WATER BUDGET ANALYSIS FOR THE EVERGLADES AGRICULTURAL AREA DRAINAGE BASIN

WATER RESOURCES BULLETINVOL. 30, NO.3 AMERICAN WATER RESOURCES ASSOCIATION JUNE 1994

WATER BUDGET ANALYSIS FOR THE EVERGLADESAGRICULTURAL AREA DRAINAGE BASIN'

Wossenu Abtew and Nagendra Khanal2

ABSTRACT: Water budget studies are essential for water resourcesand environmental management. In this study, a water budgetanalysis is presented for the Everglades Agricultural Area (EAA) inSouth Florida for the period from 1973 to 1991. The EAA is a high-ly productive irrigation/drainage basin that has a high water tableand organic soils. Water quality problems are associated with thedrainage discharge from the basin. During dry periods, supplemen-tal water is used for irrigation and in rainy periods excess waterwith relatively higher phosphorus content is pumped out of thebasin to Lake Okeechobee and the Everglades ecosystem. Elevatedconcentrations of phosphorus in the runoff/drainage that is dis-charged from the EAA basin have created water quality problems.The mean surface water inflow to the basin was 63,990 ha-rn, andthe outflow was 131,447 ha-rn per year. On the average, supple-mental surface water use was 47,411 ha-rn, and runoflYdrainagewas 114,816 ha-rn per year. The mean annual basin rainfall was120.9 cm. A general trend in the decline of the wet season rainfallis observed.(KEY TERMS: agricultural hydrology; drainage; irrigation; watermanagement; water quality; Lake Okeechobee; water budgetmodel.)

INTRODUCTION

The Everglades Agricultural Area (EAA) basin islocated in South Florida between Lake Okeechobeeand the Everglades ecosystem (Figure 1). The basinarea is over 2,849 square km, lying in western PalmBeach, eastern Hendry, and western Broward Coun-ties (Cooper, 1989). The area analyzed in this studyincludes the subbasins S2, S3, S5A, S6, S7, and S8,which cover 2,400 square km (239,971 ha). Richorganic (muck) soils that range from a few centime-ters to more than 2.44 meters in depth lie over a rela-tively impervious layer known as the Fort Thompsonlimestone. Lake Flirt marl, an impervious layer, lies

between the muck and limestone over most of thearea (Gallatin and Henderson, 1943).

Originally, the EAA was part of the natural Ever-glades system that consisted of custard apple and wil-low-elderberry swamps and sawgrass marshesinterspersed with tree islands, wet prairies, andsloughs (Davis, 1943). The flat swamps that gentlysloped to the south have been drained to provide oneof the nations richest agricultural basins. Gravitydrainage started in 1916 and drainage by pumpingcommenced in 1925, resulting in soil shrinkage andsubsidence of as much as 4.3 cm per year when thewater table depth is 91 cm below the surface and 1.0cm per year when the water table depth is 30.5 cmbelow the surface (Clayton, 1943). In 1916, at a benchmark at an area about 6.4 km south of Lake Okee-chobee, at the intersection of North New River Canalwith the Bolles Canal, the sea level elevation of thesurface of the land was 6.1 m (Allison, 1939). At pre-sent the elevation of the area is about 3.97 m (SouthFlorida Water Management Model, 1993).

The commercially grown crops in the EAA are sug-arcane, vegetables, sod, rice, ornamental nurseryplants and improved pasture. At present, approxi-mately 84 percent of the agricultural production areais under sugarcane production. Close to 11 percent ofthe production area is used for year round vegetableproduction and about 5 percent for sod production andimproved pasture. About 6 percent of the fallow sug-arcane land is used for rice production, which is arotation crop. Wetlands cover 12 percent of the studyarea.

The basin is serviced by a complex network of pri-mary and secondary water supply, flood control, irri-gation and drainage systems. The primary canals

1Paper No. 93079 of the Water Resources Bulletin. Discussions are open until February 1, 1995.2Respectively, Senior Civil Engineer and Senior Professional, South Florida Water Management District, 3301 Gun Club Road, West Palm

Beach, Florida 33416.

