[1] Water is a highly mobile substance, constantly circulating in and out of global reservoirs. The reservoirs in Table 7.1 may gain or lose water but the total supply remains constant. This is in contrast to the supply of water in the reservoirs, which may store water for long, short, or intermittent periods. For example, water that infiltrates the soil via rainfall or irrigation may be taken up by plants. The water absorbed by plants is soon returned to the atmosphere by the process of transpiration, so its residence time in the soil is brief. However, the water in glacial ice does not change much so Technical Note 7. the supply is relatively constant. Ag Water Literacy I. The transfer of water from one place to another in Earth’s biosphere is called the hydrologic or water cycle (Figure 7.1.1). At any given time, only about 0.005% of Earth’s 7.0 Preamble water supply is moving through the cycle, but it is still a large amount of water. The hydrologic cycle is a continuous, open- Water, written as the chemical formula H O, is required by all 2 ended process driven by the evaporation of water via solar living organisms. Life processes such as cell division, growth, radiation. reproduction, and metabolism take place in water. Water is supplied to plants through the young root hairs as they grow into the pore space between mineral particles in the soil. Water lubricates the pore linings, allowing roots to easily penetrate. Water also gives mobility to soil microbes and dissolved plant nutrients. Without a constant supply of water during its life, no plant could reach maturity. This is true for the desert dwelling cactus as for the plants we grow for food, fiber, energy, and shelter. While plants are activity growing, even one day without absorption of water can yield substantial losses, and even cause death. About 71% of Earth’s surface is covered in liquid water. In considering solid, liquid and gaseous states, H2O ranks as the most ubiquitous molecule in nature. Each year about 111,000 km3 of water precipitates on the land surface. On average, 70% of this water returns to the atmosphere as water vapor through the process of evapotranspiration (Ajami 2020). The soil serves as a reservoir for water absorbed from rainfall or irrigation that can be used by plants in the intervals between additions. Water in the soil is, however, different from that of rain water, or water from a spigot, for two reasons: The liquid properties of soil water are modified by the attraction between water molecules and soil particles. The water in soil is never pure, but has a tremendous variety of substances dissolved in it, including 14 of the 17 known plant-essential nutrients. Despite the very large capacity of soil to absorb water, as a factor limiting terrestrial agroecosystems, the supply of water is first in importance. 7.1 Global Distribution of Water Table 7.1 breaks down the global distribution of water by reservoir. Note that the amount of water held in the soil is a minute fraction of the total water. On the other hand, ground water is the third largest reservoir yet comprises a meager 1.7% of the total water. The oceans are, by far, the largest reservoirs of water. However, we can’t drink salt-laden ocean water or use it for watering plants. Only 2.5% of the total water supply is fresh enough for human or plant consumption and about two- thirds of this is locked up in a frozen state. Figure 7.1.1 The hydrologic or water cycle. Source: USGS rev09Jul2021 [2] 7.2 Water as a Substance This creates a tetrahedral electron ‘cloud’ geometry wherein the relative force exerted by the two lone electron pairs Water is a peculiar substance with no natural substitutes or spread around the oxygen nucleus causes the bond angle replacements. It is the product of two light hydrogen atoms between the oxygen atom and two hydrogens to shrink to and a 16-fold heavier oxygen atom joined together by strong 105°, effectively bending the water molecule as shown in chemical bonds called covalent bonds. The average mass of Figure 7.2.11. The lone pairs of electrons do not participate water is 18.02 atomic mass units (amu) or 18.02 grams per in bond formation; as the name implies, they simply exist. mole. There are a number of ways to visualize a water The other two pairs of electrons participate in bond molecule but for our purposes a simple cartoon will suffice: formation with the two hydrogen atoms, represented by the line segments connecting oxygen and hydrogen in Figure 7.2.1, and by the sticks in Figure 7.2.2. Despite the aforementioned partial electron cloud geometry, the water molecule overall is electrically neutral. The larger number of closely held electrons in the oxygen atom makes oxygen attract electrons much more strongly, i.e. it is more electronegative compared to hydrogen. As a result, there is a net