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  k  EP  CP
Where,
E:
EP:
CP:
k:
Design evaporation rate
[in/month]
PAN evaporation rate for month and location
being studied
[in/month]
PAN coefficient to correct for excess
evaporation. Usually 0.8.
[-]
Weather correction factor
Relationship Between Precipitation
and PAN Evaporation
k =Actual Pan Evaporation [in/year]/Average PAN
Evaporation [in/year]
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
y = -0.0569x + 1.04 R2 = 0.1256
0.75
0.70
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]
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
Monthy Averages Report for DAVIS.C (NCDC #2294, Davis)
(Calculated from daily averages)
County: Yolo
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
Month
Precipitation
Amount
Air Temperatures
Solar
Radiation
Soil Temperatures
PAN
Evaporation
(in/day)
Max
(F)
Min
(F)
(LY)
Max
(F)
Min
(F)
(in)
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)
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:
(or ETo)
• ETo is a function of:
– Location
– Time
– Weather
CIMIS ETo Reference Crop
• The California Irrigation Water
Management System (CIMIS)1 uses the
following reference crop:
– Grass
•
•
•
•
1
Closely clipped
Actively growing
Completely shading the soil
Well watered
www.cimis.water.ca.gov
Calculating Crop Specific ETo
ETc
Kc 
ETo
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-DurationFrequency (DDF) Information
Rainfall Depth Duration Frequency
Station
Station No
Davis 2 WSW
A00 2294 00 Yolo
1
RP 2
1.75
RP 5
2.36
RP 10 2.75
RP 25 3.23
RP 50 3.57
RP 100 3.91
RP 200 4.24
RP 500 4.66
RP 1000 4.98
RP 10000 6.03
Return Period for Rainfall For Indicated Number Of
3
4
5
6
8
10
15
2.71 2.99 3.27 3.53
3.96
4.27 4.96
3.79 4.19 4.56 4.88
5.44
5.85 6.73
4.51 4.97 5.41 5.73
6.37
6.82 7.77
5.41 5.95 6.46 6.77
7.47
7.96 8.98
6.07 6.65 7.22 7.51
8.26
8.77 9.81
6.73 7.35 7.97 8.22
9.01
9.54 10.60
7.37 8.03 8.70 8.92
9.75 10.28 11.35
8.21 8.92 9.66 9.83
10.69 11.23 12.30
8.85 9.59 10.38 10.50 11.39 11.93 12.99
10.96 11.79 12.75 12.69 13.65 14.19 15.18
2
2.34
3.24
3.85
4.60
5.16
5.71
6.25
6.95
7.49
9.25
County
Lat
Long.
38.535 -121.775
Elev. Source Ob Time Yrs Rec Slope Intercept
60
CD
105
Concecutive Days
20
30
60
W-YR
5.50
6.63 9.59 16.95
7.51
9.00 12.93 21.68
8.72
10.43 14.90 24.37
10.14 12.10 17.17 27.40
11.13 13.26 18.75 29.45
12.06 14.36 20.23 31.35
12.96 15.42 21.65 33.15
14.11 16.78 23.44 35.40
14.95 17.77 24.75 37.02
17.63 20.93 28.90 42.06
Source: California Department of Water Resources, Division of Flood Management, Hydrology Branch
Step 1 – Development of
Monthly Disposal Potential
• Some calculations need to be done:
Monthly 100 RP Rainfall Events:
Rain fall1100  2.072  AverageRain fall[in / month]
Where,
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.
1 in 100 Year Rainfall Event
Pond Evaporation
Pond Evaporation1100  CP  k  PAN[in / month]
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
Infiltration [in / month]  k inf [# of days / month]
12[in/ft]
Where,
kinf = 0.01 feet/day (from equation)
Losses or Gains from Pond and
Disposal Area
Ponds:
Net loss or Net Gain from Ponds 
Rain fall  Pond Evaporation  Infiltration
Disposal Area:
Net loss or Net Gain from Disposal Area 
Rain fall  Evapotranspiration  Infiltration
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
-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
-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
Net Loss (-), Gain(+) from ponds
area a [in]
Net Loss (-), Gain(+) from disposal
area b [in]
Step 2 – Analysis of Table of
Disposal Potential
DISPOSAL
• Disposal area used from March through
October
• Disposal potential of 61 in/year under 1in-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:
Approx Annual Flow  7 months  30 days month1MGD
 5 months  30 days month1.2 MGD  390 MG / Year
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)
Annual Flow and Storage Volume
Month
October
November
December
January
February
March
April
TOTAL
Flow (MGD)
1
1.2
1.2
1.2
1.2
1.2
1
Estimated Days
Storing Water
15
30
31
31
28
31
15
Volume of Water
Stored (MG)
15
36
37.2
37.2
33.6
37.2
15
--
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
(211  10 6 gallons )  (1 ft 3 7.48 gallons )
Rough Pond Area 
(12 ft  (net gain from Oct . thru April  8.83 in  0.736 ft))
 2,504,311 ft 2 
1 acre
 57.5 acres
2
43,560 ft
Step 4 – Estimation of Disposal
Potential of Storage Volume
• Based on a net pond loss of 69.8 in/year
Disposal Potential of Storage Volume  (2,504,311 ft 2 )  69.8in/yea r 

 ft/12in   7.48 gal/ft

3
  109MG
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:
Actual Annual Volume  Storage Volume Disposal Potential 
395.2 MG  109 MG  286.2 MG per year
Calculate Disposal Area
• Recall a net loss from disposal area of 60.97
in/year during March through October.

(286.2 106 gal/season )  ft 3 /7.48 gal
Disposal Area 
60.97 in/season ft/12in 
ac
 7,529,426ft 
 173ac
2
43,560ft
2

Step 6: Complete the Water
Balance
Further Calculations
Flow to Disposal Area:
 43,560 ft 2 

Flow to Disposal Area  173 acres  
 acre 

 in  
  Disposal Potential Disposal Area 


 month  

 ft   7.48 gal   MG 


   6  [MG]
3
 12in   ft
  10 
Disposal Area from Storage
 43,560 ft 2 

Disposal from Storage  57.5 ac 
ac



 in    ft 
  Disposal Potential Ponds 
  


 month    12in 

 7.48 gal   MG 

   6  [MG]
3
 ft
  10 
Net Flow to Storage
Total Flow
 Flow to Disposal
 Disposal to Storage [MG]
Net Flow to Storage
Water Balance Table
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
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
-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
34.3
72.5
115.1
152.5
182.2
168.4
137.1
93.2
43.0
-1.0
Total Flow [MG]
Storage Net Flow [MG]
Accumulation in Storage [MG]
No Storage
Needed
Storage
Starts
Storage
Dries
Maximum
Storage
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
Disposal Area  179ac  1.15  205.85 ac  206ac
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 overirrigation).
– Prevents:
• Run-off to drainage and surface water.
Storage Area and Pond
• Add 20% more area for:
– Levees
– Roads
– Fences
Final Storage Area and Pond  57.5ac  1.2  69 ac
Recalculation of Pond Depth
• Max Storage Volume: 182 MG (August)


(182.2  10 6 gal)  ft 3 /7.48 gal
StorageDepth 
 9.73 ft  10 ft
2
 43560ft 

57.5 ac   
ac


Include at least 2 feet of freeboard:
Final Storage Depth  12 ft
Final Note
• Disposal/Pond system not used much in
normal years!
Questions? Questions??
Anyone???