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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 <q ts) “sao = | NT ee ee a a ee a ee a ee pe " . Q. 33 S S>an Oa Seen Se & <f ms 6 g E E i | t f ! rs) om oe =x eI A dy fe) fe) Q Q 2 ; B ro 8 9 e a 8 8 . o ? a a B ~ . 7 ~ wy ef P quaraiyja0d UOLUIyXe 91)!93dS fs é fa 'S Speciral Sensitivity Curves Process EA-5 ' ~ ' Development Process EAS Gremcotn oOo tehoe Formang Mogere ore \ iu; oo Cron a Formeng 0.0) Lover \ < 1. 500 a €] 700 750 500 850 400 450 WAVELENGTH (am) Sensitivity = Reciprocal oi the exposure (ergs/cm?) required to produce 2 density of 1.6 zdeve D-min Measurements were confined to the 400- to 700-nenomeier region. FIGURE 3 From: Eastman Kodak "Aerial Photography as a Planning Tool". 1976, M-128 C. Carotinoids were extracted into chloroform and read at 450 nm. This wave length was arbitrarily chosen because the majority of the over 400 carotinoids are found in the range of 400-530 nm. Margalef (1959) found the ratio of “aretingids to chlorophyll to correlate well with primary production in several ecosystems. Odum et al. (1970) applied this method to productivity and reaction to stress in a tropical rainforest. In this study we found that carotinoids ranged from e 2.3 mg/g to 2.9 mg/g and were consistg@nt per species where chlorophyll varied rather widely according to plant conditions. D. Remote sensing of reflected light was accomplished m using Nikon F Camera loaded with Kodak Ektachrome Infrared film (false color). Initially a 135 mm lens was used: but -s— MD this proved to have too poor resolution, ag in the last two sample periods an 80 mm lens was used. The film required a Wratten 12 filter to exclude blue wave lengths. Films were developed locally according to the manufacture?S specifications. IV. Results Soil moisture levels were measured on three occasions, once, a weegk after a 4 inch rain. The moisture blocks and Samples were at the surface and 36 inch depth. It was found that the surface soils varied considerably according to soil composition, rainfall, and environmental factore{ (Table ca a ~ ee en Ad ae At Oo, —— — faa) = ies) a) TABLE I TROPICAL TREES AND SHRUBS - ST. THOMAS Chlorophyll 4” (mg/g) Top/Bottom Leaf Varia- Species Sun Leaves Shade Leaves tion in color 1. Sloanea berteriana 4.5 5.3 2. Picteta aculeata owe) 6.1 3. Exostrema caribaeum 7/2 7.6 a 4.3 4.6 Prosopsis juliflora Guaicuma officinale 4.9 5.5 ey Amyris elemifera 6.1 6.1 4.3 7. Andropogon sp. 4.3 8. Bucida buceras 5.5 6.3 = 9. Canella winterana 4.9 5.7 5.5 ss) 10 Thouinia sp. 3.3 11 Rhizophora mangle 5.0 6.0 = 12. Luceana glauca 4.3 4.8 -] m= | 10 ao] =A — = = = = an a that at 36 inches there was little difference in moisture even after moderately heavy rains. There was no ap “eH correla- x Fale, tion between deeper soil moisture and aquifers Dor tiivea 7. SS ae eX Lameshur). Fa Kekio, iin. Ht 7 SLE HEX T Soil moisture measured at 0.15-0.37 parts/of ary —— weight (15-37%) at the surface and 0.5-0.13 parts/dry, weight (5-13%) at 36 inch depth. The available moisture was both lower and less variable at the depth feeding most shrubs and stall trees. Plant pigments were found to vary with species, sun A adaptation and age of the leaf. Chlorophyll *& (spectrum maxi- ma at 665 nanyfometers , nm) ranged from 4.5-9.3 milligrams per gram (mg/g). Of the 400 known carotinoids., over 80% have a 2/sTHD spectrum maxima between 425 and 480 nm. Following (Margalef, == 1962) and Odum, et al, 1970 @ arbirary median spectral point, ——— 450 nm of carotinoids in chloroform,was used as a reference. Total