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er, LAND USE, RUNOFF AND RECHARGE ON SELECTED WATERSHEDS IN THE U.S. VIRGIN ISLANDS by Henry Smith Owolabi Ajayi Project No. A-012-VI Agreement No. 14~—34-0001-~-2150 September 1983 The work upon whteh this report ts based was supported in part by funds provtded by the United States Department of the Interior, as authorized by the Water Research and Development Act of 1978 * 13 Technical Report No. Caribbean Research Institute College of the Virgin Islands St. USVI 00802 Thomas, DISCLAIMER Contents of thts publication do not necessarily reflect the piews @rd policies of the U. S. Department of the Intertor, nor does mention of trade names or commercial products constitute thetr endorsement or reconmendation for use by the U.S. Government, Li ABSTRACT Three watersheds with different land use ‘characteristics on St. Thomas, Virgin Islands were instrumented and monitored to study the effects of various land use patterns on runoff and groundwater ~ recharge. The. water crop (combined runoff and groundwater recharge) for each watershed was calculated using two different nethods and the runoff determined independently using a Soil Conservation Service method. While results illustrated the effect of different land uses on recharge, the wide discrepencies in results - according to the method applied highlighted the need for extensive data collection for such a study to be conclusive. Lii ACKNOWLEDGEMENTS We would like to express our appreciation to those who assisted us in carrying out this project to completion. In particular Mr. Pedro Diaz of the Caribbean District Office of The United States Geological Survey in Puerto Rico provided us with some of the daily water level records. Mr. Clement Browne and Mr. Philmore Andrew, student research assistants at the Caribbean Research Institute, tirelessly monitored campus rainfall and water levels. The typing of the final document was a burdensome task well executed by Ms. Medina Rogers ‘and. Ms. Cynthia Rymer of the Caribbean Research Institute. iv iT. IV “TABLE OF CONTENTS » ABSTRACT 2... ee ee ee ee ACKNOWLEDGEMENTS . 2... eee LIST OF FIGURES .........0.. LIST OF TABLES es ree INTRODUCTION . 2... 0... 2. we wk ee Location . 2. 2. 2. 2. 3 ew ww ww ee ew Need for Study .........00.4 Methodology cee ee ewe ee FACTORS INFLUENCING RECHARGE ...... Rainfall. . 2... 2... ee ee ‘Evapotranspiration. wee ee ek wee Groundwater and Geology ..... 4... Soils . 2... 2s ee ww ew kw ek kl STUDY SITES 2... ck ee ee ee ke Lockhart Elementary School Study Site . College of the Virgin Islands Study Sites CONCLUSIONS AND RECOMMENDATIONS ..... REFERENCES . 2... 0.0. ee ee ew ee el * iv vi vii fon) Oo WO WO 18. 18 BT LIST OF FIGURES Figure , ; 2 Location of St. Thomas, Virgin Islands... Rainfall Running Average, Accumulated Departure from the Average and Annual Rainfall at Charlotte Amalie, St. Thomas Precipitation and Evapotranspiration at Charlotte Amalie, St. Thowas see Generalized Geologic Map . of St. Thomas °. Estimated Recharge to Groundwater Areas” in St. Thomas . 2... . 2. ee Lockhart Elementary School Study Site . Rainfall and Groundwater Levels at Lockhart Elementary School Study Site .- Location Map of College of the Virgin Islands Study Sites 2... 2.2 2 eee General Soils Map a ae Rainfall and Groundwater Levels at CVI Study Area 2. ww ee ee kk ke ee vi 13 15 16 19 30 33 40 47 ' Pable LIST OF TABLES Supply of Water Available from Various Sources in St. - ‘Thomas, 1979 10 | V4 Ultimate Disposition of Rainfall - Engineering Classification and Estimated Soil Properties in Lockhart Elementary School Study ATGQA 2. 6 we wk wk ek ew Water Level and Rainfall Data for Lockhart Elementary School Study — Site e e 2 e e . 8 . ° oe o 8 * . a 2 _ Lockhart Elementary School Runoff Calculations and Estimates, January to September 1983... . . Engineering Classification and Estimated Soil Properties in College of. the. Virgin Islands Study Area 2 2. 6. 2 we ew ee wl College of. the Virgin Islands, St. Thomas Rainfall October 1982 to September 19835 . 2... 2 eee eee Groundwater Production at CVI Study Sites, 1982-1983....... Site I: Reichhold Center Rainfall- “Runoff Calculations October 1982— “September (4983 Pe ee a Site Il: Golf Course Rainfall- ‘Runoff Calculations. October 1982- September. 1983 i Summary of Hydrological Conditions at the College of the Virgin Islands Study Sites a vii. 14 21 34 41 45 44 46 49 51 INTRODUCTION. “Location - | } The United States Virgin. Islands is an unincorporated territory of the: United States located in the Caribbean Sea, approximately 1,100 miles east- southeast of Miami, Florida and 500 miles northeast of Caracas, Venezuela. The U.S. Virgin ‘Islands consists of ‘three large islands and more than forty small islands -and cays. The three. largest inhabited islands; St. John, St. Thomas, and St. Croix, have respective land areas of approximately 19, 32, and 84 square miles. All of the islands are characterized by steep rocky. , mountains of volcanic origin. The islands also display diverse ecological systems ranging from beaches and ary thorn scrub of the lowlands to the deciduous. forests of the higher: elevations. | “St. Thomas is the second largest of the three — major islands of the U.S. Virgin Islands and is located. 50 miles east of Puerto Rico, (See F Figure 1). the island is. approximately 19 miles long and 2 to 3 miles wide. Flat land is generally rare on St. Thomas for. most of the land surface is sloping and extends seaward bh i7930° Figure 1. ATLANTIC OCEAN ANWEGADD %, . - of by ‘Fe m2 76° Sng TORTOLA 0. . jr rrr bv. s> Oy,8 my enava 4> ~ &s aaceak ewes a . ST sOnn, | : | ST. THOMAS | se .* oo e f wHE QUES . CARIBBEAN SEA. a “PUERTO RICO ST CROIX - ry 1 20 so 40 Brlemeters ieee nena iorenonpnaeebenerireneh ° 20 ae 30 40 mise rf s L i . 67°00" . S630 7. . 65°00" ri . 1 cee + : §3°30° | ssro0 , 64°30" ‘ : ~ Location of St. Thomas, Virgin Islands from a central ridge, 800 to" 4, 200 feet high, that yuns almost the entire length of. ‘the island. The flat areas are. found for the most part in Charlotte Amalie, the seat of government of the Virgin Islands, and a few alluvial-filled embayments. These embayments are seldom more than a few acres in area with the thickness of the alluvial deposits at a maximum being generally less than 90 feet. | In addition to. rain water harvesting, ground— / water is the only other Significant "natural" water source .on St. Thomas. “Surface water supplies are negligible. As a result of the topography none of the streams in St. Thomas are truly perennial. Bonne ‘Resolution Gut and Turpentine Run in the north and — eastern parts of .the island respectively, although often described as perennial, have been known to go dry during extreme drought periods. “It has been estimated that in . the. peréfinial reaches of these streams about one-half to three- fourths of the flow is storm runoff. and the. remainder is pase ‘flow contributed by groundwater . (1) “Groundwater, though not abundant, is withdrawn - throughout the island in varying amounts. In St. Thomas most of the groundwater occurs in fractured rocks, joints