429 WATER RESOURCES BULLETIN

Abtew and Khanal

Figure 1. Everglades Agricultural Area Flow Control Structures and Rainfall Stations.

(Miami, North New River, Hillsboro, and West PalmBeach) are approximately 16 km apart and run fromnorth to south and southeast (Figure 1). The Bollesand Cross canals run east and west and interconnectthe four primary canals. With the aid of pump sta-tions and water control spiliways, water can be movedeither north or south according to complex watermanagement and flood control schedules that includeLake Okeechobee and the Kissimmee River System tothe north, the Water Conservation Areas, the lowereast coast, and the rest of the Everglades ecosystemto the south. The South Florida Water ManagementDistrict and the Corps of Engineers are responsiblefor operation and maintenance of the primary system.The primary system is designed to remove 1.9 cm ofexcess water per day from the EAA.

The secondary system consists of farm pump sta-tions, mains, farm laterals, field ditches, and culverts

WATER RESOURCES BULLETIN 430

that are owned and operated by the farmers. Thepurpose of the secondary system is to supply supple-mental water from the primary canals to the farmsand to remove excess water (drainage/runoff) from thefarms to the primary canals. The water table is high,and subsurface storage of water in the soils is limited.Generally, the stages in the primary canals are higherthan the water levels in the farm mains, and irriga-tion water is withdrawn by gravity, while excesswater is pumped out.

The excess water (drainage/runoff) that leaves thefarms has higher phosphorus concentrations thanunimpacted areas, such as interior marshes in theWater Conservation Areas (Lake Okeechobee Techni-cal Advisory Committee, 1989). The phosphorus inthe drainage water has resulted in acceleratedeutrophication of Lake Okeechobee and in degrada-tion of the Everglades ecosystem (State of Florida

j1o

MILES

Water Budget Analysis for the Everglades Agricultural Area Drainage Basin

Department of Environmental Regulation, 1986;South Florida Water Management District, 1992a).For the period from 1978 to 1991, an estimated meanannual load of 201.6 metric tons of total phosphoruswas transported by EAA drainage/runoff water with85% going to the south (South Florida Water Manage-ment District,1992b). Periodic water budget analysisof the area is essential for water resource manage-ment. In the past, studies were conducted on supple-mental water use in the Everglades AgriculturalArea. Mierau (1974) analyzed water use in the areaand estimated a dry period irrigation applicationmean rate of 21.3 cm for the period 1962 to 1972.Ortel (1992) reported a mean irrigation application of5.3 cm in the wet season for the period 1971 to 1987.

In this study, a detailed water budget analysis ofthe EAA is presented for the period from January1973 to September 1991. Time series and seasonalcharacteristics of water budget parameters are docu-mented; a method for parameter calculation from theSouth Florida Water Management District databaseis developed; and a brief trend analysis of monthlyrainfall is included. The rates of drainage/runoff gen-eration, supplemental water use, net water yield gen-eration, basin inflows, basin outflows, rainfall, andevapotranspiration are quantified. The objective ofthis study is to present the hydrologic system of theEverglades Agricultural Area and the detailed waterbudget for the period 1973 to 1990 using the databaseof the South Florida Water Management District.

WATER BALANCE COMPONENTS

This water balance study is a systematic represen-tation of measured, derived and estimated hydrologicparameters for the EAA basin. The study basin iscompletely levied around its perimeter, and theinflows and outflows are gaged (Figure 1). Verticalseepage loss is assumed to be negligible, as the layerunder the muck is relatively impermeable. Net hori-zontal seepage is also assumed to be negligible for thewater budget analysis.

