COO/4339-3 SOLAR ENERGY CONVERSION : AN ANALYSIS OF IMPACTS ON DESERT ECOSYSTEMS Final Report for Period June 1, 1977 - December 31, 1977 Duncan T. Patten Arizona State University Tempe, Arizona 85281 May 1978 Prepared For The Department of Energy under Contract No. EC-77-S-02-4339
COO/4339-3 SOLAR ENERGY CONVERSION : AN ANALYSIS OF IMPACTS ON DESERT ECOSYSTEMS Final Report for Period June 1, 1977 - December 31, 1977 Duncan T. Patten Arizona State University Tempe, Arizona 85281 May 1978 Prepared For The Department of Energy under Contract No. EC-77-S-02-4339
SOLAR ENERGY CONVERSION: AN ANALYSIS OF IMPACTS ON DESERT ECOSYSTEM ARIZONA STATE UNIVERSITY DEPARTMENT OF ENERGY CONTRACT NO. EC-77-S-02-4339 FINAL REPORT EXECUTIVE SUMMARY 1. The desert region of North America is considered a prime area for the development of solar energy conversion facilities. Development of these facilities will both create disturbances and alter the environment of the construction area. The desert ecosystems are under stress and can be expected to respond dramatically to any major environmental change caused by solar energy conversion system construction and operation. 2. There are solar energy conversion systems in the Southwest that are functional while others are in the testing or planning stages. Two solar powered irrigation pumping systems are functional using parabolic mirror solar concentrators. The solar thermal power system concept ("power tower") is being tested at a 5 MW thermal test facility near Albuquerque, New Mexico prior to actual construction of a 10 MW^ solar thermal power system near Barstow, California. A 100 MWe solar thermal system (.central receiver system) is being planned as a follow up project to the 10 MWg facility. Photovoltaic solar dispersed power systems are also planned for construction in the next decade. 3. Impacts resulting from construction and operation of solar conversion facilities have the potential of altering the complex desert ecological systems. These ecosystems are composed of two parts, the biotic or organism component and the abiotic or chemical and physical components. Within the biotic component energy is transferred through the food chain and organisms interact in the process of growth and survival. The abiotic
factors such as temperature, precipitation, soil, etc. establish the environment within which the organisms function. Interrelationships between organisms and abiotic factors generate flows of nutrients and other ecosystem constituents through biogeochemical cycles. It is these interacting processes and cycles that may be altered by solar conversion facilities. Based on the ecosystem concept, theoretical ecological impacts of solar conversion systems have been developed. These are discussed under the impact categories of shading, wind deflection and physical disturbance. Ecosystem factors that are considered to have significant changes resulting from shading are solar radiation, soil and air temperatures and soil moisture. These factors greatly influence the functional processes of plants and animals and these changes are expected to cause changes in growth diversity and density of plants and animals and behavioral patterns of animals. Wind deflection by solar conversion facilities is expected to cause changes in air velocity and turbulance, air temperature profiles, heat flux patterns, soil moisture and evaporation and humidity. Organism response will relate directly to development of stress or optimal conditions as these factors change. Construction and maintenance of solar conversion facilities will cause physical disturbances which may change soil structure, runoff and soil erosion patterns, moisture holding capabilities of the soil and energy flux patterns due to surface albedo changes. Plant and animal changes in density diversity, growth, reproduction and behavior can be expected as a result of disturbance alterations within the solar collector array. 4. Research needs to study the impacts of solar energy conversion systems are based on the research required to test hypotheses developed from an analysis of the theoretical ecological impacts of solar collector systems. Hypotheses on biotic changes in solar collector arrays are based primarily on the concept that abiotic changes are the driving variables governing biotic changes. General biotic changes will be in plant productivity, plant species diversity and animal species composition.
