Utility of Thermal-based Two-Source Energy Balance Model for Estimating Vineyard Evapotranspiration at Field and Regional Scales

The prolonged drought in much of California over the last few years, particularly in the Central Valley region, has caused severe reduction in water reservoir levels and a major depletion of ground water by agriculture. Dramatic improvements in water and irrigation management practices are critical for sustainable agriculture in this region. This, in turn, requires the development of tools and technologies for monitoring water use and at both the field and regional scale. California growers devote significant acreage to the cultivation of orchard crops and vineyards, at over 3 million acres, with much of it irrigated. These crop types share a unique canopy structure and row spacing. The architecture of wine-grape vineyards is characterized by widely spaced rows (∼3 m) and tall plants (∼2 m) with most of the biomass concentrated in the upper one-third to one-half of the plant. This wide row spacing and canopy architecture facilitates sunlight interception, air flow, and field operations and results in two distinct management zones: the vines and the inter-row. Often, the treatment of these two management zones is further complicated by a cover crop grown in the inter-row. A remote sensing-based land surface model that captures the micro and macro-scale exchanges between the vine, inter-row and atmospheric boundary layer is needed to operationally monitor vineyard water use and both vine and inter-row plant stress. A remote-sensing-based modeling system has been developed with these capabilities and is called the Atmospheric Land EXchange Inverse (ALEXI) model. Additionally, a Disaggregation module (DisALEXI) using high resolution thermal remote sensing data has also been developed.  The ALEXI/DisALEXI land surface scheme is based on the Two-Source Energy Balance (TSEB) formulation that addresses the key factors affecting the convective and radiative exchange within the soil/substrate–plant canopy–atmosphere system. An experiment being conducted at vineyard sites in California since 2013 called GRAPEX (Grape Remote sensing Atmospheric Profiling and Evapotranspiration eXperiment) has involved the collection of ground validation data under a full range of environmental conditions and vine phenological stages. These data included in-situ measurements of the water and energy fluxes, meteorological conditions, soil moisture, and biophysical properties along with high resolution airborne imagery for determining inter-row and vine cover fractions and thermal temperatures. A description of the field experiment and initial results applying both the TSEB with local/airborne remote sensing data and ALEXI/DisALEXI with satellite observations will be presented.  The capability of the TSEB scheme to quantify vine plant and inter-row water and energy fluxes for the unique vineyard architecture will also be discussed.