chloroform-soluble carotinoids measured at this point were calculated at 2.3-2.8 mg per gram of leaf. These measures were not so variable as the chlorophyll contents and seemed to remain within 15-20% of a mean regardless of the situation of the plant. Aerial and satellite photos of the watersheds were evaluated in terms of known plant pigment ratios. The 10 satellite Infra-red photos and 72 aerial false-color infra-red photos showed no discernable pattern in plant/pigments despite li ~ = =— = 7 os 7 ao | — we —% TABLE 2 SOIL MOISTURE, AS PARTS OF DRY WEIGHT Sampie DE Pou Sample Site Number 3-6 inches 35-38 inches Dorothea I SET I 1 0.15 0.05 2 0.28 0.08 Elevation; + 18 MSL S O25 0.07 Soil Sandy Roam 4 0.23 0.07 SET 2 5 0.17 0.05 6 0.24 0.08 7 0.34 0.12 amt 8 0.33 0.10 9 O37 0.43 Mean —- 0:26 0.08 Dorothea Il 1 0.23 0.08 SET I 2 0.16 0.06 3 0.18 0.07 4 0.21 0.07 SET 2 bi 0.27 0.09 6 0.15 0.05 ye 0.19 0.07 SET 3 8 O22 0.08 9 0.18 0.06 Mean - 0.20 0.06 Lameshur I aL. 0.12 0.05 Sandy Loam SET I 2 0.09 0.03 + 15' MSL 3 0.13 ND LZ ‘= =] — Ss =e (continued...) Sample DEPTH Sample Site Number 3-6 inches 35-38 inches 4 0.07 0.02 SET 2 5 0.09 0.03 6 0.10 0.03 7 0.08 0.05 SET 3 8 0.09 0.07 9 ©...L1 0.03 Mean — 0.09 0.03 Lameshur IT L 0.13 0.06 Rocky clay, SET I 2 O.c:2 0.04 Loam 3 0.15 0.07 4 0.13 0.05 SET 2 5 O.11 0.05 6 0.10 0.02 7 0.09 0.04 SET 3 8 OuLL 0.05 9 0.08 0.07 Mean - Oued. 0.05 L3 —_— 6 =—=q on site differences which were measured. Possible reasons for this will be brought forth during the discussion section of this paper. V. Discussion Remote sensing has become a more and_ more common rae ore oe s tool for resource studies ((USGS/NASA, 1980, Jensen and Meyer, 1976). This includes various estimates of water resources (U.S. Nat. Research Council, 1980; Striffler and Fitz, 1980). This techniques is especially attractive to planners where ———— large areas a distant and difficult terrain is involved. In the case of tropical ecosystems plant pigment indices were suggested by Margalef as indicators of primary peof DEPEL RE = production and stress in 1959 and 1962.( Odum, McConnell, and ‘Abbott (1959)) verified these findings in temperate systems and Odum, et al, (1970) using the Richards and Thompson methods. BE (1952) for chlorophyll analysis, extended the study to a tro- pical rainforest. Here they applied the Margalef ratios to the El Verde research site in Puerto Rico in a study which combined actual measurement of pigments with estimates from remotely sensed data. The agreement between remote data and actual measurements on a spot by spot comparison was not conclusive$ but, if the chlorophyll measurement was sonrep ted to an areal value (multipling the amount per gram of Leag to the calculated leaf biomass per meter) of chlorophyll per square meter, the agreement was quite good. ce. Fine (Plant pigment analyses (Table 2) compare well to those pre- Gul Pe < Studies by Canoy (1970, 1972)) showed that the plant . . 2 Oo, sa: . pigment ratio per m™ was a Bix indicator of stress, production, and biochemical potential (as DNA) in rainforests, tropical dry forests, and mangrove forests. The present study attempted to relate on-site pig- ment analysis directly to aerial data and soil moisture. The A. SA we tiv O4S7Y, viously found in Puerto Rico (Table 4) and are probably accu- rate. The direct measure of free soil moisture was checked against samples dried in the labl and these measures seem accurate. Furthermore the mean soil moisture data correlated well (y = 0.93) with the plant pigment ratios. There is however no discernable pattern in the remote photographs relative to soil, moisture or aquifers. The aim of this study Is tO AE xP rthtdine mits pote seers the question of remote sensing for aquifers and there- fore did not cover some expensive or time consuming studies such as biomass or potentiometric sensing for really deep (over 36'') water sources. Ve The new data hag yielded several results. 