Plant and animal density and diversity should increase with time while plant productivity is increasing. Density of soil organisms should also increase over time. Diurnal animal behavior should be altered. Plant and animal responses will vary depending on whether the desert community was preserved or the site was cleared. Abiotic hypotheses are based on the premise that solar radiation or energy input will be altered and air movement will be changed. It is hypothesized that more of the radiation available to organisms will be diffuse, soil will be compacted changing soil moisture availability, and air temperatures should be cooler on the average within the collector array. Other hypotheses are related to these. The research needed to test these hypotheses is designed around sampling changes in plant and animal processes and monitoring the abiotic changes within and outside of a solar collector array. There are established techniques for measuring the suggested biotic and abiotic changes and these should be used. 5. The short and long range research program to establish the impacts of solar energy conversion is based on background studies and the hypotheses on potential impacts. The research program is considered in two parts, the first a chronological sequence of events that should lead to an adequate understanding of the environmental problems resulting from various solar conversion systems and the second, a presentation of specific types of research manipulation and monitoring to be done at various solar conversion facilities and/or collector simulation sites. The chronological research program is a six step sequence. The first step is to develop a theoretical base of ecosystem parameters that might be altered due to construction and maintenance of solar collectors. This theoretical base is presented in the body of this report and should be used as a background when considering any further research or monitoring program. The second step is validating the theoretical responses proposed in step one. This could be done, in part, as actual solar energy conversion systems are constructed but it would be better to be able to project potential environmental changes prior to establishing a monitoring system
or research program at any actual site. This pretesting or prior validation can be done by simulating a solar collector facility and then studying those ecosystem parameters that are expected to change under this form of altered environment. Step three is directly related to step two. Once expected ecosystem changes are validated, alternative methods of monitoring should be tested to establish the level of detail desired from future monitoring programs. This can be developed at a simulation site prior to establishing a monitoring program for an actual solar collection facility that is planned or under construction. Step four is the establishment of a baseline study and monitoring program for the first functional solar thermal power system (STPS) to be constructed near Barstow, California. This program should be based on all the background material developed in the theoretical impact review as well as at a solar collector simulation facility. Data from the monitoring program at the 10 MWe STPS at Barstow will be used as a guide to develop construction and operation procedures at future solar conversion facilities. For this reason step five should include various forms of manipulation of site characteristics at the Barstow facility. These should include such modifications in procedures as changing the soil surface and using different heliostat washing techniques. Monitoring these manipulations then becomes an integral part of the overall monitoring program. Step six is the establishment of monitoring programs for future STPS and photovoltaic facilities. These programs may be limited based on the data obtained from the ecosystem parameter validation tests (Step 2) and the monitoring and manipulation program at the Barstow STPS. Specific monitoring and manipulative programs are the second part of the research program as presented and should be developed for each solar conversion or simulation facility to be studied. The ecosystem parameters to be studied at each facility are defined and the specific length of time of measurement to enable adequate determination of parameter response is presented. Three solar conversion or simulation facilities are discussed in this section. These include a solar powered irrigation pump system at Willard, New Mexico, a solar collector simulator facility such as one being studied
near Phoenix, Arizona, and the 10 MWe STPS to be constructed near Barstow, California in the next few years. The specific data collection research needs at the Willard irrigation facility should be limited to only plant density and diversity and a few abiotic variables such as solar radiation, heat flux, temperatures, wind, precipitation and soil water. These factors should be measured only periodically over the next three years to compare with data taken from other solar collector facilities. The selection of these parameters and those described for the simulation site and 10 MWe STPS facility are based on the background theoretical impact data and are only a few of the large number of biotic and abiotic variables that could be measured. Research at the solar collector simulation site is more intensive because the data are to be used as guides for future monitoring programs and possible modifications in construction or maintenance procedures. The period of study should last at least three years, if not longer, because of the response time lag of the biotic components of the ecosystem. Plant and animal density and diversity as well as plant productivity and animal behavioral pattersn should be closely monitored within the collector simulation array and in an adjacent control site. Soil organisms should also closely be measured for possible changes resulting from abiotic parameter modification. The abiotic factors that should be measured include those that are involved in energy flux, the water cycle and soil characteristics. After getting baseline data on changes resulting from collector influences in all of these factors for two years, site manipulation should be done to determine the impacts of altering construction or maintenance activities on ecosystem parameter responses. These manipulations should include altering vehicular traffic density and patterns, modifying the ground surface through use of different surface materials and use of alternative heliostat washing techniques. Study of the impacts of these alterations in procedures should continue for at least two years. The monitoring program at the 10 MWe STPS at Barstow should be established in two steps. The first step is to determine the baseline characteristics of the ecosystem parameters to be monitored. This baseline study should occur prior to site preparation and should last for at least
six months and preferably a year. The monitoring of these parameters should begin following construction except for soil characteristics that should be monitored throughout the construction phase. The factors to be monitored should be essentially the same as those measured at the simulation site, although the intensity of measurement might not be as great. When the monitoring begins, manipulative studies should also be started at the 10 MWe STPS site. The manipulations would include those used at the simulation site, i.e., vehicular use variations, soil surface changes, and washing procedure changes, as well as altering the stow position of the heliostats and monitoring ecosystem variables in areas with different heliostat packing factors (densities). The ecosystem parameters that are expected to respond to these changes in operation procedures are indicated in the text. The monitoring program at Barstow should continue for the life of the facility (i.e., to 1984 or 1985). The large amount of data obtained from this monitoring effort will give guidance to the type and intensity of any monitoring program at a future solar energy conversion system.
