A GENERAL OVERVIEW OF HYDROLOGY AND



THE ANALYSIS OF RAINFALL DATA



Dy



Henry H. Smith, Ph.D.



Introduction



Hydrology can be detined in general terms as the science

which deals with the waters of the earth, their distribution

on the surtace, underground and the cycle involving evapora-

tion, precipitation and flow to the seas. All components of

this cycle are of equal importance. The linkages between

subsystems in the hydrologic cycle are illustrated in Figure

1 while the hydrologic cycle and asscciated water quality

parameters are illustrated in Figure 2.



The hydrologic cycle is perhaps the best vehicie to use in

a general discussion on hydrology. However, it is erroneous

to give the impression that water moves at a constant rate

through this cycle. It is instead, an erratic movement, both

temporally and spatially. Furthermore, it 1s important to

have more than a qualitative understanding of the cycle and

knowledge of how to measure quantities of water at various

stages in the cycle. It is necessary to be able to deal



quantitatively with the iterations between the various fac~



—j-

ATMOSPHERE



/

/ f NS



A « rey

wy OY Wo

ra oy en



ey we VR



& we 2,

Q ey Xe,



f L linfiltration = _

SURFACE pn) SOIL

WATER i WATER

SUBSYSTEM SUBSYSTEM



a “ *, “Al ace coon Pad aS) Pa



Rs "oO, Ta, ND WP 5) os

No Oy . a a “oy P — a!



OQ, ye Ne”. g Q"



“3 RD ‘ ‘ ‘, Pi ef



* OQ, Se \ Ne . of rd

“aN GROUND

SS) WATER

N SUBSYSTEM



Linkages Between Subsystems in the Hydrologic Cycle



Figure l.

(From: McWorther and Sunada, 1981)

(O861 ‘Saepueg iwoig)

Slajyaneieg



ARTTeNY Asem pPpaqersdossy pue aTo9AH BDIsoOToOApAY su] *7 sansry



1 -



uoIsns0doAz



WwDEHS



SPOS peajossig

‘eprion, 4s ‘aseunBbuoyw ‘ wos] ‘ @o9 ‘sseupsoy

1ajOM pUNoID



JID PY PHOS

‘eobjy ‘iawipes ‘ piuejopg Ja;JOW papuedsns (27)



‘Q ' spioy '%o f ly

10°7OrS ‘sjusinnN ' Gog (JeUDW pasjossiq {1}

19D. FIDpANS



DIVIOD “pouspoW AojD ‘ysAG “SeIDinoy4dg (¢)



SED LH PO * 20S ° 19 [DteyOW pasrjossig (2)

"ay7p UaBO HY JO SApIXG

"*N" O05 f09 ‘SH '*Q lsasspg paajossig (1)



JOJDM jOIbojosoayaw Fo siaydsouiy



ager JsloM punoig



a4 07 ¢ \ )

} +” UCHDIOIad

UdHDIOUDAR ~——

jyouny

go0png

UOMOAL EU]

; vorjosuidsuory

{933 MOUS *UIDY }. i i

“OND YGINe8sd | |

(DIJO; paaossiq

sossny

SOON IDN

PNT.)



M4



ae MN . 7

tors in order to make assumptions on the intluence of man-

made works on the relations. It is necessary to be familiar

with methods tor evaluating the frequency of various extermes

ot the cycle. The final section of this discussion will

examine some of the methods relevant to us.



Of the average annual rainfall in the Virgin Islands, (38-

45 inches), it has been estimated that 87% returns directly

to the atmosphere as evapotranspiration, 5% goes to groundwa-—

ter recharge and 8% occurs as runoff. in Figure 3 the move-

ment ot water through the hydrologic cycle for an average

year in the Virgin Islands is illustrated. It is indeed a

challenge then with our high slopes, cooling ever-present

tradewinds and limited tlat areas, to intercept for use a5

needed, the maximum possible amount ot water that can be

teasibly withdrawn from each source,



Rain Water Harvesting



When atmospheric water vapor condenses, hydrometeors are

formed. These hydrometeors include damp haze, fog, ice fog,

snow, frost, drizzle and rain. For purposes of clarifica-

tion, drizzle consists of tiny liquid water droplets, usually

with diameters between 0.004 and 0.02 inches, with such siow

settling rates that they often appear to float. Drizzle

rarely exceeds 0.04 inches/hour. Rain on the other hand

consists of liquid water drops mostly larger than 0.2 inches

in diameter.



Raintall may be measured conveniently by any open recept-—



acle with vertical sides. The standard gage of the JU. 5.

f

) ioe tr

' I f

‘ b ge qt

fr : | . ;

F) t +

* F ef, ee aera Bag eh

} q

4

Ciauds

a cu ew .

or a. 4 an ad .

. MAS arene ‘ \

A

4 *

Rain

Qo

oF:

Gn

a

Ns at “r

( g ey a

we KL FE LAND

Hischarge and - < Ong fom is

a rr 7



¥



ao wor . ,;

Setns OPP Ery ae she Se

1 a ae ait oe ee oe ne ne ii a ee bre Level

ee Fras Water iy ice

en Ny Aauiter . SOIL fda

Uf: Salt Water Wedge —} . o oT eee



—- =1



SLA



a

- 1 oa be

acy - * ee - oo

-



WRAG ? ap? 7



Figure 3. The Virgin Islands Hydrologic Cycle

(From: Peebles, 1979)

National Weather Service consists of a collector which passes

the collected rain into a cylindrical measuring tube which

stands in an overflow can. The cross-sectional area of the

measuring tube is one-tenth that of the collector. In this

way, 9.1 inch of rain fills the tube to a depth of 1.0 inch.

This makes it possible to measure rainfall to an accuracy of

0.01 inch with the calibrated measuring stick.



Other types of raingages in common use are the tipping

bucket, the weighing type, and the float recording gages.

These gages are all best suited for particular applications

and, for the most part, used where a continuous record is

desirable.



The preferable network density of rain gages in an area is

highly dependent on the use to which the data will be put.

The density recommended by the World Meteorological Organiza-

tion for small mountainous islands with irregular precipita-~-

tion is one station for each 10 square miles.



Rain water harvesting has been the traditional source of

water in tne Virgin Islands and still is the preterred source

of water for most residents. On each of the islands the

greatest annual rainfall occurs in the northwest and the

least on the southeastern coasts. AS can be seen from Figure

4, which shows average rainfall data for a location in St.