and fault Zones. The major portion of the island is underlain principally by. fractured volcanic tuff and breccia of the Louisenhoj Formation. | Soil depths of one to two feet have developed in this formation. While the underlying bedrock, especially in the flat areas, has been tapped and can be a reliable source of water, extreme care must be exercised due to the ever. present danger of “salt water intrusion. if over-—pumping occurs. Although slopes along the central ridge commonly exceed 35 degrees, at one time (during the early 1800's), almost all of the land on St. Thomas was under cultivation. Because of the ‘decline in profitability, of producing ‘the principal crops (sugar cane and cotton) agriculture has declined now to the point where only small areas on the north central portion of the. island are used for farming. Furthermore, tourism has replaced agriculture and livestock production as the mainstay of the economy and the once cultivated land has reverted to | “brush and secondary forest. | An ever increasing population brought | on by an “anflux of retirees” from the U.S. mainland, workers in. “the tourist trade from the U.S. mainland and other Caribbean islands, increased water use by businesses and increased standards of living have caused water demands . to often exceed available supply. “Since: groundwater was a major source of water when the island was under eculti- - vation attempts are being made to incorporate it into the urban supply as well. Such efforts have met with ~ Little success leading to the speculation that changes in land use have adversely affected the available quantity of water from this source. Examination. of available records reveals that - " streanflows in St. Thomas. have declined over ‘the years. The change in availability of groundwater is not readily apparent as groundwater records are rare. This is the primary objective of the present study. The effects of various land use practices in the Virgin Islands on groundwater availability will be examined. Results are expected to be useful in the deter-_ mination of appropriate management practices for rural areas in the island that ‘are presently not under cultivation. -Need For Study With rising energy costs and — increasing demand for potable water, desalination of salt water is becoming a less economic. option than groundwater for _ the supply of potable water. -Rain water. on the other hand is fairly unreliable. Consequently efforts are "being directed towards improving both the quanti ty and -quality aspects of groundwater as a source of water supply. . | a , Several researchers have studied the effects of changing land use practices on groundwater availability. Filippini and Krotne(2) studied the impact. of urbaniza— tion on a flood aquizer while attieck(3) — studied the effects of phreatophyte management on water levels in Arizona. While no study along these lines have been ‘eonducted recently -in the ‘Virgin Islands, Jordan(4) analyzed hydrologic data collected over | several years +0 quanti fy the effect of land use on the water regimen of. the. U. S. Virgin Islands. In a later “publication Jordan and Fisher()) attributed the historical decline in (groundwater levels to changes in land use. The. authors also discussed the effects of transpiration: by the dense growth of brush and trees along a stream channel on the north side. of st. Thomas. Peeblés, and others(6) suggested “that. the loss of. water to recharge and streamflow be reduced by controlling the - vegetation growth on watersheds. © All the above studies done in the Virgin Islands ‘formulated conclusions based on either results of the | Sordan(7) study or were. not confirmed by field data. ‘Thus there is a need for efforts directed to field determination of the. effects of and use on groundwater in. St. Thomas. This study was undertaken in this — regard. | Methodology , Three small basins ‘were investigated as ; part of this study. However, the available: data are extremely limited. Although long-term daily rainfall and temperature records were available either on-site or at adjacent locations, corresponding water level records were generally not available. In the case of the two basins with pumping wells, daily water production — records were not available. Estimates of total. production were made from ‘periodic readings. 7 Groundwater level ‘monitoring at all three basins was initiated as part of this study in order to. establish correlation between groundwater recharge and rainfall events. Evaporation data collected at one of the three basins are inadequate for use in computing oS water balances using hydrometeorological methods. Consequently daily water balances could not be. obtained. "The water crop, defined as combined surface runoff and groundwater replenishment, is calculated using the equations(8) - P NAS (0.9+B2/12)1/2 a = 300825740.0572 (44) _ where: R = average annual water crop (mm/yr) ; B = average annual precipitation (mm/yr ) f= average daily temperature (centigrade) Recharge calculated -using (4) is converted to inches by using the conversion factor 1 inch = 25.4 mm / - Runoff for various storm events have been calculated using methods suggested by the Soil Conservation Service(9: 10) basea on the equations: Qs (R025) (444) P+0.85 - ce S = 1000 =~ 10. Be, (iv) “CN where: Q= runoff (in) P= precipitation of runoff producing event (in) S = potential abstraction, (in) CN = curve number, (dimensionless) water crop defined as The R = Rs + Rg . (vy) = P - Ea . where: R= water crop (L/T) Rs = surface runoff (L/T) Re = groundwater replenishment (L/T) P = precipitation | , - Ea = evapotranspiration is also used to estimate groundwater recharge. II FACTORS INFLUENCING RECHARGE Rainfall | ‘The average annual rainfall of 44 inches is the “principal ‘natural source of potable water in St. Thomas. Table 1 shows a comparison of the sources of. water supply on St. Thomas in 1979. This table is intended to provide an indication of the relative amounts of water. available from various sources. In Figure 2 the ten- year running averages, accumulated departure from the average and annual rainfall for the period 1920 to 1967 at Charlotte Amalie are presented. ; . Evapotranspiration “The climate in the Virgin Islands promotes a very high rate of evapotranspiration. Rainfall showers are often intense, of short duration followed by sunshine and the ‘continual trade winds. Since the vegetation and upper soil layers generally hold water for a long time - without percolation, the exposed water is evaporated _ -@irectly while the capillary action within the upper. soil layers acts like a wick to bring the percolating © water back to the surface for evaporation. Concomitantly, the vegetation moves soil water by transpiration through leaves while deep-rooted plants withdraw and transpire water from the lower depths. Table 1 Supply of Water Available from Various Sources. in St. Thomas, 1979 (11) + Source ce . Amount (MGD) POTABLE ‘IMPORTED eo Barged - ~ 0.50 MANUFACTURED | _ Desalted — 1.50. Surface on . 