transfer of charge away from the hydrogen atoms, and towards the oxygen atom, giving rise to an unbalanced distribution of electric charge within the water molecule. Due to these unbalanced charges and the Figure 7.2.1 A water molecule, chemical formula: H2O. Note bent structure of water, the positive and negative centers of that the size of the atoms represent relative differences in charge are non-cancelling. In liquid water, this produces a charge density, not atomic radii which are actually quite similar net dipole moment or force. These partial charges, for hydrogen and oxygen atoms based on their positions in the symbolized by the small delta plus δ+ or delta minus δ- in periodic table. Figures 7.2.1 and 7.2.2, explain the apparent polar nature of the water molecule. There are several things to note in Figure 7.2.1. First, water In a water molecule, the separation of charge is located is a bent molecule, i.e. its molecular geometry resembles within the H-O bonds. Such bonds, called polar covalent that of boomerang or the letter ‘v’. This arises from the bonds, allow the molecule to interact with the dipoles in interaction of two atomic properties: (1) the relative other molecules, creating a dipole-dipole moment. The differences in charge density on the oxygen atom nucleus opposite charges on hydrogen and oxygen atoms also allows which has 8 positively charged protons (8+) and an equal water molecules to attract neighboring water molecules via number of closely held electrons, compared to hydrogen hydrogen bonding2. The electrostatic force of attraction in which has only one (1+); and (2) the presence of two, hydrogen bonding, though relatively weak compared with mutually repellent, lone pairs of electrons above and below covalent bonding, is strongest when the O-H bond from one the plane of hydrogen atoms, which are better visualized by molecule points in the direction of a nearby oxygen atom this three dimensional stick and balloon figure: such that the three O-H-O atoms fall within a straight line: Figure 7.2.2 Three-dimensional representation of a water molecule. The two lone, mutually repellent, pairs of electrons spread out around the central oxygen nucleus while the shared electron pairs only partially cancel out the positive charge on the hydrogen atoms. This electric charge structure approximates a four-cornered tetrahedron with the oxygen atom near the center and two positively charged hydrogens on two corners. The lone pairs occupy the remaining two corners. 1 More precisely the bond angle given is 104.5°, considered the most Image source: modified from Tro 2011. thermodynamically stable. The bend and stretch (length) of water molecules may exhibit values away from this depending on energy state. Bond angles other than 104.5° are considered “mean” or ‘approximate” values for a given energy state. 2Another force of attraction between molecules, called London dispersion forces, depends on the probability distribution of electrons within their orbitals at a given moment. These are very weak forces relative to hydrogen bonding and are not considered here. rev09Jul2021 [3] One water molecule can participate in four hydrogen bonds Transparency to visible light (~390-780 nm wavelength) with other water molecules. This attraction between adjacent and opacity (i.e. absorbing) to ultraviolet (<200 nm), water molecules, combined with high density of molecules near- to -mid infrared (~1 um – 10 um) and far-infrared due to their small size, produces strong cohesion between (~10 um – 1 mm) and microwave (~1 mm – 15 cm) molecules that is responsible for water’s liquid nature at radiation (Figure 7.2.4). Water’s properties of standard pressure (1 atm) and ambient temperatures 0° to transparency and opacity are exploited via microwave 100° C. ovens, heat exchangers and in remote sensing applications. It also permits photosynthesis and oxygen The combined effects of cohesive hydrogen bonding and generation by plants in aquatic environments. polarity impart special properties to water, for example: Water’s high heat of fusion (6.02 kJ/mol @ 0° C, the Capillarity, from the combined effects of adhesive and melting point) is exploited for frost protection via internal cohesive forces, the last contributing to the overhead sprinkler irrigation, particularly fruit crops. This “surface tension” of water molecules. Capillarity works for two reasons: (1) when water changes from explains the behavior of water retention in soil, and in liquid to solid ice, heat of fusion is released to the air it the conducting (xylem) tissue in plants. Actively growing comes into contact with at a rate of 335.2 J/g of water; land plants maintain a continuous flow of water and (2) heat of fusion can maintain temperature at or between the soil and roots, throughout their conductive very close to 0° C, i.e.