Croix, February and March are the driest months and September

is the wettest. Almost half of the average annual rainfall

occurs between August and November. Generally, rainfall

occurs as brief intense showers of less than a few tenths of



an inch.

ef



RAINFALL, IN INCHES



2b.8@2 INCHES



ANNUAL, 42.18



20.36 [RHCHES



INCHES



Figure 4. Mean Monthly Rainfall at Anna's

1920 ~ 1967)



Hope,



ot.



Croix,



r

(From: Jordan, 1975)

oA tht 4 morning Gamer th+ A oh

| L P Bi, M3 % 5 4 el

eu]

I

rae | 1 Pcp

Key R-F-P i Buy coutth alan 1 Su Pasa ct uz

P. AAW rq

t- 3 HEAL Pele) Pavel {AAs

bUDG iw clu

: any CHIE 1400

| '

see! Bohs . rattle Ben

4 BOO

ol Fides: uf SCJ boc 14

tA 40 fag 400 Gu BOO

SUCKS : Mauty

,. eon i

! sou 600 a8

¢ FU . 400

eu SUNG Psa S00 fu

*, . a 54 GH

Cah cy}.

LI

Tages cn Ea cz EEE - 3th}: digi}

Ba aks es i Sats

" q .

st ad he) Ca v: ire 17th F

. a:

3 Kee wn } a 40) 400

™ E: = 1

tt u ca =" Ho! chia?

. Tat . Zt,

c ‘, ° e108 -_ : ret “oe

i * i i .

w fu : \, x 4 we PU Ad he re etal] - ay

a aF all 3 :

€ fy ; : Hi He)

= Py i : ‘ .

- 1s Bui ies : IG ; a Be

x md) ay bell : . Wij { aCe ;

= 3 fo vo) | re

a f Ms a : on

~ , = sah 44} - | 1b

o J a)

3 "4

Pal ar ob:

4 = ro iy

. - Pas) aan

Lm) :

u 4G

aut a

4] 30 rs

#0 30F

5a5

if

ai : ae

. 329

O4 ad 4 EC 1r- ZU vue

; Pao

t EEE

nae I my



Figure 35.



Nomograph of Rainfall and Runoff from Catchments



(From:



Cosner,



1972)

Methods utilized to determine the amount of rainfall

that may be harvested from a purpose built catchment use

essentially the same approach as those to determine runoft

from other surfaces (Explained further below). Results. ob-

tained are often expressed in nomographs (Figure 5) or design

curves (Figure 6).



Cisterns are required by law in the Virgin Islands and



sizing requirements are presented in the table below.



Table 1. Cistern Sizing Requirements in the Virgin Islands

¥*



Type of Structure Required Cistern $1ize

Single story dwelling 10

Multi-story dwelling 15

Churches and warehouses 4.5

Other buildings Exempted



* gallons per square foot of roof area



In St. Thomas, rainfall harvesting satisfies 20% of the

daily potable water demands and in St. Croix and St. Jonn

13% and 75% of the respective demands are satisfied by rain-

fall harvesting. |



Surface Water



During a rainfall event, certain abstractions occur

before there is any direct runoff from the ground surface.

These initial abstractions include interception storage by

the covering vegetation and depression storage due to

surface unevenness. A schematic representation of this pro-

cess is presented in Figure 7. Because of the mountainous

relief and high evaporation rates, surface water in the



Virgin Islands is very limited. Streams are for the most

| 1050 1000 950 900 850 800 700



REQUIRED CISTERN STORAGE (Gals./Ft. 2 of Roof area)



% RELIABILTY



Figure 6. Cistern Sizing Curves for Sprat Hole, St. Croix

, (From Smith, 1981)

<I



Volume of Rainfall Excess



ee



ee



ee ee



rors &



- 2 bo moro



a



ee ee)



wr ee |



+ meena



4 af 2 8



Z



oe oaie



_ . - me moe



a2 5 6 np foe



= 2 f# es 2 oe



Infilrrartion Rate



xT



YZ



CH



.



g,



tat



Intttak Wasinactisns



ee =e



aoe ore



ee ee



.



Rainfall Excess = Dinect Runolf



Pr ee ee



er ee |



a ee



Pn



- Ds



.



re ee



ee



i ee ee



Pe ee ee



ee ee



_ ee



Volume of Dinect Runoff



a



.



i



-_ to Fr #



re ee ee



Pe ee ee ee



ee ee



ee 2



=: 4



es al



ee ee



1 ee ee



re



~_ = ' * & 8



a



a a



ee



a



rr



rr ee al



| ee eee



re ee



ee



ee ee



a = mw fF FTF



ia



wea aa



is



ee



Pe i,



- ee eee



=) 8 a & wot



ee al



a



ee ee



oa or 2 ee



ee ee



-



ee tr F



a ee



oF



i a a a a ai a



ne ee



a



' we



eos 2



+ eee ee ee



aoa Fie FF



er eit



4



rar ara are Oe Lares a ue



a a



_—



aie



Tine



Figure 7.



Relationship of Rainfall, Runoff, Infiltration and



Initial Abstractions



(From:



Schulz, 1978)



— | O-

‘part ephemeral - they go dry during periods of little or no



rain. Steep slopes and clayey soils lead to rapid runoft.

Several methods are used to estimate discharge of water



from an area. For the most part they are all based on the



rational formula

QO = ciA



where Q is the discharge in cubic feet per second (cfs), ¢c 1S

a runoff coefficient, i is the rainfall intensity in inches

per hour and A is the watershed area in acres.



While the rational method provides an easily computed

estimate of discharge from an area, there are certain factors

associated with its use that must be considered. Most impor-~

tantly, results obtained are greatly influenced by the size

of the watershed being considered. Roth the rainfall inten-

sity and the runoff coefficient can be expected to vary as

the size of the area increases. Values used in the tormula

then should be an average value and thus the estimated dis-

charge is significant only as an estimate for the whole area

and results should not be used to approximate conditions for

a smaller area in the watershed. A commonly used “Rule of

Thumb' is that the rational method should not be used for

watersheds greater that 200 acres or where there is a great

variation in the value of the runott coefficient.



Runoff from an area may also be be calculated using

methods suggested by the Soil Conseravtion Service and des~-



cribed in a publication popularly referred to as Technical



—~|1-

Bulletin No. 55. This method depends principally on the use

of runoff curve numbers. Runoff curve numbers for land use

and treatment practices for hydrologic soil groups were

developed from daily rainfall records for small agricultural

watersheds. By using land use patterns found in various

areaS and accounting for the impervious areas, a weighted

curve number representing runoff porentzial for the watershed

can be determined. When only some sections of the watershed

is urbanized, peak discharge downstream is determined through

combining the component hydrographs and routing then to the

outlet at the discharge point.



