Thirsty Plants – Riparian Groundwater Use in the SPRNCA
Gerald R. Noonan PhD © 2019
Water is the all-important resource essential for the survival of the beautiful ribbon of green that stretches along the San Pedro River. Much of the water that plants use in the riparian corridor along the river comes from groundwater. The riparian corridor is a transitional area between aquatic and terrestrial ecosystems that require the existence of surface or subsurface water flows. The riparian zone is partially vegetated by plants that use groundwater. When rain falls to the ground, some of it seeps into the soil and clings to particles of soil or roots of plants just below the land surface. Water not used by plants moves downward through empty spaces or cracks in the soil until it reaches a layer of rock through which water cannot easily move. The water then fills the empty spaces and cracks above that layer. The top of the water in the soil is called the “water table” while water that that fills the empty spaces and cracks in the soil is called “groundwater.” Maintaining a high water table of groundwater along the river is essential for their survival of many riparian plants such as cottonwoods and willows.
A 2006 scientific study provides detailed information about the use of groundwater within the SPRNCA (Leenhouts et al., 2006). The 174-page paper is a multidisciplinary investigation by 16 different scientists of vegetation water needs and uses in the riparian zone. Scientists prepared the report in cooperation with the U.S. Geological Survey, Bureau of Land Management, Arizona Department of Water Resources, City of Sierra Vista, U.S. Department of Defense, the Agricultural Research Service of the U.S. Department of Agriculture, and the U.S. Environmental Protection Agency. The fourth chapter (Scott et al., 2006) of the report estimates the amount of groundwater lost through evapotranspiration
As part of their metabolic processes plants transpire water. Transpiration involves the movement of moisture through plants from roots to small pores on leaves where it changes to vapor and is released into the atmosphere. Water can also evaporate from the surface of water bodies such as rivers or from wet soil. Evapotranspiration is the sum of both transpiration and evaporation. The evapotranspiration discussed in the fourth chapter includes that from the free water surface of the river and that from plants. Researchers studied evapotranspiration at five different sites from March 2001 to December 2003. They ultimately determined evapotranspiration for the 2003 growing season because measurements for that period were available from all of the study sites. The researchers estimated evapotranspiration for each of the major cover types (Freemont cottonwood—Goodding’s willow trees along perennial and intermittent reaches, mesquite woodlands (primarily mesquite but also including other shrubs), sacaton grasslands (also including some other grasses), and direct evaporation from the stream).
The complex techniques for estimating evapotranspiration are beyond the scope of this article. However, to provide a glimpse of the complexity of procedures, a brief summary is here provided of part of the techniques used for measuring evapotranspiration from cottonwoods. Two thermocouple needles were installed as a vertically aligned pair 4 cm apart within the sap-wood of each of four different cottonwood test trees at each study site from April to November 2003. One of the needles was constantly heated and the other was unheated. A technical device measured the difference in temperatures between the two needles. The rise in temperature of the heated needle was inversely proportional to the velocity of the sap flow that carried away the heat. That is, the greater the sap flow, the lower the rise of temperature within the heated needle. A series of additional measurements and complex mathematical calibrations made possible the calculation of transpiration from a given tree. Additional calculations provided estimates of transpiration from cottonwoods within different regions of the SPRNCA and within the SPRNCA as a whole.
Water is the all-important resource essential for the survival of the beautiful ribbon of green that stretches along the San Pedro River. Much of the water that plants use in the riparian corridor along the river comes from groundwater. The riparian corridor is a transitional area between aquatic and terrestrial ecosystems that require the existence of surface or subsurface water flows. The riparian zone is partially vegetated by plants that use groundwater. When rain falls to the ground, some of it seeps into the soil and clings to particles of soil or roots of plants just below the land surface. Water not used by plants moves downward through empty spaces or cracks in the soil until it reaches a layer of rock through which water cannot easily move. The water then fills the empty spaces and cracks above that layer. The top of the water in the soil is called the “water table” while water that that fills the empty spaces and cracks in the soil is called “groundwater.” Maintaining a high water table of groundwater along the river is essential for their survival of many riparian plants such as cottonwoods and willows.
