Seasonal energy and evapotranspiration partitioning in a desert vineyard

Authors: KOOL, D. - Collaborator; Kustas, William - Bill; BEN-GAL, A. - Israel Agricultural Research Organization (ARO); LAZAROVITCH, N. - Collaborator; HEITMAN, J. - North Carolina State University; Sauer, Thomas; AGAM, N. - Collaborator

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2016
Publication Date: 1/30/2016
Citation: Kool, D., Kustas, W.P., Ben-Gal, A., Lazarovitch, N., Heitman, J., Sauer, T.J., Agam, N. 2016. Seasonal energy and evapotranspiration partitioning in a desert vineyard. Agricultural and Forest Meteorology. 218–219 (2016) 277–287.

Interpretive Summary: Partitioning of energy and water in vegetated systems determine productive use of water through plant transpiration (T), which is relevant for food production, ecosystem functioning, and climate. In arid areas, vegetation is generally sparse and evapotranspiration (ET) tends to dominate the water balance, with potentially large water losses due to evaporation from the soil. For many agricultural row crops determining E and T is critical for assessing water use efficiency, particularly grape vines which are the world's most economically important horticultural crop and are often grown in arid regions. There is therefore a need to study energy partitioning at a seasonal scale, taking into account variability across the soil surface as well as between the surface and the canopy. The objective of this research was to study the effects of canopy growth, water status, and changes in atmospheric conditions on energy partitioning, as well as assessing the productive and unproductive allocation of water through ET partitioning.The below canopy energy balance approach based on below and above canopy measurements of radiation and temperatures used in this study allowed continuous assessment of E throughout the season. Seasonal ET partitioning indicated total E amounted to 7-8% of ET. Crop coefficient methods had significant day-to-day variability and therefore was not a suitable technique for monitoring daily E and T loses. The energy balance approach required a modification to the wind factor to reproduce estimates in agreement with below canopy E measurements. This measurement technique will improve models estimating E and T contributions from row crops, vineyards and orchards having a significant area of bare soil in arid regions. Results will be useful to irrigation and water resource managers for improving water use efficiency in water limited environments.

Technical Abstract: The challenge of partitioning energy and evapotranspiration (ET) components was addressed over a season (bud break till harvest) in a wine grape vineyard located in an extreme arid region. A below canopy energy balance approach was applied to continuously estimate evaporation from the soil (E) while system ET was measured using eddy covariance. Below canopy energy balance was assessed at the dry midrow position as well as the wet irrigated position directly underneath the vine row, with E calculated as the residual of measured net radiation, soil heat flux, and computed sensible heat flux. The variables used to compute sensible heat flux included soil surface temperature measured using infrared thermometers and below-canopy wind speed in a soil resistance formulation which required a modified wind factor. The E derived from below canopy energy balance was reasonable at daily intervals although it underestimated micro-lysimeter E measurements by as much as 20% suggesting there may have been advected energy from the midrow to the below-vine position. Seasonal partitioning indicated that total E amounted to 7-8% of ET. In addition, empirical functions from the literature, relating plant size to crop coefficients (Kcb), appeared to give reasonable results under full canopy but underestimated Kcb during the growing period.