200. Ground Water 0.40 — Rain Harvesting © .0.50 SUBPOTABLE | NATURAL | ao oo _ Ground Water _—.. .00 Salt Water oe 1.00 _. MANUFACTURED | Recycled | ~ 0.01 10 Figure 2 2 80 xo = LA 10-year runhing overage. - -f co fr 20K —A . & Fam =x 0 . — — | NAY A cumuloted “eporture from pveragé. y \ & 20 N . 40 fe] oO: wm tad : : gol aye mean 44 ws | 40 ty ve Py = watt = 6 1320 1930 1949 950 ~ 1360 1970 mo, oe % ; oO oe ‘Rainfall Running Average, Accumulated Departure from the Average and Annual Rainfall at Charlotte vee (12) 0 0- , Amalie, St. Thomas (4 ) “i 7 Using 20. years of data, Buzdugan(13) determined average monthly values of evapotranspiration for. Charlotte Amalie (Figure 3). The ultimate disposition of rainfall ‘on nearby St. Croix is compared with that of the continental United States in Table 2. Groundwater and Geology . Groundwater is the only other significant natural source of potable water and is withdrawn throughout the island in varying amounts. A generalized map of the geology of St. Thomas is shown in Figure 4. The principal ground- water supplies are contained in a mantle of fractured and weathered rock, approximately 300 feet thick. Fractures beneath this mantle are very few and small and— thus .contain very little water. For most of the island the estimated storage capacity of the rock. is one percent or less. (14) | The bedrock. aquifer is estimated to receive recharge only about three times ayear and varies from ‘location to location because of the variability in rainfall quantity and in the characteristics of the rocks and soil throughout the island. On the east and west ends of the island, annual’ recharge to the bedrock aquifer is estimated to be 0.2 inches while in the Outer Brass Limestone in the Lovenlund Valley in the north, recharge has been estimated to more than five inches annually. - Jordan and Pisner(15) estimated recharge to groundwater areas in St. Thomas as shown in Figure 5. 12 Figure 3 75 150 Estimated ET o~ x Co 125 100 Af NY Normal precipitation 75 ard 50 25 3 ¥ a Aa x J J A s ce) N DB MON TH Precipitation and Evapotranspiration at Charlotte Amalie St. Thomas (76), - 13 > Pa Table 2 — | Co 17) _ Ultimate Disposition of Rainfall So ‘Evapotranspo- : Ground- _ - Location ration (4%) - Runoff (3) water (%) St. Croix's Central Se a _ oO Coastal Plain = 96 Poo BB Continental ne oe - USA . 70 20° 10 . Figure 4 Paces ATLANTIC T OCEAN Gromtic rocas Mune a «paren he am ie > no) b-re*z0 > “A, Wn RES Te : OT area CA R I BBE EA NV. Ss E A i} : 2 3 mien | eee 4 4 : ; ; oe 83700 : : aera’ Geology generohizad after TW. Donastiy, 1960 batted Pleistocene (?) and Holocene = { EYEE MRR ARHRY TRRHHRAARS . EXPLANATION ~ ALLUVIUM - Sitt, cloy, ond thin, discontinuous beds of sond ond gravel. includes beech sond. ; stimated 3 moximum thickness 50 ft. - TUTU FORMATION - Tuffaceous -conglomerctic misture derived from ofder — rocks. Contams some limestone, especially nécr the top. Moximum thickness greater thon 6,000 ft. OUTER BRASS LIMESTONE - Thin-bedded siliceous limestone ond o few thin beds of tuff. Estimated moximum thickness 600 ft LOUISENHOJ FORMATION -Woter-laid tuft, breccia, and a few thin beds of limestone Mozimum thickness. known {3 090 fi. - ‘waTeR ISLAND FORMATION -Lovo flows, flow breccio, ond woler-loid tuff intruded by dikes and plugs Maximum thickness greoter thon 15000 f1 - Contact Inferred foull, doited where concealed Generalized Geologic Map of St. (18) Thomas posed in Beets Figure 5 6500 b2°5> ATLANTIC OCEAN 5 NX 4-- ———~ ae — ~+ “Soe —~ An @ oT = —— y.* @) 1E20 | ba een Y CARIBBEAN SEA 0 MA 3 1 ‘ wee ¥ Se a 2 3 @ RLOMETERS 1 ’ AREA “RECHARGE Nap Ssuare 10® gallons Inchee No miles per year per year 364 OT 2a . 36 2.3 130 3:3 2b hi 18 1.6 3a 20 2.3 3b 43 £1 1.2 36 5.3 10.0 36 2 undwater Estimated Recharge to Gro Areas in (19) St. Thomas 16 These estimates of recharge rates were determined based on the geology, soil, topography, rainfall and exposure of the regions. Soils A detailed discussion of the soils of St. Thomas | will not be presented here. Comprehensive descriptions — of the soils of the United States Virgin Islands are described elsewhere.(20, 21) mye soil characteristics of areas of particular interest in this report will be discussed in detail at the appropriate places. 17 Tit SEUDE SITES The. Lockhart Elementary School Study Site The Lockhart. Elementary School (LES) study site is located at latitude 18°20'38", and longitude 64°55:03 It isa subwat ershed of 110 acres within the Sugar Estate Basin (Figure 6) which « covers a » total area of approximately 530 acres. Only the areas most Likely to influence recharge to the observation well are _ included for rainfall-runoff calculations. Consequently downstream watershed Areas 7 and 8 in Figure 6 have ‘been excluded as are the rest of the subwatersheds within. the _ basin. 7 The study site may be separated into two distinct ‘regions for descriptive purposes; the flat lower .area in’which the observation well is located and the upper. hilly region which serves as the principal catchment area. The’ observation well is drilled and cased to an “unknown depth with six-inch PVC (plastic) pipe. The well is owned. by the Department of Public. Works. | It is. Situated midway in a small flat meadow located between the | rearnost buildings of the Lockhart Elementary School , and the Raphael QO. Wheatley Vocational Training Center, — ‘The meadow is about 150 feet wide, covered principally with guinea grass, giant milkweed (Calotropis procela). and wild tamarind (Leucaena: Glauca). It is mostly dry 18 Figure 6 =F rr AAA x ™, NEE. SAYS \ =~ (a SAO 2N Ww Sf ~™N \: ff NaS VR Lpsy ey 7S wes 4 Sorpesned ha 1} YRS Seate in Feet —~ wy ~~ Contract Lisnits lS Ye ® 1606 aN P20 ANS aN ~~ of this Stauty Pa ooo WY) ie —, Ve Sats ——— SA Sot. —_—_ oo SS I~ —- aa —~ |Hospetal Sie Watershed | — NE a me 23 4 and 5) J DQM a Yr = SS x SE RN = - 2 ~~ ~~ oe — ria ~-=T SS Nf erg oN of NP P£ ~~ t= NIN 7. re =3)° — See eS, —aoe —— ~- —_ -— “7 SS —— —- ‘4, oy, SHAFLE SNARE . f 200% ~~ —— —-— ——-. ~SM semecn' ss —_— — oe al —. -= mite ~e- card a Bhom Den a. = a Th war = “ESV ATE tS >) ~ 77 Ree io. aT Ee ~NG ff XY ~~. Ne pe Ny SX x SY > UICIND Satur pat AR lore: S ws / FX Wi id sy SoNSS SY Ls ~s ~™ eS Li a , ea wR QAst ALS HAPRESO oe ~“-GARDENS if aN if) owt SE-2 N\« XE AN nas Le AN aS \ j.: NG -—— _ codHi <A og if wy i THU NE ty Ca a a a <a _ eink q oe SiS Ce oo} Read West incian Compery ~- {Siuowatershed Ho. 6) il AY, Aen i €stawe Thomas Watersned No. 7.and 8 VSS 7 ye. we = ° a ——- —— =r — f ee a go —~ ~~ _ 865 SS a i] ob I~ <e oy —~ — es aed an fo = Pot LIS —— i > 7 “> . — Ne ae Arroeds show the vos general Hrection of flow. =o Te —— tay tee Ae C ) Lockhart Elementary School study Site 19 except: for ‘periods after “neavy’ rainfall. when ponding occurs and the area becomes ‘temporarily swampy. The soil ‘in this meadow is predominantly GyB (Glynn clay loam) which occurs on gently sloping alluvial fans where - the upland drains have deposited their sediments on ‘the coastal plain. ‘The water holding capacity of this soil is high and it absorbs water readily. Glynn clay loam typically is classed as a moderately permeable soil with a plastic, slowly permeable layer at 4 to 6 feet. The area of the upper