Flow discharge in an open channel may also be estimated



using the Manning formula

(2/3) (1/2)

O = (1.49/n)AR S

where 0 is the flow discharge in cubic feet per second, nh is

a roughness coefficient available rrom several handbooks, A

is the cross-sectional area of the channel, R is its hydrau-

lic radius and S is the slope.



While there are several small livestock ponds in the

islands, Creque Dam on St. Croix is the only dam in the

Virgin Islands constructed to serve for municipal supply.

This dam was constructed by the U.S. Navy in 1920 and is a

concrete arch dam with a design capacity of 9 million gal-

lons. Creque Dam is 40 feet high and 200 feet long. it is



presently not in use.



_}o-

Steep slopes also make erosion a concern. Estimates can



be made of the potential soil losses from these slopes using



the universal soil loss equation



A = RK LS C P



where:

A = average annual soil loss (tons/acre)



ze

i



rainfall factor expressing the soil erosion

potential of average annual rainfall in the

vicinity.



va

If



soil erodibility factor. The average soil

loss per unit R and is a function of soil

type and percent organic material. K ranges

from 0.02 for sand with 4 percent organics

to 0.60 for silt with less than 0.5 percent

organics.



LS a dimensionless topographic factor that re-

presents the combined effects of slope length

and steepness. For a slope length of 300

feet, LS varies from 0.18 for a l percent



slope to 31 for a 50 percent slope.



C = cover and management factor. A ratio of the

soil quantities eroded frcm land that is

cropped under specific conditions to that

which is eroded from clean tilled farrow

under identical conditions. C ranges

from 0.001 for a well managed woodland to 1.0

for tilled continuous tallow.



P = factor for supporting practice. No sup-

porting practice, i.¢. contouring or contour

terracing, P = 1.0.



Use of this equation will give an estimate of soil ero-

sion per year due to rill and sheet erosion but should not be

used aS an estimate of the sediment delivered to the channel

since much of the eroded material will be redeposited before

reaching the channel. To determine the amount of sediment

reaching the channel, the sediment eroded is muitilpied by a



sediment delivery ratio factor which has been founda co pe a



i 3~

function of drainage basin area. Detailed discussion on this

process can be tound in the manual "Control of Water Pollu-

tion from Cropland" by W. C. Walton.



Groundwater



The process by which water sinks into the soil 1s known as

infiltration. The rate o£ infiltration is dependent on the

characteristics of the soil material and the type and density

of the vegetation growing or iying on the soil surface.

Because there are larger spaces between the grains, sandy

soils tend to have higher infiltration rates than silty or

clay loams which are fine grained.



As a soil becomes wetter during a raintall event, the

infiltration rate (measured in units of inches per hour)

decreases. The first reason for this is that wetting of the

soil causes granules of silt or clay to expand and close some

of the pore spaces between granule=. The second reason 15

that the tilm of water that surrounds each grain 1S more OF

less continuous and forms a three-~dimensional network of

interconnected viens of water. The liguid continues to flow

downward through the network out encounters Frictional re~

sistance that increases as the depth of the network in-

creases. The rate of downward movement is slowed with the

increase in frictional resistance as the depth of wetting

increases and thus the rate of entrance ot new water from the

surface is impeded.



Rainfall that enters the soil suffers From one cE three



fates:



mide

a. It may evaporate directly from the soil



b. It may be transpired by vegetation



c. It may enter the saturated zone

In the upper strata of the soil, the openings are only par-

tially filled with water. This zone of aeration is divided

into three belts - the belt of soil water, the intermediate

belt, and the capillary fringe. The belt of soil water

furnishes most of the water for plant growth. The inter-

mediate belt is principally a passage for water from the soil

belt to the capillary fringe. The capillary fringe holds

water above the zone of saturation by capillary force acting

against the force of gravity. Phreatophytes, which grow

without dependence upon the belt of soil water, get their

water from the capillary fringe and the water table. In the

saturated zone, the pores spaces are filled with water. The

transition between these belts and zones is gradual and their

thickness varies according to local geology, the availability

of pores or openings in the formations, the recharge and

movement of water within the zones from areas of recharge

toward points or areas of discharge. The general subsurface

distribution of water is illustrated in Figure 8.



In the Virgin Islands, soils are for the most part rela~



tively shallow with high clay contents and cap a rock base.

Groundwater recharge on an average amounts to 5% of the



annual rainfall. The occurence ot feasible extractable water



is summarized in the foliowing table.



~15-

cf



Ground surfoce



a att



=—— alt



ea



fon ae



1 ad



bg



+



eh anal



nny



Fs 7



wt



Sod water



sor



2 Sol mos



ure =



bei?



‘egy! +>



Oe



ak ewes Pee



ta Lor hh ae



aa a



fh



Payee



ee a ee



as 4



te A



a hing!



1



infer -



Pore spaces partially



.



mediate



filled with woter



q



=



belt



ta)



.



es



_



Capillary fC : “Capillary rise



>



Ground weoter



Lane



Figure 8:



Subsurface Distribution of Water



Johnson Division,



UOP Inc.,



1982)



(From:



~1 6

Table 1. Estimated Yeild of Virgin Islands Aquifers

*



Aquifer Characteristics Potential Yield

Thin sand and gravel bed 10 - 30

Solution cavities 20 — 60

Fractured rock 2~ 5



* gallons per minute



Tt is useful to examine the detinition ot several of the

terms most commonly used in discussions of groundwater. The



porosity of a soil is the proportion of its volume not oc-



cupied by solid material. An aquifer is a water saturated

geologic unit that will yield water to wells or Springs at a

sufficient rate so that the wells or springs can serve as

practical sources of water supply. While porosity represents

the amount of water an aquifer will hold, it does not indi-



cate how much water the porous material will yield. The



specific yield of a soil is the quantity of water that a unit



volume of the material will give up when drained by gravity,



while the corresponding volume retained is its specific reo



tention. The capacity of a porous medium for. transmitting



water is its permeability. The rate at which water may be

withdrawn from an aquifer without depleting the supply to

such an extent that withdrawal at this rate is no longer



feasible is the safe yieid.