SOLAR ENERGY CONVERSION: AN ANALYSIS OF IMPACTS ON DESERT ECOSYSTEMS TABLE OF CONTENTS EXECUTIVE SUMMARY ............................................... i I. INTRODUCTION ................................................... 1 II. TYPES OF SOLAR CONVERSION SYSTEMS ............................... 3 Systems in Existence ......................................... 3 Systems in Planning ........................................... 4 III. THEORETICAL ECOLOGICAL IMPACTS OF SOLAR CONVERSION SYSTEMS. ... 7 Ecosystem Structure ........................................... 7 Biotic Components ........................................... 7 Abiotic Factors ............................................. 9 Ecosystem Cycles ........................................... 10 Environmental Modifications ................................... 11 Shading......................................................... 12 Solar Radiation............................................... 12 Temperature................................................... 16 Soil Moisture................................................. 18 Plant and Animal Response.....................................19 Wind Deflection................................................. 22 Velocity and Turbulence ..................................... 22 Temperature and Heat Flux..................................... 24 Soil Moisture and Humidity................................... 25 Plant and Animal Response..................................... 26 Physical Disturbance ......................................... 27 Soil Structure............................................... 27 Runoff and Erosion........................................... 28 Soil Moisture................................................. 30 Temperature and Heat Flux..................................... 31 Plant and Animal Response..................................... 31
IV. RESEARCH NEEDS: A FUNCTION OF ECOSYSTEM CHANGES...............35 Biotic and Abiotic Hypothesis ............................ 35 Biotic Hypotheses: General ............................. 36 Biotic Hypohteses: Site Preserved ....................... 38 Biotic Hypotheses: Site Cleared ......................... 40 Abiotic Hypotheses ....................................... 40 V. RESEARCH PROGRAM...............................................43 Chronological Program ..................................... 43 Ecosystem Parameters: Establish a Theoretical Base. ... 43 Ecosystem Parameters: Validate Responses ............... 45 Monitoring Ecosystem Parameters: Alternative Methods. . . 47 10 MWe STPS: Baseline and Monitoring Program .......... 48 10 MWe STPS: Manipulations and Site Variables.............49 Future and Photovoltaic Systems: Monitoring Program ... 50 Specific Monitoring and Manipulation Programs ............ 51 Solar Powered Irrigation Pumping Site .................. 51 Solar Conversion System Simulation Site ................. 53 10 MWe STPS: Barstow, California ..................... 57 100 MWe STPS: Proposed...................................59 VI. LITERATURE CITED............................................... 60 APPENDICES..................................................... 67 Glossary...................................................A-l Appendix Table 1...........................................A-5 Data from Solar Collector Simulation Site Phoenix, Arizona ........................................... A-8 Field Set-up.............................................A-8 Air Movement (Wind) .....................................A-8 Air Temperatures.........................................A-ll Soil Temperatures.......................................A-ll Soil Moisture...........................................A-14
SOLAR ENERGY CONVERSION: AN ANALYSIS OF IMPACTS ON DESERT ECOSYSTEMS I. INTRODUCTION The deserts of North America are considered to be prime areas for the development of solar energy conversion. The northern or cold deserts have limited potential; however, the deserts of the Southwest have an environment with most of the characteristics necessary for construction of economically feasible solar energy conversion facilities. The deserts of the Southwest, including southern California, Arizona, New Mexico and western Texas, have extensive periods of sunshine, little cloudiness and precipitation and very few days with any appreciable amount of snow. These typical characteristics of the Southwest deserts create an environment that is marginal for sustaining a high level of species density and productivity. Because of this marginal level of productivity, the desert ecosystem is extremely fragile to any types of disturbance. Disruption of the plant community will cause a shift in species dominance which requires an extremely long period for ecosystem recovery extending for decades or possibly centuries. The response of plants and animals to disturbance in the Southwest deserts is not the only slow process of this arid region. The soil, especially the soil surface, has taken an extensive period to develop and stabilize. The surface, through wind and water erosion, has become a stable layer of small rocks called desert pavement. This layer prevents accelerated erosion by water and development of dust through wind turbulence. Disruption of the surface layer through any form of perturbation will increase dust in the area, cause changes in the texture and microtopography of the soil surface and alter surface albedo. The generally clear, warm, dry climate of the Southwest which makes the deserts so fragile and slows down any biotic recovery following disturbance, also attracts many visitors and new residents. This increasing population has created a demand on the limited energy resources of the region. Solar energy has become a real potential for helping solve the predicted energy shortages of this rapidly developing area.
Design and construction of solar energy conversion facilities in the arid Southwest will create a form of perturbation which is new to this region. Farming, grazing, industrialization, and urbanization are established forms of perturbation in the Southwest. Development of large solar powered electrical systems will add to the ecological disruption of a desert area that is already near the critical edge of an environmental shift. Although the Southwest has been dominated by deserts for centuries and the ecosystems appear stable, many areas have been shown to have been modified within a short time period due to man’s activities (Martin 1963, Hastings and Turner 1965). If the solar energy conversion systems are to be developed on a large scale basis in the Southwest, then it is important that the ecological disruption created by these systems be understood. It should be emphasized that the ecological changes caused by the solar conversion systems are not necessarily deleterious and might be used in an economically beneficial fashion. There are many types of solar conversion systems from thermal powered solar concentrating systems to photovoltaic dispersed power systems. Considering the wide variety of these systems and the great diversity within the Southwest desert ecosystems, the potential for different types of ecological impacts is unlimited. This project was designed to review the potential ecological sensitivity of the desert and correlate the types of environments to be created by the various solar conversion systems with the desert variables. Through developing theoretical responses of the desert ecosystem to perturbations similar to solar conversion facility construction and operation, and proposing hypotheses that, if tested, will show the desert ecosystem response, a research program can be established that will enable prediction of ecological impacts of solar conversion systems on the desert ecosystem. This report in a stepwise fashion attempts to establish a properly designed research program to accomplish this objective.