hilly ‘region draining onto the meadow is about 105 acres with a 34% slope. The “maximum elevation in this area is 960 feet. and is located 2; 800 feet away. from the observation well. The lower third of this region is covered with thick shrub consisting principally of false or wild ‘tamarind (Leucaena glauca), Casha (Acacia Sp. ),. Maran (Goton discolor), Cactus (Cephalocereus royeni), Yucea (Yucca Sp-), Century” plant (Agacie | Sp-), Thistle (Argemove mexicana), Catch-and- Keep (Anthacanthus spinosus), and Sage (Lantana. Sp. ye ‘The upper two thirds of the area is dense forest, devoid of buildings, roads, or any other man-made intrusion. The dominant soils in this area are summarized in Table 3 along with pertinent properties. of the soils the most dominant is Cramer gravelly clay loam. This soil is generally found in 12-40 percent ‘slopes of strongly rolling to steep hills, foot slopes, narrow ridges and hilltops in the mountainous areas: 20 MOT! OZ'O*STOIC9O°O'T | 'AT'TD'UD | 0S9T2 0s » desg + oUON | deeg AzoA OVS MOT | OF'O$T'O 19°0-0'T - T'TD'UD-1 lee . ° . MOT | O2°O*ST°O|S9°O-0°T | weoT ABTD | 6-0 os - dea Fetig _ cot ‘aA ‘beady | daaqg Ara, _Wwys oo: weoy . Avy] uozyuy uss MOT | STOSOTOlED'O=O'T WOT HD | Lote @2B29PON | OZ'O-ST'O] £9'0 ABTD | wT2-8 - . MOT | OZ'O-ST'O}E9'O“O'T | “WITTOTHD | 8-0 06 » deeq + 9uON | deed ‘POH BAD : . : . weo7d . Aeqg ATTOARs9 pecs] -lawelg ° . “O'T | ABTD+x204 | 28-02 06 - daag - suo | moTTeYs | ad) DIELIPON |. O2'O-ST'O1L9'O-0'T ABTO | O2-HT 06 - daeq = GUON MOTTBYS zhse) @IeLOpOW | O2°O-5T'O O'T wT TOUS | kT -0 06 - daag - SUON moTTeys ‘DUO : . a wBoy AlTaaetg s2euelzy Tepquarod | (*Ur/'ur)|('4H/*ur) | | eanaxay | yadeg (*ur/*utH) voyyea uofiel | x20y paeH aseyd: pue trams) | Agpoeded aqty yasn 1adkeq -- agny -ng ysdaqg j -ng ‘berg | oO, yadaq | addy ‘sozaag Tyos *XUPLYS | tayeM AVFTTG UOT? (u8tH piezeH a aTQUT Peay] - Beaute UOTILIFITSSETD -UTOI1ad Ap yRuos eos) pooty aTqul 10284 (yz ez) Pet¥ Apnys Tooyos AleqUeWeTY JIEYYIOT uy Satjziedoig [TOS pazyewtysg pue uoT eITFTSseqO 6 eTqel sutteeutsug 21 throughout St. Thomas. Though it absorbs. water. readily, because of. its. shallow depth it has a low water holding capacity and is subject to ‘rapid runoff ‘and severe erosion where the surface is not protected by vegetation. The forest consists for the most part of - phraeatophytes common to such areas; Mampoo (Corcho - blanco), White and Yellow Cedar (Roble blanco and Roble amarillo), Turpentine (Bursera simaruba), Tamarind (fTamarindus indica), Genip (Milicoccus bijugatus), Calabash (Crescenta cujete) and Silk Cotton (Ceiba pentandra). ‘Along the. lower western portion of the drainage area. is a small perennial north-south gut which is intersected by. an even smaller channel which: runs “parallel to the ‘training center and the elementary school ‘and between the observation well and the training ‘center. Between the ‘larger gut and the observation well (a distance of about 50 feet) is a capped six inch well. Aside from the capped well the closest well known to be in the vicinity is located on the grounds of the adjacent public high school, about one quarter of a mile away from the abservation: we11.(25) “his well is inactive. 22 ‘Data Water devels in the observation well were... monitored monthly from March 1982 until April 1983. Weekly” observations were made from April 1983 until June 1983 when a ‘continuous water level recordar was installed and hourly water levels could be obtained. Rainfall was not monitored directly at the study site. However daily rainfall records for an area ‘adjacent to the study site (about one mile north east). are used. Rainfall data are available for much longer than the groundwater level data. | In Table 4, water level data collected at the study site ‘and rainfall data are presented for. ‘the © ) period January 1, 1983 to September 30, 198%5.. This data is also plotted in Figure Te Analysis and Results: The response of the water level to rainfall input in the Lockhart School study area is shown in Figure 7. The first portion of the plot, January 1; 1983 to. April Dd, 1983 cannot be meaningfully used in this analysis because of the gap between water level data points which were measured monthly in this case. It is not possible _to determine with any confidence whether at any point, the water level was rising or falling between data points. Data points are too widely Spaced to suggest any trends. This portion of the plot is useful in that it 23. Table 4 Water Level. and Rainfall Data for Lockhart: Elementary School Study Site . oo, Water Level Rainfall Date (Feet below ground surface) (Inches) 1-19-83 © 7 7 23.500 ne 0.00 2-22-83 ng a7 ee 9.01 3-22-83 ae 27.42 , ee 0.00. 4-06-83 32.000 9.13 4-13-83 31.90 a 4-16-83 3 oe 0.00 4-18-8300 | ; a 7 (14.33 4-20-83 ASB2 ; 0.26. ; 4-22-83 50 , are 0.00 4-23-83 Sn ee re 0.02 4-24-83 5.46 sss 00 4-26-83 1B 95. oe 9.00 . 4-27-83 a - “1g.12 0.00 (4-28-83 , — ne Ce 0:00 5-03-83 15.70 9.90 5-05-83 16.06 0.02 5-10-83 : (17,0000 : 0.00 5-13-83 - (17.02 | ns oe. 5-14-83 672 0.00 2h continued... . Water Level ae | Rainfall Date . (Feet below ground surface) . (Inches) _ 5-16-83 = ~—-—«&A,20 os , 0.07 “$-19-83 BS ss—~—~S~=~i $-24-83 a8 5-26-83 ee ee 0.00 5-28-83 Se 12.220 0.00 S-31-83 (13.28 oe 0:13 6-02-83 ee 14.00 0.00 6-04-83 es oI 0,07, 6-07-83 15.09 0,00 6-08-83 1s.46 —~—si8 6-11-83 6 SS 0.00 6-14-83 _ 18.16 en 0) 6-18-83 Mets ne 6-27-83 eee 11.68 _ he 0.46 6-28-83 oe ne 11.6500 0.16 6-29-83 6 9.00 6-30-83 coe, ne 0.00 7-01-83 agg 0.04 7-02-83 ae 12.04 os 0.04 (7-03-83 s—<—~s‘«w AS 9.03 7-04-83 a 12.250 003 25 continued... . Water Level Rainfall ‘Date (Feet below ground surface}. (Inches) 7-05-83 Ce, 0.43 7-06-83 ees 12.2460 agg 7-07-83 12.66 : 0.83 7-08-83. 0.97 a ce goles 14.49 — | “0.42— 8-02-83 ast : 1.16 8-03-83 age22 0.02 8-04-83 14dd 0.12 8-05-83 - ee Ces ee 0.12 8-06-83 aS 0.14 8-07-83 t—<—Ss—s«s TG - 9.00. 8-08-83 1497 00 8-09-83, = SA 01 8-10-83 15.40 0.00 “8-11-83 15.59 0.05 8-12-83 eon oe 15.77 .00 8-13-83 592 87 8-14-8360, 17 8-15-83. 16.18 01 8-16-83 16.31 —— 04 18, 6 6 6 8 6 8-17-83 | 1645 26 © continued... Water Level Date Rainfall (Feet below ground surface) (inches) 8-18-83 (16.53 0 91 8-19-83 16.62 00 8-20-83 16:70 00 8-21-83 16.70 92 8-22-83 16.22 Ol 8-23-83 15.89 00 8-24-83 15.99 00 8-25-83 16.08 00 8-26-83 16.26 00 8-27-83 16.46 00 16.61 08 8-28-83 8-29-83 16.63 06 8-30-83 16.62 02 8-31-83 16.67 00 16.74 9-01-83 ‘ 9-02-83 16.86 00 9-03-83 03 17.97 9-04-83 17.08 06 9-05-83 17.19 00 9-06-83 11 17.31 287-8 17.44 00 27 continued... Water Level ‘Rainfall (Inches) ‘Date (Feet be low ground surface) 17.57 9-08-83 0.81 9-09-83 0.00 17.76 9-10-83 0.00 17.90 9-11-83 17.99 0.00 9-12-83 18.10 0.05 18.22 0.00 9-13-83 18.37 0.00 9-14-85 9-15-83 18.60 0.19 19.20 9-16-83 0.02 19.74 9-17-85 0.00 9-18-83 20.20 0.00 20.54 0.15 9-20-83 20.82 0.03 9-21-83 21.04 0.09 9-22-85 21.22 0.09 9-23-83 21.36 0.16 9-24-83 21.46 0.26 9-25-83 21.53 0.14 9-26-83 21.61 0.29 21.70 0.00 9-27-83 28 continued.. “Water Leyel Rainfall 29 ~ Date ~ (Feet below ground surface) - (Inches) 9-28-83 21.79 0.01 9-29-83 21.84 0.00 _ 9-30-83 21.93 0.00 => on | -3 "i =| od oe —S —- ees bes ba) af at Wye iis tue = a pop batt [ose pos i= 4 Hd errr oe aes ca --$ Ir je bee. sj + aoel in t aa =p =2 ru rs) Ko gi ‘| —s oa ] ehat foe res 4 ubtar je “| -- y ry E°g F:4 at ih — = Mi iH TL Al gripe hte it th i cry Hy i pt fhe il a i Hie thik oA a si ———— ee uw it att 14 iy th Ra ii 14 ah >3 ty a i it pare aydpesi 3 oa: it il si ait! fi i vy ny Hy ul " ar ord re] iff qf at Gi uw is it] cal st ates yt pr Ye 3s if Ty nu it vy <f7 3. HT his Ti ii i]: poe Fores wF ub hi ai 4 Xi i} io & ~f in i fe bay ores) il ins ae La ri Fi uv il Be Tepes Hy hi tt Ay hit Mat 57 iii Ww Re Hy it if Hae at i 22 ot 232 uy th hifi 13 iti il hh if 3! qi wit TH Hahn + i sty TF roy 7 sifottye 1 tt i itt od He Hpbnptinfe TE hy < --) @ ° o o (aaa) 3577p 02 wedoa (seyout) reser 30 ‘points to the need, ina 2 study such as this, for water level measurements to be taken more frequently, at least weekly, throughout the study period. , Water level monitoring continued twice every week after April 5,1983. On April 18, 1983 a storm dumped over 14 inches of rain on the study area and on April 20, 1983 when the water level was measured it had risen over 18 feet above’ the previously known level. There were OQ. 64 inches of rain on April 19 through April 20, 1983 and a corresponding 0.3 foot rise in the water levels in the well. Subsequent to that, until May 13, . 1985 there was very little rain and the groundwater level . dropped ata rate of approximately 0. 42 feet per day. On May 13, 1983 nearly two inches of rain fell and the. groundwater level immediately started to rise again. It can be noticed that because of the antecedent noisture conditions since the April 18th rainfall event, the water level responds quickly to ‘subsequent rainfall events. | . . (On June. 18, 1983, another major rainstorm ‘deposited over 6 inches of rain. on the: study area and the water level rose to nearly 7 feet above its former height. “Examination of the plot in Figure 7 shows: that the water level "following June 18, 1983 continued to respond to rainfall events until the duly 6, 4983 rainfall of approximately 1.60 inches. After this time the water level continued to fall even though there were several significant rainfall events. -— There may be 31 several reasons for this. Among these are: a. The rainfall events occurred at the site being monitored but not at the Lockhart b. Failure of the water level recorder. Total rainfall for the period January to September 1983 trom Table 5 is 32. 16 inches. Estimated rainfall for the three- month period from October 1982 to ‘September 1983 is 19.62 inches based on correlation with data from the Harry S. Truman International Airport, for. a. total rainfall of 51.78. inches for the period October 1982 to September 50, 1983. The average temperature during this period as recorded at the airport for. 1981 is 81.1°F or aT. 53°C. ‘Using the figure of 51. 78 inches derived above. for annual - rainfall and the average annual temperature of 27. 3°C as recorded at the Harry 5. Truman Airport in equation (i) yields a water crop. of 19.44 inches for the one year period, October 1, 1982 to September 30, 1983. - , ‘Runoff is calculated from daily rainfall for the period of record from January 1, 1983 to September 30, 1983 using equation (47i).— The runoff for the previous three months is estimated from the cumulative rainfall estimates as a fraction, using the same fraction calculated from. the period January to September 1983. From experience of. storms in this area, “only rainfall of 32. 0.75 “inches or more during a 24- hour period are included in the computations for runoff. he results are shown | in Table 5. The water crop {combined runoff and groundwater recharge) calculated for the LES study area using equation (i) is 19.44 inches. If we substitute the actual evapotransporation for St. Thomas, estimated at 43 inches a year . (26) in equation (v), we obtain a value of 8.78 inches for the water crop for the LES subbasin. | Remarks Two methods were used to estimate the water crop (combined runoff and groundwater recharge) in the LES. One method gave an estimate of: 19. Ad inches while the other method gave an estimate of 8.78 inches. Neither . ‘method gave an: estimate which would account: for the runoff estimate which was. calculated using SCs methods. Ses methods yield a runoff estimate of 22. 14 inches. The discrepancy in the water crop estimates can be attributed to the "global" nature of the formulas, neither of which has. been proven $0 be valid locally in the study area. “Another factor affecting the estimates of the water crops is the lack of actual data on both evaporation and runoff. These observations point to the need for more direct observations of these parameters. - The rainfall-runoft plot of Figure 7 indicate the influence of groundwater levels by rainfall. However, “lack of data on porosity and extent of the groundwater 33 Table ce) Lockhart Elementary School Runoff Calculations and Estimates, January- September 1983 a Fts Gals ‘Date -—- Rainfall In Af (1000) (1000) 4-18-83 14.33 12.00 110.0 4,783.2, 35.8 5-13-83 = 1.71 0.47 4.3. -185.8 5-19-83 .-:0.96 0.10 0.9 39.8 0.3 7-06-83 1.59 0.40 3.6 157.9 1.2 7-07-83 0.83 0.06 0.5 23.6 0.2 8-02-83 ‘1.16 0.18 1.6 © 71.1 0.5 8-18-83 0.91 0.08 0.8. 33.2 0.2 8-21-83 1.92, 0.60 5.5 237.9. «1.8 8-28-83 1.08 0.14 «1030 57.8 9-08-83 . 0.81 0.05 OS 24 0.2 10-01-82 25.22 14.08 129:0 5,611.6 (42.1 12-31-82 15.51 8.66 69.4 3,458.0 - 25.8 Totals 40.73 22.74 ~«-208.4 9,069.6 67.9 (1) Annual rainfall. 51.78 inches Average annual temperature 27.3°C * : . Total rainfall January 1, 1983-September 30, 1983. = 32.16 inches Runoff producing rainfall _ January- September = 25.22 inches. . . 34 . continued... Fraction — 25.22 37.16 = «0-78 Total rainfall October 1; 1982-December 31, 1982 = 19.62. ae Runoff producing rainfall = 19.78 x .78 = 18.51 350 basin again preclude direct estimation of recharge. "This observation reflects the need for geological and geophysical studies to delineate the extent of the aquifers, as well as hydrogeological studies to deter- mine the water-bearing characteristics of the sub- surface formations’ in the area ‘to enable a reliable determination of groundwater recharge in the area. 36 College Of The Virgin Islands Study Sites Groundwater recharge in two small watersheds in the vicinity of the College of the Virgin Islands (CVI) was investigated as part of this study. These are the --Reichhola watershed, Area I, covering approximately 165 acres and part of the Brommer Hill watershed, the Gulf Course watershed, Area Il, covering approximately 140 acres. The Brommer Hill watershed itself covers appro- ximately 350 acres. However, the basin boundaries are such that the Golf Course area ean be isolated for study of rainfall-runoff-recharge as shown in Figure 8 | The CVI study sites are Located in the vicinity of the College of the. Virgin Islands, north of the Harry ‘s. Truman International Airport, approximately latitude 18°20' north and 64°58" east. The elevation of the watersheds range from. sea level to nearly 1,400 feet at the top of Hawk Hill. , Site I is mostly undeveloped in the upper reaches except for a few homes and a radio station and tower. at ‘the top of Hawk Hill. The area has steep gradients; a 1,400 feet rise in elevation from sea level in less than 4,000 feet. Consequently, the area is characterized py -papid runoff following significant storm events. ‘There. 