A 2006 scientific study provides detailed information about the use of groundwater within the SPRNCA (Leenhouts et al., 2006). The 174-page paper is a multidisciplinary investigation by 16 different scientists of vegetation water needs and uses in the riparian zone. Scientists prepared the report in cooperation with the U.S. Geological Survey, Bureau of Land Management, Arizona Department of Water Resources, City of Sierra Vista, U.S. Department of Defense, the Agricultural Research Service of the U.S. Department of Agriculture, and the U.S. Environmental Protection Agency. The fourth chapter (Scott et al., 2006) of the report estimates the amount of groundwater lost through evapotranspiration
As part of their metabolic processes plants transpire water. Transpiration involves the movement of moisture through plants from roots to small pores on leaves where it changes to vapor and is released into the atmosphere. Water can also evaporate from the surface of water bodies such as rivers or from wet soil. Evapotranspiration is the sum of both transpiration and evaporation. The evapotranspiration discussed in the fourth chapter includes that from the free water surface of the river and that from plants. Researchers studied evapotranspiration at five different sites from March 2001 to December 2003. They ultimately determined evapotranspiration for the 2003 growing season because measurements for that period were available from all of the study sites. The researchers estimated evapotranspiration for each of the major cover types (Freemont cottonwood—Goodding’s willow trees along perennial and intermittent reaches, mesquite woodlands (primarily mesquite but also including other shrubs), sacaton grasslands (also including some other grasses), and direct evaporation from the stream).
The complex techniques for estimating evapotranspiration are beyond the scope of this article. However, to provide a glimpse of the complexity of procedures, a brief summary is here provided of part of the techniques used for measuring evapotranspiration from cottonwoods. Two thermocouple needles were installed as a vertically aligned pair 4 cm apart within the sap-wood of each of four different cottonwood test trees at each study site from April to November 2003. One of the needles was constantly heated and the other was unheated. A technical device measured the difference in temperatures between the two needles. The rise in temperature of the heated needle was inversely proportional to the velocity of the sap flow that carried away the heat. That is, the greater the sap flow, the lower the rise of temperature within the heated needle. A series of additional measurements and complex mathematical calibrations made possible the calculation of transpiration from a given tree. Additional calculations provided estimates of transpiration from cottonwoods within different regions of the SPRNCA and within the SPRNCA as a whole.
The cover type “Open water” refers to shaded and unshaded water that stands or flows along the surface of the riverbed within the other cover types. Ground-water use for the remaining cover types refers only to water lost by transpiration from plants and does not include water lost by evaporation from open water. Because of the small amount of tamarisk in the SPRNCA, the water use patterns by that plant were assumed to be similar to those of mesquite. The ranges of ground-water use given for mesquite, sacaton, and tamarisk reflect the uncertainty of measurements concerning the actual vegetation areas of these cover types.
Figure 2 graphs the water use in acre-feet per year for the various cover types, based on the data in table 49. For the three cover types with estimated ranges of water use, I converted the range for each cover type into an average use by averaging the low and high ranges. The greatest amount of evapotranspiration from a cover type is that from mesquite, because of the very large amount of area covered by these plants. Figure 2 shows that the 684 acre-feet of water evaporation from open water is much less than the 3072 total evapotranspiration from cottonwoods—willows of perennial and intermittent reaches and tamarisk.
Figure 2 graphs the water use in acre-feet per year for the various cover types, based on the data in table 49. For the three cover types with estimated ranges of water use, I converted the range for each cover type into an average use by averaging the low and high ranges. The greatest amount of evapotranspiration from a cover type is that from mesquite, because of the very large amount of area covered by these plants. Figure 2 shows that the 684 acre-feet of water evaporation from open water is much less than the 3072 total evapotranspiration from cottonwoods—willows of perennial and intermittent reaches and tamarisk.
Figure 3 graphs the water use in acre-feet for all riparian plants versus open water, based on the data in table 49 and using averages for cover types with ranges. Most of the riparian evapotranspiration water loss within the SPRNCA is from plants, as opposed to from open water. The much greater water loss from plants is because of the much greater surface area covered by vegetation relative to open water.
Measurements and calculations suggested that shading of water by trees, arroyo walls, and the sides of active water channels might reduce evaporation by approximately 65%. However, the highly heterogeneous degrees of canopy shading and of amount of river entrenchment made it difficult to derive a precise estimate of the amount of water evaporated from open water. The authors noted that, “the amount of open-water surface is small (table 49) compared to the vegetation community amounts, so additional refinements in the open-water evaporation estimate were not warranted.” If shade did reduce the amount of water evaporated from open water by 65%, then the amount of saved water would have been 1270 acre-feet—a figure much less than the 3072 acre-feet of evapotranspiration from cottonwood-willow and tamarisk.