Rainfall in the EAABasin

The South Florida Water Management Districtdatabase, DBHYDRO, was the source of daily rainfalldata for nine rainfall stations in and around theEAA that were used for the water budget analysis(Figure 1). Missing daily data for periods of not longerthan one week were estimated based on the nearestneighbor method, where the value of the nearest

station was used as the estimate for the missing day.Basin areal rainfall was estimated using the Thiessenmethod (Shih and Hamrick, 1975).

In the EAA, the wet season extends from the begin-ning of June to the end of October and accounts for,on the average, 66 percent of the annual rainfall. Wetseason rainfall is from convective rainfalls, localizedthunderstorms, tropical depressions, and hurricanes.The dry season (November through May) rainfall ismainly frontal rainfall, and on average comprises 34percent of the annual rainfall. The mean historical(1929-1990) annual rainfall for the EAA was 132.9cm: 45.6 cm for the dry season and 88.4 cm for thewet season.

The mean annual rainfall for the study period(January 1973 to September 1991) was 120.9 cm (dryseason 44.8 cm and wet season 77.8 cm). Relative tothe historical data, a decline in wet season rainfallwas observed. The deviation of rainfall for the studyperiod from the historical mean is —10.6 cm for thewet season and 0.26 cm for the dry season.

Statistical analysis was performed on the monthlyvalues to determine if there was a significant differ-ence in monthly rainfall between the early 31 years(1929-1959) of record and the recent 31 years (1960-1990). A least significance difference (LSD) test at 5percent significance level (a = 0.05) was performedbetween the means of the two groups for each month,with the assumption that each group of data is inde-pendent (Table 1).

In flows and Outflows

As stated earlier, the EAA is levied on all sides,and inflows and outflows are gaged. Inflows are fromLake Okeechobee, Hendry County, the three WaterConservation Areas (WCA1, WCA2, WCA3), and fromthe S5A complex. The S5A complex is a water controlsystem on the eastern limit of the EAA at the junctionof the L8 and West Palm Beach canals. It is composedof one pump station, three gated structures, and anopen connection to the L8 canal. The S5A complex canobtain water from the West Palm Beach Canal, the L8canal, WCA1, or any combination of these threesources. Outflows from the EAA occur to Lake Okee-chobee, to the S5A complex, and to the three WaterConservation Areas. The outflow from S5A complexcan flow to the West Palm Beach Canal, the L8 canal,WCA1, or any combination of the above.


Abtew and Khanal

TABLE 1. Means and Standard Deviations (SD) of Monthly Rainfall in theEverglades Agricultural Area in cm (1929- 1990).

Month1929-1990 1929-1959 1960.1990 LSD

Mean SD (a = 0.05)Mean SDMean SD

January 4.42 3.92 4.30 4.39 4.54 4.48 1.44

February 4.72 3.65 4.47 3.66 4.98 3.71 1.34March 7.13 5.96 7.42 5.82 6.84 6.18 2.19

April 6.61 4.67 8.20 4.79 5.02 4.03 1.62**

May 12.30 5.35 12.17 4.71 12.43 6.01 1.97June 21.64 9.40 22.25 10.73 21.03 7.97 3.45

July 18.87 5.43 19.41 5.77 18.32 5.11 1.99

August 18.65 5.45 19.22 6.00 18.07 4.88 2.00

September 19.06 7.91 21.22 7.88 16.91 7.46 2.80**October 10.48 6.50 12.15 6.95 8.82 5.66 2.31**November 4.81 4.43 4.69 4.71 4.93 4.19 1.63December 4.31 4.39 4.61 5.13 3.99 3.56 1.61

*Data for 1929-1972 (Sculley, 1986)**sigthficant decline.

Water Management

In the EAA, water is managed by farm operatorsaccording to their needs. Generally, different cropsrequire different depths of the water table at eachstage of growth. Apart from crop growth, there areother factors that determine the depth of the watertable on the farms. The water table may be raised tothe ground surface level to control muck soil oxidationand subsidence, fire, frost, and for insect control.Drainage is usually initiated by excess rainfall occur-rence, anticipation of excess rainfall, and desiredchanges in preferred water table depth. The optimumwater table depth below the ground surface for sugar-cane is 45 to 60 cm, 30 cm for sod, and more than 60cm for vegetables (Stone and Legg, 1992).