II. TYPES OF SOLAR CONVERSION SYSTEMS The types of solar conversion systems in operation, under construction and planned for the desert Southwest are variable enough that a brief description is presented to enable evaluation of the potential habitat changes created by the systems and thus analysis of the ecological impacts. This section is not designed to offer details of the existing or planned conversion systems for these are available from the contractor’s engineering specifications or from the Department of Energy. This section is limited to direct solar energy conversion systems and does not include home units cr other conversion systems such as wind or ocean thermal. Systems in Existence There are very few functioning solar conversion facilities in existence. Two types that are of interest to this study include the solar powered irrigation pumping facilities which are considered dispersed solar powered conversion systems and the 5MW thermal solar power tower at Sandia Laboratory near Albuquerque, New Mexico which is termed a solar powered central receiving system. The solar powered irrigation pumping systems use parabolic-trough collectors to concentrate solar radiation on a pipe containing oil. The heated oil is used to "power" a pump which moves water for irrigation. The parabolic collectors are each between 1.5 and 3 meters wide and no more than 25 meters long, depending on the system. They track the sun, rotating on a horizontal north-south axis. During tracking the edge of the reflectors come within about 0.5 meters of the ground. The series of reflectors are spaced to avoid shading each other after about 9 AM in the summer. Other times of year, shading may continue later into the day. The sites of the solar powered irrigation systems are small. Neither of the two sites observed near Gila Bend, Arizona and Willard, New Mexico exceeded one hectare. In both cases the substrate of the site had been prepared by scaping and compacting. The Gila Bend site had a builtup, compacted substrate because it was located in the middle of irrigated fields. The 5MW solar thermal test system at Albuquerque, New Mexico consists of an array of reflectors or heliostats which concentrate reflected solar
radiation at a point on a tower. This system is designed primarily to test different heliostat styles and configurations. The area around the tower has been thoroughly disturbed during construction and will remain that way and therefore is of little interest in terms of ecological impacts or changes over time. The total impacted area where the heliostats and tower are located does not exceed 30 hectares. This area has been asphalted for convenience of heliostat testing and therefore all potential ecological responses on site have been eliminated. The heliostats at the 5 MW solar thermal test facility, as of July 1977, were manufactured by Martin-Marietta and consisted of a 7 x 6.5 m (22* x 21’) panel of 25 mirrors (each 4’ x 4’) with spacing between the mirrors. When horizontal the panel was approximately four meters off of the ground surface but when vertical the lower edge was only one meter off of the ground. The heliostat tracked the sun by pivoting on both vertical and horizontal axes. When stored horizontally the heliostats were spaced approximately three meters apart (E-W) and six meters north-south. Shading of adjacent panels occurs in the early morning or late afternoon. The array of heliostats at this site is not very extensive so that the packing factor varies only slightly from near the tower to the area farthest from the tower. Systems in Planning A variety of solar conversion systems are in the planning stage to be constructed in the next few years for smaller units and in the next decade for larger systems. These systems include larger solar thermal power tower systems and small photovoltaic solar electric systems. The system that is due for initiation of construction in the next year or two is a 10 MWe solar thermal power tower (central receiver collector) system near Barstow, California. This system is not unlike the 5 MW STPS at Albuquerque. It does have some differences in design that will have a different influence on ecological responses. The overall area being impacted is three to four times as large (about 100 Ha). Because of the large area the heliostats near the power tower have a higher packing factor than those on the perimeter of the array. The ground surfaces of the site will probably not be permanently sealed (e.g., asphalt) but the surfacing has not been
determined. The heliostats are designed by McDonnell Douglas and consist of two rectangular reflectors (each approx. 6.5 x 3 M) spaced 0.7 M apart. The heliostats rotate on both vertical and horizontal axes. When vertical the lower edge of the heliostats will be 0.7 M off of the ground. There will be 1760 heliostats on the site. A 100 MWe central receiver collector system is planned to be started in the next decade. This system may be composed of two 50 MWe power towers but the general design will probably be very similar to the Barstow 10 MWe STPS. Basic differences will be the same as the differences between the Albuquerque 5MW STPS and the 10 MWe STPS. The area will be considerably larger creating a very large altered habitat that has a greater potential of becoming ecologically stable than some of the smaller sites. The larger site size will also require a greater variation in packing factors of the heliostats as they will be located farther from the central receiving tower. Because of the size of the location selected, the terrain will probably be less disturbed as a result of construction and operation but this is difficult to predict. Photovoltaic solar dispersed power systems will probably also begin to be constructed in the next decade. These will initially start out as small units such as the 500 kWe system planned for the Phoenix, Arizona airport. The area disturbed is small and because of the location the ecological impact of this particular system is not worthwhile evaluating. However, larger systems will be planned and ecological consequences will then be worth considering. The photovoltaic panels may well look like those described by Johnston (1977). The panel is 3 x 5 M with 135 plastic Fresnel lenses concentrating the sunlight on small photovoltaic cells. Peak electric output is about 1 kW per panel. The panel can adjust slightly both vertically and horizontally to track the sun. It does not have a horizontal stow position. In time, both solar thermal central receiver collector systems and photovoltaic systems may cover hundreds or thousands of hectares. These extensive systems will not only have major ecological impacts within the array of collectors but perhaps more importantly will have a significant environmental consequence on the surrounding region. Although the microhabitat changes may be predictable at the time of construction of the very large
systems, the regional impacts can only be conjectured and these changes will have to be measured after the systems are completed.