Cottonwood-willow forests were dense and of multiple aged composition in places where the maximum ground-water depths averaged less than approximately 3 m, the streamflow permanence was greater than about 60%, and intra-annual ground-water fluctuation was less than about a meter. The cottonwood-willow forests gave way to tamarisk stands as the site-average ground-water depths across the floodplain became greater than 3 m. Conditions were too dry within intermediate-dry streamflow regimes sites to allow the establishment of cottonwood and willow seedlings.
Mesquite—primarily velvet mesquite—was the most abundant vegetation type within the SPRNCA. The plants were deep-rooted and could survive in areas where they did not have access to groundwater. However, mesquite formed denser stands in riparian locations. Mesquite was widely distributed within the SPRNCA and was abundant at both dry and wet sites and on floodplains and terraces. The growth form and abundance of velvet mesquite was related to the elevation of the site, with the mesquite being more abundant and forming larger trees at lower sites.
The root system of mesquite is well adapted for acquiring and storing moisture. It has a deep taproot that typically is 5 to 13 m but sometimes reaches as deep as 58 m (Sosebee and Wan, 1989, p. 106, 112 abs.). It also has an extensive system of lateral roots with radii extending to 17 m. Unlike many other plants, velvet mesquite has a root system that is able to take moisture, such as that from precipitation, and transport it downward into deeper soil where it is less likely to evaporate and is unavailable to plants with shorter roots (Hultine et al. 2004). The transport of water downward can occur even during the winter when the portions of mesquite above ground are dormant. The storage of water in deep soil layers can provide a reservoir of moisture to buffer plants from water deficits during the initial stages of the growing season.
The perennial grass sacaton occupied more area on the floodplain than any other herbaceous plant species and was also abundant on terraces. Bermuda grass and Johnson grass also were abundant on floodplains. The grasses probably used a variety of water sources such as groundwater, precipitation and floodwater, depending in part on seasonal availability.
There was a relatively small amount of tamarisk in the SPRNCA, mainly restricted to places north of Fairbank. The abundance of tamarisk increased at dry sites, probably because of reduced competitive interactions with cottonwoods and willows.
The abundance of mesquite near the river accords with early historical records for the San Pedro River and for riparian areas in southern Arizona in general. For example, Cooke wrote (1849, p. 37, 58 abs.) in his journal that in 1846 there was a vast thicket of mesquite near where the San Pedro River and Greenbush Draw joined. Guy Keysor also noted (Standage and Golder, 1928, p. 193) the large areas of mesquite there. On December 11, 1846 some members of the Mormon Battalion hid behind mesquite while fighting off bulls (Tyler, 1881, p. 219, 228 abs.). On December 12, 1846 the Mormon Battalion marched down the San Pedro River from a camp slightly northwest of the junction of the Babocomari and San Pedro rivers to a camp southwest of the present town of Saint David (Talbot, 2002, p. 46-48). Cooke noted (1849, p. 38, 59 abs.) that the mesquite along the march took on the appearance of a small tree and along with others gave a wooded appearance to much of the valley bottom. He also wrote that there was plenty of mesquite at that night’s camp. On December 13, 1846, the Battalion marched to a new camp approximately ¾ of a mile northwest of current day Benson (Talbot, 2002, p. 47-48). In places the mesquite along the route took “the exact resemblance of orchards. . . .” (Cooke, 1849, p. 39, 60 abs.). On December 14, 1846, the Battalion marched northwestward, leaving the vicinity of the San Pedro River and ascending ground that for the first approximately two miles was difficult to pass through because of the need to clear a route through plants that consisted mainly of mesquite and palmetto (Cooke, 1849, 39, 60 abs.).
Mesquite, like cottonwoods and willows, affords important habitats for birds and other animals. The sad history of the Santa Cruz River demonstrates the importance of these habitats. The river south of Tucson used to have The Great Mesquite Forest, one of only two mesquite bosques in North America that were named in the scientific literature (Webb et al., 2014, p. 25, 92-111). People came long distances to view the rich assortment of birds and other wildlife at the mesquite bosque and in other areas along the river. The cutting of mesquite for fuel and especially the lowering of water tables by pumping destroyed the mesquite bosque and its wildlife habitats.
The survival of the beautiful riparian ribbon of green in the SPRNCA depends upon the continued endurance of a high water table there. Citizens of the San Pedro River Valley have a choice—they can maintain the ribbon of green by seeing that urban development is not allowed to further deplete groundwater or they can decide to convert the ribbon of green into an ugly barren drainage ditch as was done with the Santa Cruz River in Tucson.
Abbreviation abs. = Page number as indicated by Adobe Reader.
Literature cited
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