WATER BALANCE MODELS

The EAA is an intensive agricultural area that hasa network of water supply, drainage, and flood controlsystems. Inflows to the EAA are composed of irriga-tion demand (supplemental water use) and flow-through. Supplemental water use is defined as thequantity that supplements rainwater to meet cropwater requirements, to raise the water table, or tosatisfy any other water demand in the EAA basin.Flow-throughs are releases from Lake Okeechobee tothe south and southeast through the Miami, NorthNew River, Hilisboro and West Palm Beach (wpb)canals. These releases can be used for flood control,water supply, or canal and Water Conservation Areastage maintenance.

The outflows are composed of the flow-through anddrainage that is pumped out of the farms into the pri-mary canals. Net yield is the volume of water that iscontributed by the EAA to the region's surface water.Net yield is the difference between drainage and sup-plemental water use. A schematic model of the hydro-logic system of the EAA and the discharge capacity ofthe water control structures is shown in Figure 2. Thegeneral water balance equations for the EAA basinwater budget analysis are presented as follows:

STORAGE = INFLOW- OUTFLOW (1)

where STORAGE = change in water storage in thebasin, INFLOW = total inflow into the basin, andOUTFLOW = total outflow from the basin;

INFLOW = QL + Q WCAs + QS5A + QL1 + QL2

+ RAIN + SEEPAGE (2)

where QL = Lake Okeechobee inflow, QWCAS = WaterConservation Area inflows, QS5A = S5A complexinflows, QL1 = inflow from Hendry County throughstructure G136, QL2 = inflow from Hendry Countythrough structure G88, RAIN = rainfall over thebasin, and SEEPAGE = seepage into the basin;

OUTFLOW = QL + Q WCAs + Qs5A

+ ET + SEEPAGE

(3)

where QL = Lake Okeechobee outflow, Qwc = WaterConservation Area outflows, QS5A S5A complex out-flows, ET = weighted evapotranspiration losses basedon land uses, and SEEPAGE = seepage out of thebasin;



Structure Type

S2 Pump

S35 I

S6

Spillway

Pump

Si Pump

BoxCulvert

S150 Culvert

S5A Pump

S5AE Box Culvert

S5AS Spillway

S5AW Box Culvert

S352 Spillway

S3 Pump

S354

S8

G136

Spillway

Pump

Culverts

Culvert

Design Discharge(m3/s)

101.88 (4 units)

67.92

82.78 (3 units)

70.47 (3 units)

11.32

28.30

135.84 (6 units)

19.81

56.60

19.81

35.38

75.56

56.60

117.73

14.15

24.06

ET= (E(.))where, ET = evapotranspiration loss, i = crop type orland use, A1 = area covered by crop or land use i, andA = total area.

The daily difference between inflows and outflowsat the two ends of the major canals is used to calcu-late supplemental water use and drainage for thearea serviced by each canal. Since the North NewRiver and Hillsboro canals join near Lake Okee-chobee, daily difference in inflows and outflows arecalculated together. Daily flow data in the SouthFlorida Water Management District database, DBHY-DRO, has the following conventions for the daily flowsthrough the various structures: (a) inflows from LakeOkeechobee to the EAA are positive while backflowsfrom the EAA to the lake are negative [S2, S3, S351,S352, S354(HGS5)]; (b) outflows from the EAA to theWater Conservation Areas and the S5A complex arepositive, and backflows to the EAA are negative (S5A,S5AW, S6, S7, S8, S150); (c) inflows from HendryCounty to the EAA are positive (G88, G136), andthere are no outflows to Hendry County.

The daily water budget equations for each canal(4) are as follows.