III. THEORETICAL ECOLOGICAL IMPACTS OF SOLAR CONVERSION SYSTEMS Ecosystem Structure In order to study the response of an ecosystem to some form of perturbation, the components of the ecosystem as well as ecosystem processes and functions must be taken into account. An ecosystem has two major components, the organisms (biotic part) and the environment that controls or influences the functions and processes of the organisms (mostly abiotic). Because ecologists are biologists by training, they set a primary emphasis on the organisms, considering the other parameters as controlling, limiting, or trigerring factors. Changes in organism behavior or function within an ecosystem is, in all probability, a response to a change in one or many of the influencing factors. For this reason great interest might be placed on physical or chemical factor changes but only because these changes ultimately cause biotic change. Biotic Components of the Ecosystem. The biotic part of an ecosystem can best be described by following biological energy flow through the system. The organisms in an ecosystem are divided into producers and consumers. One set of organisms produces food or energy for another set which is the consumer. The plants are the primary producers. They convert solar energy to usable biochemical energy that is then available for all other food (trophic) levels in the ecosystem. Only those few types of ecosystems that have an inflow of biochemical energy from outside the system, such as an
heterotrophic stream system, can function with few plants. Plants, in a sense, are the energy driving force of an ecosystem, second only to the sun. The plants are of many types. Some grow for many years and are woody while others carry on their whole life cycle in a few weeks and survive only in a seed form. In the desert ecosystem both of these types are common. In addition, plants in the desert have adapted in many ways to drought or water stress conditions. These adaptations include, for example, deciduousness, small leaves, heavy pubescence on the leaves and succulence. For each plant species, the period of primary production varies. Some produce year around while others are seasonal. Any composition change in the plant species within an ecosystem may alter the periods and amounts of primary production. The organisms in an ecosystem that are dependent on the primary producers for energy are the animals and the decomposers. The animals that eat the plants (herbivores) are often referred to as secondary producers because the energy they store is available for the next trophic level, the carnivores. Animals play many roles in the transfer of energy through the ecosystem. These roles can be partly related to the types of animals, that is, whether they are large browsing or grazing mammals or small sap sucking or detritus eating invertebrates. Many of the animals also play an active role in the below ground portion of the ecosystem. In the desert ecosystem the larger animals are found in both the herbivore and carnivore levels. Some of the major herbivores are the lagomorphs (e.g., jack rabbits) and rodents (e.g., ground squirrels, wood rats and kangaroo rats). Insects such as grasshoppers also may play an important role in energy flow from plants to consumer. The carnivore consumer level varies widely from large carnivores, such as coyotes, to smaller vertebrates, such as the lizards and snakes, to small invertebrates such as beetles and spiders. Birds in the desert function in both herbivore and carnivore roles. Extremely important in the function of the desert ecosystem is the role of the detritivores, for example, termites. In order to have a constant energy and, nutrient turnover, primary and secondary production must be reduced to a form again usable to the primary producers. Often the dead organic and waste material is too large for rapid breakdown by decay organisms. The organisms that reduce the organic waste material and speed up the potential decay are the detritivores. These organisms are found both above and below ground.