37 2. as SS WIE “Tropace Fowmt DIO Ee WA, sie ope tS 4s tal) << ar —" de STF Ay ‘ail Bay he : Dorothea & >= SS ae Sat = Neltjeberg eres ae Fi prede ORGS ees Bay roe yee Hull psc je SAN Y EN ‘oars. <2, NSOMNE WS a foe (WS SIA RS SSG « Gs hy, OB I NY Ate os N EQTIE S\ (mh 2A Ca es ve Ss AB S SS SeRsSQE oo 8 Se “os a Ae MIEN 6 f ——— NS wen aS SES a SN Q DSS SI376\ Sas ye RIOD mm SSS SS a NS ~ SS Nit (PF LAE XR \\ =e NaN een a Ws = =) SS; @¢ ye 2k aN a= oe. Lo a> See ay7 RN bi ~Y re ENDAHK ~ S =e O77 ) ox RAE SA Ss 8 £PERAK, S SS : RNS SS = a 8, =T Sa SAWS x Ke oS; ante ad SS PSN ZN \\ SS N= —— YN S= yy ey Ea LS nee Bay ae eZ WN GI.-NN YE % I) AS A iG Aw ) ass ff——, SigZ Bleck Point * he Og) ” ——"S QO = SSF YZ Brewers Bay ote 2b ERG) u ay i weinews os TT eR A SS. yi —— Sea oe. S33 == NC ae © wen, oe Harry S. Truman Airpert on meee, ae 0 oneness Gr 20 trees nate Ay; womwens. Crown Bay = pny USER ance onet ~~ Banna CABRITABERG Ip ie ak tLe Petm = Wie = Rad Point GEE ot sj Hitt? Wy, . Ruyter, Gra AN! Can ) sy} p SS tna (A e en Reais Point oo Mosaquite Point ea Dy big tne VF ye we Sig FZ Figure 8. Location Map of the College of the | Virgin Islands Study Sites (27) 38 isa beach in the lower reaches. ‘The Reichhold Center for the Arts is located in the valley. . The vegetation of the area is predominantly water mampoo (Corcho blanco), tamarind (Tamarinous indica) cactus (Cephalocereus royenii) and genip (Milcoccus bijugatus). The area around the Reichhold Cultural Center has been landscaped and planted with ornamental trees and shrubs. | The soils belong to the Cramer Isaac Association, Figure 9. The soil characteristics are shown in Table. 6. The predominant soil type is Cramer Gravelly clay loam covering approximately 90% of the area. Sixty percent of the area is covered by this soil. type on 40- 60% slopes while the eroded variety of the same soil type is found on the lesser slopes of 10-40%. The rest of the flat area, approximately ten percent, is covered by Jaucas beach sand found on less than 10% slopes formed in marine deposits of sand-sized particles of ' corals and seashells. Site II contains most of the campus development of the College of the Virgin Islands. It includes a grass covered golf course on approximately thirty percent of the site. About ten percent of the site, mostly on the ridges, is built. up and consists of office. buildings, classrooms and dormitories. ‘The developed area is landscaped and covered with ornamental trees and other vegetation. The rest of the area is covered with water mampoo (Corcho blanco), tamarind (Tamarindus Botany Pt e- ‘ | BE GE 4 Pelican Cay P : ittie Hans i TLA NTIC O CEAy ; Lollik {sland Outer Brass »3 Pic. Inner Brass i , ~~ pt : : Tropaco SS Stumpy Pt 5. Saba Island SOIL ASSOCIATIONS Descalabrado-Jacono association: Strongly sloping to steep, , Back | 55 well-drained. soils; clay loam to clay subsoil; shallow ond | Frenchcap Cay moderately deep over volconic rock; on mountainsides ond Capelia islands = — —18° 14! . foot slopes ; . ; d -Si :. Genrly sh 64°51! Aguilito-Fredensborg-Sion associction:. Gently sloping to steep, well-drained soils; clay. loam ond silry clay loom meterial below the surface layer; “shallow over soft marly” limestone; on hills, foot slopes, and terraces . Fraternidod-Aguirre-Glynn associction: Nearly level to gently | sloping, well-drained to poorly drained, deep, mainly clayey - soils on alluvial fons Southgate-Parasol associotion: ‘Steep to sloping well droined ; soils; gravelly loom to clay subsoil; shallow ond deep over : ‘ weathered granitic rock; on mountainsides, foot slopes, ond alluvial fons Cromer-isaoc association: bat d steep to strongly sloping, well- drained soils; clayey In subsoil; shallow and moderately desp over volcanic tock; on mountainsides and foor slopes Dorothes-Victory-Mogens association: Steep ond very steep, well-drained, deep soils; clay to clay loom subsoil; on ° mountainsides -@ Cornhiill-Coamo-San Anton association: Nearly level to gently sloping, moderately well drained and wel! drained, deep soils; cley t to clay loam subsoil; on alluvial fons and flood plains ‘Figure o- (28) General Soils Map in WT'19'HD | 08-12 MOT] O2'O-ST'O €9'0-0'T Aoy O%*0-ST'O €9°0-0'T VIOUS | yTe-6 ; MOT | Of'O-ST'O eg'o-o't | moot Korg] 60 0S daag doog £194: _ Ys aroy ‘ a A819 uoquy uss | Bpta apy / oe 4BzYy Mott eug YBry zorag . Kot faq | OT'0-$0'0 O'OZ-O6°9 | BIFTOAdeS | 62-92 0s J°TIG "A "POH A. thaag | deeq A204 ang teeoner * Ao" ST'O-O0T'0 £9°O-O'T. |. WROT "HO | L272 ve ‘ aIeTaPOR | OZO-STO |. £9°0 ABTD | iT2~8 , “AOL | 0270-ST"0 (E9O-O°T [ME TOHD | 8-0 06 ~ daag- = SUON dag “POH aA . . m¥07 dvqo Attearag \ ; ‘ OUEST ~l9NRID - + o°t. |ketodyooy | ize-02 | 06 ~_ daag ~ auoN | MoTTRUS ERR) aerapol | O%'O-ST'O €9°0-0'T Avt | ,0%-"t “06 ~ daag ~ auoN AOTT RUS: 9 . * weet A¥T9 ATT @AvIg sewwIg eTiusio eupstu “Cranstur) | eanaxen | yadeq (1OL/ UTR) wora “YoFT "| YOON PreH ‘wend pos eens. Cady ¢ id yasn aakety , eley “wang yidaq wing ‘berg oL yideg eddy feazaas THOS “yupays 19284 AMT ; i (U8TH PABZeH . . 1. BTUPTTBAY ~TqQuauteg UCFIBIFIFIRETO s[oosed App euosess) POT aTqeL, 18324 e8eT loo uy 0 85% -g eTaBy weary Apnyg SPpuBISI UTsaATA ayy fo Old [FOS pe,BMTIsS_ pus UuoTABoTFTSseto suzta aouTtsaug 41 indica), cactus (cephalocereus royenii), and genip (Milcoceus bijugatus) trees. Other unidentified species also occur. , , The area is characterized by a flat "g"—shaped valley on which the golf. course is Situated. The upper reaches of the basin contain steep slopes of over 40%, and rises to nearly 1,400 feet at Hawk Hill. Rapid runoff from the steep slopes recharges the groundwater in the valley. There are several wells on the golf course, two of which are used by the College for water supply- The soils on this site also belong to the Cramer-_ Isaac Association, Figure 9. The soil characteristics are shown in Table 6. The predominant soil type is Cramer Gravelly clay loam covering approximately ninety ' five percent of the area. The other five percent is covered by soils of the Jaucas beach sand. , Data ‘Daily rainfall and temperature data are available from the adjacent Harry S. Truman International Airport | for nearly 20 years. The College of the Virgin Islands - also.maintained daily rainfall data during 1982-83. Rainfall data at the College of the Virgin Islands | during 1982-83 and at the adjacent Harry S&. Truman International Airport are shown in Table 7. However, water level records are only available for the last two years. Groundwater production records for wells in the area are also available and are shown in Table 8. 42 Table 7 Rainfall at the College of the Virgin Islands, St.Thomas October 1982-September 1983 ee 1982 = 1983 . } aga3 4 pay = : Oct Nov Dec Jan | Feb Mar { Apr May June ‘July Aug Sepe 2 | 0.01] 0.22 0.62; . 0.05 | 0.03 |-0.14 2.