Energy and Water Balance Determination in An Advective Environment: From Direct Measurement to Microclimate Based Estimation.

The contributions of John Monteith are perhaps nowhere more widely acknowledged than in the community of scientists and engineers concerned with estimation of evapotranspiration. His addition of surface energy balance flux resistance formulations to the evaporation estimation formula of Penman presaged an explosion of scientific and engineering exploration implementing the resulting so-called Penman-Monteith formulation in estimation of evapotranspiration (ET) from reference crop surfaces, development of crop coefficient functions for estimation of crop ET for irrigation scheduling, estimation of canopy surface temperatures and many other fields of enquiry. In 2008, a team of USDA and university researchers assembled to study the surface energy balance and associated energy and water fluxes within, below and above the internal boundary layers over irrigated and dryland cotton crops in the hot, windy and advective environment of the Southern High Plains of the Texas Panhandle at Bushland. Subsurface measurements of soil water storage accomplished using both weighing lysimeters and networks of deep neutron probe readings were used with measurements of irrigation applications and precipitation to determine the ET based on mass balance, which was then compared with ET estimates derived from eddy covariance (EC) and surface energy balance models. Gaps in knowledge and limitations of all methods were evidenced and accentuated due to the severely advective conditions and the spatial variability of ET, which was related to plant cover and water balance variations in space. The EC methods were shown to consistently under estimate ET in this environment, which likely routinely violated assumptions of equilibrium conditions. Weighing lysimeters were shown to be not routinely representative of the surrounding fields. We will share lessons learned and avenues of enquiry elucidated by this multi-institutional study.