In their initial development, welis are tested by

pumping under controlled and well monitored conditions to

determine the productive capacity of the completed well and

to provide data on which the selection of the pumping equip-



ment to be used can be based (Refer to Figure 9). The



-|7-

ij



Deoth to

static woter we!



,



Crawdown

~



N



\



Pumping \



Pal

“



a Drowdows?

|



ieyer or

dynamic

water ‘eyel



/



re Le |

aceutee



ePFP eb pea



Well screen



.o =

cn a oe a

Oe i i a



a ee |



1

Pe ee



. v

. 1 \ . wa r

. 7 Te “oe oo. oa pore i

L . ata sat ohy7t anos . a

oa i a i es ere rt - . pen a

- - at r "=. 7 oboe =-_4 0" .

mae! re a Batoat . ™ sak ee

satya a Fy oy L

"=a a . - 7 a ot a

wete aytlere : r corey :

ute aaa ee rou wate may *

~ Teta an - cate ano? ut

. W reales toa tee . eeu,

“aor . "oe Poet 1a r

eee et tar, a tata ty t

a Sie a -.

. wee *. i ee Tae atte vate! ae et

i ee ee otha re 7 He

; tes - rhe rae re a : fe

ae Ls Veena erate ay "Y a

cee ye Fe, a "

ee a er ee Satyr Ee bowa 7

er cr ar tale tte +, re Cr Tye ;

i re ee he te

a ee a oe ee grb ’ uy

ee Tt tee ae ea tae ay 1 ra I

a i a re a i i i ee

i i i | i ee "aa .

ah ead te . i ee eee ee . a TE

a 2, a Ce .. - . rs . F ;

ah asl wea -_ oo. re er or cs i a oe on .

“ 4 7: . 5 .

. ‘i =

a'F

* .

rh

1

Lo



r 1

a ee



(cai



I A |



_. Mpervieus stratum 2000 0 LESS



—_—



Figure 9. Measurements Related to Well Performance and Pumping

Tests of Wells and Aquifers

(From: Johnson Division, UOP Inc., 1982)



~]18—

Observation Production

Piezometer | Wel!



Weil



| |

piezometnic



1 _—— surface f



Bal a (



_

water

! _table~, v |



tei eee



tightly



pe



seated



| x STURN RG

confining forat ; ‘ S

stratum pertoragHons fF



aa



sealed S



ee" OF SCTEENS f S



| nie {| RSS

- gravel

pack



confined aquifer



Figure 10. Common Facilities for Observing Water Levels in

Aquifers



(From: McWorther and Sunada, 1981)



1 9-

static level of the well is the level at which water stands

in the well when no water is being removed from the aquifer.

This level is normally expressed as the distance from the

ground surface to the water level in the well. The pumping



dynamic level is the level at which water stands in the well



when pumping iS in progress. When pumping stops, water in a

well rises and approaches the static »rter level. At any

particular time during this recovery period, the distance at



which the water level is found is the residual drawdown. The



volume of water per unit of time discharged from a well is



the well yield. The yield of a well per unit of drawdown is



known as the well's specific capacity.



Groundwater levels are commonly monitored by piezometers,

observation wells and production wells. These are shown in

Figure 10. A piezometer consists of a casing, perforated

near the terminal point only, that is installed in such a way

that the casing fits tightly against tne geologic formation.

The height to which water rises in the piezometer is the

water-pressure head at the terminal point of the piezometer.

The observation well is a perforated casing simply placed in

a bore hole with no attempt to provide a seal in the aquifer.

In installing the observation well, care is taken not to

penetrate the underlying confined aquifer in order that the

observation well properly indicates the position of the water

table. The production well may penetrate any confined layers



present and indicates an average water-pressure head.



—-?Q—

Elementary Statistics in Hydrology



In hydrology a hydrologic variable is defined to be any

variable which measures or defines the magnitude or quantity



of some element in the hydrologic cycle, The mean is an



estimate of the most likely observed value, the average. It



is a measure of central tendency as are the median, mode and



the weighted mean, The middle value > a set of observed



values arranged in order of magnitude is the median while the

mode is the value which occurs most commonly. , The weighted

mean is derived based on the association of weighting factors

to the observed vaiues. The mean (Xx) is computed as

4 } | .

Am = zx /N

where N is the number of observations.



The standard deviation (S,) 1s a measure of the dispersior



of various values about the mean. [It i:. computed as



Sy = \2 Cain Ka) /N



Since the mean was used in the equation above we would have a



biased estimate of the standard deviation. When there are

only a few observations or when there is a great variability

in the values, the effect of this bias is great. The un-



biased standard deviation is preferred and is talculatec as



52 [EO



~?j-

Multiplication of the biased estimate by the ratio

N / (N - 1)



will produce the unbiased standard deviation.



The coefficient of variation (C,) is measure of the dis-



persion of a variable about the mean expressed in dimension-



less units. ft is calculated as



It is often desirable in hydrology to determine if there



is some change (trend) in the value of observations over a



period of time (time series). This may be difficult to

observe because commonly there is a high variability and 4a

large scale random variation about the mean vaiue in most

hydrologic time series. Variations may be seasonal or diur-

nal resulting in cyclic components in the long term record.



Stationarity in a hydrologic time series occurs when the



statistical parameters (mean and variance) do not change with



respect to the origin. Nonstationarity occurs when a trend



or cyclic component is present. Nonstationarity also results



when there is a sudden change in the hydrologic eco-system or



in the method of observation or reduction of the data ({nonho-



mogeneity or inconsistency).



One way to determine trends is by plotting the data on a

scatter diagram as well] as the mean of the full time record

and observing if there any obvious trends. It a trend is

suspected the data should be divided into two or three seg-



ments of equal length and the mean for each segment computed.



2?

The moving mean is another method of determining trends.

The mean is calculated for periods of consecutive data of

equal length but with successively different starting points.

While periods of different length may be used, in the analy-

Sis of precipitation records the five-year moving mean is

usually satisfactory in damping out the effects of random

variation. Use of the moving mean method to examine 80 years

of annual precipitation data for the British Virgin Islands

is illustrated in Figure ll.



Probability analysis may be used to determine the probab-

ility that an event may be equaled Or exceeded in any year.



The return period or recurrence interval is used to signify



the average number of years within which a given event will

be equaled or exceeded. The return period and the exceedence

probability are reciprocals. To determine the return period

the data is first ranked according to magnitude and assigned .

an Order number (Table 3). The plotting position of the data

is determined using a chosen plotting position -formula. The



one most commonly used is Weibull's

n=m/ (n + 1)



where nis the number of years of record and m is the rank.