1) Plant —— pigment ratios for 12 common trees in St. Thomas can serve as indicators of water deficiency or adequacy. 2) The plant pig- ment ratio is a good indicator for agricultural purposes. 3) There is , at this point, no simple way to relate water availability to remote data via plant pigments. 15 TABLE 3 SOIL MOISTURE AND PLANT PIGMENTS [ Mean Deep a Cartonoids . Soil Chlorophyll (@ 450 nm) Ratio lr Site Set Moisture A mg/g EE ng/g - Dorothea IL 1 0.07 5.5 2.8 0.51 2 0.07 6.2 2.3 0.37 3 0.11 9.3 2.3 0.25 ia Dorothea II 1 0.07 6.2 2.5 0.40 2 0.08 6.3 2.5 0.40 ~ 3 0.09 7.8 2.4 0.31 L Lameshur I 1 0.03 4.5 2.9 0.63 2 0.03 4.5 2.6 0.58 r 3 0.05 5.1 2.3 0.45 - Lameshur ITI 1 0.06 6.0 2.6 0.43 lr 2 0.04 4.8 2.8 0.58 3 0.05 5.3 2.5 0.47 i | 16 TABLE 4 he 4e REPRESENTATIVE CHLOROPHYLL X AND B FOR SELECT TROPICAI., SPECIES Js Beckman Chlorophyll Bayech & L omb Chlorophyll %, _ Tree and leaf data Tree No. Sample No. 645 630 665 wK, mg/liter (665) mg/liter Daeryodes exucelsa 2695 Sun leaves, new 1 0.185 0.179 0.462 600 0.83 70 Sun leaves, new 8 0.154 0.143 0.396 570 Shade leaves, new 22 0.118 0.108 0.325 422 45 Shade leaves, old 39 0.378 0.334 0.978 14 10 86 Mantlkara bidentata 2680 Shade leaves, old 47 0.375 0.357 +925 13 00 78 Sun leaves, new 51 0.254 0.245 -641 725 14 67 0.285 0.275 742 10 850 84 Sun leaves, old Euterpe globosa 2421 Sun leaves, new 98 0.253 0.245 -650 9.4 780 04 Shade leaves, new 105 0.353 0.334 -950 13.8 05 35 Sun leaves, old 113 0.153 0.145 »415 6.0 520 57 Ase *Ratio of chlorophyll & to B is 8.9 From: Odum and Pigeon, 1970. "A Tropical Rain Forest." USAEC 17 & a £ E E. E Ee. E&W & €&_. — of remote 1) 2) 3) 4) The parameters necessary to clearly define the use data for water "prospecting" include: Leaf biomass per Square meter Root, particularly tap root, depth. The species diversity index (species/acre) Other physiological reactions of these species to water stress such as leaf folding, leaf abcission, or release of gums, oils, etc. As a result of this study it is shown that Virgin Island§foresters or agriculturalists can use plant pigment ratios as an indicator of productivity or stress. It has further indicated that remotely sensed pigment data can be used, given the data listed above, to locate shallow aquifers during dry periods. 18 a | i re a ns Se Sn Sc Se Sn Se Se rr rr en ns ne | 10. ll. References Anderson, E.A., ''National Weather Service River Forecast System -- Snow Accumulation and Ablation Model," NOAA Technical Memorandum NWS HYDRO-17, U.S. Department of Commerce, Silver Spirng, Maryland, 1973, 00 217. American Society of Photogrammetry, Falls Church, VA 20046, "Manual of Remote Sensing", 1982. Aruga, Y., and Monsi, M., 1963, Chlorophyll Amount As an Indicator of Matter Productivity in Bio Communities, Plant Cell Phystol., 4 29-39. Burnash, R.J.C., Ferral, R.L., and McGuire, R.A., "A Generalized Streamflow Simulation System; Conceptual Modeling for Digital Computers", U.S. Department of Commerce, NWS, and State of California, Department of Water Resources, Sacramento, California, March 1973, p. 204. Canoy, M.J., 1969. "DNA in Ecosystem Stress and Diversity". Proc. Amer. Assoc. Adv. Science. , 1970. "DNA in Natural Ecosystems. Ph.D. Hsasertation, University