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2 Background

3 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.

4 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

5 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

6 Other Water Balance Considerations In addition to the applied effluent, Don’t forget the rainfall!

7 Infiltration

8 Definition:Infiltration is the movement of water through the soil surface and into the soil itself.

9 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.

10 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.

11 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.

12 Evaporation

13 Definition:The transformation of water from the liquid to the vapor state. Source:

14 How Do You Get Evaporation Data?? Evaporation data from open water (PAN) surfaces are available from local/state authorities. An evaporation pan Source:

15 Calculating Site Specific Evaporation Rates E:Design evaporation rate[in/month] E P :PAN evaporation rate for month and location being studied[in/month] C P :PAN coefficient to correct for excess evaporation. Usually 0.8.[-] k:Weather correction factor Where,

16 Relationship Between Precipitation and PAN Evaporation Source:Data points obtained from the Statewide IPM Program, Agriculture and Natural Resources, University of California (

17 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.

18 Example of PAN and Rainfall Data for Davis, CA

19 Evapotranspiration Source:

20 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

21 Factors Affecting ET Location Crop Type Season Irrigation Practices

22 Finding ET Information Government agencies will usually have something called the: ETo is a function of: –Location –Time –Weather (or ETo)

23 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

24 Calculating Crop Specific ETo Where, kc:Crop coefficient (ratio of ETc to ETo) ETc:For crop of interest[in/unit time]

25 CIMIS Normal Year ETo Zones for California Source: images/etomap.jpg

26 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.

27 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.


29 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 AWWF 1-100 ~ 1.2 MGD ADWF = 1.0 MGD

30 Some data needs to be collected…

31 Average Water Year Data for the City of Davis

32 Rainfall Depth-Duration- Frequency (DDF) Information Source: California Department of Water Resources, Division of Flood Management, Hydrology Branch

33 Step 1 – Development of Monthly Disposal Potential Some calculations need to be done: 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:

34 1 in 100 Year Rainfall Event Pond Evaporation Where, C p = 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

35 Monthly Infiltration Where, k inf = 0.01 feet/day (from equation)

36 Losses or Gains from Pond and Disposal Area Ponds: Disposal Area:

37 Voila! A Disposal Potential Table

38 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.

39 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).

40 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)

41 Step 2 (Cont.) – Analysis of Table of Disposal Potential 1-in-100 year storage pond disposal: –69.8 in/year

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

43 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)

44 Annual Flow and Storage Volume 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.

45 Estimating Pond Area Assume typical pond depth of 12 feet

46 Step 4 – Estimation of Disposal Potential of Storage Volume Based on a net pond loss of 69.8 in/year About 109 MG of stored effluent can be disposed of per year.

47 Step 5 – Estimation of Disposal Area Need: –Disposal area capable of accommodating approximately:

48 Calculate Disposal Area Recall a net loss from disposal area of 60.97 in/year during March through October.

49 Step 6: Complete the Water Balance

50 Further Calculations Flow to Disposal Area:

51 Disposal Area from Storage

52 Net Flow to Storage

53 Water Balance Table No Storage Needed Storage Starts Maximum Storage Storage Dries No Storage Needed

54 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)

55 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)

56 Step 7: Final Design Considerations for Land Disposal System

57 DISPOSAL AREA Add 15% more area for: –Roads –Fences –Regulatory setbacks

58 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.

59 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.

60 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.

61 Storage Area and Pond Add 20% more area for: –Levees –Roads –Fences

62 Recalculation of Pond Depth Max Storage Volume: 182 MG (August) Include at least 2 feet of freeboard:

63 Final Note Disposal/Pond system not used much in normal years!

64 Questions? Questions?? Anyone???


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