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EVAPOTRANSPIRATION in a DESERT ENVIRONMENT By Evapotranspiration in a desert environment Item Type Thesis-Reproduction (electronic); text Authors Shields, Suzanne Jean. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 30/09/2021 20:46:17 Link to Item http://hdl.handle.net/10150/191771 EVAPOTRANSPIRATION IN A DESERT ENVIRONMENT by Suzanne Jean Shields A Thesis Submitted to the Faculty of the DEPARTMENT OF HYDROLOGY AND WATER RESOURCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE WITH A MAJOR IN HYDROLOGY In the Graduate College THE UNIVERSITY OF ARIZONA 1982 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg- ment the proposed use of the material is in the interests of scholar- ship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: /0) /cP D. D. EVANS Date' Professor of Hydrology and Water Resources ACKNOWLEDGMENTS The author wishes to extend her sincere thanks to the Department of Hydrology and Water Resources for the opportunity to research and present this thesis. Special thanks are extended to Professor Daniel D. Evans for the aid in the topic selection and help in preparation of this research. Without his assistance, this work could not have been performed. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS vi LIST OF TABLES viii ABSTRACT ix 1. INTRODUCTION General Description 2 Scope 5 2. LITERATURE REVIEW 7 Background 7 Lake Evaporation 11 Evapotranspiration under Irrigated Conditions 13 Evapotranspiration under Natural Desert Environments 16 3. PROJECT DESCRIPTION 24 Objective and Scope 24 Site Description 25 Physical Setting 26 Instrumentation 28 4. METHODS 33 Equipment 33 Lysimeter 33 Thermocouple Psychrometer 35 Raingauges 36 Procedures 36 5. RESULTS 40 Previous Investigations 41 Spatial and Temporal Variations 41 Plant Growth and Water Transfer 43 Water Balance 45 iv TABLE OF CONTENTS -- Continued Page 1977-1978 Investigations 51 Direct Evapotranspiration Measurements 51 Soil-Water Potential 54 Soil Characteristics 59 6. DISCUSSION 66 7. SUMMARY AND CONCLUSIONS 70 REFERENCES 73 LIST OF ILLUSTRATIONS Figure Page 1. Evapotranspiration and precipitation for the Curlew Valley site 19 2. Evapotranspiration and precipitation for the Washtucna site 20 3. Precipitation record at the Santa Rita site 22 4. Evapotranspiration and related soil moisture data at the Santa Rita site 23 5. Location map for the Silverbell site 40 miles northwest of Tucson, Arizona 27 6. Diagram of site I showing approximate location of measurements at Silverbell site 29 7. Diagram of site II showing approximate location of measurements at Silverbell site 30 8. Lysimeter containing creosote bush 32 9. Diagram showing approximate location of raingauges and psychrometers used during the 1977-1978 study at site I, Silverbell site 37 10. Cumulative soil water content in 1973 for bursage (A), creosote ([]), and open (0) cover at Silverbell 42 11. Evapotranspiration results using water balance method at Silverbell 47 12. Total precipitation for the same period at Silverbell . 48 13. Evapotranspiration data by the lysimeter and water- at the balance methods, and precipitation for 1976 Silverbell site 50 14. Evapotranspiration data gathered with the lysimeter at the Silverbell site 52 vi vii LIST OF ILLUSTRATIONS -- Continued Figure Page 15. Precipitation record for Silverbell site in 1977 and 1978 53 16. Soil-water potential data at various depths under a creosote bush for the Silverbell site 56 17. Soil-water potential data at various depths within the lysimeter for the Silverbell site 57 18. Soil-water potential for open area at Silverbell site 58 19. Characteristic curve for Silverbell site 60 20. Comparison of evapotranspiration rates from the lysimeter to evapotranspiration rates determined using psychrometer data 64 LIST OF TABLES Table Page 1. Energy balance and diffusion resistance of the creosote bush 44 2. Evapotranspiration rates for selected periods without rainfall 49 3. Computations of a soil characteristic curve for the Silverbell site 61 4. Calculation of evapotranspiration rates using soil- water potential data and a soil characteristic curve . 