The data and the corresponding exceedence probabilities may



then be plotted on probability paper (Figure 12).



3 3-

Won



X iy



WSSU



Se



4



b Ga



=o



qd}



{3



fo



4



ic:



en



| &*



ry



by



0



ay



fy



Q@ 0



ee ord



oq 43



= 4



: G*



OD be



fs



La



tm



43



‘a



“4 @



cd



us



Ri



ue



Lynd



a



Ph



=F ty



BS



yet



wt



Ju



bet



oy TD



Pe =



we



q)



oh



(S34 200) IIMS



-~2h~

Table 3. Determination of Return Periods and Exceedence



Probabilities for the British Virgin Islands



YR



RALN



RET.



ORDER



PX.



1933



94.26



Gl



1.23



1932



88.52



40.5



2.4/



L931



75,03



2/



344



1960



41.52



20.25



4.94



19Q9



70.64



lo.2



6.17



1916



609.05



13.5



7,41



1969



67.11



Il .5/



G.64



1936



05,505



IG.13



9.8/7



LO2/



63.63



9



ii.ll



1952



63.14



10



Sed



12,35



L915



63



O4



1]



7.36



13.59



1942



O2



G2



12



6.75



14.61



1979



62



if



6



23



16



O05



1970



52



O2



14



5



79



lL?



2/7



1956



Og



Cris



m? ©



15



3



di



18



92



-



rc



ra



19350



one



O&



16



0



GO



19



7



rw



1905



59



28



1?



fy



706



At



oO]



L9OL



29



O°



iJ



A



3



22



22



are



19



&



20



2



“y



ad



1949



96



1902



Qf



69



20



&



7



fom



24



69



L951]



56



2d 1



-)



O



6



25



91



193¢



59



ae



,



on



22



3



=



20



3



Af



i?



25



23



3



32



ZO



4]



1937



56



1975



56



Ll



2 &



3



39



29



59



1974



22



41



22



—



3



24



_



-,



QO



)



O



6



197



23



G2



26



3



i?



32



O05



55



Od



Zf



3



33



33



1924



O



1947



23



99



Z



i



2



2 OF



34



T9111



23



35



29



2



79



35



~34



1909



33



GO



30



2



7



3/



» O4



Z



61



38



31



19035



23



S/



34



a)



1903



53



2



32



he



53



39



223



1934



52



35



2



hy



2



E



40



82



19]s



i



+



34



2



230



4.2



O2



52



Aa



2



43



1923



5



2



3



4



31



29



19653



32



36



2



0 22



4 4



44



10354



i



c



"



4



/



2



o 19



45



66



LG44



=



re)



3



}



2ei3



Af



95



38



4



{_}



O08



L950



3]



Oy



39



2,06



LG4s



ome



OZ



iQ



2,03



49



26



~25—

(Table 3 Continued )



ge



en, rata



— Se



ti



RAL



RET.



PER.



ur 1 ar.



E x .



a



L922



50.72



& I]



98



50.51



SU



54



42



93



591.81



1904



1919



5G



306



43



&



393.19



1930



49



D2



dy dy



B34



35



1943



4%



93



ra



a”



Cc



&



53



2)



{



6



46



7



56



82



1935



1985



3%



4]



V2



56



14



19/7



O/



AS



69



59



1/



1962



é7



Q



“



5



AQ



65



60



61



mn



73



1953



A]



71



a)



tT



tad



62



61



Lola



A6



ere



a)



l



39



62



89



193¢



46



5



Poa



OQ



t



c



56



64



1



1912



41



>



4



Fae



=f



~4



2



23



LG4v



dy



1



}



54



2)



66



67



1o2i



A,



95



35



47



68



Q3



L946



45



93



56



AS



68



97



45



23



a/



42



ras



4.72



1950



1945



ad



5%



fs



74



43



1955



ty



/



oh



37



f2



99



LSS



43



26



60



35



7 &



0?



Oi



33



75



19



L929



43



25



LO7f



é 3



Cf



62



34



76



34



-



1963



i?



0



>



L



Th



29



fi



7



2



LY10



oh yt



&y



OG



D3



27



74



2.)



BO



1920



42



Ll



65



ad



1961



ta



DO



23



8]



1923



1



+



67



24



L976



30



i #



c



OS



1%



84



O03



ia



85



4/7



Lol?



3U



w?



}



9



1/



1925



36



rae



16



36



21



1907



ua



26



ai



14



72



3/



f



72



13



WO



ofa



1O59%



a



”



LOS/



—



7



a



Jl



90



O9



19S



36



V7



wad



7é



O9



91



74



G72



3



™ Th



50



7



2»)



co



OS



92



a9



4 /



76



ii



/



93.406



1o4]



1367



34



2



if



i



OS



95,24



1964



5



10



i



O4



90,15



1925



Ol



/



C



1.03



97.09



e



9.01



30



OD



oe



1.Q1



ee



oR Se



Lor



ERP De ae eS AS



MEA



51.41



DEV.



11.81]



ee ee ee



ess 8



RAINFALL:



EAR Fa IL el CY SSS oe Tee



Se Sad ata a, ke fale eg



~26

a



ee er ere



12.083



iaee ©



La



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666 3°66

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it 1 : a ; 1 1 ' . ee | 1 . - ee en = _ 4. H ee woop at . peepee ee

: : ! : meee aca ae r —_--= 1 ' r seeene

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: I t ' ' T by —4 whee d ' soe t ._ L L . : 1 " 1 . + . | F 1 aoe. : 1 . ce euae

r oj ‘—— -$-4.- : oe. : . jets - epee I : 7 - .

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: "7 we _— ue ee . . . . 7 . . ae seeds

; | riat nan eee sin song a nracd andes Pigott voeediee dege feces “e ie i _ foie ese bop kes Se Se : 1 . es 4

t . Lal . . . - = —_— ie = . 7

aa ; rd a i --: : es ne re EB cme t er . bets Do, iaideed ae !