8miami = MAX [0, (Qs3 + Qc88 + QG136 — Qs8)1 (5)

Dmiami = MIN [0, (Qs3 + QG88 + QG136 — QS8)] (6)

Fmiami = MIN ([MAX(0, QS3),

MAX (Qss — QG88 — QG136), 0]) (7)

Shilisboro = MAX [0, (Qs2 Qs6 —Qs7— Qs15o)} (8)

Dhillsboro = MIN [0, (QS2 — Qs6 — Qs7— QS150)1 (9)

Fhillsboro = MIN ([MAX (0, Qs2)1,

[MAX (0, Qs6 + QS7 + Qs150)]) (10)

Spb = MAX [0, (QHGS5 — QS5A + QS5AW)] (11)

Dwpb = MIN [0, (QHGS5 — Qs5A + Qs5Aw)1 (12)

Fb = MIN ([MAX (0, QHGs5)],

[MAX 0, (Qs5A + QS5Aw) 1) (13)


G88 Culvert

Figure 2. Schematic Model of the Hydrologic System of the Everglades AgriculturalArea and Discharge Capacity of WaterControl Structures.

Abtew and Khanal

where S = water for supplemental use drawn from thespecific canal, D = drainage/runoff outflow throughthe specific canal, F = flow-through the specific canal

Subscripts miami, hilisboro, and wpb (West PalmBeach) indicate canals where water is drawn or pass-ing through; Q is discharge through the structureshown in subscript (Figure 2).

Within the annual cycle of agricultural operations,the annual changes in storage and the net seepageare assumed to be zero, and annual ET losses can becalculated as follows:

ET = RAIN + CANAL INFLOW

- CANAL OUTFLOW.

RESULTS AND DISCUSSION

The water balance model as described in Equations(1) through (14) was used to complete the water bud-get study for the period from January 1973 toSeptember 1991. Surface water inflows, outflows,runoff, supplemental water use, and flow-through thearea are computed. The EAA is an area with a shal-low water table where excess water has to be pumpedinto the primary canals, as no substantial retention orwater storage facilities exist. The excess water comesfrom current excess rainfall, from previously storedrainfall, or from irrigation water pumped out to lowerthe water table.

In this study, areal rainfall was calculated usingthe Thiessen method based on data from nine pointrain gages. Basin inflows and outflows are measureddata from the South Florida Water Management Dis-trict database, DBHYDRO. Supplemental water use,drainage/runoff, net basin yield, flow-through, andbasin consumptive use were calculated from equa-tions 1-14. Dry season (November through May) andwet season (June through October) analyses are pre-sented separately for the benefit of understandingseasonal variations in the water budget parameters.

Rainfall

In the water budget analysis of the EAA, it isappropriate to use total rainfall as effective basinrainfall, since net seepage and deep percolation lossesfrom the system are assumed to be negligible. Theirrigation and drainage network is designed to supplyirrigation water on demand and to remove excesswater. Therefore, any rain in the basin is eitherstored in the soil to satisfy evapotranspiration and/or

to raise the water table, or it generates drainage. Theannual weighted rainfall in the area during the studyperiod ranged from 99.7 cm (1990) to 152.2 cm (1978)with a mean of 120.9 cm per year and a standarddeviation of 16.1 cm.

Canal Inflows and Outflows

Annual inflows from canals into the EAA basin forthe study period ranged from 30,053 ha-rn (1978) to111,512 ha-rn (1989) with a mean of 63,990 ha-rn anda standard deviation of 24,206 ha-rn. The lowest

(14) inflow corresponds to the highest annual rainfall of152.2 cm. The highest inflow corresponds to the sec-ond smallest rainfall of 101.3 cm in 1989. The largestmonthly inflow was 27,565 ha-rn in May 1989. For thestudy period, 90.4 percent of the inflow was fromLake Okeechobee; 5.4 percent was from the S5A com-plex; 2.9 percent from Hendry County through struc-tures G88 and G136; and 1.3 percent was from theWater Conservation Areas through structures S6, S7,S8, and S150. The percentage distribution of inflowsto the EAA is depicted in Figure 3a.