| 0.04) 0.07} 0.24 | 9.02 0.08 | 0.22 | 0.81 3 0.07 | 0.02:}- o.orf ~ fo 9.01 |0.01 | 0.25 | 9.02 4 0.01| 0.03 | 9.05] 0.02 | - } 0.02| 0.15 5 — | 0.04] 0.07 | 0.14] 0.01 | 0.08] 0.28 | 9.02 | 0.12 | 0.01 6 0.76| .. | 0.28 "| 0.07 ; 0.96 | 0.04} 0.05 7 0.02}. - | O.11f 0.05 — ~{ 0.21 -| 0.30 8 : 0.01 | 0.29] 0.01 | 0.27] 0.06 | 0.02 | 0.14 0.20 " -4 0.25 9. 0.05 “1 o.03] | oon 0.80 | 0.31 10 | 0.01 0.01 12 foazh |. | 0.05 | 0.06 12: 0.254 0.05 0.20 7 0.27. | 0.07{ 0.02 13 0.03 ae ) 1 0.451 0.25.1 0.04 14 | 0.06{ 0.02 | 0.03 0.05} 0.16 | 0.35 | —‘| 0.07 15 | 0.06 0.54} = f | 0.04 . | O10 fF | 8.04] 0.06 16 |. (| 0.59} 0.06] — . 0.09 | om} — | 0.04 _ ‘10.06. | 0.03 y1iy [> | 0.02 ~{ e.10] ~ | 0.02 : o 0.19 18 | 0.88] 0.32 . : . | 0.08. | 0.35] 0.20 ig | 1.44] 0.16] | - -fo.on [5.80 | 0.02 | 5.52 | of 0.03 20 | 0.66} 0.16.|.0.88 _ $9.25 1 0.50- 0.03. | 0.05| 0.14 21 | 0.03} 0.01] | | 3.50. 0.2% | 3.43 | ‘| 1.42} 0.48 22] 0.08] 0.05 0.02 0.44 | 0.04 | 0.01] 0.04 23 | 0.49) 2.20| 0.12 0.02 1 0.33 2 |o.o1je.cs}o.os| | | | - . 0.93 25 | 0.07} 0.01 0.02 | ft o.or | 0.88 [ 0.01 |. | 1.02 }26 fo.os]° - |o.02z | o.osf 0.01 | 0.09 | 0.02 | 0.23. 27 4 0.12| 0.13} 2.40 fe | 0.70 28 0.90 | 0.04]. 0.02) - | 0.01 | 0.30 29 0.38}0.09} oor], |. | 4 10.68 | 0.16 30 | 0.57] 0.23 | 0.02 fo es ee en 31 | 0.03 ode - ‘0.30 4.60| 6.921 4.46 |. 1.61 | 3.78 | 1.30]26.99 | 6.91 | 7.64 | 4.40 | 3.56] 2.67 43 TABLE 8 GROUNDWATER PRODUCTION AT CVI STUDY SITES, 1982-83 Total Production (x10° gals.) Year Month - Site I “Site II 1982 October 0.290 0.36 , “November 0,13 0.28 December | = --0- 0.40 1983 | January = 0- 0.34 , February 0.06 |. 0.13 March , 0.02: — : , -0- April 0,02, 0.40 May yo gate Lesa june -0- , 0.24 July a Pa 0.56 August — pf Lg. (0.67 “September . = 0- . 0.61 TOTALS 0.99 453 44 Analyses And Results Site I: Reichhold Center Area . The total rainfall during the year (October 1, 1982-September 30, +983) as recorded at the College is 68.98 inches. During this period, the total rainfall producing significant runoff amount to 39.47 inches. The water crop (combined runoff and groundwater re-. charge) calculated using equation (i) is 34.30 inches. The runoff calculated using scs-methods(3') is ~ 16.48 inches, Table 9, or about 210 acre-feet per year over the area. ‘The groundwater recharge therefore is 17.82 inches (34.30-16.48). The influence of rainfall “on groundwater levels in the area is shown in Figure 10. Water production from wells at the site during the same. period is shown in Table 8. ‘Site II: Golf Course Area | The total rainfall for Site II is assumed to be , equal to that of Site I, and is taken as 68.98 inches. Rainfall producing Significant runoff is also 39.47 inches. The water crop (combined runoff and groundwater — recharge) is also 34.30 inches. The runoff calculated using scs-methods(32) is 18.65 inches, Table 10, or nearly 220 acre~feet per year _ over the area. the amount of water available for “groundwater recharge is 15.65 inches. _ Groundwater production during this period is shown in Table 8. 45 “~ SITE I: REICHHOLD CENTER RAINFALL - RUNOFF - TABLE 9. CALCULATIONS OCTOBER 1982-SEPTEMBER 1983 (2). Omittéd from totals 46 Runoff Date Rainfall | INS ar | Ft? 109 | Gals. (x106) 1.| 10-01-82 0.010) | o 0 9 0 2.) .10-18-82 ° o.sgt) | 9 0 0 0 3. 10-19-82 1.44 0.07} 0.96 41.7 0.3 4. |} 11-06-82 0.76) | 9 0 o- 0 5. | 11-23-82 2.20 0.32] 4.41] 191.6 1.4 6. | 02-21-83. | 3.50, 1.01] 13.86 603.0 4.5 7.|-04-19-83 | 15.80 —}11.61 | 159.67 | 6,945.6 52.0 3. | 05-09-83 oso) | o | oo 0 0 9. | 05-21-83 3.43. | 0.97 | 13-30 | 577.3. 4.3 10. | 06-19-83 5.52. 2,43 | 33.41 | 1,453.3 10.9 ms 11. | 07-06-83 0.962) | 9 | oo 0 0 12. | 07-24-83 0.93) 0 Oo) 0 0 13. | 08-02-83 0.81%) | o 0 0 0 14. | 08-21-83 “1.41 0.06} 0.87 37.8 0.3 15. | 09-25-83 1.02. 0.01} 0.08 | 003.6 (0.03) (7) 16, | 09-30-83 0.00 0 0 9 0 TOTALS 39.47. 116.48 | 213.65 | 9,853.9 73.7. Notes (1) Insignificant runoff generated by this storm event ..-. a hf i i Aa 1 ise ral Thi seoumnan (saqpet) CLesared 47 (33343 32244. 02 q3daq cra Tee TE Be FA EU MAUHT UU TT i nit iid Han FTE a TT HIER itl i HH ani TON TMU ' gi]! fi i me HTL Hef ETE as Feat tere TT OTH LOTTO nt fem ETE GE sr AULA eon STD OTILLR TTT UTL TT e ory Fas i n ait HH 2 UAHELOESLETH FL iH RET TH RU (it TUTTI IEEE = anTEH aot | fs H UH tit 22} ft iif HET s tHE] i ITT RTT a: EHENCTUTY TH WL ii jitTeline it il (plz nu i Hl] it He 2 HA hh mlilning RHP i i | za 4 RHE IES Se ) at tee | HEE age ‘ LUT TA a ni i oo Fe aT AMMA i RETA WTI tives: MN MTT | iis iA HTD Hit PFSTMTN MT nT TTT hile ttl mii : Wa th i Hii FUERA ATT RE TET 4 =A tail UOT ae Tinea i LCT HER ie HUE HepyianL — A TTT EE HeETT TE ATLA NOTITIn — H Hi i EGTA TTE a tt ' OT WUT j i THAT TE f iI ETH AIT mn ’ Hi PA if tl Hine a ats TET | i ae SE ; 3 Bit aA NIOn ly {ay il} | J : ATE TY ii { ii THT i | i 7 + . i te 1 VHT i i HIE | Hi HW i i uf u Wy EAH i ith | iu | huh iki Ae ii i au li i Mh = | if nin tt hil 1 Wi RE WT uh | EE Ho ARETE AAU i it 1 tial ahbel Hit t i HET ae mult on Ny | ee RE Hy HEATER What i Hitt BETH TTHH Pets HU AT descr Oa as bce inte LTT ~~ co o - : - " Figure 10 Rainfall and Groundwater Levels at CVI Study Area ng page.) : aL (Continued on the follow STS Tra Tle up peel =< Ty 3th a: 4 Buy 1 ey anit t ait HH ae " Tas mii TH Hitt ‘al ie MU sur ant | Be Oe i ih | IN neon, - en Oe Ce nt iV AEE: cod bbb? jhiay }. eta } ft IND t +3 | ft } af STACI eae ie be = wat 2 = boos pocedow} 4: 7, ft = coas pure Man, /PRSTESE ‘aoe em =a = sage E i it EAH i a4 enc COeR nT ey HBE fi can | ety HESITUTEES TT UGE TATE Ui HESS ei Q8aqgonyy TIEIAwE L i ETH HEATH HUGE TBH THITTH Ian eo Hut lia fl faba thi ST PAA THEE EEA Th Hifi: Vig eed Enea HET SU LE BE Hfesstnaytt eafagd feces pest Hn RBs aul Ate culos = 2 P=4 : o (3792) 2329, ©? wading 48 Figure 10 (continued) Rainfall and Groundwater Levels at CVI Study Area. SITE IT: TABLE 10 GOLF COURSE. RAINFALL — - CALCULATIONS OCTOBER 1982-SEPTEMBER 1583 RUNOFF Runoff Date Rainfall INS AF rt? 103 |Gals.(x108) 1. |10-01-82° 0.01 0 0 0 0 2. {10-18-82 | 0.886) 0 0 0° 0 3. [0-19-82 1.44 0.15 | 1.70 73.9— 0.6 4. [11-06-82 9 0 0 0 0 5, [11-23-82 2.20. 0.48 | 5.64 245.2 1.8 6. [02-21-83 3.50 1.30 | 15.19} - 660.7 49° 7. lo4-19-83 | 15.80 12.40 |144.69 6,293.9 47.1 8. [05-09-83 o Ch). 0 0 0 0 9. |o5-21-83 3.43 1.25 | 14.61 635.6 | 4.8 10. |06-19-83 5.52 2.88 | 33.57| 1,460.2 | 10.9 11. {07-06-83 0.96 0.02 0.28 12.0 0.1 12. |07-24-83 0.3301) 0 0 0 0 13. |08-02-83 0.81 (2) 0. 0 0 0 14. |08-21-83 1.41 0.14 1.58 68.8 0.5. 