It should be obvious from this brief discussion of the biotic aspect of an ecosystem that all organisms play an important role. Any alteration in the number or diversity of the various groups of organisms can have a dramatic effect on the normal function of the ecosystem. Abiotic Factors. The functions of plants and animals in an ecosystem are directly related to interaction with other organisms as well as the physical and chemical environment in which they live. For example, variations in abiotic factors control the reproductive, mortality, and physiological rates and behavioral patterns of the animals. All organisms require moisture to survive. This moisture comes from the environment or other organisms that are consumed. For this reason moisture availability above and below ground in an ecosystem is extremely important to the survival and functioning of both plants and animals. This is especially true in semi- arid and arid regions such as the deserts of the Southwest. Most of the functions of plants and animals are directly influenced by the temperatures of their surroundings. For example, the poikiolotherms (cold blooded animals) maintain the correct body temperature by moving to the proper environment. Rates of physiological function as well as behavioral and distributional processes are also directly correlated with temperature variations. Environmental temperatures are, in turn, a result of many other abiotic factors. For example, soil temperatures are influenced by soil moisture, soil color, wind movement over the soil surface and shading or sun exposure. Air temperatures at any one location are also a result of many of these same factors, i.e., amount of solar insolation, air movement, humidity, etc. Moisture and temperature are probably the two most critical abiotic environmental factors in controlling the functions and diversity of plants and animals. They are obviously closely interrelated with other factors. Some have been mentioned such as solar radiation, air movement and soil type, but other factors can also be important. These factors can be characteristic of a particular habitat. Topographic and geological features, for example, influence the amounts of solar radiation soil moisture retention and potential nutrients in the soil. The interaction of all of these factors as well as many others that are difficult to measure produce the interwoven aspect of the environmental complex. They can be separately studied but
they must be analyzed in relation to the complexity of the ecosystem. Ecosystem Cycles. Each function or response of a plant or animal in relation to abiotic factors is not totally separate from the responses of other organisms or other factors. All processes within an ecosystem are, in some way, related and cyclic in nature. For example, solar energy flows in from outside the earth’s atmosphere, is transformed into different physical or biochemical forms and through various conversions is ultimately lost again as heat to outerspace. The rate of flow of the energy through these cycles controls the physiological and growth processes of the organisms as well as many abiotic factors such as evaporation, temperature and air movement. Although energy and the energy cycle are driving forces in the ecosystem, other cycles are of great enough significance to be considered in research on ecosystem responses. In the arid regions the water or hydrological cycle is of great importance. In this cycle both physical and biological factors play an important role. For example, air temperature controls the amount of moisture in the air and the potential for precipitation as well as evaporation; soil texture and structure control the amount of water maintained by the system while plants intercept precipitation, control runoff and remove quantities of water from the system through transpiration. The water cycle can thus be considered the flow of water from a physical state through a biological function and back into a physical state with all facets of the ecosystem acting on the processes. Other cycles such as the nitrogen or carbon cycle are important to proper functioning of an ecosystem but they are closely related to energy and water flows. It should be obvious that an understanding of ecosystem response to perturbation such as construction and operation of solar collectors is not dependent on analysis of the response of a single animal or plant species or a single physical factor but requires a study of those factors that most significantly influence biotic functions and an analysis of the accompanying biotic changes. These changes occur most often as changes in animal and plant species composition (diversity) as well as species density. These two measurements are indicators of the organisms ability to grow and function within its original or newly created environment.
Environmenta1 Modifications Environmental modifications are of two types, short and long term. Short term or immediate changes in the microenvironment of a structure in the desert can be readily demonstrated by appropriate instrumentation. These are, however, only physical changes and might quickly be reversed if the modifying structures were removed. On the other hand, if these short term microenvironmental changes are permitted to continue for any length of time, they will then trigger, both directly and indirectly, longer term modifications of the ecosystem that are not so readily reversible. Long term modifications are considered to be ecological changes because not only is the microenvironment altered but the biotic components of the ecosystem are also changed. Although ecological changes may take many years to occur, the ecosystem once stabilized under new conditions will remain changed unless man again alters the overriding controlling conditions. Ecological changes resulting from construction and operation of solar collector systems are impossible to quantify without taking some actual measurements. It is possible to estimate changes in such factors as solar input, energy budgets and soil moisture using equations based on hypothetical structures and environmental processes, but every ecosystem is variable and it is this variability that will cause hypotheses based on theoretical calculations to be in great error. Other ecological factors such as wind movement, soil erosion and biotic processes are impossible to predict. One can draw hypothetical conclusions based on the literature and the theoretical calculations of variations in physical processes, but this is only a preliminary step. From these early predictions one must design tests to prove or disprove established hypotheses.
