Plants play an important role in moving water across the land surface between the atmosphere above and the soil below. Some plants can extend their roots substantially below the surface to take advantage of ground water, giving them a moisture reservoir that persists through dry seasons and droughts. When plants tap into this reservoir they transpire moisture through their leaves, providing a source of moisture to the atmosphere at a time when the air may be at its driest. The extent to which this transpired groundwater influences meteorological conditions such as precipitation, cloudiness, and atmospheric stability is not known, nor is its dependence on region, season, and other factors.
The transpiration of groundwater involves a complex set of biological and physical processes which are difficult to observe and simulate. But the PIs have developed a scheme in which the bulk effect of these processes can be approximated using two observationally-motivated assumptions. First, the extent to which plants extend their roots to tap groundwater depends on their position relative to the local topography. In dry or seasonally dry climates plants on a hilltop are typically too high above the water table to effectively access groundwater, so we can assume that they rely exclusively on near-surface soil moisture. At the valley floor the water table can be so close to the surface that plant roots have to be shallow to avoid excessive salinity and waterlogging, so they also rely exclusively on near-surface moisture. Thus maximum groundwater uptake occurs at mid-hillslope locations, and groundwater usage depends on the Height Above Nearest Drainage (HAND). The PIs have developed a "giant hillslope" method to quantify this dependence in terms of a five-bin representation of small-scale HAND topography.
Second, roots respond dynamically to the vertical profile of soil water. The PIs argue that root dynamics can be simply represented by assuming that roots actively extend to reach available groundwater, taking up water from whatever level offers the greatest moisture access for the least effort. This assumption is formalized using a scheme in which the transport of moisture through roots is analogous to the movement of electric current in a circuit: the roots act as "wires", through which a "current" of moisture flows from a specific soil layer to the plant leaves, driven by the "voltage" difference (i.e. water potential difference) between plant leaves and the soil layer tapped by the roots. The flow of moisture from a soil layer to the surface is then given by the ratio of the layer-to-leaves voltage drop to the resistance of the wire, in exact analogy to Ohm's law (electric current equals voltage divided by resistance).
The PIs implement their root-groundwater scheme in the Noah land-surface model, which is coupled to the Weather Research and Forecasting (WRF) model to form a coupled land-atmosphere model. The model is then used to test the impact of groundwater transpiration on the continental-scale hydrological cycle. Among the scientific questions to be addressed is the extent to which groundwater transpiration promotes precipitation, both by making a substantial contribution to the moisture available for precipitation, and by reducing atmospheric stability.
The research has societal value due to the importance of the hydrological cycle for water resources. The work is of particular value for building bridges between the research communities concerned with the separate but closely connected fields of land surface hydrology, continental-scale hydroclimate, and plant ecology. The implementation of the new scheme in the WRF model will make it available for operational use, as WRF is widely used for weather forecasting. The PIs also conduct educational and outreach activities in K-12 schools, and the project supports two graduate students.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