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: +— Saale tak: eee aan il eae ‘i youre wopete pope Aan . aa * wi ob H ew an ns —e— : , ‘ soe . . ;

1 ! 1 I Pu 1 1 1 1 . ; ee eee ry . 1 . + i i, Oo a) Hi . ; 1

fot : 4 ' pf ee Poh yt pire mememe sees ‘ — ™ Ps ih “t ts aa * fe

: H : T + : ' : a iatetatiaiiat hetierteia eh to: rr or pororpetd a+ ¥ . = bs Z 4 bi aie " j

— 1 1 . 1 road 4 . I i : . . iaeie hae aia cee . . eee eda : . . . 1 a al “| -—h wt n =, 1 |

| raeé * Sti alee rot , Peeters poe ses t : 1 weber cmes ! = - “ ae

1 ' . 1 : : ' H agers deee I I ' ! wedeeeee Ae eeee pe ee : ake d-- ae y t ¥ i na . i +. 1 . 1 . i

; ri} ; : _ : = | To tog pe con es H a ee ; ‘ a ‘ art ™ TTT il + - wal : :

oe re ee eee nena ne Perpegc ine M S S3IUSHSsed? : “A eG

. = ———= cope coo r i

ii tty Dot dS dont bap nt ceee bee geeeeen 7 ~ iL _3 pra girs _ oY ao OF - .

1 + | t I : 7 . . ' roo: ae Pe ee eee ’ . ~fo4 . . . ° heme te 7 I Poroy to oqyoe ah } et mt ~ seems

| . : ' hieieieiel intent . =: . ' : t i - - -

semrece ra: . =e H 4 1 Poms ta eos eae wuivh . ! . ; 1 r i . 1 ' . ae . 1 . 1 ato 1 f ; . : ;

. eee of . ems dog oor t 1 *- oc. . 1 - .

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. | La 4 . 4-—4 : ot » Fo: weeeree ee eee . 1. hoe eek : : \ 1 aw ee 4

: ee | a i ; i wry 1. . Pomnn oe oo ' : ae ae : ‘ | | 1 ron ott j

! ' en ! ' H ++ : r Lan sore ee . "ot in | i tog 1 1 ' too l " a 4 ' , of |

1 H : : : eae a > |: : ' vbacus r : r pass we deeper epee ah re Ber H * i re . 4 : 1 ' re f

H t 4 1 : . reba re . 1 : : wereqgees ede i... . . . : ; of ath Rta Al ’ acm oes 5 - . r " + H ' _. 1 : . ; _ : . . _ .

——+-- i . . i" = ose . - .

' 1 1 : 7 rou i | H : 1 —t rt-+—- i : r ™— whe be eee - ’ en ee weeed . 1 Foca ee toy : _— \ to eo ' - 1 i ; a

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' * 1 * Ce . . 4 .. . 7 Oo . . . . . .

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i ' | i 3 ; ee eat a eee ai ; bem 4 : Le ee . . - . . : t eee epee H i . . 1

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‘ l ' 1 . . iy een dale ae _ - - . - : + * ' re - saiald

t 4 L | toa r } L Pe | l Lena ewes, eee eee pe . . = r ana El ee ae sehen eae - : k 1 - . . . : 4 ' ' ' =_ ' moo = j loam qed | asa rs -: . "

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t ' ! - = 1 oa / ! 7 ee eo ' ' St te ee Citi : 1 oe ee ' ae a ee : a - . woth ores 1 toe ; ' rors Pe /

L k 7 : 1 . wpe ace . : sees es peeee: : y ee oe es ] FT ee ere ee 7 sca —— . 1 Tr 1 , . F 1 . eee . 1 r T . 7 .

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+ . . EI 1 2 1 : 1 1 . ese corse ----- 1 ees ee eed = 8 . dee L L . 1 1 oe aa! - L . . H . - Lene

Hi 7 L. . . 4 1 : wee eee . . Ae oe ee ee . 1 a . . . ey / 4 . r weed . woe .

! 1 ' ae " ' : I a dealale J aah r Fe mo as peste ag . ' ' woe ee : ‘ coe aetey pqs eee ' dee . ene @ nee e ee

+ I. ! ' A / 1 F pot iil ' te pce eta i : 7 TS mrss sss F Rossa 200-4 A 1 4 so 4

| ; + A 4 : ¥ . L ” i] 1 . : 1 : + . at smo mee lege I t: 4 - . L 7 ' . . k eee eee ee . Pee ee ee ee

' i k 1 ' : = : . : 1 : I : . ' o™ corm L - . sm te iH iad ! Powter ee * 1 “ soot 4

A to 4 ' ror d : " — ! I stoa -hoom j a0 j- . 4 1 weet peer = be .

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1 ! + L : ' — ss leat i halen Seen ea tad nie : | t t i, a we we te a - ; . | i . ta 5 : ee |

1 = pooen -t—r a . . .

dot 1: : veer di nboanpes deed eh nn ae ° saiieieiartaieae inal Siar i sono wt . ' 1 1 "oat so : pest or oko

: i - : . ninhelL-tall itieia el: aE ou : oe ee ee i 7 i tHe Pe : ' es et eT . . : * 7 . 4 : a . tees

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1 7 ‘ : I 4-4- - I 1 .

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T + : . ! fpr k ' a eee ae sot . i Soros om kon sores t ' , + : .

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‘ 1 : a ee . . -—_. a ' - . . 1 _ : a)

- J . ? _ . . con 1 a0 . . . - . 1 . . .

i L : . : I | ' . ee : . t — i 7 wg t H - . roc, ‘ 1 . T . : ve kre

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H 1 ; r ior H t ng poe oot ' ree a Cer ‘ rots ™ fas r 74 . a . . boc 3 . 1 - i : 1 Eine

T it c : " to matea da rdas aot soo: : oo a aie aaeee lagk . oped etme i 7 + Lm owe . t . 5 + ke 1 Bos omet - . k so - t Co ate

1 r : . Jenne tod. : 1 : ‘ - ae . : 1 1 ee ne ee : ‘pease ‘ . - . : .o7 - ‘ IF —- L .

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7 b— ' r r + 1 Doe PF . erence rete ena Fe : , . ‘ i . = * coeiianiar iain ' i

’ t we al weet ee 4 “r : te : ae

i | f Preasapananrinry ; iol | ~ -a44- , \ 1 a ‘4 a a r j ee oe me mt are 4

te ae (ee Cl bee ome r . l + . ! 1 , ' . 1 a ea eeeer

: | t tat | a ae a i iad Peeve B : : = toot : meee ed

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r 1 ! . ' : —_ 1 ' roa 7. 4 1 2 +s

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1 . a ee . - 1

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T | 1 hd 1 1 i] . -a0c-- -_-- . 1.7 + 4 + 2 - : L 1 1 + 1

: . ater 7 . - rf .

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3 denudnes ' | | Dot oe oe o fo, i er a ! popet yea. yg ; ; i, aes

l ’ : +—} \ . wees pee ee . . r . . 1 1 ope . i 1 - a] . i

: k I ' ‘ Ij , + ___. . * - _ os L ved ; . ' .