As a first step in developing hypotheses, the literature was thoroughly searched for all relevant data or ideas that might help with analysis of solar conversion system impacts. This literature search was not limited to solar collectors or to the Southwest but reviewed all worldwide literature that might pertain to structures, either natural or artificial, and their influence on arid or semi-arid ecosystems. In addition, the basic and applied ecological literature as well as appropriate engineering literature were reviewed for conceptual matter that might be even peripherally considered pertinent to this study. The literature review presented many differing views on the possible consequences of construction and maintenance of solar collector arrays in a desert ecosystem. Not all data agreed but by using the most reliable information, theoretical impacts of solar collectors were developed. The approach taken was to break the impacts down into major categories and subcategories and develop theories within each of these. The major categories are: 1. Shading 2. Wind Deflection 3. Physical Disturbance Shading Solar Radiation Solar radiation, the environmental variable which is probable in least demand to living organisms in the southwestern deserts, will possibly be the most affected by arrays of solar collectors. How much of the incident solar radiation is intercepted by collectors, and thus redirected away from the
ground surface, will depend on the types of collectors used, and the spacing (or "packing factor") of the collectors along east-west and polar axes. Black and Veatch (1977) analyzed a hypothetical photothermal collecting facility, and determined that an array of heliostats covering between 42% and 56% of the ground surface would intercept 59% and 78% of the annual daily direct radiation, respectively. Their calculations were made at the equinoxes, and thus did not account for changes in azimuth of the sun with season. They assumed that interception of diffuse radiation would be equal to ground coverage of the heliostats (i.e., 42-56%), although acknowledging that this is probably an underestimate because tracking mirrors should intercept a greater proportion of the more intense diffuse radiation from the sunward part of the sky than would a horizontal surface. Assuming total incident radiation in the desert Southwest to be 80% direct and 20% diffuse (Liu and Jordan 1960), between 55% and 73% of the total incident radiation will be intercepted by heliostats and removed from the desert ecosystem (based on Black and Veatch 1977). A reduction of such proportion is highly significant. A more recent study of the "power tower" type of solar collection facilities indicates that only 25% of the ground area will be covered by heliostats (Hildebrandt and Vant-Hull 1977). The heliostat coverage will vary from 40% near the tower to approximately 10% near the periphery of the heliostat field, because with increasing distance from the tower greater amounts of blocking of reflected sunlight by adjacent heliostats occur. With reduced ground cover there will be less shading than the figures given by Black and Veatch (1977). The design suggested by Hildebrandt and Vant-Hull (1977) would probably result in removal of less than 50% of the total incident radiation from the system. Unfortunately, we were unable to obtain any values for ground coverage of projected photovoltaic arrays. Photovoltaic arrays may suffer greater penalties from shading loss due to adjacent collectors, and thus should be installed at reduced packing factors compared to photothermal systems (Arizona State University 1977). If so, solar radiation losses would be reduced in photovoltaic arrays but still should not be substantially below 50% of total incoming radiation.
Since solar arrays are designed to operate year around, packing factors should be designed to enable the solar facilities to produce sufficient quantities of energy in the winter, when shading losses from neighboring collectors will be highest. During the peak growing season in late spring to early fall, the amount of intercepted radiation, and thus shading of the ground, would be expected to be less than values estimated at the equinoxes due to the higher azimuth of the sun. On the other hand, shading of the ground surface in the winter months, also a very productive season in the Southwest desert ecosystems, could be nearly complete, especially during the morning and afternoon. The types of solar collectors used will play an important role in how much of the ground surface is shaded, and how that varies on a temporal basis. For instance, heliostats (designed by Martin Marietta) tested at the Sandia Laboratories, Albuquerque solar thermal test site had spacing between the mirrors while others to be tested may be solid. This difference in reflectors may not cause significant differences in absolute incident radiation reaching the ground surface, but could be important with regard to duration of shading of a given area of ground, as sun spots will move across the desert floor as the system tracks the sun. Different types of proposed photovoltaic collectors will produce different seasonal patterns of shading of the ground surface (Arizona State University 1977). Collectors which rotate on a single horizontal axis will produce almost complete shading of the ground surface within the collector array in the winter (with collectors rotating on a polar axis) or on a diurnal basis (early morning and late afternoon for east-west axis rotation of collectors). Conversely, a collector tilted at 45° from the zenith and rotating around a vertical axis to track the azimuth of the sun (similar to the simulators constructed in Phase I) will not exhibit significantly different seasonal shading (Arizona State University 1977). Collectors rotating on vertical and horizontal axes simultaneously exhibit the smallest shading losses, and thus can be placed at the highest packing factors. It is evident that knowledge of specifics involved with collector design is a necessary pre-requisite in order to predict interception of incident radiation, and thus energy loss from the ecological system.
