Ecosystem water balance
Keeling plots may also be used to determine the isotope ratios of D/H or 18O/16O in evaporated and transpired water. The flux of water vapor from ecosystems to atmosphere contains both evaporation from the soil and transpiration from plants. These fluxes are important parts of the global hydrological cycle that affect both climate and water quality and availability. Because environmental conditions can affect each source of water vapor differently, ecologists and meteorologists wish to estimate the components of evapotranspiration separately. Isotopes may be one method of doing this. As long as the isotopic signature of evaporation and transpiration are significantly different, it is possible to partition evapotranspiration into its component fluxes.
Leaf water is isotopically enriched by evaporation according to equation 2.9. This calculation assumes conditions of steady state, that is, water entering and leaving the leaf have the same isotopic composition, equivalent to the isotopic composition of water at rooting depth. While this may not be true over short time periods, it must be true over long periods or plants would continually accumulate heavy isotopes over time. In contrast, evaporation from soils takes place only at the surface. Because of evaporative enrichment, soil water is far "heavier" in the first 5cm or so of soil, and rapidly becomes lighter, or more negative, at depth. Plants extract their water from these lighter regions, while evaporation takes place in the heavier surface layers. Therefore we should be able to distinguish between evaporation and transpiration isotopically. In reality, this is difficult to accomplish in ecosystems that are dominated by transpiration such as forests, but it has been successful where evaporation plays a greater role in evapotranspiration, such as grasslands and agricultural fields.
Water sources of plants
Another interesting ecological question is where plants obtain water for transpiration in a given ecosystem. Different plants can have vastly different rooting depths depending on the species, the soil depth, and the environmental conditions. Some plants access the groundwater table, particularly riparian species - species that grow near rivers and streams. We may wish to know if and when a plant uses groundwater versus moisture in the upper soil layers. For example, a species that depends purely on streamwater may be adversely affected by the changes in streamflow caused by a dam. An understanding of the dynamics of water uptake in various plant species can help us predict how changes in climate and environmental conditions will affect ecosystems.