of North Carolina. VOY) '7 B- Trolf, T.R., and Vandnais, K.G. "Operational Airborne Measurement of Snow Water Equivalent Uisng Natural Terrestrial Gamma Radiation", Proceedings, Western Snow Conference, Laramie, Wyoming, April 1980, p. 10. Crawford, Norman H., and Linsley, Ray K., "Digital Simulation in Hydrology: Stanford Watershed Model Iv'', Department of Civil Engineering, Stanford University, Technical Report 39, July 1966, p. 210. Harborne, J.B., 1973. Phytochemical Methods. Halsted Press, New York, N.Y. Itten, K.1., "Possibilities for Remote Sensing of Sur- face Characteristics", Chapter 15, Land Surfaces Processes in Atmospheric General Circulation Models, WCRP, Publication Series, WMP, Geneva 1982, (in press). Jackson, T.J. Ragan, R.M. and Fitch, W.N., "Test of Landsat-Based Urban Hydrologic Modeling", Journal of the Water Resources Planning and Management Division, ASCE, Vol. 103, No. WRI, May 1977, pp 141-158. 19 1 Bx 1o.. 14. LS. 1. Ld s 18. iG. 20.. a Jensen, Mark S. and Meyer, Merle P., 1976. A Remote Sensing Applications Program and Operational Handbook for the Minnesota Department of Natural Resources and Other State Agencies. Remote Sensing Laboratory, Univ. of Minnesota, St. Paul. Margalef, R., 1959, Pigmentos Asimiladores Extraidos de las Colonias de Celentereos de los Arrecifes de Coral y Su Significado Ecologico, Invest. Pesquera, 16: 81-101. National Research Council, "Remote Sensing for Water Resources and Hydrology, Recommended Research Emphasis for the 1980s, "Panel on Water Resources, Space Appli- cations Board, Assembly of Engineering, NRC, Washinton, D.C. 1980, p. 34. s Odum, HLT... and Pigeon, R.F., 1970. <A Tropical Rainforest. Div. of Tech. Info., U.S. Atomic Energy Commission, Washington, D.C. Peck, E.L., "Catchment Modeling and Initial Parameter Estimation for the National Weather Service River Forecast System", NOAA Technical Memorandum NWS HYDRO- 31, U.S. Department of Commerce, Silver Spring, Mary- land, June 1976, p. 64. Peck, E.L., McQuivey, R.S., Keefer, T.N., Johnson, E.R. and Erekson, J.L., "Review of Hydrologic Models for Evaluating Use of Remote Sensing Capabilities", NASA CR 166674, Goddard Space Flight Center, Greenbelt, Maryland 1981, p. 99. Ragan, R.M. and Jackson, T.J., ''Runoff Synthesis Using Landsat and SCS Model", Journal of the Hydrologic Division, ASCE, Vol. 106 No. HY5, May 1980, pp 667- 678. “ Schmugge, T.J., "Microwave Approaches on Hydrology", In Photogrammetric Engineering and Remote Sensing, Vol 46, No. 4, April 1980, pp. 495-507. Sittner, W.T., Schauss, C.E., and Monro, J.C., "Continuous Hydrograph Synthesis with an API-Type Hydrologic Model", Water Resources Research, Vol. 5, 1969, pp- 1007-1022. Striffler, W.D. and Fitz, D.C., "Applications of Remote Sensing in Hydrology", Completion Report OWRT Project #B-160-COLO, Part I, Colorado State University, Water Resources Research Institute, September 1980, p 39 plus appendices. : ray Se U.S. Army Corps of Engineers, "Program Description and User Manual for SSARR - Streamflow Synthesis and Reservoir Regulations", U.S. Army Engineer Division, North Pacific, Portland, Oregon, Program 724-D5-60010, Revised June 1975. 2b Bi. U.S. Army Corps of Engineers, "Storage, Treatment, Over- flow, Runoff Model, "STORM", The Hydrologic Engineer- U.S. Army:, Davis, ing Center, Corps of Engineers, California, Program No. 723-S 8-L2520, July 1976. 24. U.S. Army Corps of Agriculture, "CREAMS: A Field Scale Model for Chemicals, Runoff and Erosion from Agricul- tural Management Systems", Conservation Research 640. Report No. 205 1980, p 21, , a