Solar collectors will also affect the thermal (long-wave) radiation emitted by the ground to the sky. In a normal open desert system, intense heating of the ground surface during the day is followed by a high rate of reradiation at night from the ground to the ’’cold" sky (Sellers 1965), resulting in rapid cooling of the ground surface. If 10 to 40% or more of the sky is blocked by solar collectors, then approximately the same percentage of long-wave reradiation would be intercepted. If the collectors are in a stow (inverted) position at night (Hildebrandt and Vant-Hull 1977), then the thermal radiation intercepted will be redirected back toward the ground. Therefore, solar collector arrays will substantially decrease incoming solar radiation in the daytime, as well as reduce outgoing thermal radiation in both the day and night. The result will be reduced net radiation under the collectors on a daily basis, and a substantial reduction in average hourly radiation flux. Several related studies support these predictions. Bajza et at. found an irrigated landscape (i.e., with trees and large bushes) to have lower incoming radiation at the ground surface than a desert landscape during the day, as well as having lower long-wave emitted radiation at night due to less sky exposure (as well as cooler surface and subsurface temperatures). Net radiation on the irrigated landscape never fell below zero, while on the desert landscape it was very high during the day, and fell well below zero at night. Results by Lowe and Hinds (1971) are similar. They found a palo verde tree in winter in the Arizona desert to reduce incoming radiation by 50% compared to the open, while also reducing effective outgoing radiation at night by the same amount. The result was a maximum diurnal net radiation flux under the tree of just 44% of that observed in the open. Patten (1975) found similar reductions of incident radiation by palo verde trees in the Sonoran Desert near Phoenix. Of potential importance is the amount of radiation a given area of ground will receive as bands of sun and shadow move across the desert surface. When an area is in shadow (i.e., shielded from direct beam radiation), the area will still be receiving diffuse radiation. How much will depend upon
elevation of the sun. As the sun increases in elevation, the amount of diffuse radiation received in shade spots should increase. A given area of ground in a patch of sun may also receive greater amounts of solar radiation than it would in the open desert. For example, Rosenberg (1974) found areas near a tree shelterbelt to receive amounts of daily radiation equal to an area remote from the shelterbelt. Although shaded for several hours in the morning, the site near the shelterbelt received additional reflected energy off the shelterbelt in the late afternoon. Furthermore, Patten and Smith (1975) note that solar radiation can be extremely high (approaching the solar constant) when scattered cumulus clouds provide large amounts of diffuse radiation in addition to direct radiation. Such a phenomenon will possibly occur due to reflection off nearby collectors, and could have significant ecological effects. Although solar radiation under collectors will be greatly reduced on a daily basis, it may be intense at certain times of the day. Temperature Daytime air temperatures under heliostat fields and photo thermal and photovoltaic arrays will probably be only slightly lower or not significantly different than in the open desert. Air temperatures in shaded microhabitats are generally slightly cooler during the day (Patten and Smith 1973, Hanson and Ravzi 1977, Bajza et at. 1977), with the differences being more pronounced in the dry season (Patten and Smith 1974). Daytime air temperatures near the ground surface should be substantially cooler under solar collection arrays (Black and Veatch 1977); this has also been observed under large Sonoran Desert shrubs (Patten and Smith 1974). Also of potential importance in collector fields is the observation of Tuller (1973) that areas shaded from morning sun have a higher mean daily temperature than areas shaded from afternoon sun. In winter months, air temperatures may be warmer than the open desert due to reduced losses of thermal radiation. Night temperatures in areas shaded during the day are not as well documented. Bajza et at. (1977) observed nocturnal air temperature to be higher in a shaded microhabitat, evidently because the tree canopies which produced the shade also reduced the amount of escaping long-wave radiation at night.
Patten (.1975) obtained similar results for microenvironments beneath palo verde trees. Nighttime temperatures under collector field may not be as easy to predict due to the complication of wind deflection by arrays of collectors, and their effect on nocturnal inversions, which will be discussed later. Although early evening temperatures should be cooler under collectors (assuming cooler temperatures during the day), interception and possible reradiation of long-wave emitted radiation should retard nocturnal cooling relative to that in the open desert. ERDA (1977b) predicts no significant difference in nighttime temperatures. When comparing temperature and organism function, Terjung et al. (1970) state that total environmental radiant temperature is more important in determining organism heat gain or loss than the popularly used air temperature. Radiant temperature is the temperature that a black body would have to be at to produce thermal radiation equal to the downward counterradiation of the sky (Terjung et al. 1970) , and is calculated from the total radiation flux using the Stefan-Boltzmann equation (Schmidt- Nielson et al. 1965). Studies using this parameter have shown significant reductions in radiant temperatures in shaded desert habitats. Dawson and Denny (1969) report radiant temperatures beneath the canopies of black oak and mulga {Acacta} trees in arid Australia to be 55% and 49% of that in the open, respectively. Lowe and Hinds (1971) found palo verde trees in Arizona to reduce this parameter an average of 37% in the winter with respect to daily maximum value. Furthermore, Lowe and Hinds (1971) found winter nocturnal radiant temperatures associated with the infra-red flux to be much higher under a palo verde canopy than in the open, once again due to the canopy blocking the sky and thus limiting loss of thermal radiation from the surface. Surface and soil temperatures should be influenced far more than air temperatures due to shading. Cloudsley-Thompson (1965) and Patten and Smith (1974) observed desert surface temperatures in the shade to be similar to air temperatures, while exposed temperatures may reach 80°C (Cloudsley- Thompson 1965), with a diurnal range of up to 55°C (Geiger 1965) . Although trees and shrubs in arid regions are known to significantly reduce surface temperature in the warm season when they are fully leafed out (Shreve 1931,
RkJQdWJsaXNoZXIy MTU5NjU0Mg==