REMOTE SENSING OF SUBSURFACE WATER RESOURCES IN THE U.S. VIRGIN ISLANDS by Michael J. Canoy Project -No. A-016-VI Agreement No. 14-34-0001-1150 September 1983 The work upon which this report is based was supported in part by funds provided by the United States Department of the Interior, as authorized by the Water Research and Development Act of 1978 Technical Report No. 14 Caribbean Research Institute College of the Virgin Islands St. Thomas, USVI 00802 ~~ ij P [ r [ [ l [ [ [ [ DISCLAIMER ia) Contents of this publication do not necessarily re- Gal flect the views and policies of the U.S. Department of the Interior, nor does mention of trade names prt or commerical products constitute their endorse- Fret ment or recommendation for use by the U.S. Government P 11 [ [ f a rr ne lhe, renee oe a poe yj “3 “F “yoy | SF 7 ABSTRACT Location of areas for groundwater exploitation has been a long standing problem in the Virgin Islands. In response to this need a project was designed to briefly assess the possibi- lity of using plant pigments surveyed from aircraft or satellites to locate aquifer. The results of this project indicate that: (a) plant pigments can indeed be used to detect soil moisture changes, (b) the chlorophyll/carotinoid ratio is especially useful, (c) technical difficulties prevent the type remote sensing tried from being useful. Recommendations are made for design changes. iii Fy “~sJ TC" ~~ ?Z |? ee ee re ee ee ee oe iio iio ire Abstract List of Tables List of Figures Introduction Thesis Methods Results Discussion . References TABLE OF CONTENTS iv Page iii- Vi 14 19 Table LIST OF TABLES Page Chlorophyll Content of sepa} Trees and Shrubs - St. Thomas... a oe ee 10 Soil Moisture, as Parts of Dry Weight ...... 12 Soil Moisture and Plant Pigments .......-. 16 de de Representative Chlorophyll 4 and B for ———eT Select Tropical Species . . s 6 we wle os we 17 LIST OF FIGURES Figure Page 1 Location Map « « 6 © woe ew mw 8 om om mm em 2 Le de 2 Absorption Spectra of Chlorophylis & and & ee Curve A, Ghlorophyll a; curve B,“€hlorophyll b in 90% acetone Soe ie Ss en Me Me 7 5 Spectral Sensitivity for Kodak EA-5 ..... . 8 yi I. Introduction and Background St. Thomas, a Caribbean Island in the U.S. Virgin Islands, relies on a variety of sources for its domestic water supply. In the early 1970s, these sources included wells on the eastern part of the island, three desalinization plants, water barged from Puerto Rico, and a large number of individual household cistern systems. At one time, large municipal cisterns were operating; however, these systems have not been maintained j and were functioning at the College of mae Virgin Islands(only) at the i ine of the study. St. homes is 4.8 km (3 miles) wide and 19 km (12 miles) long. It has a backbone ridge of mountain which rise to approximately 456 m (1,500 ft.) above sea level. The y climate of St. Thomas is essentially marked by constant oy easterly trade winds and maximum average temperatures about 27°C (80°F) in the winter and 30 to 32°C (87-89°F) during the summer. Average relative humidity is above 80%. Rainfall erty accuve in the form of brief showers, with the .~ higher elevations on the island tending to receive greater amounts of rainfall, on the order of 102-187 cm (40-80 in.), per year. Average monthly rainfall for the month of December though June is 5 to 7.5 cm (2-3 in.), while for July through >? NS = ATLANTIC OCE AA F | | — ~ Y —— — ec’ or % BAHAMAS CUES HAITI: DOMINICAN wie Ctr a2 HONDURAS Pears —e ee: BRITANICA JAMAICA vemee iz HONDURAS CARIBBEAN SEA “Ng EL SALVADOR NICARAGUA SD PACIFIC SArriniDap OCEAN OSTA RICA COLOMBIA, af ak — VENEZUELA ATLANTIC OCEAN ST. TERS 2 -s = a_i . CHARLOTTE AMALIE o * ee ST. JOHN CARIBBEAN SEA CHRISTIANSTED FREDERIXSTED ST. CROIX THE U.S. VIRGIN ISLANDS LOCATION MAP FIGURE fi a) November it is on the order of 10 to 12.5 cm (4-5 in.) of which 80% is during July and November. The population of the Virgin Islands is approximately 110,000 persons, having doubled in 15 years and being expected to double again in the next 10 years. The water problem is expected to parallel this. Fresh water has always been in critical supply in St. Thomas. Rain collected on roofs and stored in cisterns is still the source of water for most rural and urban domestic supplies. Before 1960 hillside rain catchments and a few dug ' wells were the major source of water for public supplies. Since then, desalted water has become the major source of water for public supplies, and water barged from Puerto Rico is a close second. Charlotte Amalie has a dual public water system. Fresh water is used for drinking and general household needs, and salt water is used for sanitary and