' : ean ! I - : tT * 7 . ery See ere ae ! 1 1 r 4 ~ <I 1 1 tote gd T eerteapet ase oe 1 1 r . . '

L 1 : H i ' nr a : f t pomeecpesss ome aon . roo he as ediooass i . waoBres be emeee . -_ .

, | ! T jk 4 | H 4 hea ceneetea be erd 7 | 1 Ca a) - 4 a ' 1 ' - - - , . og ee eee ' . . ; . 1

H J 7 1 F * oo : res pruric —meee . k : em, _ r Focco™ -- 4 . “ : 7 . ; _ Fi

i 1 Lud | : + 4 toh oe oo ' T 4 : - or te . ete ee .

a . “ 1 Fj : ' H - ep erase op op 1 L a ee ee oe J “hh . | r F t . L - - oo. - ' . . eer of Toe r

oe ‘ : ra ‘ io 2 ok aientiaetietetentel 4 . [ cee wep reer ae ob : ahah . ross : . i

i " * tr —_. : 1 ' . a eee eee ' 1° 7 toe oe . ee es 2 a o-oo . ' -

1 5 : + LJ 1 1 : 1 a ee ae . / ee ee ee . = 08 - . ie . - . . on

i i 1 I : ee pee Saree eee ak he Pa ne a 7 TT meee teed a ob oo te. : r a _ 4 ' non pee unenectest r == . . : — “ i. ‘

' 1 ==t 1] I 7 1 ro" —- ed en i r wee oe a 4 . irr es ' r ' : -

Hi : l c .t +t | | ‘ Ff 1 . . -- 4. ee ‘ pom otros 4 . . TOO 4 f ere em Te a

T fae anal wedunde-d I 1 1 | eu ' aap eee A acum on of , ' 7 OT - . . . 771 ‘ L r

i : tt t . ' : “Ty Sep a ne eer i “ rc ‘ . . cc FoF . - * . . = t

. : | r I ' ‘ : : . i . +f +e t . . ' . rowers Ta mee Bom neee -_ 4% * ' r- . - - ’ a!

1 1 . i] I op eeepeee—e aed = . . . . inns iene t gore eo . . sosteea . . os . ' . i a

1 1 Tt = | J 1 . 1 a ae seo pee 1 a ptt wea . ' . - 1 ae | Ea

+ | : ' " + " . descemeeme doe . ro 4 . 1 s 4 ee po 1

+-4 7 . 1 er ee es 1 ' -- Lo. :

i , oso ! . . 1 oe

T a : ! Ls oT ab ta pee ede ' ’ ' 1 oes 4 . _..

— ar. 1 1. . en | 1 1 1 a re eee 1. I —- 1 mI mae _

+ 24 : . Tore safes bose Fl 1 _ oT - me a i ne “— . ee we coer

' ' Pori? = nara : i fore pre “4 ' . ™ . 1" oe Toe ee ee ee ‘ ‘ ° 1 . ’ ve

ee ere . “ . . mae '

+ | ae: . rope to: bd 1 vee 4 a ee | _ ; ; ' au meee oe es . .

1 = ee I 1 —f eee . —1 } - d - . . 1 . |

1 I tat a ee I 4-4 cok i 7 bee . L wee _. 1 ; 1 |

: : ' -_

_ ioe ros ay ™ = wherry + ' ’ c + . : ; roaoatoe ee ee ee — . Ls .

Lo rit ay Ee Pr. oo: UT saan ; ~ be ee : bore 7 ; toon eee ee - Oe

L I L.: 4 Pema e wakes om . dee Mee 1 roe Fa # . wom.—e .

. one a - .

. ' : * =n a ee 1 . ees ee Se a 1 i ee oe es 1 . .

4 — . . ’ - . ;

T + whanehes peep ! —o on re ’ - ao ; ae ns . ' - ae eee oc ee

ot: 7 Fr - - .

ba sare | ' ot 7 wd t \ . * a eT | L aoe 4 ., _ . - ' eee OU

" | ‘ t -—-§-— i : ' " ' . . 1 1 at

. * }—+—4 Beau. cee 4 -. . . ae . . . \ . a

L ' otal atalale id | r ' H - +r + © . ' - .

. Foi. t ron wrap ke oe 1 . - \ 4 ; - 1 1 . . \

I ! ' 1 ae | | Hi a .e ot & " - r - 8 .

1 H con eet i i * r - r , - * ' . - chooses ors ' 14 _

o_o eee . . .

: ' l t t pr bd nt : ! T # — '

: ti wee ee pees : meee -

oe oe 4 poop deia Poa panes cece ot

L ' : 1 1 oH ee ae ter . 1 a an ora _—— =r- .

! t tag : oe ce . 1 woe eer nk oe 4 " . Se el i el

: ' L Pe ee) ee 4 -+ +: L . : ee ee _. 1 . = = .

1 1 wees I wee ee at

“ r jy bat. srr rr bee . k q 1 an hence vets i Mee

a ak jot ot “ : . .. Sa.

H ! L . aerate . : . = 4.) 6 4 4 . r

. es . ” i . =— = 8 - 1 . . . v . .

. - ro ee ee t . . - weet eee *

t ¥ * r i+ ] . che ee fe ee Los : r oof aerooqeeta

| 1 . _ 7 ! mien | : a | r a - _ . -

. : r _o - . - a - - - “1 .

: 1 : ee presser ee t -4- i ° ‘ - * ‘ 7 il r _ #

+ ib 7 . : I ——S res * reece or rT . ° ne

1 1 ieliel Metall es . 1 tt: +--+ 2 oe wr ee -a4 rier -— 414 -

1 " ' 1) eee a a teow or od eel . . saan tare

+ r : ' ! . ro ae oa o- ' “4 . - . a owwo}.f wee . . r -owess \

. 1 ale (alielid tia thd Oa a ' 4 . became oo { . " . 4 4 ~ en wo eee

: \ roa i 1 7 a | ' . eos L ° ° ror . ' . +r oe

cote 1 . --

. . bitte ends ab —- —+ | 1 ° ie Os 4 7 ' * 4 ' . . 1 . 1 . . mw

i "4 a a | 4 ' 1 1 1

' . t I ‘oF - .