The inflow water to the EAA contributes to thephosphorus load of the basin. A ten year (1978- 1988)water quality data and flow analysis showed that themean total phosphorus load from Lake Okeechobee tothe study area was 39.5 metric tons per year. Theinflows from Hendry County contributed 6.2 metrictons; inflows from the S5A complex contributed 6.5metric tons and inflows from the Water ConservationAreas contributed 0.70 metric tons annually (SouthFlorida Water Management District, 1992b).

Annual outflows from the basin ranged from 56,111ha-rn (1990) to 203,092 ha-rn (1982) with a mean of131,447 ha-rn and a standard deviation of 34,069 ha-m. The smallest outflow corresponds with the small-est annual rainfall of 99.7 cm, and the largest outflowcorresponds to the third largest annual rainfall of142.6 cm. The largest monthly outflow was 53,185 ha-m in June 1982, when there was 30.6 cm of rainfall inthe month. The outflows on the average are composedof 87.4 percent drainage/runoff and 12.6 percent flow-through. The amount of drainage is determined byrainfall magnitude, rainfall distribution, and watermanagement practices.

Outflows to Lake Okeechobee from the EAA aver-aged 19,916 ha-rn annually, 15.4 percent of the totaloutflow from the EAA. The Water Conservation Areasreceived 59.6 percent of the EAA outflow throughstructures S6, S7, S8, and S150. The remaining 25percent passed through the S5A complex. The per-centage distribution of outflows from the EAA isdepicted in Figure 3b.



drainage/runoff pumped out of the EAA. The meanannual outflow to Lake Okeechobee from the EAA, 24percent of the total runoff from the EAA, has beenreduced to 12.9% since 1979. The decline is attributedto the Interim Action Plan and the relatively dryyears of the eighties. The Interim Action Plan was anoperating permit issued to the South Florida WaterManagement District by the Florida Department ofEnvironmental Regulation (South Florida WaterManagement District, 1989). The purpose of this per-mit was to limit pumping into the lake from the EAAand to reduce input of nutrient-enriched runoff fromthe EAA.

Drainage generation from the EAA was analyzedfor the wet and dry seasons. On the average, 44 per-cent of the drainage/runoff is generated during thedry season (November through May) (Figures 4a and4b). On average, annual drainage/runoff amounts to42.5 percent of the annual rainfall. Factors affectingthe relationship between rainfall and drainage aremagnitude of rainfall, spatial and temporal distribu-tion of rainfall and water management within thebasin.

Net Yield and Supplemental Water Use

Figure 3. Percentage Distribution of Canal(a) Inflows to and (b) Outflows from the

Everglades Agricultural Area (1973-1991).

For the period from 1978 to 1988, the outflows toLake Okeechobee from the EAA contributed 29.7 tonsof total phosphorus per year (South Florida WaterManagement District, 1992a). Although the outflowfrom the EAA to the lake is only 34.6 percent of theinflow from the lake to the EAA, the phosphorus loadis 75 percent of the inflow load due to the higher con-centrations of total phosphorus in the drainage/run off.

Drainage/Runoff

The annual drainage/runoff from the EAA rangedfrom 33,982 ha-rn to 191,172 ha-rn in the study peri-od. The annual mean was 114,816 ha-rn. The lowestrunoff matches the lowest annual rainfall of 1990,and the largest runoff matches the third largest rain-fall of 1982 in the study period. The largest monthlyrunoff was 50,579 ha-rn in June 1982. Outflow fromthe EAA to Lake Okeechobee — from drainage/runoffpumped from the EAA — is 17.3 percent of the total

The net yield is the volume of water that is con-tributed by the EAA to the region's surface water. It isthe difference between runoff and supplemental wateruse. The mean annual net yield is 66,912 ha-rn.