fire-control purposes. The fresh-water supply, obtained from salt-water distillation plants, hillside rain catchments, and wells, is supplemented by water barged from Puerto Rico. Potable water use and the sources of the water not only show the increasing demand for water but also the shift in sources of the water. Barged water became the major source of supply in the early 1960s, but by the late 1960s, desalted water became the principal source of supply. Il. Thesis The technological explosion stimulated by military and space research has made available a diversity of airborne sensors to record information about ecosystems. These sensors capture energy from various portions of the electromagnetic spectrum. The information 50a Faby: remote sensors may be primarily a function of plant surfaces,oy the environment or of a complex interaction of both. In any case, the biolo- gical implications of the signals received must be interpre- THEE /S * ted, and herfe is alittle prior experience to guide the ———— interpreter. Three basic types of sensor systems for remote sensing from airborne or satellite platforms are available; all three are processed to present two-dimensional or picto- Yial displays. Photography is used in the visible and far-red spectrum, 300 to 1000yum; optical-mechanical scanners are employed in the infrared wavelengths, 1 to 40um; and passive microwave and radar are developed for selected bands from .lum up to lum. To date, by far the greatest amount of information about vegetation has been derived from photography. Among the most promising wavelengths are 700um to 900ym in which plants reflect 80% to 90% of the impinging light. Frequently information from several spectral bands surpasses the sum of each band considered separately. Ecological research with remote sensors is predicated on the necessity of developing these tools for the study of entire populations and ecosystems to understand and manage the consequences of our technology. (Eanoy, 1978) FLE= _ 4 Water resources have been evaluated by remote sens- ing for years in terms of surface water, submerged lands or wetlands, or snow cover. In the dry tropics, however, most water resources are subsurface and often in rugged terrain. This makes it attractive to have a method for remote sensing of subsurface moisture, or at least an elimination of certain areas in water prospecting. This study was designed as a quick survey to deter- mine if there is a quick and easy way to use remote sensing for this purpose. III. Methods A. Soil Moisture at the surface and three (3) foot depth were determined in the 4 study sites. Moisture was read from a Bouyoucos Moisture Meter, Model BN- 2B ,Beckman Cedar Grove, N.J. Three (3) sets of samples were taken September 22- 26, 1982 December 21-28, 1982 and April 18-24, 1983. Leaf samples for pigment analysis were taken at the same time from mature trees within a 10 meter radius of the moisture meter site. B. Plant Pigments were analysed on fresh material from the site. Extraction and chlorophyil per gram of leaf were determined as follows. Chlorophyll Estimati The determi phyll content is frequently required in s always better to extract fresh tissue an ts immediately, al- though extracts can be st in the dark in acetone containing traces of Na2CO; at -20 to -30°, without appreciable loss. As a general precaution, it is es to work in dim light to avoid pigment losses. The fresh tissue is ground in a mortar or macerator in the presence of excess acetone or re methanol until all the colour is released from the tissue. (Mg)—— Mp COs is added to prevent pheophytin formation and the extract is filtered on a Buchner funnel, the brey being washed with fresh acetone until colorless. The extract and washings are then made up to a known volume and stored in the refrigerator. Measurement of chlorophylls a and b can then be made by direct determination of the absorpance at different wave-—— 2 lengths, using a standarg spectrophotometer. Assuming an PT AMEE 90% acetone extract, the absorBace should be measured at 663 and 645 nm in 1 cm cells. The concentrations can then be cal- culated from the following fomulae: 20.2 Agys'+ 8.02 Aces Total chlorophyll (mg/1) Chlorophyll a (mg/1) 17.7 Ace3 — 2.69 Agus Chlorophyll b (mg/1) 22-9 Acgys — 4-68 Aces can be converted to chlorophyll content on a fresgh weight basis as follows: a mmc’ 12.3 Aces — 0.86 Agus X V a x 100 xy W Chlorophyll a (mg/g) = \ oe? Fi \ye wr epee one ite tents mane Swe me Oo a + F 2 Se —_—_ ge = 5 0 0 rs fe) . > 2 ‘So e-28 Eigg on Oo R @ 8 ~ oO dg 4, ron 2 Or qo o 2 sb « Ga 0) SN g 88 3 Ww VHD Re - a au p>} eee eeeeeen wad peewee errr ne s 3 * mM ee eee —“— Ho . Y : ae T $B an Oa Seen Se &