15. |09-25-83 1.02 0.03 0.40 17.2 0.1 16. [09-30-83 0.00 0 0 0 0 torats | 37.81 18.65 |217.66| 9,471.40 | 70.8 49 () Insignificant runoff generated by this storm event _ Remarks | The summary of rainfall- runoff-recharge condi- tions during 1982-83 at the two study sites adjacent to the College of the Virgin Islands are shown in Table i1. The available data indicate approximately 69 inches of rain was recorded during the past year. This is signi- ficantly much more than. the average annual rainfall generally expressed in the literature as 45 inches. Consequently, it can be concluded that 1982-83 waa a wet year in the historical record. Using a global formula, equation (i), the water crop defined as combined runoff and groundwater recharge, is calculated to be 34.30 inches for the year. Assuming the validity of this formula for the Virgin Islands, approximately half of the incident rainfall resulted in runoff and groundwater recharge. , | , Runoff is calculated using established scs - methods. (33) Runoff calculated for Site. Ir is 19 inches and slightly more than runoff calculated for. Site I which is 16 inches, despite Similarities in topography © and a smaller area for Site II. | This result can be attributed to greater development on Site II as compared with Site I. | | Water available for groundwater recharge in Site II is 16 inches and in Site I is 14 inches. Potential groundwater recharge at Site I is approximately 81 50 wer ee ee nel tw |e |. est | ot tof ste ist | ot ec | 69 ort z°0 | esanog 7109 | tr “ty se | 18 tt ane et | oe 66 | 4T? ‘ot 6¢ 69 | ‘sot £°0 za3uaQ proyyotey | 1 (e292 aed) | (ur)! (01) [(g0T) | (739 249v) | (AT) (OT) |(0t) | Craz-ezey) | (UE) | (uz) suaued | (uz) (sa22¥) | C,H) | HOFIP9OT | ON F ul suoT Te) (,34) , _[PUOTTEO (,3a) - ome zpouny | TIL , aits RassO VOTI , adaeysey m yyouny THEZUTIY woay, -wapdsusajodaag seits Apniag Spue{sy UTSIFA OUR FO adOTTOD. au} 7B SUOTZTpPUOD TeoTZOTOApAR ZO Azeuumng Il ATdBL 51 ‘million gallons, whereas less than 1 million gallons were pumped during the year. Potential groundwater “recharge at Site II is 61 million gallons whereas less than 5 million gallons were pumped. Consequently, there is significantly more groundwater in each area than is presently being withdrawn. , - ‘These results would indicate that the slightly more developed Site II receives less water for groundwater recharge and generates more runoff than the less developed Site I. This result is to be expected. However, more definitive research needs to be undertaken in order to establish the role of different vegetative cover on the groundwater regime in the area. 52 IV CONCLUSIONS AND RECOMMENDATIONS ‘The Virgin Islands are undergoing changes in land use patterns from predominantly rural to urbanized. These. changes are bound to affect the ‘groundwater regime. Three Small basins were investigated in order to determine the effects of changes in land use on the availability of groundwater. These basins, while being Similar in area, topography, and geology and sub ject to the same hydrologic influences were not all similarly developed. , All methods applied ‘in using the collected and very limited available data, though not in agreement as- to the magnitude, did coneur “that urbanization negative- ty affected the quantity of groundwater. recharge. | One of the- principal. difficulties encountered in "ths undertaking was the lack of basic hydrologic data not intrinsic ‘to this study. Estimates had to be made of soil porosities, long term values of annual evaporation, rainfall and runoff. The latter parameters — are routinely monitored by local and state and federal agencies on the mainland of the United States. This is not the case in the United States Virgin Islands. While an extensive data collection program may ~ 53. ‘seem expensive, the worth of it will be realized when : studies such as this one are conducted. Studies of this nature are crucial in formulation of plans and. | policies for land and Weiter usage. | , It is recommended that further research be conducted to more specifically define the role of not only land use effects on groundwater recharge but also the effects of various types of vegetative Gover on groundwater recharge in the Virgin Islands. When these effects are known, proper management strategies for undeveloped areas in the islands will result in substan- tial enhancement of the already scarce and precious groundwater resources. It is strongly recommended that priority be given to establishment of an extensive basic’ hydrologic data collection. network in the Virgin Islands. Without data from this source; ° water resources planning efforts are pointless. 54 REFERENCES Jordan, Donald G. and 0.Jd. Cosner, "A Survey of the Water Resources of St. Thomas, Virgin Islands," U.S. Geological Survey Open-File Report, 1973. Filippini, M.G., and Noel C. Krothe., "The Impact of Urbanization of a Flood-Plan Aquifer: Bloomington, Indiana." Purdue University Water Resources Research Center, Indiana, -March 1983. Affleck, R.S., "Potential for Water Yield Improvement in Arizona Through Riparian Vegeta- tion Management" PhD. dissertation. University of Arizona, Arizona 1975. Jordan, Donald ‘Gey: ‘“Land- -Use Effects on the Water. Regimen of the U.S. Virgin Islands," ‘U.S. Geological Survey Professional Paper 800-D., 1972. Jordan, Donald G. and Donald W. Fisher. "Relation of Bulk Precipitation and Evaportranspiration | to Water Quality and Water Resources, St. Thomas, Virgin Islands" in Contributions to Hydrology of Latin America and the “Antilles, Geological Survey Water-Supply Paper 1663-I, 1977. Peebles, R.W., A.E. Pratt and H. Smith, "Waterplan: A Comprehensive Water Management. Framework for the U.S. Virgin Islands." Technical Report No.2, Caribbean Research Institute, St. Thomas, June 1979. Te . see Supra, Note 4. 8. Mandel, S.T. and Z. L. Shifton, Groundwater Resources Investigation and Development, . Academic Press, 1981. g. - United States Department of Agriculture, Urban Hydrology for Small Watersheds, -Soil. Conservation Service, Technical Release No.55. 10. McCuen, Richard H., A Guide to Hydrologic Analysis Using SCS Methods, Prentice Hall, 1983. - -41. -Buzdugan, Stefan C.,--Written Communication, Virgin | Islands Agricultural Extension Service College the Virgin Islands, St. Croix, July, 1983. | 12. See Supra, Note 1. we 13. See Supra, Note 11. 14. See Supra, Note 1. 415. See Supra, Note 5. 16. See Supra, Note 11. 17. Buros, 0O.K., "A Water Management Plan for St. os 2525 Croix, United ~ States (“Virgin Islands." Gainesville, Florida: Black, Crow and Eidsness, Inc. 1976. 18. See Supra, Note 1. 19. See Supra, Note 5. 20. Rivera, L., W. McKinzie and H. Williamson. "Soils and Their Interpretations for Various Uses - ‘St. Thomas. and St. John, American Virgin Islands." ‘U.S. Department of Agriculture, Soil Conservation. 56 of Service, Caribbean Area, 1966. “2h. Rivera, lL. et. al. "Soil Survey, Virgin Islands of. the United States. " U.S. Department of Agriculture, Soil Conservation Service, Washington, D.C., 1970. , | 22. Buros, Q.K., "Planned Drainage Basin Studies for the Protection of Roads from Flood Drainage in the “Virgin Islands, Volume. One: St. Thomas." Prepared for the Department of Public Works by CHOM Hill Southeast, Inc., 1982. 23. See Supra, Note 20. 24. See Supra, Note 21. 25. Stevens, Ken. Fernando Gomez and Jose Alcea. , "Water Wells in the U.S. Virgin Islands. Part One: St. Thomas." U.S.¢.8. Open- -Pile Report 82-82, December 1981. 26. See S Supra, Note 4. | 27. Black, Crow &. Bidsness, Inc., CHOM H11 Southeast, , Inc., "A Sediment Reduction Program.” Prepared ie the Government of the United States Virgin Is- lands, Gainesville, Florida, January 1979. a 28. See Supra, Note 21. _ a , 29. ‘See Supra, Note 20. a, 30. “See Supra, Note 21. 31i- See Supra, Note 9. 32. Ibid. 33. ‘Ibid. 57