62 5. Maximum evapotranspiration rates 67 viii ABSTRACT The natural evapotranspiration rate in a desert environment was studied at a site near Silverbell, Arizona, as part of the Desert Biome Program. Evapotranspiration rates were measured directly using a mono- lith weighing lysimeter. Indirect determination of evapotranspiration rates by a water-balance analysis was made using two different methods to measure soil moisture. The soil moisture data were initially gathered directly using neutron probes, but during 1977 soil-water potential data measured with thermocouple psychrometers were translated into soil moisture information using a soil characteristic curve developed for the soil. The annual evapotranspiration rate that was measured or calcu- lated was equivalent to the natural evaporation rate from bare soils. The annual precipitation was approximately equal to the water loss due to the annual evapotranspiration with little or no change in soil mois- ture storage between years. Peak evapotranspiration rates of 3-5 mm/day occurred only after rainfall events, while during dry periods the evapo- transpiration rate approached zero. ix CHAPTER 1 INTRODUCTION An investigation of desert ecosystems was undertaken to deter- mine the natural soil moisture conditions and evapotranspiration rates, and to evaluate methods of measurement in an arid environment. This study attempted to describe in quantitative and qualitative terms the existing conditions using a weighing lysimeter, neutron probes, and thermocouple psychrometers to measure the evapotranspiration rate, soil moisture, and the soil-water potential, respectively. The site for this study is located at Silverbell, Arizona, approximately 70 km northwest of Tucson, Arizona, within the Sonoran Desert. The desert vegetation studied at the Silverbell site included creosote (Larrea tridentata) and bursage (Ambrosia deltoidea). The information gathered at this site was then correlated with another warm desert site in the Santa Rita Mountains, Arizona, and with two cold desert sites located in Curlew, Utah, and Washtucna, Washington. All sites were a part of the Desert Biome Program of the International Biological Program to study plant and water relationships in the major desert environments of the Western United States. This research, which was conducted from 1972 to 1977, was funded by the National Science Foundation and administered by Utah State University. 1 2 General Description Arid regions or deserts were defined by Meigs (1953) as an area where the precipitation is not adequate for regular crop production. Most deserts lie within 15 to 35 degrees of latitude north and south of the equator. Arid and semi-arid regions are generally located on the leeward side of the prevailing winds or so far inland that they are isolated from the moisture source provided by the oceans. While hot deserts are typical, deserts at northern latitudes or high altitudes may be quite cold. Both hot and cold deserts experience extremes in daily diurnal minimum and maximum temperatures because they lack sufficient vegetation cover to prevent the loss of heat at night and sufficient cloud cover to reflect outgoing radiation back towards the earth. The precipitation that is received yearly in arid regions is nominal, yet an individual rainfall event may be quite large and of a high intensity. Total rainfall received overall is small, but the main stress placed on vegetation is the length of time between precipitation events, which may be months or years in length. This variation of long dry periods and short-duration, high-intensity storms has a marked effect on the soil, landforms, and vegetation found in deserts. Desert soils are considered "young" soils, not from their lack of age but because the soils have generally not been leached and weathered as they would be in a more humid climate where water and vege- tation are more pronounced soil-forming factors. Desert soils will, instead, have a high salt content and little organic matter. During most of the year, the soils are extremely dry with soil suction values of 50 bars or greater. The high-intensity rainfall and lack of 3 vegetation cover will sometimes result in compaction of the soil surface layer, limiting infiltration. What moisture does infiltrate may trans- port salts which form calcareous or, in some cases, siliceous layers once the soil moisture evaporates. The low range of soil moisture normal to desert soils also means that vapor movement, heat gradients, and the soil
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