)

1.0 MGD: Flow during disposal months (ADWF)

YearMGMGDmonthdaysmonths /3902.1305 MGDmonthdaysmonthsFlowAnnualApprox 1307

Annual Flow and Storage Volume

Month Flow (MGD)Estimated Days Storing Water

Volume of Water Stored (MG)

October 1 15 15November 1.2 30 36December 1.2 31 37.2January 1.2 31 37.2February 1.2 28 33.6March 1.2 31 37.2April 1 15 15

TOTAL -- 181 211.2

Note: Flows used for this estimate are the flows provided in the initial assumptions. A more thorough approach must correlate rainfall and flow into the plant to include infiltration/inflow phenomena.

Estimating Pond Area• Assume typical pond depth of 12 feet

))736.083.8.(12(

)48.71()10211( 36

ftinAprilthruOctfromgainnetft

gallonsftgallonsAreaPondRough

acresft

acreft 5.57

560,43

1311,504,2

22

Step 4 – Estimation of Disposal Potential of Storage Volume

• Based on a net pond loss of 69.8 in/year

r69.8in/yea)ft (2,504,311 Volume Storage of Potential Disposal 2

MG109gal/ft 7.48ft/12in 3

About 109 MG of stored effluent can be disposed of per year.

Step 5 – Estimation of Disposal Area

• Need:– Disposal area capable of accommodating approximately:

PotentialDisposalVolumeStorageVolumeAnnualActual

yearperMGMGMG 2.2861092.395

Calculate Disposal Area

• Recall a net loss from disposal area of 60.97 in/year during March through October.

ft/12inin/season 60.97

gal /7.48ft)gal/season 10(286.2 Area Disposal

36

ac17343,560ft

act7,529,426f

22

Step 6: Complete the Water Balance

Further Calculations

acre

ft 43,560acres 173Area Disposal toFlow

2

Flow to Disposal Area:

month

in Area Disposal Potential Disposal

[MG] 10

MG

ft

gal 7.48

12in

ft63

Disposal Area from Storage

ac

ft 43,560ac 57.5Storage from Disposal

2

12in

ft

month

in Ponds Potential Disposal

[MG] 10

MG

ft

gal 7.4863

Net Flow to Storage

Flow TotalDisposal toFlow

[MG] Storage toDisposal

Storage toFlowNet

Month O N D J F M A M J J A S Total

Flow Rate [MG] 1.0 1.2 1.2 1.2 1.2 1.2 1.0 1.0 1.0 1.0 1.0 1.0 13.0

Days of the Month [days] 31 30 31 31 28 31 30 31 30 31 31 30 365.0

Total Flow [MG] 31.0 36.0 37.2 37.2 33.6 37.2 30.0 31.0 30.0 31.0 31.0 30.0 395.2

Flow to Disposal [MG] -28.6 0.0 0.0 0.0 0.0 -5.9 -32.6 -45.6 -53.6 -60.0 -55.4 -42.2 -323.9

Disposal from Storage [MG] -10.9 -1.7 1.1 5.4 3.8 -1.5 -11.3 -16.6 -20.3 -21.3 -19.6 -15.9 -109.0

Storage Net Flow [MG] -8.5 34.3 38.3 42.6 37.4 29.8 -13.9 -31.3 -43.9 -50.2 -44.1 -28.1 -37.7

Accumulation in Storage [MG] 34.3 72.5 115.1 152.5 182.2 168.4 137.1 93.2 43.0 -1.0

Water Balance Table

No Storage Needed

Storage Starts

Maximum Storage

Storage Dries

No Storage Needed

Storage Accumulation

• Storage starts accumulating in first month having net flow to storage.– In this example, November is the first month.– Can be October in wetter areas (e.g. in the

mountains, along the coast)

Initial Conclusions and Questions

• Maximum Storage Capacity Required:182.2 MGD

Why is this lower than preliminary estimate of 211 MG?– Preliminary estimate did not consider disposal

capacities of storage area.

What to do about it:– Consider extra capacity as safely buffer– Recalculate pond depth (which we will do later)

Step 7: Final Design Considerations for Land Disposal

System

DISPOSAL AREA

• Add 15% more area for:– Roads– Fences– Regulatory setbacks

acac 205.851.15acArea Disposal 206179

Types of Irrigation – Disposal Area

• Due to flatness of area around Davis, can attain flood irrigation.

• A sprinkler system can be selected if flood irrigation is not cost effective.

Sprinkler-type Tip:

Use an inline basket strainer after sprinkler pumps to remove material that might obstruct sprinkler nozzles.

Crop Considerations – Disposal Area

• Crop Type: Grasses– Need additional disposal land for drying and

harvestings

• Different crops– Use crop correction coefficient for

evapotranspiration rate calculations.

Other Disposal Area Considerations

• Must have run-off catchment with return to storage area (in case of accidental over-irrigation).– Prevents:

• Run-off to drainage and surface water.

Storage Area and Pond

• Add 20% more area for:– Levees– Roads– Fences

ac 691.257.5acPond and Area StorageFinal

Recalculation of Pond Depth

• Max Storage Volume: 182 MG (August)

ft 10 ft.

ac

43560ftac 57.5

gal /7.48ftgal) 10(182.2 htorageDeptS

2

36

739

Include at least 2 feet of freeboard:

ft 12Depth StorageFinal

Final Note

• Disposal/Pond system not used much in normal years!

Questions? Questions?? Anyone???