Integration of Remote Reference Surface Meteorological Data with Field-Specific Observations for Improved Parameterization of Actual Evapotranspiration.

Accurate parameterization of reference evapotranspiration (ET₀) is necessary for optimizing irrigation scheduling and avoiding costs associated with over-irrigation (water expense, loss of water productivity, energy costs, pollution) or with under-irrigation (crop stress and suboptimal yields or quality).  ET₀ is often estimated using meteorological data gathered over a reference surface, usually short grass.  However, the density of suitable ET₀ stations is often low relative to the microclimatic variability of many arid and semi-arid regions, leading to a different microclimate at an individual farm/field and potentially inaccurate ET₀.  In this study we investigated 11 ET₀ products from five reference meteorological stations, a non-reference meteorological station located on field with multiple reference ET equations, a satellite ET0 product, and integration (merger) of the on farm data with the  closest ET0 station.  We intercompared the ET₀ products and evaluated the ET₀ against lysimetric ET observations from two lysimeter systems (weighing and volumetric) and two crops (wine grapes and Jerusalem artichoke) in Southern California.  The results show a major difference (50% or ~0.9 m s^-1) in wind speed despite the short distance between the stations (<3 km) and a systematically lower ET₀ when the field-specific wind speed is incorporated.  The merged reference ET had the best stability of crop coefficients for the wine grape field and had the closest agreement with parameterized Kc (highest r2=0.91 and 2^nd lowest RMSE =0.17).  The results indicate the potential importance of on farm/field meteorological sensors for ET₀ parameterization; particularly in variable microclimates and/or irrigation water is expensive.