. . . LJ ae. . - oo”

i I aap BH ae dee or = I iH " . ' ‘ . . ‘

j e ' Do. Porc =a j -_

lee ar poo tot. i ta elie erlen eiol 4. a : a 1 . r ' -

1 4 abt + pee i . zs om ou4 - — 1 1 ‘

i . 1 . be armen were tere I 1 . = Fr ee r = ee é

ce ee 4 4 al ala ope spot a 4 . ee eee ee 1 = ee .

1 ‘ t el teie ieee deleted -_—+ caoa 4 . " " + SS ee ane 1 - “* co. -

poe yearn 4 ... ee ee er on “4 ' . - 1 eae —._ . ' ‘ 1

17 = ' 4 . . .

. ™ bowed ts ee . ' . ato - * - + . ™ . r - . . 7 oF - wee ee . .

i . t + l ' a- 8 . 1 4 L --0t L 1

' 1 ' ied J . I i ° ' - 4 ~™ twos '

: whee ! 1 | ee a a er iy oe ' . 1 :

ane: ! ! | +-" ee eee eee | . rept te ‘ F Fy : ‘ . - et . -

rac hoe . Io: . cote oF soot -- . . . . tee oe 1

H —-t- 4 ' ccm ae = . mere . ' oe a

Tore prc ae ' roma boos [rrr oF

J I 1 ' ann som teen . . ' ' neers

ee ed ee ee . aot - + -b r . . . a .

1 1 rey eoceegee re _.

oe 1 : F we pee eee | r . . . —

: ae | . . ,

. ! . —_ - wr: . a ' r 1

i = pone + oh 3 Loe “VO : |

- I . some ete . 1 - 4 . F

i Ly . Ly : t 1

* - a

goo #10 : l a a

iy : ' —_

Zo G8 , a

" — _

' r wouee pee 1

“r 1 ress ee

- 4 — eee le 1

“7 ros ot ™ 1 .

. = cof -

1 '

tog . '

- -. . ' 4

. ak

meee . .

og ee pee ee

7 ee eed ot u

a” -

1 f -

a

ao a4



FIG, ff RR ET



=

‘<4



(sava0y) Pha



ae



triad .

Conclusion



The basics ot hydrology relevant to the Virgin Islands

were presented, While this examination was not exhaustive,

it is expected to serve as an overview and source of general

information to persons with interest in the development and

management of our critical water resources, A list of



suggested readings is included for use if more detailed



discussions are required.



7 3-

Useful References



Ajayi, O. and Smith, H. ii. 1981. Compendium of Water

Resources Data for the U. S&S. Virgin ZIslands. Technical

Report No. 12, Caribbean Research Institute, College of the

Virgin Islands, St.Thomas, Virgin Islands.



Bowden, M. J., et. al. 1970. Climate, Water Balance and

Climatic Change in the North-West Virgin Isiands. Caribbean

Research Institute, College of the Virgin Islands, o5t.

Thomas, Virgin Islands.



Chow, V. T.fCed.) 1964. Handbook of Applied Hydrology - A

Compendium of Water-resources Technology. McGraw-Hill, New

York.



Cosner, Oliver J. 1972. "Water in St. John, United States

Virgin Islands." United States Geological Survey, Caribbean

District Open-File Report. |



Davis, S. N. and DeWiest, R. J. M. 1966. Hydrogeology. John

Wiley & Sons, New York,



Fisher, D. W. and Jordan, D. G. 1977. “Relation of Bulk

Precipitation and Evapotranspiration to Water Quality and

Water Resources, St. Thomas, Virgin Islands.” U. Se

Geological Survey, Caribbean District Open File Report.



lem, J. oD. 1970. Study and Interpretation of the Chemical

Characteristics of Natural Water. U. S$. Geological survey

Water Supply Paper 1973. :



Hydrology Subcommittee 1982, Guidelines for Determining

Flood Flow Frequency, Bulletin #17B. U. S. Geological

Survey, Reston, Virginia.



Johnson Division, UOQP Inc. 1982, Ground Water and Wells.

Saint Paul, Minnesota.



Jordan, D. G. 1972. "Land-Use Effect on the Water kegimen

of the Virgin Islands." U. S. Geological Survey Professional

Paper 8O00-D.



Jordan, D. G. and Cosner, O. J. 1973. "A Survey of the

Water Resources of St. Thomas, Virgin Islands." U. 5.

Geological Survey, Caribbean District Open File Report.



Jordan, D. G. 1975. "A Survey of the Water Resources of St.



Croix, Virgin Islands." United States Geological Survey,

Caribbean District Open-File Report.



-79-

Linsley, R. K. et. al. 1975. Hydrology for Engineers.

ricGraw-ilili, New York.



McWhorter, D. and Stunada, OD. kK. 1981, Ground-Water

Hydrology and Hydraulics. Water Resources Publications, Fort

Collins, Colorado.



Office of Water Research and Technology, "The A+B-C of



Desalting" U. S. Department of the Interior, Washington,

D.C,



Peebles, RR. We. et. al. 1979. Waterplan: A Comprehensive

Water Management Framework for the U. S&S. Virgin Islands.

Technical Report No. 2, Caribbean Research Institute,

College of the Virgin Islands, St.Thomas, Virgin Islands.



Rivera, L. H. et. al. 1970. Soil Survey, Virgin Islands of



the United States. Soil Conservation Service, Washington,

D.C,



Rivera, L. W. et. al. 1966. Soils and Their Interpretations

for Various Uses - St. Thomas and St. John, American Virgin

Islands." Soil Conservation Service, Washington, D.C.



Robinson, T. M. 1972. "Ground Water in Central St. Croix,

United States Virgin Islands." U. S&S. Geological Survey,

Caribbean District Open File Report. |



Robinson, T. M. et. al. 1973. “Water Records of the United

States Virgin Islands.” U. S. Geological Survey, Caribbean

District Open File Report.



Sanders, T. G.f€ed.) 1980. Hydrology for Transportation

Engineers. U.S. Department of Transportation - Federal

Highway Administration, FHWA-IP-80-1.



Schulz, £. F. i978. Problems in Applied Hydrology. Water

Resources Publications, Fort Collins, Colorado.



Science and Education Administration 1979, Field Manual for

Research in Agricultural Hydrology. Agriculture Handbook



Number 224, U. S. Department of Agricuiture, Washington,

D.C.

Smith, H. H., 1981. Effects of Various Factors on the



Sizing of Rain Water Cistern Systems. Technical Report No.

19, Caribbean Research Institute, College of the Virgin

Isiands, St.Thomas, Virgin Islands.



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~3]~