Supplemental water use is the amount of waterrequired to fill the demand that is not fulfilled byrainfall. The largest annual supplemental water usewas 78,964 ha-rn in 1989, which was a dry year. Thesmallest annual supplemental water use was 22,506ha-rn in 1978, which was the wettest year in the studyperiod. The largest monthly supplemental water usewas 19,308 ha-rn applied in May 1989, when the rain-fall was 5.4 cm. The mean annual supplemental wateruse was 47,411 ha-rn. Annual inflows, supplementalwater use, flow-through, outflow, and runoff areshown in Figure 5.

Available rainfall is defined in this study as the dif-ference between rainfall and runoff. For the studyperiod the mean dry season available rainfall was27.1 cm with a coefficient of variation of 0.17. Themean wet season available rainfall was 42.9 cm witha coefficient of variation of 0.21. Land use data wereused to convert volumetric supplemental water useinto depth of water in centimeters. The mean supple-mental water use for the dry season was 23.6 cm witha coefficient of variation of 0.42, and for the wet sea-son the mean was 7.0 cm with a coefficient of varia-tion of 0.73. Figure 6a depicts the distribution of theannual available rainfall and supplemental water use


70

c. 60

0 50

b40

3020

10

Dry Season Rainfall in cm

Figure 4. (a) Dry and (b) Wet Season Rainfall and Ru nolT (drainage)in the Everglades Agricultural Area.


Abtew and Khanal

80

83 78

9184z

7982

8776 8886

60

50

r).E 40

0a30

0

20I-

10 -

0

80

89

0740

850

900

20 30 40 50 60 70

820

86 00750

85

78

83

088 79

76 8189

84 00

80870

900

50 60 70 80

Wet Season Rainfall in cm90 100 110


Figure 5. Annual Inflows, Supplemental Water Use, Flow-Through,Outflows, and Runoff in the Everglades Agricultural Area.

in the dry season, and Figure 6b depicts the wet sea-son distribution. The sum of available rainfall andsupplemental water use provides an estimate of sea-sonal consumptive use.

Consumptive Use

Consumptive use of the basin occurs mainly fromevapotranspiration of agricultural crops. The meanconsumptive use for the dry season is 50.6 cm with acoefficient of variation of 0.12, and the mean for thewet season is 49.9 cm with a coefficient of variation of0.20. The mean for a water year (November throughOctober of the following year) is 102.4 cm with a coef-ficient of variation of 0.11.

Actual water budget parameters were presented involumetric units (ha-rn) as calculated by the waterbalance model. Since rainfall is measured in depth ofwater (cm), the rest of the parameters needed to beconverted to depth of water units to compare withrainfall. Land use and land cover information wasgathered from various sources, including GIS landuse maps. In converting volumetric supplementalwater depth into depth units, 90 percent of the irri-gated land was estimated to be irrigated at one time.

In converting volumetric units of drainage into depthof water, 93.4 percent (1973-1988) and 87.5 percent(1989-199 1) of the study area was estimated to con-tribute to the basin drainage.

SUMMARY

In this study, the water budget analysis for theEverglades Agricultural Area was presented for theperiod from 1973 to 1991. Rainfall analysis showed ageneral declining trend in wet season rainfall in theEAA. For the period January 1973 to September1991, the mean annual rainfall was 120.9 cm with 63percent of the rainfall occurring during the wet sea-son. A least significant difference test (LSD) betweenhistorical monthly rainfall of the period 1929 to 1959and the period 1960 to 1990 was performed, and theresult showed that April, September, and Octoberhad significant declines in rainfall during the last 31years (1960-1990) when compared to the earlier 31year period (1929- 1959). A summary of the testresults is shown in Table 1. Estimated mean con-sumptive use for the dry season was 50.6 cm and 49.9cm for the wet season. The mean annual inflow to the


80

70

60

50

0

30

20

10

0

Abtew and Khanal

• Available Rainfall Supplemental Water

80

70

60

50()

b

30

20

10

0

• Available Rainfall •Supplemental Water

Figure 6. (a) Dry Season Available Rainfall and Supplemental Water Use in the Everglades Agricultural Area (1974-1991); and(b) Wet Season Available Rainfall and Supplemental Water Use in the Everglades Agricultural Area (1973- 1990).


74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91Dry Season (1974-199 1)

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Wet Season (1973-1990)


EAA was 63,990 ha-rn with a coefficient of variationof 0.38. The rnean outflow frorn the EAA was 131,447ha-rn a year with a coefficient of variation of 0.30.The mean annual supplemental water use was 47,411ha-rn with a coefficient of variation of 0.31. The meanannual drainage/runoff generated from the EAA was114,816 ha-rn with a coefficient of variation of 0.31.The mean annual net yield was 66,912 ha-rn.

LiTERATURE CiTED

Allison, R. V., 1939. The Soil and Water Conservation Problem inthe Everglades. The Soil Science Society of Florida Proceedings1:35-57.

Clayton, B. S., 1943. Nature and Extent of the Surface Subsidenceof the Organic Soils of the Everglades. Part I. Shrinkage andSubsidence Due to Drainage and Cultivation. Soil Science Soci-ety of Florida Proceedings V-A: 119-125.

Cooper, Richard M., 1989. An Atlas of the Everglades AgriculturalArea Surface Water Management Basins. Technical Memoran-dum. South Florida Water Management District, West PalmBeach, Florida.

Davis, J. II., 1943 Vegetation of the Everglades and Conservationfrom the Point of View of the Ecologist. Soil Science Society ofFlorida Proceedings V-A: 105-115.

Gallatin, M. H. and J. R. Henderson, 1943. Progress Report on theSoil Survey of the Everglades. Soil Science Society of FloridaProceedings V-A:95-103.

Lake Okeechobee Technical Advisory Committee, 1989. InterimReport Submitted to the Governor of State of Florida, the Secre-tary of the'Florida Department of Environmental Regulation,and the Governing Board of the South Florida Water Manage-ment District, March 1989.

Mierau, Ronald. 1974. Supplemental Water Use in the EvergladesAgricultural Area. Technical Publication 74-3, South FloridaWater Management District, West Palm Beach, Florida.

Ortel, Terry W., 1992. Supplemental Water Use in the EvergladesAgricultural Area 1970-1987. Technical Memorandum, SouthFlorida Water Management District, West Palm Beach. Florida.

Sculley, Shawn P., 1986. Frequency Analysis of SFWMD Rainfall.Technical Publication 86-6, December, South Florida WaterManagement District, West Palm Beach, Florida.

Shih, Sun-Fu and R. L. Hamrick, 1975. A Modified Monte CarloTechnique to Compute Thiessen Coefficients. J. of Hydrology27:339-356.

State of Florida Department of Environmental Regulation, 1986.Final Report Lake Okeechobee Technical Committee, August,1986.

South Florida Water Management District, 1989. Interim SurfaceWater Improvement and Management (SWIM) Plan for LakeOkeechobee. Issued in Compliance with the Surface WaterImprovement and Management Act (Chapter 87-89, Laws ofFlorida) and Rule 17-43.035, F.A.C (Florida Department of Envi-ronmental Regulation), Feb. 9, 1989, West Palm Beach, Florida.

South Florida Water Management District, 1992a. Surface WaterImprovement and Management Plan for the Everglades. Plan-ning Document, March 13, 1992, West Palm Beach, Florida.

South Florida Water Management District, 1992b. Draft TechnicalDocument in Support of Chapter 40E-63, F.A.C., Works of theDistrict Within the Everglades, March 3, 1992, West PalmBeach, Florida.

South Florida Water Management Model, 1993. Grid TopographyInput. South Florida Water Management District, West PalmBeach, Florida.

Stone, J. A. and D. E. Legg, 1992. Agriculture and the Everglades.Journal of Soil and Water Conservation, May.June:207-215.


Documents

WATER BUDGET ANALYSIS FOR THE EVERGLADES AGRICULTURAL AREA DRAINAGE BASIN